Small Conductors and Conductor Ampacity Derating

[Elect117]

I wanted to make sure I am reading these things correctly before I reply to a contractor. I think I am confusing myself. The contractor ran 33 phase conductors of #10 in one conduit (like a 5in). The conduit is a 15' long run up to a box and doesn't meet any of the exceptions of 310.15(C)(1). So, I notified him that even if the loads were 1A, having the that many conductors in one conduit would require derating the conductors and needing to reduce the OCPD size from 30A to 20A (THWN-2-> 40A * .4 = 16A, rounded up to 20A).

Is that right?

I got back to the office and was reading 240.4(D) and noticed that it says:

240.4(D) - "Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied."

I don't know if I am confusing myself and was correct in my correction or if 240.4(D) means that even after all of the correction factors, you can still use a 30A OCPD.

 

[infinity]

having the that many conductors in one conduit would require derating the conductors and needing to reduce the OCPD size from 30A to 20A (THWN-2-> 40A * .4 = 16A, rounded up to 20A).

What size are the 33 CCC's, #10 AWG?

 

[Elect117]

What size are the 33 CCC's, #10 AWG?

Yes.

 

[augie47]

What is the nature of the loads (receptacles,lighting etc??)

 

[infinity]

having the that many conductors in one conduit would require derating the conductors and needing to reduce the OCPD size from 30A to 20A (THWN-2-> 40A * .4 = 16A, rounded up to 20A).

Your calculation is correct. Given that the original OCPD was 30 amps I'm assuming that these are not multiple receptacle circuits.

 

[wwhitney]

240.4(D)

This requirement of max 30A on #10s (except where allowed by 240.4(E) or 240.4(G)) is in addition to the base requirement of the first sentence of 240.4, not instead of.

So if the ampacity works out to 16A, you still need to use a maximum 15A OCPD on each circuit, or 20A using 240.4(B) (if the circuit supplies at most one receptacle). And the calculated load needs to be 16A or less.

Cheers, Wayne

 

[Elect117]

What is the nature of the loads (receptacles,lighting etc??)

Single 480V 3ph receptacle for each circuit. They were welders.

This requirement of max 30A on #10s (except where allowed by 240.4(E) or 240.4(G)) is in addition to the base requirement of the first sentence of 240.4, not instead of.

So if the ampacity works out to 16A, you still need to use a maximum 15A OCPD on each circuit, or 20A using 240.4(B) (if the circuit supplies at most one receptacle). And the calculated load needs to be 16A or less.

Cheers, Wayne

Thanks. That is what I thought.

Your calculation is correct. Given that the original OCPD was 30 amps I'm assuming that these are not multiple receptacle circuits.

Thank you. I was worried I misinterpreted the last part of the section and was basically telling them to do quite a bit more work fixing it.


I told them the max they could have in a single conduit with 30A breakers was 6 CCCs. (40*.8 = 32A). But now I am realizing they could have gotten away with 9 CCCs before the OCPD would need to be reduced ( 40*.7=28 , round up to 30A). I will email them.

 

[suemarkp]

Welders is one of the exceptions in 240.4(G), just like motors and HVAC units. So the breaker doesn't matter as much as the welder nameplate loads. They could be 15A effective welders with 30A circuit breakers.