Feeding 1/0 to sub panel...what ground size?

[66DDart]

Hoping someone can give me a quick answer. In short, I'm looking to run 75' of 1/0 from the main panel to a 125A sub on the far side my garage (~75' up, over, and back down). I know 1/0 is over kill...but, I lucked into 250' of THHN much cheaper than I even think of buying #1. My question is, what size ground would you run with 1/0? I _think_ I could get by with #4 on a 125A circuit but, I'm thinking #2 would be best...if I ever sold the building for for commercial use (or want to upgrade the power later). Suggestions?

Thanks in advance!

 

[pattenp]

Aluminum or copper? #4 if Al, #6 if Cu. if feed is over 100A.

 

[66DDart]

Aluminum or copper? #4 if Al, #6 if Cu. if feed is over 100A.

My bad...copper. I'm kind of surprised it's that small but, I guess it's just being used to trip the breaker, not necessarily carry a load at all.

Thanks for the info!

 

[wyliesdiesels]

how much did you pay for the 1/0?

 

[66DDart]

how much did you pay for the 1/0?

$0.85/ft

 

[wyliesdiesels]

$0.85/ft

yeah thats a really really good price.

 

[sberry]

My bad...copper. I'm kind of surprised it's that small but, I guess it's just being used to trip the breaker, not necessarily carry a load at all.

Actually,,,not only not bad but good understanding.

 

[bjcouche]

Correct, 6awg copper ground for a 125A subpanel feed when the feed breaker is rated 101-200A.
Keep in mind that if you size your breaker for 90A and still use your 1/0 wire, you are essentially oversizing the wire. This is usually done for voltage drop over long distances. HOWEVER, Oversizing the wire for any reason requires you to ALSO oversize your ground above the minimum size. For example, if you use a 90A breaker, it would require a 8awg (copper) ground, but if you are oversizing your hot conductors, then the ground wire would be required to be larger (than 8awg) as well.
I point this out because you were asking about being code compliant, and I didn't know if you were considering less than a 125A feed breaker.
Brian

 

[Aceman]

Like bjcouche said, your ground wire needs to be upsized to account for the oversized ungrounded conductors being used for a 125A feeder.

1/0 is 1.26x larger than #1.

Your ground wire needs to increase proportionately.

A standard #6 CU EGC then becomes a #4 CU in this case.

 

[pattenp]

Like bjcouche said, your ground wire needs to be upsized to account for the oversized ungrounded conductors being used for a 125A feeder.

1/0 is 1.26x larger than #1.

Your ground wire needs to increase proportionately.

A standard #6 CU EGC then becomes a #4 CU in this case.

I have never understood that code requirement. I don't know why the code makers didn't just say the EGC needs to be sized from table 250.122 based on the max ampacity of the ungrounded conductors. I just don't get why using #1/0 with 125A panel should require a #4 Cu EGC.

Edit: I should say in this case where the oversized conductor is not because of voltage drop.

 

[sberry]

Can use 6 for 200 and 3/0 copper. Agree that it doesnt appear its being upsixed for drop here.

 

[Norcal]

Can use 6 for 200 and 3/0 copper. Agree that it doesnt appear its being upsixed for drop here.

Does not matter if it was upsized for voltage drop or not, you increase the ungrounded conductor size, you increase the grounding conductor size. It used to only be required for conductors increased in size for voltage drop.

 

[sberry]

Ok. So any time we use a smaller breaker on a bigger wire we need to upsize?
If we have a number 2 alum wire it takes an 8 copper with a 90 but if we use a 60 we need a 6.

 

[sberry]

I do see a 2/2/2/4 and I figured they include a bigger ground as its often used on long circuits with smaller breaker but in that case it was ' drop.