Article 240.4(b)

[Ben Trueblood]

Can someone please clarify the intent of this Article. I know it is standard practice to run 500 MCM CU and set the OCPD at 400 Amps or parallel 500 MCM CU on an 800 Amp OCPD. With that said would it be allowed to install a # 6 THHN Cu conductor on a 70 Amp breaker because 65 Amps is not a standard size OCPD? Or is it understood to apply to larger size conductors only?

Thanks for the input.

 

[cpal]

Loading a No 6 75 deg C Cu to 65 A is compliant with the ampacities of 310.16 T as long as the other parameters are maintained.

240.4 (B) would allow a 70 A ocpd where as 240.6 does not list 65 A device.

Read 240.4 D small conductors. this section for general application actually limits the ocpd for conductor sizes 14 -10 CU and 12-10 AL

 

[winnie]

The intent of this article is to permit a circuit breaker of fuse that has a 'standard' trip rating to protect a conductor of lower ampacity than that trip rating. However the _load_ supplied by this conductor must be less than the ampacity of the conductor.

A 65A load on a 65A conductor on a 70A breaker: good to go.

A 68A load on a 65A conductor on a 70A breaker: no go.

(Note: there are more details involved in the application of continuous versus non-continuous loads, the various '125%' rules, specific application, etc.)

-Jon