15 or 20 amp for gas furnace?

[mbrooke]

New residential development. Im kind of stumped as to whether a 15 amp or 20 amp circuit is appropriate for the gas furnace installs. The builder wants 14 gauge wire where ever code allows but I would think 20 amps would be more appropriate considering the name plate rating of the units. The rating plate does say 14.4 amps but I would think the 80% rule applies to all HVAC equipment, the other sparky says it doesn't and has always been putting them on a 15 amp without issue to date. Who would you say is right here?




Here some pics of the furnace:

 

[Dennis Alwon]

Does it not have a max overcurrent protective device? I usually pull a 20 amp circuit but occasionally I have seen some small units that stated max 15 amp ocpd

 

[mbrooke]

Does it not have a max overcurrent protective device? I usually pull a 20 amp circuit but occasionally I have seen some small units that stated max 15 amp ocpd

No minimum or maximum is stated. The install instructions don't seem to say to much either.

 

[edlee]

I always put residential forced-air gas furnaces on a 15a circuit unless specifically otherwise called for, and that's what I almost always see when there is one existing. I don't see how the 80% rule would apply for what is really mostly a motor load.

 

[kwired]

I don't see how the 80% rule would apply for what is really mostly a motor load.

The general rules for motors require supply conductor ampacity that is 125% of the motor FLA.

What we don't really know based on how things are worded on that nameplate is whether that 14.4 amps is the actual rated full load amps or if it is the equivalent of "minimum circuit ampacity".

The manufacturers seem to have a better standard of marking HVAC units that have hermatic compressors in them but other equipment seems to vary from one to the next and is not as consistent through the industry.

 

[LEO2854]

New residential development. Im kind of stumped as to whether a 15 amp or 20 amp circuit is appropriate for the gas furnace installs. The builder wants 14 gauge wire where ever code allows but I would think 20 amps would be more appropriate considering the name plate rating of the units. The rating plate does say 14.4 amps but I would think the 80% rule applies to all HVAC equipment, the other sparky says it doesn't and has always been putting them on a 15 amp without issue to date. Who would you say is right here?




Here some pics of the furnace:

A 15 amp circuit should be fine,I'll bet it does not even draw 10 amps.

 

[gar]

131115-0931 EST

My 1/3 HP furnace blower motor runs at 5 A 123 V 615 VA 410 W.

I do not know the mechanical load on the motor. Assume it was 1/3 HP, then output wattage is 250 W, and efficiency is 250/410 = 61%. Not good.

.

 

[mbrooke]

The general rules for motors require supply conductor ampacity that is 125% of the motor FLA.

What we don't really know based on how things are worded on that nameplate is whether that 14.4 amps is the actual rated full load amps or if it is the equivalent of "minimum circuit ampacity".

The manufacturers seem to have a better standard of marking HVAC units that have hermatic compressors in them but other equipment seems to vary from one to the next and is not as consistent through the industry.

I agree even the instruction manual just says to look at the listing and to determine wire size based on local and national electrical codes...what ever that means:lol: I am thinking the same that the 80% rule applies, at least in cooling where the unit could easily go for more than 3 hours.

A 15 amp circuit should be fine,I'll bet it does not even draw 10 amps.

I did amp clamp one that was already hooked up in the finished model and got around 7 amps to 8 max on high fan only. However, there is a draft induction fan and a hot surface igniter since the unit is a High efficiency PVC vented type so I would imagine that adds to the load.

 

[RICK NAPIER]

I have come across this before with french made units the "maximum input amps" is actually the minimum circuit ampacity.

 

[templdl]

Just curious, how much cost is to be saved by installing a 15a rather that a 20a circuit? The manhours it took to respond to the OP must more that cover the cost of the #12 wire and a 20at breaker costs the same as a 15at. If you have to ask the question the addition cost of running a 20a circuit can break the bank unless the length of the circuit is significant.

 

[augie47]

Just curious, how much cost is to be saved by installing a 15a rather that a 20a circuit? The manhours it took to respond to the OP must more that cover the cost of the #12 wire and a 20at breaker costs the same as a 15at. If you have to ask the question the addition cost of running a 20a circuit can break the bank unless the length of the circuit is significant.

On a 50 ft run the difference might just be a dollar or so, however, if you are G/C on a 500+ unit apartment complex, that adds up.

 

[iwire]

On a 50 ft run the difference might just be a dollar or so, however, if you are G/C on a 500+ unit apartment complex, that adds up.

You bet it does and when the getting the job is often all about the price you quote all these little things can make a differance.

 

[mbrooke]

Just curious, how much cost is to be saved by installing a 15a rather that a 20a circuit? The manhours it took to respond to the OP must more that cover the cost of the #12 wire and a 20at breaker costs the same as a 15at. If you have to ask the question the addition cost of running a 20a circuit can break the bank unless the length of the circuit is significant.

Not much for one unit. Normally I would not think twice and just pull a 20, the run is only about 100 feet on average, however this is a minimum code job on a large new development. The builder wants savings where ever possible while still being up to code. In cases like this is either do it cheap and make a profit or add extra and loose the job.

 

[kwired]

Just curious, how much cost is to be saved by installing a 15a rather that a 20a circuit? The manhours it took to respond to the OP must more that cover the cost of the #12 wire and a 20at breaker costs the same as a 15at. If you have to ask the question the addition cost of running a 20a circuit can break the bank unless the length of the circuit is significant.

So far I don't think asking the question cost the OP anything, where should all that answered send a bill?

 

[templdl]

So far I don't think asking the question cost the OP anything, where should all that answered send a bill?

So, what's the bottom line? Is it to decide if a 15 or 20a circuit should be used out of principle. But if it is the cost of the time and materials to install a 20a ckt in instead of a 15 then it would be dependent upon the job which was not described other than what ifs.
But I must admit that it may be for the sake of discussion and not the practicality which is part of the fun of it.

 

[kwired]

So, what's the bottom line? Is it to decide if a 15 or 20a circuit should be used out of principle. But if it is the cost of the time and materials to install a 20a ckt in instead of a 15 then it would be dependent upon the job which was not described other than what ifs.
But I must admit that it may be for the sake of discussion and not the practicality which is part of the fun of it.

I was just wondering if it cost him, someone else must have earned something, since I was one of the contributors I just wanted to know where to send the bill, you can send him one also:happyyes:

You must also remember that advice from this site is worth every penny paid.

 

[templdl]

I was just wondering if it cost him, someone else must have earned something, since I was one of the contributors I just wanted to know where to send the bill, you can send him one also:happyyes:

You must also remember that advice from this site is worth every penny paid.

Yes, sometime I forget that this is is at no charge and is a great deal at twice the price. I draw from my background when I was paid for this as it was part of my job description and was held accountable. There is something to be said about you also get what you paid for taking it for what its worth.

 

[StarCat]

Gas Furnaces

Gas Furnaces

With respect to this type of unit the major load is the fan drive, as thus the fan motor. Depending on the CFM rating of said unit you may have up to 3/4 HP motor on board. As always, usually the nameplate indicates what is needed. Inrush on starup is a factor that can make 15A borderline.
Smaller units can run on 15A, but a 5 ton fan drive generally should have 20A.
While the game would seem to be " always " doing the gig as cheap as possible, HVACR is one area that its always better to go on the high side.
Always in my experience.

 

[kwired]

Inrush on starup is a factor that can make 15A borderline.

Next thing to take into consideration is what kind of drive do we have for this motor. If it is a variable speed type drive the starting inrush isn't much of a factor in needing a breaker rating of very much more than motor full load current.