Current Rating Using #8/3 NM-B Conductor

[NewtonLaw]

#8/3 NM-B wire is rated for 90°C temperature however the NEC current carrying capacity limits its rating based on 60°C. This is based on the typical 60°C rating for residential applications. My question is if I am using it on equipment that has terminal temperature ratings of 75°C or 90°C, am I allowed to use this wire at the 75°C or 90°C current rating?

 

[texie]

No.

 

[infinity]

No. You're stuck with the 60° C rating.

334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The
allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor.

 

[Dsg319]

Only thing you can use that 90degree ampacity for is for ampacity adjustments and corrections. Ex. Conductor bundling, & temperature corrections.

 

[texie]

Only thing you can use that 90degree ampacity for is for ampacity adjustments and corrections. Ex. Conductor bundling, & temperature corrections.

Or if the equipment requires 75 or even 90 degree wire due to heat from the equipment itself. But you still have to use it at 60 degree ampacity.

 

[Dsg319]

Or if the equipment requires 75 or even 90 degree wire due to heat from the equipment itself. But you still have to use it at 60 degree ampacity.

Good point. I have seen lights that required 90degree conductors.

 

[NewtonLaw]

Thank you everybody for your answers but further I want to ask - So an engineering review identifying the terminal ratings of the equipment used as 75°C then is of no value? You still have to use the 60°C value?

 

[retirede]

Thank you everybody for your answers but further I want to ask - So an engineering review identifying the terminal ratings of the equipment used as 75°C then is of no value? You still have to use the 60°C value?

Think of it this way: Once it’s all said and done, after all adjustments, you can’t exceed the 60C ampacity.

 

[James L]

What if you have a NM home run that's too short and has to be spliced in the panel?

Say you have a 6-3/g to an oven that needs a 50 amp breaker. Does the wire going from the breaker to the splice count as NM or THHN?

#8 from 50 amp breaker, connected to #6 NM

What say you?

 

[augie47]

Thank you everybody for your answers but further I want to ask - So an engineering review identifying the terminal ratings of the equipment used as 75°C then is of no value? You still have to use the 60°C value?

Basically, yes due to the wording in 334.80

What if you have a NM home run that's too short and has to be spliced in the panel?

Say you have a 6-3/g to an oven that needs a 50 amp breaker. Does the wire going from the breaker to the splice count as NM or THHN?

#8 from 50 amp breaker, connected to #6 NM

What say you?

It's not what it "counts as" but what it actually is. IF you use a section of #8 THHN and no adjustments are required and the terminations are 75°, then the 50 amp breaker would be allowed.

 

[infinity]

Thank you everybody for your answers but further I want to ask - So an engineering review identifying the terminal ratings of the equipment used as 75°C then is of no value? You still have to use the 60°C value?

Almost all terminals are rated for at least 75° C but nothing gets you around the code section in post #3.

 

[James L]

It's not what it "counts as" but what it actually is. IF you use a section of #8 THHN and no adjustments are required and the terminations are 75°, then the 50 amp breaker would be allowed.

I agree. Was thinking about a scenario I had once

 

[NewtonLaw]

Basically, yes due to the wording in 334.80

So NEC 310.14(A)(1) and 310.14(B) can not be applied? Even though310.15(A) does allow for ampacity correction up to the termination temperature rating in accordance with the provisions of 110.14(C)?

 

[don_resqcapt19]

So NEC 310.14(A)(1) and 310.14(B) can not be applied? Even though310.15(A) does allow for ampacity correction up to the termination temperature rating in accordance with the provisions of 110.14(C)?

Both the NEC and UL product standard specify that the ampacity of NM cable conductors is at 60°. Nothing in the code changes this.

 

[Eddie702]

What about when using NM on a motor load like an AC condenser with an MCA of 25 and an MOCP of 40. You can use #10 with a 40 a breaker on that

 

[augie47]

What about when using NM on a motor load like an AC condenser with an MCA of 25 and an MOCP of 40. You can use #10 with a 40 a breaker on that

Changes noting in regard to ampacity. 10 NM has an ampacity of 30 amps.
Protecting the conductor is regulated by Art 240 and 240.4(G) allows that breaker,,, doesn't change the allowable ampacity,

 

[mbrooke]

FWIW, in Canada NM can be used at 75*C- technically even 90*C is terminations allowed for it.

 

[don_resqcapt19]

What about when using NM on a motor load like an AC condenser with an MCA of 25 and an MOCP of 40. You can use #10 with a 40 a breaker on that

Yes, but the required ampacity of the conductors for that circuit is 25 amps and the 10 AWG NM has an ampacity of 30.

If you were using MC cable you would be permitted to use 12-2 MC for that same circuit with the same 40 amp breaker.

 

[James L]

If you were using MC cable you would be permitted to use 12-2 MC for that same circuit with the same 40 amp breaker.

And blow the minds of some lesser informed wiremen on the job. I've had that conversation more than once

 

[mbrooke]

And blow the minds of some lesser informed wiremen on the job. I've had that conversation more than once

Wouldn't the wires over heat though on longer runs or if someone brought a generator on the property?