Sizing Furnace Circuits

[winterovereverything]

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

 

[infinity]

What is the MCA?

 

[winterovereverything]

What is the MCA?

9 and 11.4 Amps respectively with the information I've been given. Even in consideration that they are continuous loads, we are far off from a 20 amp overcurrent device, aren't we? This could also be a simple case of me overthinking things, but I just want to make sure.

 

[infinity]

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.

 

[winterovereverything]

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.

Ah I figured. Thank you!

 

[LarryFine]

Wouldn't it be more appropriate to have both be protected with a 15 A breaker?

If it will start without tripping, sure.

 

[winterovereverything]

If it will start without tripping, sure.

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.

 

[LarryFine]

"Miyagi have hope for you!"

 

[infinity]

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.

With the MCA's you've listed it won't nuisance trip.

 

[suemarkp]

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.