Proper SSU fuse for gas furnace

Questions on how we spend our money and our time - consumer goods and services, home and vehicle, leisure and recreational activities
Post Reply
Topic Author
Californiastate
Posts: 1268
Joined: Thu Feb 04, 2021 11:52 am

Proper SSU fuse for gas furnace

Post by Californiastate »

We recently had a new HVAC system installed. It didn't pass city inspection due to the SSU fuse for the furnace. The maximum listed amps for the furnace is 12.8A. The current fuse is 20A and the inspector wants a 15A. The furnace installation manual lists a maximum 20A fuse. I'm not up to date on current NEC. I thought a continuous motor (furnace blower) is calculated at 125% of maximum amps for minimum overcurrent protection. Therefore the 12.8A would calc out as a minimum 16A fuse. I'm looking for a current specific NEC section detailing this calculation.
I know I can satisfy the inspector with a 15A and then satisfy the manufacturer and code with the 20A. I'd prefer to have the specific section to educate the inspector or myself.
suemarkp
Posts: 909
Joined: Sun Nov 12, 2017 8:18 pm
Location: Somewhere in WA State

Re: Proper SSU fuse for gas furnace

Post by suemarkp »

A furnace usually has a Minimum Circuit Ampacity (MCA) and a Max Breaker listed on its nameplate. The MCA includes the 125% factor for the motor, and then adds whatever the controls are to that.

The problem is the SSU assembly. It looks to me like it is limited to 15A. A problem with edison type fuses is you can put in larger ones than the system is rated for in the fuse holder (old houses many times had 20A and 30A fuses in places where the wiring was limited to 15A).

I wonder why someone even bothered with that SSU fuse device. Does the furnace say "max fuse", "max fuse or circuit breaker", or "max circuit breaker"? Most modern ones say breaker or "fuse or circuit breaker" so the circuit breaker feeding it from the panel is sufficient for overcurrent protection. If it says "fuse only", then you must have a fuse and can't rely on the panel circuit breaker. If it can be a breaker, and the circuit breaker in the panel is 20A or less, then replace the SSU cover with a switch cover and use a 15A switch labeled "AC Only". The furnace needs a local disconnect within sight, so you need something there to turn it off.

Since the MCA is 12.8A, I think a 15A slow blow fuse will hold. But I'd be sure to have spares, and if it blows randomly then replace it with a 20A rated fuse cover or do away with the fuse.
Mark | Somewhere in WA State
Topic Author
Californiastate
Posts: 1268
Joined: Thu Feb 04, 2021 11:52 am

Re: Proper SSU fuse for gas furnace

Post by Californiastate »

Thanks for the info. I’ll check the nameplate again tomorrow.
Topic Author
Californiastate
Posts: 1268
Joined: Thu Feb 04, 2021 11:52 am

Re: Proper SSU fuse for gas furnace

Post by Californiastate »

The nameplate has one line for electrical information.
Volts Hertz Phase Max Amps/ Amps Max Indoor blower motor FLA blower motor HP
120V 60 1 12.8 12.8 1.0

I didn't see any mention of MCA. The installation documents show the 20A maximum overcurrent protection. It allows either fuse or breaker. I didn't see any printed information on the SSU switch. I didn't remove the fuse or pull it out of the wall. I'll assume it's a 15A SSU. It was the SSU from the original 1993 furnace. It had a 20 amp fuse. That circuit is from a dedicated 20A breaker.

The strange thing is that the blower FLA and the unit max amps are the same per the nameplate. The igniter and controls aren't a consideration. Regardless the contractor installed a time delay 15A fuse in the SSU. Inspection has been rescheduled.
suemarkp
Posts: 909
Joined: Sun Nov 12, 2017 8:18 pm
Location: Somewhere in WA State

Re: Proper SSU fuse for gas furnace

Post by suemarkp »

The SSU assembly also has a horsepower rating. I think it was 1/2 HP when I googled the SSU specs. So that is also a problem if the motor is 1 HP.

Id be inclined to replace the SSU cover with a switch cover. Id probably install a 20A rated AC Only switch just because its a motor load and because the nameplate seems inconsistent.

The circuit breaker in the electrical panel would be the short circuit protection.
Mark | Somewhere in WA State
User avatar
AllMostThere
Posts: 491
Joined: Sat Dec 31, 2016 2:04 pm

Re: Proper SSU fuse for gas furnace

Post by AllMostThere »

If it were me - I would just install a 15A slow blow fuse as wanted by the inspector to get it approved. If it blows, then bump up to the desired 20A fuse. Really, the 15A will be fine, so move on and get'r done. :wink:
It is not about how much you make, it is about how much you keep and how well you invest it. - Author Unknown | Don't buy any annuity. Your future self or your heirs will thank you. - Author AllMostThere
Topic Author
Californiastate
Posts: 1268
Joined: Thu Feb 04, 2021 11:52 am

Re: Proper SSU fuse for gas furnace

Post by Californiastate »

suemarkp wrote: Wed Aug 03, 2022 10:19 am The SSU assembly also has a horsepower rating. I think it was 1/2 HP when I googled the SSU specs. So that is also a problem if the motor is 1 HP.

Id be inclined to replace the SSU cover with a switch cover. Id probably install a 20A rated AC Only switch just because its a motor load and because the nameplate seems inconsistent.

The circuit breaker in the electrical panel would be the short circuit protection.
It's been decades since I installed a furnace. At that time a SSU switch was required at the unit. I don't have a copy of current NEC or UMC to verify.
Topic Author
Californiastate
Posts: 1268
Joined: Thu Feb 04, 2021 11:52 am

Re: Proper SSU fuse for gas furnace

Post by Californiastate »

AllMostThere wrote: Wed Aug 03, 2022 6:12 pm If it were me - I would just install a 15A slow blow fuse as wanted by the inspector to get it approved. If it blows, then bump up to the desired 20A fuse. Really, the 15A will be fine, so move on and get'r done. :wink:
It was signed off this morning with a time delay 15A. It works fine but it also hasn't been running at full tilt. Regardless I have an extra 20A fuse.
ItzaHoot
Posts: 53
Joined: Thu Aug 27, 2020 12:05 pm

Re: Proper SSU fuse for gas furnace

Post by ItzaHoot »

Check the actual wire size from the breaker panel to the furnace. If it is 12 gauge (12/2-with ground) a 20 amp fuse should have been fine with the inspector. If, on the other hand it is 14 gauge (14/2-with ground the wire is undersized for the 20 amp breaker and should be replaced or the breaker should be replaced with a 15 amp breaker.

It is not uncommon to find mismatched breakers and wire sizes in older homes.
Topic Author
Californiastate
Posts: 1268
Joined: Thu Feb 04, 2021 11:52 am

Re: Proper SSU fuse for gas furnace

Post by Californiastate »

ItzaHoot wrote: Thu Aug 04, 2022 2:54 pm Check the actual wire size from the breaker panel to the furnace. If it is 12 gauge (12/2-with ground) a 20 amp fuse should have been fine with the inspector. If, on the other hand it is 14 gauge (14/2-with ground the wire is undersized for the 20 amp breaker and should be replaced or the breaker should be replaced with a 15 amp breaker.

It is not uncommon to find mismatched breakers and wire sizes in older homes.
It’s a 20 amp dedicated circuit for the furnace up to the SSU switch. The question is whether the SSU is rated at 15 or 20 amps. I’ve looked and haven’t found one online. I haven’t pulled the switch to look at the back. I’m assuming it’s standard and rated at 15A
suemarkp
Posts: 909
Joined: Sun Nov 12, 2017 8:18 pm
Location: Somewhere in WA State

Re: Proper SSU fuse for gas furnace

Post by suemarkp »

The specs are here: https://www.eaton.com/content/dam/eaton ... -units.pdf

Ratings are on page 1, cover variants on page 2. I believe it is no different than buying a breaker panel rated at 70A. Even though the larger breakers will fit the bus stabs, you can't go over 70A because of the bus. The rating seems to be 15A for all these Bussman box cover fuse variants even though larger fuses fit. SSU is just one of many variants.
Mark | Somewhere in WA State
whomever
Posts: 1146
Joined: Sat Apr 21, 2012 5:21 pm

Re: Proper SSU fuse for gas furnace

Post by whomever »

Out of my lane, but I'd make sure any switch is rated for the HP of the motor:

"The difference is, it's cheaper to make a switch with a 20A rating for purely resistive loads, but switching an inductive load like a motor means the switch must be capable of interrupting and extinguishing the arc that forms as the inductor tries to keep the current flowing. But just saying "Commercial grade" "Industrial grade" or even "Spec grade" does not necessarily mean you get that motor rating, you must look for it to be explicitly stated on the switch or the spec sheet. For example Leviton Spec Grade 20A toggle, yes, it's rated for up to 1HP. P&S / LeGrand Spec Grade 20A toggle, no HP rating at all."

https://forums.mikeholt.com/threads/def ... ch.144233/
suemarkp
Posts: 909
Joined: Sun Nov 12, 2017 8:18 pm
Location: Somewhere in WA State

Re: Proper SSU fuse for gas furnace

Post by suemarkp »

That is why I mentioned a switched marked "AC Only".

NEC 404.14 (A) Alternating-Current General-Use Snap Switch. A form of general-use snap switch suitable only for use on ac circuits for controlling the following:
(1) Resistive and inductive loads not exceeding the ampere rating of the switch at the voltage applied
(2) Tungsten-filament lamp loads not exceeding the ampere rating of the switch at 120 volts
(3) Motor loads not exceeding 80 percent of the ampere rating of the switch at its rated voltage

Most switches you buy today are labeled "AC Only" on the front strap near where the toggle is. The AC-DC type switch needs a HP rating when controlling a motor and these typically make a solid thunk sound when you work the toggle whereas AC Only ones are silent. Generally, you'll find those AC-DC ones in older homes from the 50's.

I'm not sure what switch type is in the SSU cover, but the SSU version is only rated at 1/2 HP which is considered a 9.8A load per NEC tables.
Mark | Somewhere in WA State
Topic Author
Californiastate
Posts: 1268
Joined: Thu Feb 04, 2021 11:52 am

Re: Proper SSU fuse for gas furnace

Post by Californiastate »

The SSU was originally installed when the house was built in '93. The existing 20A fuse had never been replaced.

edit
Thanks for the link.
Post Reply