Sizing Furnace Circuits

W

winterovereverything

Member
Location
California
Occupation
Electrician
Hey Folks,

Hope all is well. The gist of it is this: I have a number of furnaces handed to me that I have to size and install but their MOPs, their Maximum Overcurrent Protections, seem ludicrously high given the load. For example, I've got a nameplate whose Total Current is 9 Amps but has a MOP of 20 A, and another who has a Total Current of 11.4 but a MOP of 20A. Quite high, no? Wouldn't it be more appropriate to have both be protected with a 15 A breaker? And how do we determine the MOP anyways? My initial thought was to go to 210.21(B)(2) which gives the maximum load with respective breaker and receptacle size, but I'm curious to find out what everyone thinks.

Cheers,
Jack
 
The MCA already has a 125% factor added to it so the actual current is likely lower. The MaxOCPD is just that the maximum. You could use a 15 amp circuit also.
 
LarryFine said:
If it will start without tripping, sure.
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I was just going to follow the nameplate rec to overcome the nuisance tripping caused by both of them. We have a lot of spare 15 Amp breakers in the shop so I thought it might be more efficient to try and use them but with the labor and time I think we'll just go with the manufacturer rec.
 
winterovereverything said:
I was just going to follow the nameplate rec to overcome the nuisance tripping caused by both of them. We have a lot of spare 15 Amp breakers in the shop so I thought it might be more efficient to try and use them but with the labor and time I think we'll just go with the manufacturer rec.
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With the MCA's you've listed it won't nuisance trip.
 
Furnace curcuits are typically 20A because furnaces can change and some need more power than others. Most appliances, from table lamps to toasters, are listed to work on 20A circuits because many common circuits are 20A even with 15A receptacles.

So if you have an existing 20A furnace circuit you can use it. If it is only 15A, it will probably work. If it's 20A, you don't have to drop it to 15.
 
Many furnaces have air handlers also as part of it. While either motor running load is quit low and even when both are running simultaneously, If both start at same time the starting load can be higher sometimes much higher. Thus some mfg are suggesting the higher overcurrent protection. but the MOP listing is just saying "No Larger Than This". Sizing to the Max starting loads combined may be advisable to avoid nuisance tripping. Depending on duration of the starting load the typical standard trip breaker may trip if load is too close to the max potential.
 
Some furnaces may have a max of 15 amps. Years ago I always used 20 amp circuits and then one day the inspector called to tell me the unit says max 15 amp. So unless the max is 15 a 20 amp may be fine.
infinity said:
The MCA already has a 125% factor added to it so the actual current is likely lower. The MaxOCPD is just that the maximum. You could use a 15 amp circuit also.
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Are you sure about that with furnaces? I assume these are either gas units or just blowers for another heat source
 
Dennis Alwon said:
Are you sure about that with furnaces? I assume these are either gas units or just blowers for another heat source
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What about this?

ARTICLE 424 Fixed Electric Space-Heating Equipment
Part I. General
424.1 Scope.
This article covers fixed electric equipment used for space heating. For the purpose of this article, heating equipment includes heating cables, unit heaters, boilers, central heating systems, or other fixed electric space-heating equipment. This article does not apply to process heating and room air conditioning

424.4(B) Branch-Circuit Sizing.
The branch-circuit conductors for fixed electric space-heating equipment and any associated motors shall be sized not smaller than 125 percent of the load.
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Dennis Alwon said:
I thought you were saying the nameplate had 125% already. I know for a/c units that is true but it appears this just had mop and amps not mca.
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If the nameplate lists the MCA and the conductors are required to be sized at 125%, wouldn't the nameplate current already include the extra 25%. I'm not seeing how you can call something the minimum circuit ampacity if it does not include the requisite additional 25%.
 
infinity said:
If the nameplate lists the MCA and the conductors are required to be sized at 125%, wouldn't the nameplate current already include the extra 25%. I'm not seeing how you can call something the minimum circuit ampacity if it does not include the requisite additional 25%.
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I agree but there was no mention of MCA that's where I was confused by your statement.
 
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