# Furnace circuit 20 or 15 amp?



## DangerMouse

I JUST asked the inspector the exact same question 2 hours ago, (as I only had ONE space left in my panel.... a 20a) he said either is fine. Po)

Hope this helps!

DM


----------



## acerunner

DangerMouse said:


> I JUST asked the inspector the exact same question 2 hours ago, (as I only had ONE space left in my panel.... a 20a) he said either is fine. Po)
> 
> Hope this helps!
> 
> DM


well i'm glad i'm not the only one who was wondering this. Thanks :thumbsup:


----------



## kenmac

If it's a gas furnace 15 or 20 should be fine.. Always check the rating plate


----------



## beenthere

Only a few furnaces that have 5 ton drives in them need a 20 amp circuit.

Most furnaces list a 15 amp.


----------



## acerunner

beenthere said:


> Only a few furnaces that have 5 ton drives in them need a 20 amp circuit.
> 
> Most furnaces list a 15 amp.


ok its decided then. I'm using 15amp.


----------



## nap

> What size does the circuit need to be?



do you guys want to know the actual answer?



It is:

the circuit must be large enough to serve the intended load. 

be aware that the furnace itself my actually give direction that must be followed. If you feed the furnace with a larger than needed circuit, you may have to provide overcurrent protection at some lower level.

You guys never specified if it was a gas or electric unit.:wink:

If your house is large enough where beenthere's statement might be of concern, I would just go ahead and put the 20 amp in. Better too large than too small.


----------



## acerunner

its a gas unit. exact model has not been specified, but will be a basic model. The house is small, 1500sqft.


----------



## nap

acerunner said:


> its a gas unit. exact model has not been specified, but will be a basic model. The house is small, 1500sqft.


You should be fine with a 15.

Now, if it were me, it would be a 20 as I don't install anything new, in my house, smaller than a 20 but that's just me.

I figured it was a gas unit or the possible choices would have been very differenct as would the responses, especially from beenthere.


----------



## beenthere

nap said:


> You should be fine with a 15.
> 
> Now, if it were me, it would be a 20 as I don't install anything new, in my house, smaller than a 20 but that's just me.
> 
> I figured it was a gas unit or the possible choices would have been very differenct as would the responses, especially from beenthere.



If you would use a 20 amp breaker.
then you would need to use a utility switch to fuse it down to 15 amp, on a furnace with a small blower.


----------



## DangerMouse

Mine is propane. The inspector said nothing about fusing it down.

DM


----------



## beenthere

DangerMouse said:


> Mine is propane. The inspector said nothing about fusing it down.
> 
> DM


He probably didn't look at the furnace's electrical rating tag.

Look on it. It probably says 15 amp.


----------



## nap

beenthere said:


> If you would use a 20 amp breaker.
> then you would need to use a utility switch to fuse it down to 15 amp, on a furnace with a small blower.


only if it states max OCPD 15 amps. Other than that, there is no requirement to add OCPD.


----------



## fabrk8r

FWIW, I always install a fused switch plate at the furnace...it serves 3 purposes. 1st, by code, there needs to be a disconnect for the furnace anyway, 2nd, it's an extra layer of protection (why take the chance on frying a board or blower motor when I can replace a 75 cent fuse) and 3rd, like beenthere said if the unit happens to be on a 20 amp circuit at the panel I can use the fused switch plate to fuse it down.

As beenthere stated, it's rare to see a residential furnace that needs 20 amp protection unless it's a large system. Too large a fuse or circuit breaker isn't a good thing.


----------



## nap

fabrk8r said:


> Too large a fuse or circuit breaker isn't a good thing.


I'll agree with the other points but since the breaker is to protect the wire and not the appliance, the size of the breaker is irrelevant to the appliance.


----------



## hvaclover

DangerMouse said:


> Mine is propane. The inspector said nothing about fusing it down.
> 
> DM


that's because propane is magic lololololoolol


----------



## hvaclover

beenthere said:


> Only a few furnaces that have 5 ton drives in them need a 20 amp circuit.
> 
> Most furnaces list a 15 amp.



What been and Nap said are gospel. Over size the breaker and you run the chance of an electrical fire.


----------



## beenthere

nap said:


> only if it states max OCPD 15 amps. Other than that, there is no requirement to add OCPD.


Those furnaces that don't list one on the data plate.
Have the note in the install instructions to install an OCPD.


----------



## nap

beenthere said:


> Those furnaces that don't list one on the data plate.
> Have the note in the install instructions to install an OCPD.


and somewhere way back when, I did agree that if the unit limits the OCPD it must be followed, didn't I?

why, yes I did in post #7, which happened to be my very first post:



> be aware that the furnace itself my actually give direction that must be followed. If you feed the furnace with a larger than needed circuit, you may have to provide overcurrent protection at some lower level.


I would still put in a 20, or at least the #12 wire. You can change the breaker to a 15 is a couple minutes. I have never regretted oversizing the circuit. On the other hand, I have regretted undersizing a circuit due to lack of information.


----------



## beenthere

nap said:


> and somewhere way back when, I did agree that if the unit limits the OCPD it must be followed, didn't I?
> 
> why, yes I did in post #7, which happened to be my very first post:
> 
> Never said you didn't, your paranoid.
> 
> Just said you need to install a 15 amp at the furnace if you use a 20 amp at the panel.
> 
> I would still put in a 20, or at least the #12 wire. You can change the breaker to a 15 is a couple minutes. I have never regretted oversizing the circuit. On the other hand, I have regretted undersizing a circuit due to lack of information.


Using the next size larger conductor never hurts.


----------



## nap

beenthere said:


> Using the next size larger conductor never hurts.


I didn't say you said I said I didn't say I said you needed to follow the manufacturers directions. . I was just reinforcing the fact that I am not really in disagreement with you on this.:thumbsup:


----------



## beenthere

nap said:


> i didn't say you said i said i didn't say i said you needed to follow the manufacturers directions. . I was just reinforcing the fact that i am not really in disagreement with you on this.:thumbsup:



lol...


----------



## hvaclover

nap said:


> I didn't say you said I said I didn't say I said you needed to follow the manufacturers directions. . I was just reinforcing the fact that I am not really in disagreement with you on this.:thumbsup:


been hitting the home grown again? save me some!!!!


----------



## DangerMouse

I think MY problem could be solved if HD had had a 15/20 tandem in stock... (it was on my list)

They didn't, so I got a 20/20... do they MAKE a 15/20 tandem? (Siemens)

DM


----------



## DangerMouse

Yay! Found one! (not at any of the BB stores either, I needed to call an electric supply store)

DM


----------

