What size ground wire for 8 gauge THHN?

[thammel]

I'm installing a standby generator (actually re-doing the wiring so it is correct and per code). The current power feed wire from the genset to the transfer switch is in PVC conduit but is NM 8 gauge. I know this should be THHN but here's my question - what gauge should the green ground wire be? I think 10 gauge is ok, but want the experts' advice. I checked my stock of NM and found some interesting info - I have some 10 gauge nm that has a 10 gauge ground wire and is so labeled. I also have some 8 gauge nm that has a 10 gauge ground wire. I even measured them with a micrometer to verify they are the same size. Both are 3 wire plus ground. Interesting!

Thanks!
Tom

 

[TheEquineFencer]

Just out of curosity, what's the genrator rated, Amp/Volts, generator breaker amp rating ? How far away from the the transfer switch is the generator mounted?

 

[thammel]

It's a Winco 8kw. 240 volts. Max current at 240v with propane is 33 amps. Been working fine but need to change from nm to thhn in conduit.

Tom

 

[ishiboo]

I'm installing a standby generator (actually re-doing the wiring so it is correct and per code). The current power feed wire from the genset to the transfer switch is in PVC conduit but is NM 8 gauge. I know this should be THHN but here's my question - what gauge should the green ground wire be? I think 10 gauge is ok, but want the experts' advice. I checked my stock of NM and found some interesting info - I have some 10 gauge nm that has a 10 gauge ground wire and is so labeled. I also have some 8 gauge nm that has a 10 gauge ground wire. I even measured them with a micrometer to verify they are the same size. Both are 3 wire plus ground. Interesting!

Thanks!
Tom

I believe you'll need a #10 for #10 wire. A #10 will still work if you are running #8 hots/neutral.

 

[thammel]

I know that thhn has a slightly higher ampacity rating than nm sheathed. From what I see 10 thhn is good to 35 amps and 10nm is good to 30. 8 thhn is good to 50 but 8 nm is good to 40. Now, my genset puts out 33 amps max. So you'd think 10thhn would be ok. But the Square D on-line table says to limit 10thhn to 30. So I'm perplexed. I think it's safer to stay with 8,but cheaper to use 10!

Please comment on ampacity for 240 volt 33 amps!

Thanks!
Tom

 

[Aceman]

A #10 copper ground is good for up to a 60 amp breaker. You're fine.

 

[ishiboo]

I know that thhn has a slightly higher ampacity rating than nm sheathed. From what I see 10 thhn is good to 35 amps and 10nm is good to 30. 8 thhn is good to 50 but 8 nm is good to 40. Now, my genset puts out 33 amps max. So you'd think 10thhn would be ok. But the Square D on-line table says to limit 10thhn to 30. So I'm perplexed. I think it's safer to stay with 8,but cheaper to use 10!

Please comment on ampacity for 240 volt 33 amps!

Thanks!
Tom

I don't think there is a separate spec for generator feeds as there are service entrances, nor does the generator feed qualify as service entrance, but if there is I may be wrong with the below.

Code would dictate a 30A max for #10. I would run #8 with a #10 ground. As you know, that's good for 50A in conduit. You'd be at the VERY top of #10's capability (and, by code beyond) and the #8 would give you some decent expansion room.