6-2 not large enough wire for 60 amp furnace

[Stevenfyeager]

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 ?

 

[texie]

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 ?

Yes and a wildly common violation.

 

[James L]

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 ?

10kw heat strip is only 41.6 amps

Multiply by 1.25 and you're at 52.08 amps

Wire good for 3 amps more

Nobody makes a 55 amp breaker, so you move up to 60

 

[texie]

10kw heat strip is only 41.6 amps

Multiply by 1.25 and you're at 52.08 amps

Wire good for 3 amps more

Nobody makes a 55 amp breaker, so you move up to 60

I agree if you just are powering the heat strip. As I recall most AHs for 10 KW will need a blower that will put it over 55 amps. The only sure way is to look at the MCA and Max OCPD for the equipment involved.

 

[infinity]

#6 NM cable is permitted on a 60 amp OCPD when the calculated load is 55 amps or less.

 

[augie47]

For many years a "standard" air handler had a 9.6 kw heat strip and a single speed blower and a MCA slightly less than 55 amps this allowing a #6 NM so that became the "norm".
The advent of "efficiency" systems brought about variable speed motors and on many units now exceed the 55 amp MCA so the #6 NM is not sufficient.
Most of the HVAC guys here have switched to using #6 SE cable with an ampacity of 65 amps,

 

[infinity]

Most of the HVAC guys here have switched to using #6 SE cable with an ampacity of 65 amps

Good solution but stupid code rule. Why would SE cable be any better than NM cable for this load?

 

[Stevenfyeager]

thank you all

 

[Fred B]

Good solution but stupid code rule. Why would SE cable be any better than NM cable for this load?

I believe it is related to the insulation material of SE vs NM. Most SE is XHHW-2

 

[James L]

many units now exceed the 55 amp MCA so the #6 NM is not sufficient.

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 attics

 

[Dennis Alwon]

There is absolutely no difference in NM or SE cable in this case however se is allowed at 75C for other reasons. Many felt that se feeding a range or heating systems has never been an issue so no need to keep it at 60C. I believe it started at 75C went to 60C and now is back to 75C

 

[tortuga]

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 exceptView attachment 2561706

Perhaps if it was not NM cable, 334.80 limits the ampacity of NM to the 60C column, so the way I read it no up-rating allowed. Another type of cable that is gaining popularity in dwellings is tray cable.

 

[James L]

Perhaps if it was not NM cable, 334.80 limits the ampacity of NM to the 60C column, so the way I read it no up-rating allowed.

I'm not following how you come to that.

 

[tortuga]

I'm not following how you come to that.

334.80 second sentence states

The allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor.

The 90°C (194°F) rating shall be permitted to be used for ampacity
adjustment and correction calculations, provided the final calculated
ampacity does not exceed that of a 60°C (140°F) rated conductor.

So the way I was taught you can't ever uprate nm cable to an ampacity above that in the 60C column.

 

[James L]

334.80 second sentence states

So the way I was taught you can't ever uprate nm cable to an ampacity above that in the 60C column.

The ampacity in the chart is only applicable under a specified ambuet temperature condition - 30⁰C or 86⁰F

If your ambient temperature changes, so does the ampacity of the wire

Adding 8% is atill within the 60⁰ column "IF" your ambient temperature is lower

 

[wwhitney]

Don't forget about ambient temperature adjustment, which allows an 8% ampacity increase for NM if temp is between 69⁰ and 77⁰ Fahrenheit

I guess that is perfectly reasonable for a furnace, if the thermostat can be configured to lock out temperatures above 77F, and the wiring is within the same ambient.

And if your MCA is above 55A but by less than 8%, you can get away with a higher temperature than 77F by using the formula given. The table just calculates the exact results for multiples of 5C; each table entry is the exact result for the high end of the temperature band given, and the multiplier will become a bit bigger as the temperature decreases over the 5C range.

Cheers, Wayne

 

[tortuga]

If your ambient temperature changes, so does the ampacity of the wire

Cant argue with that, I stand corrected.

 

[Rick 0920]

Reference 110.14 (C) (1), 334.80, 240.4 (B), T.310.16. These will help you understand why #6-2 w/ground NM has to be considered as a 60 C conductor and why if derating is necessary, we can use the 90 C numbers. 240.4 (B) will tell you when you can go up to next higher standard OCPD.

 

[Dennis Alwon]

That is a great point @James L . In many case 8/2 nm cable is used and the furnaces ampacity is not usually more than 57 amps.

 

[Dennis Alwon]

We use to use se cable for all situation where we had ampacities higher than 30 amps. Ranges when 3 wire was allowed was always se cable but once they went to 4 wire it seemed that all the suppliers stopped carrying se cable especially copper.