Ground Wire Size

[kingchevy]

OKay, so I am going 160' from my 100 amp service to my 100 amp subpanel in the detached garage I'm building. I have 1 1/2 PVC conduit between the two panels with a steel 14x14x8 pull box in the middle of the run. I am planning on using 1 awg copper thhn to keep voltage drop around 2%. And yes there is a UFER in the garage.

Question 1: Is #3 the correct gauge for the ground?
Question 2: Should I make a loop with all the wires in the pull box or splice them?
Question 3: I would prefer to loop all the wires in the pull box, and if I do that, is a lay in ground lug the proper way to ground the box.

Thanks in advance.

 

[sberry]

I believe number 6.

 

[Ribbed Doors]

Why upsize to #1. 2 thhn is rated to 115 amps
#8 is sufficient for 100 amp ground. I always leave a loop in J boxes, and you dont need to cut and splice them, just a loop. And no extra bond bushing required if you dont splice

 

[kingchevy]

Wow, I guess I was way off on the ground size. Are you saying that I don't need to ground the metal pull box if there are no splices in it? I thought that the #2 would have voltage drop over 3% over that distance is why I was planning on #1 awg.

 

[alfredeneuman]

A #8 copper or #6 aluminum is the proper size ground wire for a 100 amp circuit.

 

[sberry]

That depends on the conductor size, if that's upgraded you need to bring the ground up a size. I would skip a pull box if I could. 3 number 2 and a 6 will go 1 1/2 if there are only a couple of bends.

 

[Aceman]

OKay, so I am going 160' from my 100 amp service to my 100 amp subpanel in the detached garage I'm building. I have 1 1/2 PVC conduit between the two panels with a steel 14x14x8 pull box in the middle of the run. I am planning on using 1 awg copper thhn to keep voltage drop around 2%. And yes there is a UFER in the garage.

Question 1: Is #3 the correct gauge for the ground?
Question 2: Should I make a loop with all the wires in the pull box or splice them?
Question 3: I would prefer to loop all the wires in the pull box, and if I do that, is a lay in ground lug the proper way to ground the box.

Thanks in advance.

1. At 160' I wouldn't worry about voltage drop. I would use #3 Cu hots, #4 Cu neutral, and #8 Cu green ground. Assuming you have some 240v loads out there reducing your neutral is okay.
2. Leaving a loop isn't a bad idea, I do it when I have room sometimes. If you ever burn up a lug in a panel(it shouldn't ever happen) you will have some slack to reterminate the wire.
3. If the box is isolated and has no other metallic paths back to the panel you need to bond it, even if you didn't splice in it. A lay in lug would be the easiest method.

 

[Speedy Petey]

Wow, I guess I was way off on the ground size. Are you saying that I don't need to ground the metal pull box if there are no splices in it? I thought that the #2 would have voltage drop over 3% over that distance is why I was planning on #1 awg.

What amperage are you basing the VD figures on? Typical you do not use the full feeder amperage when figuring VD. No reason to. I'd use the calculated load, and even then for a sub like this the load will likely be very low at any given typical time. Like 50A @ 240V or less.

 

[rlitman]

If the box is isolated and has no other metallic paths back to the panel you need to bond it, even if you didn't splice in it. A lay in lug would be the easiest method.

Yeah, but if he's using PVC conduit, why not just use a PVC box in the middle?

 

[Aceman]

Yeah, but if he's using PVC conduit, why not just use a PVC box in the middle?

You'd have to ask him. He asked about a metal box so that's what my answer was for.

 

[kingchevy]

Thanks for the replies. The pull box is recess mounted in a plaster wall and I didn't want a flimsy plastic cover is why I went metal. So I will bond that with a lay in lug and it sounds like I am okay with #2 awg copper with #6 green ground. One more factor, about 80' of the conduit goes through the attic, is derating an issue?

 

[Ribbed Doors]

One more factor, about 80' of the conduit goes through the attic, is derating an issue?

Dont worry about it. The wire is rated for a higher temperature than the attic will experience.

 

[wyliesdiesels]

Dont worry about it. The wire is rated for a higher temperature than the attic will experience.

Doesnt work that way. Ampacity needs to be adjusted based on ambient temperature NOT the temperature rating of wire!!

 

[Ribbed Doors]

Doesnt work that way. Ampacity needs to be adjusted based on ambient temperature NOT the temperature rating of wire!!

He wont come any where close to 100 amps on a 100 amp breaker. Its a sub panel for a garage, not a saw mill. If that panel see's 40 amps Id be surprised.

 

[wyliesdiesels]

He wont come any where close to 100 amps on a 100 amp breaker. Its a sub panel for a garage, not a saw mill. If that panel see's 40 amps Id be surprised.

Youre still missing the point.

If the ambient temperature is x, then the allowable ampacity of the wire needs to be adjusted by multiplying the 90* ampacity with the multiplier corresponding to the ambient temperature, in the NEC table...

Sure, he PROBABLY will never pull x amps through the wire but IF he does and the ampacity(and breaker size) wasnt adjusted for the ambient temperature, then he could have problems. Since nothing was adjusted, the possibility for issues is allowed...do it right or dont it at all...