Ampacity and breaker size

[hhsting]

I have 4#12 phase and 4#12 neutral in one conduit for system furniture at 120V single phase fed from 20A breaker. System furniture load for each circuit is 4.1A.


Based on Table 310.15(B)(3)(a) my correction factor is 0.7 with 8 current carrying conductors. With #12 ampacity is derated to 14A.

Would not either the breaker need to be changed to 15A or cable changed from#12 to #10 awg?

 

[david luchini]

Would not either the breaker need to be changed to 15A or cable changed from#12 to #10 awg?

I would assume for a system furniture (dry) location, that THHN would be used. No change of conductor or breaker would be required.

 

[hhsting]

I would assume for a system furniture (dry) location, that THHN would be used. No change of conductor or breaker would be required.

Would not 240.4(D) make ampacity of #12 awg to be 20A?

 

[winnie]

The actual ampacity of a #12 conductor with 90C insulation is 30A, however these conductors are limited by the 'small conductor' rule to an OCPD of 20A unless otherwise permitted.

The actual ampacity may be used for purposes of derating and adjustment for ambient conditions.

So _if_ as is commonly the case the circuits are wired with 90C rated conductors, then the derated ampacity is 0.7 * 30 = 21A, and the #12 is still permitted on a 20A breaker.

-Jon

 

[winnie]

Would not 240.4(D) make ampacity of #12 awg to be 20A?

240.4(D) doesn't change the ampacity of the conductor. It limits the OCPD rating.

240.4(D) says 'no matter what the actual ampacity of the conductor is, your maximum OCPD for #12 wire is 20A'

You still derate from the 30A value (or 25A for 75C conductors), but if the result comes to >20A you are stuck using a 20A breaker.

If after you derate from 30A you end up less than 20A, _then_ you would need to reduce the breaker size. So if you have 10-20CCC #12 (derating factor of 0.5) then your 30A base ampacity gets derated to 15A and you need a 15A breaker. But for anything up to 9 CCC on #12 you go with a 20A breaker.

-Jon

 

[hhsting]

The actual ampacity of a #12 conductor with 90C insulation is 30A, however these conductors are limited by the 'small conductor' rule to an OCPD of 20A unless otherwise permitted.

The actual ampacity may be used for purposes of derating and adjustment for ambient conditions.

So _if_ as is commonly the case the circuits are wired with 90C rated conductors, then the derated ampacity is 0.7 * 30 = 21A, and the #12 is still permitted on a 20A breaker.

-Jon

Where in NEC 2017 it says this:

“The actual ampacity may be used for purposes of derating and adjustment for ambient conditions.”

 

[david luchini]

Would not 240.4(D) make ampacity of #12 awg to be 20A?

No, it wouldn't. 240.4(D) is about overcurrent protection, not ampacity.

 

[roger]

Where in NEC 2017 it says this:

“The actual ampacity may be used for purposes of derating and adjustment for ambient conditions.”

110.14(C)

 

[augie47]

310.15 gives details on determining a conductors ampacity.
240.4 provides that the conductor must be protected at it's ampacity (with exceptions)

 

[wwhitney]

If I understand the OP correctly, all 4 circuits are supplied by the same 20A OCPD. If so, this is a scenario in which the NEC errs in requiring derating for the circuits when in a single conduit.

Cheers, Wayne