Is #6 Romex ok on a 60 A breaker

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SunFish

NABCEP Certified
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Sr. PV Systems Design Engineer
Can I use a 60 A breaker with #6 romex? I'm confused by the allowable ampacity stated under 334.80 for NM. It states the allowable ampacity shall not exceed the 60 deg rating of 55 A but you can use the 90 deg C rating for ampacity adjustments as long as the adjustments don't exceed the 60 deg rating. My inverter has a 42 A output current so 42 x 1.25 = 52.5 A. So my breaker is 60 A. With a 55 A rating at 60 deg C the 60 deg wire rating is above 1.25 A x cont. current so the wire is able to handle the currents but am I allowed to use a 60 A breaker for this wire? Would the next size up rule in 240.4(B) allow me to use a 60 A breaker to protect the #6 romex? All my wiring is inside a wall so there are no correction/adjustment factors.

How is it the allowable ampacity can't exceed the 60 deg rating but you can use the 90 deg rating for adjustments?
 

Buck Parrish

Senior Member
Location
NC & IN
Yes, they don't make a 55 amp breaker. So you can go up to the next one.

I don't know about the second question. (How is it the allowable ampacity can't exceed the 60 deg rating but you can use the 90 deg rating for adjustments? )
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augie47

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Tennessee
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State Electrical Inspector (Retired)
I agree. With a calculated load of 52.5 (less than 55) you can use the "next size up" rule
in this case.
As I understand it the 90? rating is based on the insulation being rated at that temperature.
Many of the devices that are used with romex have a 60? rating so 110.14 would limit you in most cases even if 334 didn't.
 

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
How is it the allowable ampacity can't exceed the 60 deg rating but you can use the 90 deg rating for adjustments?

If the wire has 90C insulation then it is capable of handling more heat than the insulation. The wire is not the weak link itself it is the insulation on the wire. Since we are limited to 60C because the cable is NM then the allowable ampacity cannot exceed that limit. Since the wire in the NM is rated 90C then it is safe to use that to derate for such things as temp. or fill inside a conduit.
 

Carultch

Senior Member
Location
Massachusetts
How is it the allowable ampacity can't exceed the 60 deg rating but you can use the 90 deg rating for adjustments?
One instance is if it is in direct sunlight, or in a raceway in direct sunlight. And in the same installation, you've grouped it with another ROMEX circuit in the same raceway.

A common direct sunlight temperature correction factor I see, is 0.82. This would be with a 33 C ambient and a 17 C adder for a 5 1/2" Durablock/strut mount above a roof.
Then combine that with the 4-CCC's in a raceway factor of 0.8.Net derate is 0.656, and thus 90C wire at #6 would have a derated ampacity of 49.2 A.
 

kwired

Electron manager
Location
NE Nebraska
One instance is if it is in direct sunlight, or in a raceway in direct sunlight. And in the same installation, you've grouped it with another ROMEX circuit in the same raceway.

A common direct sunlight temperature correction factor I see, is 0.82. This would be with a 33 C ambient and a 17 C adder for a 5 1/2" Durablock/strut mount above a roof.
Then combine that with the 4-CCC's in a raceway factor of 0.8.Net derate is 0.656, and thus 90C wire at #6 would have a derated ampacity of 49.2 A.
You can't run NM cable on a roof - even if in a raceway as it is going to be a wet location and NM is for dry locations only.
 

kwired

Electron manager
Location
NE Nebraska
Can I use a 60 A breaker with #6 romex? I'm confused by the allowable ampacity stated under 334.80 for NM. It states the allowable ampacity shall not exceed the 60 deg rating of 55 A but you can use the 90 deg C rating for ampacity adjustments as long as the adjustments don't exceed the 60 deg rating. My inverter has a 42 A output current so 42 x 1.25 = 52.5 A. So my breaker is 60 A. With a 55 A rating at 60 deg C the 60 deg wire rating is above 1.25 A x cont. current so the wire is able to handle the currents but am I allowed to use a 60 A breaker for this wire? Would the next size up rule in 240.4(B) allow me to use a 60 A breaker to protect the #6 romex? All my wiring is inside a wall so there are no correction/adjustment factors.

How is it the allowable ampacity can't exceed the 60 deg rating but you can use the 90 deg rating for adjustments?

210.19(A)(1) for branch circuits, 215.2(A)(1) for feeders both tell us that conductor minimum ampacity must be at least 100% of non-continuous load plus 125% of continuous load. For a 6AWG @ 60C that means 100% non-continuous plus 125% continuous can not exceed 55 amps if it does a larger conductor is needed.

240.4 tells us that for overcurrent devices 800 amps or less we can use next higher standard sized overcurrent device above the ampacity of the conductors being proteceted can be used.

Between those sections this means for 6 AWG @60C minimum ampacity required by load calculation can not exceed 55 amps and overcurrent protection can not exceed 60 amps.

Bump conductor to a 75C conductor and it moves to load calculation of 65 amps and overcurrent can not exceed 70 amps. (you must also have 75C termination ratings - but typically not a problem with newer equipment)
 
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