6-2 not large enough wire for 60 amp furnace

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Stevenfyeager said:
Every HVAC person always tells me to run a 6-2 on a 60 amp breaker for a furnace or heat pump. But #6 romex is only rated for 55 amps, correct ?
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You need to check the name plate rating on the equipment, it will tell you the minimum circuit conductor ampacity required.
 
James L said:
Don't forget about ambient temperature adjustment, which allows an 8% ampacity increase for NM if temp is between 69⁰ and 77⁰ Fahrenheit

That makes 6-2 good for 59 amps in almost any indoor furnace setting except atticsView attachment 2561706
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That still doesn't let you run 59 amp load when your termination temp rating at 60C is 55 amps. It only lets you adjust insulation temp rating by that factor, which unless you actually dealing with older NM cable you are already able to adjust NMB with the 90C column.
 
kwired said:
That still doesn't let you run 59 amp load when your termination temp rating at 60C is 55 amps. It only lets you adjust insulation temp rating by that factor, which unless you actually dealing with older NM cable you are already able to adjust NMB with the 90C column.
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I posted this before seeing other replies.

You still can't uprate the 60C termination rating with the ambient temp correction table. Yes the terminal itself these days is probably good for 75C, but because you must treat NM cable as though it is 60C conductor that means in all cases your minimum conductor size can't be smaller than the 310.16 60C column, which is 55 amps for 6 AWG. Adjusting temp for insulation rating through the run is a different thing.

For NMB we can use 90C rating as starting point for said adjustments. So if you ran several cables in a bundle you would need to adjust ampacity accordingly, yet if had low enough ambient temp you could offset some with a positive adjustment as well.

If you run a single cable with less than 4 current carrying conductors the ambient temp adjustment is kind of meaningless as you still limited to termination temp for your base conductor size before any adjustments.
 
kwired said:
because you must treat NM cable as though it is 60C conductor that means in all cases your minimum conductor size can't be smaller than the 310.16 60C column,
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I think I argued the same point earlier, but found James rationale in post #15 compelling, the note #1 in T310.16 is technically part of the table. So I think to 'lock' the 60C table to a 30C ambient 110.14(C)(a)(1) would have to specify that.
 
tortuga said:
the note #1 in T310.16 is technically part of the table. So I think to 'lock' the 60C table to a 30C ambient 110.14(C)(a)(1) would have to specify that.
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I think I agree, but that is certainly contrary to the conventional wisdom here that temperature correction does not apply to termination ampacity considerations.

Cheers, Wayne
 
Wouldn't #4 aluminum SE be cheaper?

Also most AHU nameplates indicate copper only.
 
wwhitney said:
I think I agree, but that is certainly contrary to the conventional wisdom here that temperature correction does not apply to termination ampacity considerations.

Cheers, Wayne
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I need to chew on that one a while, maybe it is possible you could use this at 59 amps with that temp adjustment factor. Normally we only consider temp adjustment to apply to the conductor insulation and not to terminations but way things are worded this might be possible. At same time how do you prove ambient temp will never be more than 77F in the average dwelling application
 
kwired said:
At same time how do you prove ambient temp will never be more than 77F in the average dwelling application
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That's I commented on it being plausible for the case of an electric furnace--program the thermostat so it can't be set above 77F, and the furnace should never run when the temperature is above 77F. So whenever the load is active, the temp will be 77F or below.

Cheers, Wayne
 
wwhitney said:
That's I commented on it being plausible for the case of an electric furnace--program the thermostat so it can't be set above 77F, and the furnace should never run when the temperature is above 77F. So whenever the load is active, the temp will be 77F or below.

Cheers, Wayne
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Only problem I see with that is in our location about 1/3 of the air handlers are located in the attic.
 
Yeah I think its turned to more of an academic theoretical conversation, I doubt any AJH would accept the up-rating in that scenario.
 
With a conductor ampacity this close to the load, I would choose the larger size so more power becomes heat inside the furnace and less in the cable.
 
LarryFine said:
With a conductor ampacity this close to the load, I would choose the larger size so more power becomes heat inside the furnace and less in the cable.
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Keep in mind 6AWG will create same amount of heat whether it has 60 or 90 degree insulation though.

Nobody seems to know why NEC requires us to treat NM cable as 60 deg conductors even though all of it has had 90C insulation for over 30 years now.
 
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