Calculating max electrical load of your home

[TomCat96]

I'm thinking of upgrading my home electrical service, but before I do I want to be able to calculate the max load my current service can handle. In particular I'm trying to figure out as a practical matter what my home can and cannot handle.

There's a general rule that says you don't want to run more than 75% of the maximum load on a circuit for sustained periods. For example, for your average 15 Amp circuit, at 120V, you should run no more than 12Amps sustained on that circuit.

But is that really all there is? Does that mean if I have 10 15 amp circuits, I can run a sustained 12 amps on each of them, or are there other bottlenecks typically present in a house.

[MilleniumBuc]

Really don't think these type of questions should be answered by anonymous board members, vs a licensed electrician or electrical engineer that will take responsibility for sizing the system. To many things can go wrong from rule of thumb advice, including loss of life.

[Dude2]

The question confuses me. The service you buy from the electric company is the service delivered to the home, i.e. 100A or 200A, regardless of the sub-circuits in the home. This is the limit to the amount of amps your home can draw.

There's a general rule that says you don't want to run more than 75% of the maximum load on a circuit for sustained periods.

A quick Googling yielded the following (http://blog.schneider-electric.com/data ... -breakers/):

In the circuit breaker world there’s been some misunderstanding about the terms “100%-rated” and “80%-rated” circuit breakers. To dispel the confusion, Schneider Electric’s Mohamed Shishani put together a short (less than 10 minutes) podcast that does a nice job explaining the issue.

Understanding the difference between the two begins with a reading of the 2011 National Electric Code. Section 210.20(A) of the code basically says that a circuit breaker for a branch circuit must be rated such that it can handle the noncontinuous load plus 125% of the continuous load. (A continous load is one where the maximum current is expected to continue for 3 hours or more.) In other words, the breaker needs an extra 25% capacity of the continuous load for headroom. That, of course, means you need a larger, more expensive breaker.

There is, however, an exception. When the circuit breaker is listed for operation at 100% of its rating, the additional 25% requirement goes away. Instead, the device simply has to be able to handle the sum of the continuous load and the noncontinuous load.

[dm200]

I'm thinking of upgrading my home electrical service, but before I do I want to be able to calculate the max load my current service can handle. In particular I'm trying to figure out as a practical matter what my home can and cannot handle.
There's a general rule that says you don't want to run more than 75% of the maximum load on a circuit for sustained periods. For example, for your average 15 Amp circuit, at 120V, you should run no more than 12Amps sustained on that circuit.
But is that really all there is? Does that mean if I have 10 15 amp circuits, I can run a sustained 12 amps on each of them, or are there other bottlenecks typically present in a house.

I would get estimates and detailed recommendations (and answers to these questions) from several electricians.

[Sandtrap]

Most "modern" residences, except for McMansions, etc, have a 200 amp "service". Regardless of the breaker panel and what's in it.
When you mention "upgrading your "service"" this is one thing. When you address the breaker panel, that's not quite the same thing though both are related.
Every home will have the breaker panel configured to match the "service" provided. IE: 200 amp. when it was built. If you are adding a large machine shop addition with power sapping welding equipment and so forth, or installing large amperage service for several electric vehicles, and so forth, then you would maybe be calling an electrician for potentially installing subpanels and perhaps upgrading the "service" if needed.

As a previous poster has said, this might be something that you need to call a qualified licensed electrician to your home to go over. Every home is unique and demands are unique. You need as professional. This is one area where DIY is fatal.

And, absolutely never swap out a breaker of a certain amperage rating for a higher one. IE: 15 amp breaker for a 20 amp breaker. Never. Ever. Never.

(disclaimer)
Not an electrician. Just a retired, General Contractor, Home Builder, High rise builder, R/E developer.

[Jack FFR1846]

As an EE of over 30 years with a graduate degree and design experience, I understand what you're asking and could do the calculations. If this were for my house, I would ask my neighbor, who is a licensed electrician.

I don't know what I don't know in this case. The electrician will know, even if he knows no theory what the code requires and what your house currently is going to require.

[dm200]

Our house was built around 1940 and had a fuse box and 60 amp service, INCLUDING 30 amps for an electric stove! Everything "worked", but shortly after we bought the house in the late 1970's, we got bids to do a number of things and upgrade the electrical service, including a new box with circuit breakers. We were supposed to get a 150 amp box/service, but they made an installation error and put in 200 amps - and we paid the bid price.

[CaliJim]

I'm thinking of upgrading my home electrical service, but before I do I want to be able to calculate the max load my current service can handle. In particular I'm trying to figure out as a practical matter what my home can and cannot handle.

There's a general rule that says you don't want to run more than 75% of the maximum load on a circuit for sustained periods. For example, for your average 15 Amp circuit, at 120V, you should run no more than 12Amps sustained on that circuit.

But is that really all there is? Does that mean if I have 10 15 amp circuits, I can run a sustained 12 amps on each of them, or are there other bottlenecks typically present in a house.

1) If you have one of those fancy new digital lcd display utility meters on your house, one of the displays may tell you how many watts your are currently drawing at that instant, and at what voltage. If so, turn on some things, look at the meter, record the watts, calculate the amps: Watts / Volts = Amps.

2) Make a spreadsheet. In Column A list the appliance. In Column B estimate the watts. In column C list the voltage (120/240). In column D calc Amps by dividing Watts(b) by Volts (c). Sum up column D

(Use google to find typical wattage range for appliances if you want to estimate: ie google "TV WATTS TYPICAL". or https://www.frequencycast.co.uk/howmanywatts.html

Code: Select all

Light Bulbs (qty:40)      2400     120    0.5
TV...................      200     120    1.67
Dishwasher..........      1800     120    15.0  
Washing Machine...         700     120     5.8
Fridge...............       35     120     0.3
Water Heater........      4500     240   18.75
PC 
Printer
Modem
Hair Dryer
Bath Fan
Oven
Stove
Table Saw
etc

-------
Other comments:

I have a 225 amp panel. It has 40 slots for circuit breakers. If all those slots were filled with 15 amp breakers - that would be 600 amps! You can't just count breakers and multiply by some amp estimate. The panel rating is max input current, not max current draw if all circuits were drawing at 12 or 15 or whatever amps.

As people switch to CFL and LED lights, and energy star appliances... typical home electrical loads are decreasing.

For example, My house draws < 2800 watts in the evening, with led lights on, TV on, the microwave on, and gas water heater/radiant heat pumps system running, in other words... not too much really. Even at double that - it would be only 40 amps.

If you have a 60 amp panel now, a 100 amp panel should be more than adequate for future needs, unless you are going to start a grow operation in your basement.

[indexfundfan]

Question for OP: why do you think you need an upgrade? Is it because the main breaker is tripping frequently? Or are you planning on adding some new circuits, e.g. to charge an electric car? The above are two possible reasons why you need an upgrade.

[Cloud]

I'm thinking of upgrading my home electrical service, but before I do I want to be able to calculate the max load my current service can handle. In particular I'm trying to figure out as a practical matter what my home can and cannot handle.

There's a general rule that says you don't want to run more than 75% of the maximum load on a circuit for sustained periods. For example, for your average 15 Amp circuit, at 120V, you should run no more than 12Amps sustained on that circuit.

But is that really all there is? Does that mean if I have 10 15 amp circuits, I can run a sustained 12 amps on each of them, or are there other bottlenecks typically present in a house.

You already most likely have either 100, 150, or 200Amp service. This is determined by the size and length of wire from the pole to the panel. The main breaker inside your panel should have printed on it how many amps it's rated for and what your current service can handle....

It's pointless to count breakers or amps on each leg since you'll never have them all on at once. I have a 200 AMP 40 breaker panel. So that's 20 breakers per leg. I believe A 200 amp single phase 120/240 v service can provide 200 amps at 240 volts, or it can also provide 400 amps at 120 volts.

To answer your question, yes you can run 10 120V 15 amp circuits at a sustained 12 amps each for a total of 120 AMPS if you have a large enough service feed. There are no other bottlenecks.

[Big Dog]

Most residences, except for McMansions, etc, have a 200 amp "service".

Maybe most built in the last xx years or so, but our tract home build in '77, definitely does not have 200 amp service.

(The only reason that i know is that I've just started the research to see what I need to do to add a 50 amp line in the garage for 40 amp (continuous) charge for an electric car.)

[Sandtrap]

Most residences, except for McMansions, etc, have a 200 amp "service".

Maybe most built in the last xx years or so, but our tract home build in '77, definitely does not have 200 amp service.

(The only reason that i know is that I've just started the research to see what I need to do to add a 50 amp line in the garage for 40 amp (continuous) charge for an electric car.)

True. Thanks "BigDog". I corrected my post. Older smaller homes did indeed have smaller services until the code became more standardized. And even older homes like the one I grew up in had glass fuses and the old "knob and tube" wiring.
I've also did research on how much power I might have available to put in a shop building. Found out my home is maxed out. 3 stories, separate central air/heat on each floor, etc, etc. I have to upgrade to a 300 amp service or another 200 amp service $$$$$$$ so at this point I'm not sure if I really want a shop building.
Does an electric car need 220v/50 amp circuit?

[indexfundfan]

Does an electric car need 220v/50 amp circuit?

No. It depends on the EVSE equipment you want to put in.

My car does not charge any faster than 16A @240V. The EVSE I bought maxes out at 16A @240V. So I only put in a 20A circuit with #12 wire, terminating with a 6-20R receptacle.

If you have a Tesla which can benefit from a higher current, it is a different story.

[CaliJim]

The max current you can get in is limited by the main circuit breaker at your service entrance, which is sized in conjunction with the length and size of your feed from the power pole, as mentioned earlier.

Your electrical distribution panels my be rated higher. Oversizing distribution is fine. Nice and safe. Nothing will heat up and emit the magic blue smoke.

If you turn on too many things, but not enough to trip an individual 15/20/30 amp breaker, and as a result draw more than the main breaker will allow, then the main breaker will trip.

You can't draw more than your main breaker will allow. Even if your distribution system can handle it, the mains might not.

Make a spreadsheet. Estimate how much power you need. Then talk to a qualified electrician.

[pshonore]

Also, some devices require larger amounts current to start. I believe most capacitor motors are like that , so think refrigerator, AC Compressor, well pump, etc. They may draw 50% more than the rated current for a very short time (seconds). Of course the system is designed to handle that. That can also be important when sizing a generator.

[dbr]

The National Electrical Code lays out the methodology for this calculation. There is software available online to help with this.

I recommend consulting an electrician for help. In any case actually installing any new equipment will need to be done by a licensed electrician with building permits and inspection for compliance with building code.

[CaliJim]

TomCat,

You still there? Where you at with all this

[LadyGeek]

This thread is now in the Personal Consumer Issues forum (home).

[whomever]

actually installing any new equipment will need to be done by a licensed electrician with building permits and inspection for compliance with building code

The 'by a licensed electrician' part is locale dependent; in my state homeowners can do their own electrical work (you still need the permits & inspection).

(that said, a main panel isn't the job to learn on, and the OP didn't sound like he had the background to dive into a project of that complexity. I mention that DIY can be legal only because it isn't in some places; I've had out of towners amazed that it's legal)

[RetiredAL]

Tomcat,

If YOU want to do your own rough estimate of your current panel's capacity, do a Google search for NEC Load Calculation Worksheet .

Unless you have added circuits yourself without permits, your main panel is likely adequate, as reserve capacity would likely been included when it was installed . The permit people will required a Load Calc Sheet to be presented before they'll issue a permit, even when a professional is doing the work.

Your quoted 75% rule is between "somewhat to largely" erroneous, as shown if you do a Calc Sheet. What part of the 75% that is roughly correct is related to the load on an individual std 120V wall outlets, which are only rated for 15 amps even though they are on a 20 amp circuit. Thus small appliances using std plugs are limited to approx 1750 watts or less. The 75% has nothing to do with aggregating loads for service requirement.

Major appliances are calc'd on a different basis.

As others have said, if you are going to add/change any electrical equipment, get a professional to do it. I offer the above info only for you basic understanding, not as a professional service.

[iamlucky13]

Question for OP: why do you think you need an upgrade? Is it because the main breaker is tripping frequently? Or are you planning on adding some new circuits, e.g. to charge an electric car? The above are two possible reasons why you need an upgrade.

I had the same question. I don't see where this conversation is going without clarity from the OP about what he is trying to accomplish.

[TomCat96]

Thank you everyone for all the replies. (especially calijim)

I got my question answered, which was essentially an inartful way of asking "what don't I know?"
I did not intend to use the advice here to go do electrical work on my own, but rather to get some general knowledge which would allow me to explore more.

I'm essentially looking to start my own business soon (no it's not a grow operation), and wanted to know how much power I can draw, and what my options are for expanding beyond my home. It looks like I might go with upgrading my electrical service and the main breaker which would require the work of an electrician. I'm trying to keep costs down so at this stage I just want to know what options are available.

However I have found another option which I do not fully understand. Others have told me that it may be possible to install 240 volt sockets, up from the 120 volts if I need power by combining two single phase 120v lines. I was told that the limitations on the wire heating up are a function of current, not so much voltage. If that's the case, then theoretically a 240V socket would be able to provide double the power of a standard 120v, 15A socket. P = IV.

Again, that's the theory. But I don't know what I don't know. I was wondering if anyone really knows if it possible to draw 240v at half the amperage to draw the same amount of power for one's applications.

[whomever]

Others have told me that it may be possible to install 240 volt sockets, up from the 120 volts if I need power by combining two single phase 120v lines.

Ummmm... there is 'possible according to the laws of physics' and 'legal and practical'. Starting to wire between circuits to get 240V is possible according to the prior, but definitely not legal or usually even practical.

I think you need to describe what you are actually trying to do. Right now we don't know if you want to wire a clandestine grow op, a woodworking shop, a welding shop, etc. I dunno about grow ops, but the others have different requirements.

[dbr]

Thank you everyone for all the replies. (especially calijim)


Again, that's the theory. But I don't know what I don't know. I was wondering if anyone really knows if it possible to draw 240v at half the amperage to draw the same amount of power for one's applications.

Sure. Physics says that half the current at twice the potential is the same power. However, you don't just arbitrarily determine that by what sockets you install. You determine that by the electrical design of the equipment you are running.

PS It is true that you want to balance the use of both sides of the 240V center tapped power we have in our homes by using both legs of the input. Usually a breaker box uses one leg for half the 120V wiring and the other leg for the other half.

[mxs]

Short answer, yes because Watts / Volts = Amps. However, you will be using two breaker slots doing that, so you need to have room in your breaker box for that. Also, whatever you are using has to be able to handle 220/240 volts.

I had 100 Amp service with a 50 Amp sub panel in my house and we upgraded to one 40 breaker 200 Amp panel. I spoke to a local electrician company and agreed to pay them the time + permit fees if I got the materials myself. I ended up getting the weatherhead, schedule 80 PVC, mounts, a good length of double 00 wire, the panel and additional breakers, the meter box, and a few other things I am sure I am forgetting and I paid somewhere between 1/2 and 2/3 of the costs if they had provided all the materials.

[The Wizard]

...I was wondering if anyone really knows if it possible to draw 240v at half the amperage to draw the same amount of power for one's applications.

Yes, sure it's possible. But What are you Trying To Do?

Most heavy power draw items on the US use 220 volts for efficiency. You don't have much choice when installing a hot tub, for instance: you need to provide the specified electrical service...

[RetiredAL]

TomCat,

All houses built since WW2 have 120/240 3-wire 1-phase service. Two Hots and a Neutral. 120V is from either Hot to the Neutral. 240V is Hot to Hot. So, unless your house is ancient and never has been re-wired, your panel can provide either 120V or 240V. Yeah, years ago that was 110v/220V, but the utilities have upped that to today's 120/240 for distribution efficiency. Anything from 115 to 125 is generally classified as 120V.

All large consumers such as an Electric Stove, an AC, or an Electric Dryer use 240V. For motors, a Pool Pump may be 120v or 240V, and a Water Well will most likely will be 240v.

You said business, but are pretty vague. Depending on the needs for a business, your needs could be beyond what is house typical power. It could be 208v 3-phase 4 wire service which delivers 120V for outlets and 208V for large use appliances such a store size refrigeration. Beyond that, medium and heavier industrial could require 460V 3-phase.

At my previous company (I'm now retired) the utility provided us with 12KV 3-phase which we distributed to our own sub-stations across the facility for stepping down to other voltages, mostly 460V. We used a lot of power.

[IowaFarmBoy]

I'm a former electrician and most homes continuously use nowhere close to the rated capacity of their service. As someone pointed out, because of startup currents for motors in things like your refrigerator and ac unit, you need the extra capacity. We put in 200 amp services by default 25 years ago. If the house didn't have electric heat or an electric water heater, 150 would easily have been sufficient. If you are doing something unusual, the advice to get an electrician to calculate the load is a good idea.

Here's a fun calculation to give you a sense of how little electricity is used on average. Our house has a 200 amp service and our average bill is around $90/mth. I can calculate the max usage of a 200 amp service like this:

200 Amps x 240 volts = 48000 watts/hr or 48kw per hour at max capacity
48 kw/hr x 24 hours/day x 30 days/month = 34,560 kwh/month
34,560 kwh x $.10/kwh = $3456 for electricity/month ($.10 is probably low)
So we are using on average 90/3456 = 2.6% of our theoretical capacity.

I am not recommending anyone run at 200 amps continuous load but it gives a sense of how over sized most services are. We have gas heat, gas water heat and a gas dryer. Electric stove and central air.

[Theseus]

Does an electric car need 220v/50 amp circuit?

No. It depends on the EVSE equipment you want to put in.

My car does not charge any faster than 16A @240V. The EVSE I bought maxes out at 16A @240V. So I only put in a 20A circuit with #12 wire, terminating with a 6-20R receptacle.

If you have a Tesla which can benefit from a higher current, it is a different story.

My Tesla is on a 50 Amp - 240 V circuit. And when it is charged, Tesla draws maximum of 40Amps. It never goes above and I wondered why it was not drawing full current and charging faster . Thanks to this thread now I know why that may be.

[Sandtrap]

Thank you everyone for all the replies. (especially calijim)

I got my question answered, which was essentially an inartful way of asking "what don't I know?"
I did not intend to use the advice here to go do electrical work on my own, but rather to get some general knowledge which would allow me to explore more.

I'm essentially looking to start my own business soon (no it's not a grow operation), and wanted to know how much power I can draw, and what my options are for expanding beyond my home. It looks like I might go with upgrading my electrical service and the main breaker which would require the work of an electrician. I'm trying to keep costs down so at this stage I just want to know what options are available.

However I have found another option which I do not fully understand. Others have told me that it may be possible to install 240 volt sockets, up from the 120 volts if I need power by combining two single phase 120v lines. I was told that the limitations on the wire heating up are a function of current, not so much voltage. If that's the case, then theoretically a 240V socket would be able to provide double the power of a standard 120v, 15A socket. P = IV.

Again, that's the theory. But I don't know what I don't know. I was wondering if anyone really knows if it possible to draw 240v at half the amperage to draw the same amount of power for one's applications.

True, in theory.
But the majority of 220v use is a lot more than 15 amps. The size wiring in a 110 v 15 amp circuit will be designed for perhaps a lighting circuit which is most likely far far less than the use that you intend. Keep it safe. Call a pro.

[ccieemeritus]

You are being vague about why you need so much power. Keep in mind that most power use will eventually result in heat.

If you actually use 20 amps of 240V power that will result in 4800 watts of heat generated.

In modern data centers it’s easy to upgrade the power to add more servers. The hard part is getting rid of the heat before the servers melt.

So if you are (random legal guess) bitcoin mining, there’s a limit to the power consumption of servers that can be deployed in an “air cooled” environment.

[lazydavid]

We were supposed to get a 150 amp box/service, but they made an installation error and put in 200 amps - and we paid the bid price.

The only real difference between 150 and 200 amp service from an installation perspective is the main breaker, which often costs the same, but might be a couple bucks more for the larger one. Cabling coming in from the street is likely the same gauge. What I mean by that is, the main feed for your 60A circuit was almost definitely too small, and what the electrician would have replaced it with to put in a 150A service is likely the very same cable they did use for the 200A you actually got.

[dbr]

I hope people who didn't know are fully aware that the current drawn is a function of what it is that is plugged in and not a function of what the service "feeds." That remains true up to the point where a breaker trips or the wiring melts and parts or some switch or socket burns out.

That is why when you equip your home with everything from lights, to appliances, to shop equipment that you look at the current ratings on the devices rather than the current ratings on the circuits in understand what is going to be drawn. You look at the current ratings on the circuits to determine if you have an adequate and safe capability to supply the devices you plug in.

[iamlucky13]

Again, that's the theory. But I don't know what I don't know. I was wondering if anyone really knows if it possible to draw 240v at half the amperage to draw the same amount of power for one's applications.

Whether or not you need a 240V circuit depends on whether you need a 240V appliance. You don't just double up a pair of 120V outlets from opposite legs and plug a 120V appliance into them hoping to be able to plug twice as many such appliances in. In most cases, bad things happen at that point.

Figure out how much power your business equipment will need, and at what voltages. Then see what the main breaker in your panel is rated for, and how much the other large loads in you house are rated for, and whether they might run simultaneously.

Also look at what outlets are available in the space you want to operate the business from, and whether they are connected to different breakers. If you need 240V, you will probably need to have new wiring installed, as most houses only have 240V outlets in the laundry room and where the stove goes in the kitchen, plus a few hard-wired appliances like the furnace. You probably won't need to upgrade your service, but I don't know what you're actually running or what your current service is.

If you just need 120V, odds are good you won't need to upgrade your service, but you might need another circuit or two added to the space you're going to run the business if you've got a lot of power hungry stuff to plug in.

[MathWizard]

When I bought my home, I upgraded from 60 amp to 200 amp. Stove, dryer and AC are all bug electrical loads, but
I have never had a problem. Unless I started putting in a Tesla 220V charging station or some arc welders, I can't see
needing more.

My electrical use is actually going down, since I installed CFL and LED light, highly efficient LED flat panel
TV's, use laptops/smartphones rather than desktop, and generally by high efficiency appliances.

[snackdog]

Is there a way to calculate the max (or average or total) amperage used by looking at my meter or bill?

[twh]

OP....Google "article 220 sizing worksheet". Fill that out and it will tell you what you should have.

[suemarkp]

The NEC article 220 calculation can lead you astray if you are in a residence. Depending on the nature of your business and where you put it, it could be too high or too low. Here are some examples:

General use power receptacles are determined by assuming 3 watts per square foot of space. This does not count the garage. If you business uses the garage space and it is all just normal cord-and-plug connected, you may no have enough power.

At the opposite end, lets say you are putting a few racks of computer servers in the room so you are just going to treat that rack as an appliance. Computer nameplates are usually sized for a worst case computer (full of RAM, max hard drives, and maybe a beefy graphics card). Choosing to upsize the power supply for more reliability gives you a larger nameplate even though the computer load is the same. Take that oversize and multiply by 5 or 10 servers, and you can get a large error on the too big side. And then don't forget the additional cooling that a server rack may require.

Adding some large things, like an electric instant water heater or two level II car chargers could be too small using the NEC calcs. Once you have 10KW of load, and just about any house is going to have that, additional items are added with a 0.4 multiplier to their nameplate. So a 120A water heater would go in at 48 amps. Two 50A car chargers would go in at 40 amps to the calculation.

So yes, the OP should do a load calc with what they have now, and do a second one with what they are adding. Compare it to what you have. If the calc comes in at 150A and you have a 200A service, you are probably fine. If it is 198A on a 200A service and you have some some of those heavy loads that run a while, you may have issues. If it comes to 205A on a 200A service and you have a bunch of intermittent use stuff, you will probably be fine but not NEC legal.

[firebirdparts]

Is there a way to calculate the max (or average or total) amperage used by looking at my meter or bill?

The bill is the total and obviously also the average, but you really need the max for the thinking in the original thread. That's pretty hard.

I think it would fun to get a data logging ammeter and measure it (you can buy these for people that want to figure out where their money is going), but i wouldn't like to size a service based on that.

If I was going to size a service, I would probably assume all the water heaters were on simultaneously and the dryer and the oven, and any electric furnaces. The dryer and the oven are under human control, but there's no reason you'd avoid using the oven when the dryer is on. we have a hot tub, so that's essentially a third water heater. The car charger, if you have a nice one, is going to be even bigger than the dryer.

It's harder to know what to do with the 120 volt stuff. the microwave uses a lot of juice, as does a hairdryer, and space heaters if you happen to have some of those spawns of hell around. The refrigerator is on a lot. I would probably just use a made up number for miscellaneous other users and lump everything 120v into that.

The connected loads at my house far exceed 200 amps, and I think 75% of the breakers, as suggested above, would also far exceed 200 amps.

[suemarkp]

Is there a way to calculate the max (or average or total) amperage used by looking at my meter or bill?

Sometimes. Your bill with have a total KW-Hr used by month and may even show a kw-hr average per day. But those numbers are useless for sizing equipment because it is spread over too much time. Commercial customers usually have a demand factor added to their billing. This is the highest average KW used over a 15 minute window during the billing period. The size of that can change their billing tier and the demand value is listed on their bill. If the peak demand was 15KW and they have a 240V single phase service, the amps would be 15000/240 = 62.5. If it was 208V 3 phase, the amps would be 15000/208/1.732 = 41.6.

Residential customers are typically not billed for demand so it won't be listed on the bill. The power company may be able to provide the number, since they calculate it for others and the meter may automatically do it. It just has to be extracted. Call the utility and ask if they can provide the 15 minute demand value for you.

Demand is usually in KW, not KW-HR. If it is in KW-HR, you need to multiply it by 4 because it was collected over a 15 minute interval and not 60.

The NEC has provisions to do a load calculation based on this demand value if you have 1 year of history and look at a period of 30 days. It is more realistic since it is based on actual use. But it only works for adding on, since a new service will have no history behind it. See NEC 220.87 for the details.

Finally, my power meter has a wireless link to a Rainforest EMU-2 energy monitor. It shows me how much power I'm using and it updates about every 15 seconds. If you sit and watch it during high use periods, this could be useful but not NEC legal, especially if you have some high usage equipment that you can run for 10 minutes or so and watch the screen. It will do a daily total KW-Hr, but that is no where near fine enough to get a peak usage over 15 minutes. It does draw usage bar graphs with one bar per hour, so that is closer, but you can't really read the graph that accurately as there is no scale. You may want to check with your utility to see if they have some sort of device like this you can buy. I had to pay $30 to order it, but they worked with my utility to pre-register it to my meter so it worked out of the box when I got it.

[snackdog]

Is there a way to calculate the max (or average or total) amperage used by looking at my meter or bill?

Sometimes. Your bill with have a total KW-Hr used by month and may even show a kw-hr average per day. But those numbers are useless for sizing equipment because it is spread over too much time. Commercial customers usually have a demand factor added to their billing. This is the highest average KW used over a 15 minute window during the billing period. The size of that can change their billing tier and the demand value is listed on their bill. If the peak demand was 15KW and they have a 240V single phase service, the amps would be 15000/240 = 62.5. If it was 208V 3 phase, the amps would be 15000/208/1.732 = 41.6.

Residential customers are typically not billed for demand so it won't be listed on the bill. The power company may be able to provide the number, since they calculate it for others and the meter may automatically do it. It just has to be extracted. Call the utility and ask if they can provide the 15 minute demand value for you.

Demand is usually in KW, not KW-HR. If it is in KW-HR, you need to multiply it by 4 because it was collected over a 15 minute interval and not 60.

The NEC has provisions to do a load calculation based on this demand value if you have 1 year of history and look at a period of 30 days. It is more realistic since it is based on actual use. But it only works for adding on, since a new service will have no history behind it. See NEC 220.87 for the details.

Finally, my power meter has a wireless link to a Rainforest EMU-2 energy monitor. It shows me how much power I'm using and it updates about every 15 seconds. If you sit and watch it during high use periods, this could be useful but not NEC legal, especially if you have some high usage equipment that you can run for 10 minutes or so and watch the screen. It will do a daily total KW-Hr, but that is no where near fine enough to get a peak usage over 15 minutes. It does draw usage bar graphs with one bar per hour, so that is closer, but you can't really read the graph that accurately as there is no scale. You may want to check with your utility to see if they have some sort of device like this you can buy. I had to pay $30 to order it, but they worked with my utility to pre-register it to my meter so it worked out of the box when I got it.

Thanks for the advice. I am on a commercial account (cheaper than residential!) and my online account shows peak demand (w) by the quarter hour.

Looking at the past year, peak usage was on a September evening when we hit 5500 W right about sunset (just AFTER our solar panels went to zero w). It was about 80F so AC was likely running, which is 240v. Well pump could have been on also which is also 240v. Water heaters are electric but hard to know what they were up to; nobody likely showering then. Hard to know split between 240v and 120v. A cooler day (50F) may have used about 1500 w so maybe I can assign that 120v and the rest to 240v. That gives:

1500 / 120 = 12.5A
4000 / 240 = 16.7A
12.5 + 16.7 = 29.2A peak usage.

If by some miracle it was all 120v, it would be 5500 / 120 = 46A.

Is this correct? If so, that is pretty modest amperage compared to our system (600A master, 200A on each for six sub-panels).

[suemarkp]

Solar wrecks the demand value because the meter is only measuring demand from the utility. Your panel could be supplying all the solar plus the power company demand to loads off the panel. So that number isnt going to help and you will need to do the calculation instead (from an NEC point of view as you need to monitor all day long).

A typical 4 ton AC unit would pull about 5kw by itself. Water heater is another 4400 watts typically. My baseline load of lights, computers, wall warts, etc is about 1500 watts. An electric range could be 5 to 8 kw for 15 minutes.

If you had no solar, you would most likely divide the demand watts by 240 volts. But you may have 208V. Your bill will probably tell you your supply voltage (240/120, 208/120). The NEC doesnt split things by phase when doing service load calcs. You just add all the watts, assume things are perfectly balanced, and divide by 240 or 208.

[snackdog]

Solar wrecks the demand value because the meter is only measuring demand from the utility. Your panel could be supplying all the solar plus the power company demand to loads off the panel. So that number isnt going to help and you will need to do the calculation instead (from an NEC point of view as you need to monitor all day long).

But it was after sunset and my solar production was 0w so I don't see how this applies.

A typical 4 ton AC unit would pull about 5kw by itself. Water heater is another 4400 watts typically. My baseline load of lights, computers, wall warts, etc is about 1500 watts. An electric range could be 5 to 8 kw for 15 minutes.

I have a 5 ton AC but I doubt it is pulling 5kw as I would have seen it. The peak shown really stands out.

If you had no solar, you would most likely divide the demand watts by 240 volts. But you may have 208V. Your bill will probably tell you your supply voltage (240/120, 208/120). The NEC doesnt split things by phase when doing service load calcs. You just add all the watts, assume things are perfectly balanced, and divide by 240 or 208.

I had "no solar" during the peak described. My service is actually 240v 3-phase delta with a high leg, FWIW. Does that affect the calculation?

[suemarkp]

For a 240 delta service, take the demand, divide by 240, and divide again by 1.732. Again, this assumes things are evenly balanced. If you know none of the pure 240v legs had any load, then dont divide by 1.732.

But you cant just take one snapshot in time and call that your load. You need to monitor 24/7 for a year to find that one high load period during summer or winter. And solar isnt being counted, so this number is still no good.

The NEC sizing calculation is mostly to size your panel. The utility has their own rules and generally could not support a dwelling if it actually drew what the calc says. It is especially important to size correctly when you have solar because multiple sources come together at your panel and combine.

Since you have a 600A service with multiple panels, you do multiple calculations. One for the overall load to see if 600A is enough. Then you do another calculation for each panel to make sure they are properly sized.

I think the current draw of an AC compressor can vary based on temperature. So if it was not real hot, it may have drawn less than what it was designed to do. If there is an MCA value on the AC compressor, its actual current draw would typically be about 80% of that. There should also be a fan in the house that runs too which would add more amps.

[ram]

This is a cold evening in the upper midwest.
My house and water is heated by natural gas. We cook with natural gas.
At this hour energy use is:
20 LED bulbs of 10 to 15 W each .........................= 300 Watt.
Lets say the compressors on both refrigerators are on = 80 W
One computer and one monitor is on ...................= 100 W
One TV/soundbar is on.....................................=120 W
Misc...........................................................= 400 W

Total .........................................................= 1000 W (1 kWatt)

The electric heaters in each bathroom are off. The clothes drier, dishwasher, electric mini oven, toaster, food processor, air frier, iron are all off.

I have used about 12 cents of electricity in the last one hour.

Everything that is on is on 120 V.

!000 Watt/120 Volt = 8.3 Amp

I am "currently" using only 4% of the 200 Amp service.

[Valuethinker]

As people switch to CFL and LED lights, and energy star appliances... typical home electrical loads are decreasing.

For example, My house draws < 2800 watts in the evening, with led lights on, TV on, the microwave on, and gas water heater/radiant heat pumps system running, in other words... not too much really. Even at double that - it would be only 40 amps.

If you have a 60 amp panel now, a 100 amp panel should be more than adequate for future needs, unless you are going to start a grow operation in your basement.

Although domestic electrical consumption has been falling, per household, in developed countries, this may be changing going forward. The "switch to LEDs" is more or less done (given they last 4x longer than conventional bulbs). Appliances turn over every 10-20 years say, and President Bush signed a major tightening in 2006.

There are big things happening:

- Heat Pumps as an alternative to gas heat
- induction hobs (stove tops) -- I am aware that this caused a lot of fuss (over nothing) earlier this year - to note it as a possibility, here
- EV chargers

I realise that Europe is quite a bit further ahead on this than USA-- most parts of the USA gas has been very cheap (much cheaper than electricity).

Nonetheless I think a degree of future proofing is appropriate. It's very likely that EVs will be the majority of new light vehicles sold in the 2030s.

So if one is not installing a Level 2 charger now, either one will, or the next owner of the house is likely to want to.

Again I am aware that the US may not duplicate the trends in other countries, the political and economic reasons as to why, etc.

But one can definitely see the things which will drive higher electricity use on average.

PS I use about 3000 kwhr pa (Victorian house, London England). But my kettle runs 3kw ! (220v AC). Amusing that I have an appliance with a draw bigger than most items in an American home (average consumption c 13,000 kwhr pa?).

[LadyGeek]

I haven't seen anyone mention surge current. The startup current for motors can sometimes be twice as high as the steady-state (running) mode of operation. Think of anything with a motor - well pumps, sump pumps, home heater fans, and refrigerators.

This is especially important when calculating the capacity of a home generator, but the same should apply here.

[snackdog]

I haven't seen anyone mention surge current. The startup current for motors can sometimes be twice as high as the steady-state (running) mode of operation. Think of anything with a motor - well pumps, sump pumps, home heater fans, and refrigerators.

This is especially important when calculating the capacity of a home generator, but the same should apply here.

Good reminder. I suppose in 100 years we will have smart homes that intelligently load balance. They would ensure the well pump didn’t fire up at the same moment as the refrigerator or the furnace.

[LadyGeek]

Exactly. Timing of when those motors start should be considered.

An important side note - You also have to size the circuit breaker accordingly. No, don't double the breaker size. Google "circuit breaker sizing inrush current" for a ton of info.

[suemarkp]

The NEC load calculation generally ignores starting surge, but the power company will care. When I added air conditioning, the power company wanted to know its locked rotor current (LRA). It was just over 100 amps (running current is around 20A). They size their transformer to be able to source that, so a 25KVA transformer was the minimum they could install to serve my house. Same issue happens with a generator -- it needs a surge capability big enough to handle that LRA.

The odds of multiple motors starting at exactly the same time is very small (unless they are all part of one integrated machine), and the starting surge is generally less than 1 second. So usually, you focus on the one largest motor (or compressor).

The starting surge is generally why an air conditioner that draws 20A running has a 30A or 40A circuit breaker. Circuit breakers for motor only (or compressor only) circuits can generally be sized at 250% of the motor running current.