Grrrr.... upsizing ground wire based on circuit conductor size only????

[KS_Trout]

Long time lurker. First post.
I am seeing that I may need to upsize a grounding conductor for my run to a subpanel. Not because SERVICE capacity is increased, but because wire size is. I'm running a 40A service to a small outbuilding, service distance panel to panel of 131'. I chose #6 THWN for both phases plus N. Initially, it appeared that 250.122 calls out a 10AWG Cu ground wire, but because I am using #6, I'm finding references online that I have to upsize my ground wire as well, to a #8 Cu. Is this correct? How incredibly frustrating if so.

 

[don_resqcapt19]

That is correct...the EGC has to be increased in proportion to the increase in the size of the ungrounded conductor. See 250.122(B).
I your case you are using a 6AWG (26,240 circular mils) in an application that an 8AWG (16,510 circular mils) has sufficient ampacity for the 40 amp circuit. That requires that the area of the EGC be increased by a factor of 26240/16510 or 1.589. The code required EGC for that 40 amp circuit is a 10AWG with a circular mill area of 10,380 and the increase will require the EGC to have a circular mil area of 16,493. The next size larger is an 8AWG.

Not a big issue if you are using conduit and wire, but becomes a very big issue when you are using cable wiring methods.

 

[KS_Trout]

Tnx. Yes. Planning on using 1 1/4" PVC sch 40. Hoping that dragging 3 #6 and one #8 will be on easier side of reasonable effort. I have no more than 180 deg of corners between pull points. I think I will enlist my son in law to apply the necessary force. He works for ribeyes.

 

[electrofelon]

Why not just put a 60A breaker on it?

 

[infinity]

With 1.25" raceway this should be no problem installing the conductors. You might even be able to push the wire in. What are you using for the GES? Also as EF suggested why not use a 60 amp OCPD?

 

[ptonsparky]

Why not just put a 60A breaker on it?

Yes. The circuit suddenly realizes enough current will flow to trip a 60, but not a 40.

 

[kwired]

Yes. The circuit suddenly realizes enough current will flow to trip a 60, but not a 40.

60 amp breakers are just smarter than 40 amp breakers.

BTW if it were a GFCI a 14 AWG EGC likely will carry enough current to allow it to trip. Copper industry doesn't want you to know that though.

 

[retirede]

Yes. The circuit suddenly realizes enough current will flow to trip a 60, but not a 40.

Don’t conflate common sense with code compliance!

 

[wwhitney]

Don’t conflate common sense with code compliance!

250.122(B) Exception (new in the 2020 NEC) provides a way to reconcile common sense with code compliance.

IF you know that for the distance involved (131' in the OP), #6 Cu THWN would be adequate for a 60A circuit, then you know that #10 Cu would provide an "effective ground fault current path". Changing the breaker from 60A to 40A doesn't change that. Therefore you may invoke the exception and use a #10 Cu EGC with the 40A OCPD.

Cheers, Wayne

 

[LarryFine]

Nobody really knows if or why you increased the conductor size.

 

[Speedskater]

Even thou it's highly unlikely the an EGC will ever be called, in a Ground Fault condition to carry it's rated current for 3 hours.