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.

 

[KS_Trout]

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?

Thank you.

GES will be handled by two 8' 5/8 copper plated rods, at ~6' apart. That's the plan anyway. This location is sitting on top of a ledge of rock. I'm hoping to not have to use a grounding plate(s) to pass GES impedance requirements. And of course the sub is not bonded.

There will be some induction motors run out in that shed / shop, along with the usual suspect loads for lighting and switchmode power supplies for small electronics / radio gear. I have a SWAG for the full load requirements at about ~32.5A if everything is running (which it would be VERY unlikely to be), not including start up. If everything comes on at once, there might be an issue back at the main I just tend to be wary of voltage drop, especially during start up. It looks like 40A draw using #6 puts me at about 3.9% drop for 131' (pls correct me if I did that wrong). If somebody wants to swap that breaker back at the main out for 60A later, well, it won't have my fingerprints on it

I know. 4% drop is huge (YUUUUGE) margin. But it makes my OCDness feel better.

 

[solarken]

There will be some induction motors run out in that shed / shop, along with the usual suspect loads for lighting and switchmode power supplies for small electronics / radio gear. I have a SWAG for the full load requirements at about ~32.5A if everything is running (which it would be VERY unlikely to be), not including start up. If everything comes on at once, there might be an issue back at the main I just tend to be wary of voltage drop, especially during start up. It looks like 40A draw using #6 puts me at about 3.9% drop for 131' (pls correct me if I did that wrong). If somebody wants to swap that breaker back at the main out for 60A later, well, it won't have my fingerprints on it

I know. 4% drop is huge (YUUUUGE) margin. But it makes my OCDness feel better.

I get 4.3% Vdrop at 240V and 40A with 131ft x 6AWG conductors, using Southwire calculator. Are the motors running at 240V or 120V? As others have said, why don't you just put in a 60A breaker and save money on the EGC by not having to upsize? It will not change the load behavior. Seems like a win-win all the way around.

 

[KS_Trout]

Well, the exception just reads very oddly to me. Seems like the code should call out the absolute minimum and that should be it. Saying "BTW, if somebody who is recognized by the local authority has some cipherin' that indicates a smaller conductor is ok, well then have at it" just doesn't give me a warm feeling. (I'm not sure that the code is actually fully influenced by solid engineering, but I also don't want to get into a pissing match with the inspector. DEFINITELY not trying to turn this thread into another code bashing exercise, but don't trigger me).

Going with #8 will cost me a little less than $30 more. We've spent more than that on this discussion

I'm going to just suck up the extra cost and go with a larger EGC and err on the side of safety rather than savings. Apologies for my OCDness; I do appreciate and respect the input received. Thank you to all who took the time to post suggestions.

 

[wwhitney]

Going with #8 will cost me a little less than $30 more.

Presumably you mean #6 EGC.

Cheers, Wayne

 

[kwired]

Well, the exception just reads very oddly to me. Seems like the code should call out the absolute minimum and that should be it. Saying "BTW, if somebody who is recognized by the local authority has some cipherin' that indicates a smaller conductor is ok, well then have at it" just doesn't give me a warm feeling. (I'm not sure that the code is actually fully influenced by solid engineering, but I also don't want to get into a pissing match with the inspector. DEFINITELY not trying to turn this thread into another code bashing exercise, but don't trigger me).

Going with #8 will cost me a little less than $30 more. We've spent more than that on this discussion

I'm going to just suck up the extra cost and go with a larger EGC and err on the side of safety rather than savings. Apologies for my OCDness; I do appreciate and respect the input received. Thank you to all who took the time to post suggestions.

The fact that (with no upsizing of the circuit conductors) the fact that 10 AWG is the allowable EGG for 25 through 60 amp circuits sort of makes this one the most debated. The reasoning for the rule makes sense but it is a bit of a one size fits all rule though there are some situations it still doesn't necessarily fit from a technical perspective.

There are some other one size fits all rules in NEC that can be questionable at times, particularly in areas of conductor ampacity and box fill. Then there are other times they are maybe not stringent enough