Conductor Sizing

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whisperinghill

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What is the correct wire size for this installation?

If you install two 240v 30A receptacles and run conductors in same conduit, you have 4 current carrying conductors. That requires a derate to 80% IAW Table 310.15(B)(2)(a).

And you would normally select a size THHN 10 AWG for a single receptacle. IAW Table 310.16.
A 10 AWG at 75C is rated at 35A, but IAW your article Conductor Sizing and the National Electrical Code you would use the 60C column for 100A or less, well THHN is not listed in the 60C column of the ampacity charts.

So I would still use 75C column. And since 80% of 35 is only 28A this would require me to use 8AWG for this install.

But!

IAW Table 210.21(B)(2) Maximum Cord-and-Plug-Connected Load to Receptacle a 30A receptacle is only allowed to have a 24A load connected, the THHN 10 AWG derated at 80% to 28A is really a correct choice?

or is 8AWG still the correct choice?
 
Re: Conductor Sizing

If you have THHN conductors you use the 90?C rating for ampacity adjustments. This will make the #10 good for 32 amps in your case.
Don
 
Re: Conductor Sizing

whisperinghill: I agree with Don. But keep in mind the constraint of 240.4(D): Max 30 amp breaker (which I am sure you had in mind anyway).
 
Re: Conductor Sizing

Don and Charlie,
Thanks for taking the time to reply.

I have always had the issue of determining correctly if I can use the 90C column. Reason is most times I have chosen the 75C column is you can only pick the highest temp of the lowest part of your system.

IAW 110.14 Electrical Connections. C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

As for mine we have GE THQB breakers with 60/75C ratings. Does this not limit me to the 75C column?

As I read the above code, since the wire is rated at 90C and it is higher than my breaker rating, I can still use the 90C for adjustments?

But if no adjustment or correction was necessary, like installing a single receptacle I would need to use the 75C columns for this particular breaker?

Bud
 
Re: Conductor Sizing

You can use the 90C column for adjustments, but your final answer cannot be higher than the limits of your terminations. Here are two examples:

Example #1: 90C conductors (THHN), 4 current-carrying conductors in a conduit (i.e., 80% de-rating), size #10 awg, with breaker terminations rated only for 60C.

Solution: Start with 40 amps (90C column for #10), multiply by 80%, and get 32 amps. Now look at the 60C column for #10, and see ?30 amps.? You must use the lower of 32 and 30, so this circuit is good for 30 amps. Note that you do not have to start with the 60C value of 20 amps, and multiply it by the 80%, so your conductor is not limited to 16 amps!

Example #2: 90C conductors (THHN), only 2 current-carrying conductors in a conduit (i.e., no de-rating), size #10 awg, with breaker terminations rated for 75C, but this time put it in an area with an ambient temperature of 35C.

Solution: Start with 40 amps (90C column for #10), look at the temperature de-rating table for the 90C column, and see 96%. Multiply 40 amps by 96%, and get 38.5 amps. Now look at the 75C column for #10, and see ?35 amps.? You must use the lower of 38.5 and 35, so this circuit is good for 35 amps. Note that you still have to limit the overcurrent protection for the #10 to 30 amps.

Final note: As you say, if there are no adjustments to be made, that you might as well skip this setp, and read directly from the 75C column.

[ July 25, 2003, 12:34 PM: Message edited by: charlie b ]
 
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