What size ground wire for 100-amp sub-panel?

[Jason White]

I just dug an 18" deep trench from the house to my detached garage and I'm about to run 3-gauge service cable for my new 100-amp sub panel. The garage is only 10 feet away from the house. The cable will be encased in PVC conduit. The main 200-amp service panel at the house is well-grounded, and a couple of electricians here in LA have told me that I don't need any additional grounding rods for the sub-panel, just a green grounding wire that will get run back to the main service panel.

Anybody know what size wire I need for the green ground wire? I think I read 8 someplace, but I want to be sure.

Thanks!

- Jason

[Mark Bolton]

100 amp feeder (al) is 2-2-4-6

[John Coloccia]

Jason, I would suggest you get to know your local inspector and ask him what he wants to see. Municipalities don't HAVE to follow the NEC, or even the latest NEC, but as far as I know, the NEC a ground rod (sometimes 2 ground rods) is required for detached structures.

[Rollie Meyers]

Jason, I would suggest you get to know your local inspector and ask him what he wants to see. Municipalities don't HAVE to follow the NEC, or even the latest NEC, but as far as I know, the NEC a ground rod (sometimes 2 ground rods) is required for detached structures.

The State of California mandates which code & when it is adopted, currently the 2011 is being enforced. 250.56 states if you cannot prove 25 Ohms of resistance or less then a 2nd one is required at least 6 feet apart from the 1st one & that is all that is needed, because the testing equipment is expensive it's cheaper & simpler to just drive the 2nd rod.

A jurisdiction may have stricter regs, but may not have lesser requirements, so the above quote does not apply in the State of California.

[David Castor]

For a circuit protected by a 100 A breaker, the minimum ground wire size is #8 copper or #6 aluminum.

[Matt Marsh]

Jason, A separate building always requires a grounding electrode system in addition to the equipment grounding conductor that you mention. The only exception is if the separate building is fed with just a single branch circuit (2014 NEC 250.32a). If you are using rod type electrodes, you really need to drive two of them, at a minimum of 6' apart. The only way that you can get by with a single one, is if you can prove by measurement that the ground resistance is 25 ohms or less. The measurement must be performed with a dedicated ground resistance meter, something that very few contractors have. It is almost always easier and more cost effective to satisfy the NEC requirement by supplementing one ground rod with an additional one (250.53(a)(2). Although the minimum size grounding electrode conductor is typically #8 copper for a 100 amp service (it's actually determined by feeder size, not amperage), the NEC requires that conductors smaller than #6 be physically protected (usually conduit). If you run a #6 bare copper GEC, you can run it exposed on the surface unless it is subject to severe physical damage (250.64(B).

[Jason White]

Thanks for the advice, guys! Just to be safe, I went ahead and installed 2 copper ground rods and connected with #6 bare copper (sleeved in PVC conduit until is disappears below ground). I also used a #6 THWN green ground wire to connect back to the main service panel. Grounds and neutrals in the sub panel are separated.




Jason, A separate building always requires a grounding electrode system in addition to the equipment grounding conductor that you mention. The only exception is if the separate building is fed with just a single branch circuit (2014 NEC 250.32a). If you are using rod type electrodes, you really need to drive two of them, at a minimum of 6' apart. The only way that you can get by with a single one, is if you can prove by measurement that the ground resistance is 25 ohms or less. The measurement must be performed with a dedicated ground resistance meter, something that very few contractors have. It is almost always easier and more cost effective to satisfy the NEC requirement by supplementing one ground rod with an additional one (250.53(a)(2). Although the minimum size grounding electrode conductor is typically #8 copper for a 100 amp service (it's actually determined by feeder size, not amperage), the NEC requires that conductors smaller than #6 be physically protected (usually conduit). If you run a #6 bare copper GEC, you can run it exposed on the surface unless it is subject to severe physical damage (250.64(B).

[Rollie Meyers]

100 amp feeder (al) is 2-2-4-6

Please note that the OP's situation does not fit the criteria in the NEC allowing one to undersize the conductors, which is where they handle the entire load of a residence, a feeder to a outbuilding or a feed to a subpanel are not allowed to use 2 AWG AL or 4 AWG CU for 100 amperes. The applicable section is Table 310.15(B) in 2011 and earlier NEC editions, it has been deleted in the 2014 NEC.

[Jason White]

I used #2 copper for both hots and neutral and #6 copper for the grounds.



Please note that the OP's situation does not fit the criteria in the NEC allowing one to undersize the conductors, which is where they handle the entire load of a residence, a feeder to a outbuilding or a feed to a subpanel are not allowed to use 2 AWG AL or 4 AWG CU for 100 amperes. The applicable section is Table 310.15(B) in 2011 and earlier NEC editions, it has been deleted in the 2014 NEC.

[Rich Riddle]

Here you could not get a pass on installing electrical wire 18" deep unless the circuit handles 20 amps or less. Here they mandate the NEC 30" just as if it's a commercial burial. It wouldn't matter if it was encased in conduit or not. Regarding the size of the grounding wire, I prefer to go one size larger than mandated for the grounding wire. There will also be a directive on the minimal length of the grounding wire. NEC is 8' minimal depth if memory serves correctly. Not sure of California code, but they typically meet or exceed the codes of the NEC.

[Rollie Meyers]

Here you could not get a pass on installing electrical wire 18" deep unless the circuit handles 20 amps or less. Here they mandate the NEC 30" just as if it's a commercial burial. It wouldn't matter if it was encased in conduit or not. Regarding the size of the grounding wire, I prefer to go one size larger than mandated for the grounding wire. There will also be a directive on the minimal length of the grounding wire. NEC is 8' minimal depth if memory serves correctly. Not sure of California code, but they typically meet or exceed the codes of the NEC.

There is no requirement for 30" burial in the NEC for 600V & less, (low voltage), medium & high voltage are not relevant here.

[Jason White]

I actually dug the trench closer to 20 inches so that I have at least 18 inches of fill on top of the conduit 1-1/2" conduit.



There is no requirement for 30" burial in the NEC for 600V & less, (low voltage), medium & high voltage are not relevant here.