EMT wiring and shared ground

[vlocci]

I plan on running some 1/2 EMT to power two 120 volt circuits, and 1 240 volt circuit.

Each circuit is 20Amps (12 gauge wire) I believe I'm limited to 9 total (12 gauge) conductors before reducing capacity of the wiring when using 1/2 EMT.

The plan is:

1) For the two 120 volt, 20 amp circuits: Pull two 12 gauge "hots", one neutral and 1 ground. All would be 12 gauge wire. I would install a double pole breaker with a tie bar. The separate circuits would share a neutral (and ground). 4 wires in total for these 2 circuits.

2) for the single 240 volt, 20 amp circuit: Pull two 12 gauge hots, one neutral. Use the previously mentioned ground. Install a double pole breaker. 3 wires in total.

I'm at 7 conductors, which I believe is in spec.

Am I legal in using the single 12 gauge ground wire for both the 120 volt circuits as well as the 240 volt circuit, or should I pull separate grounds for both?

Vin

 

[sberry]

Just as a question does this 240 need a neutral? Actually you can do this with as little as 4 current conductors and as little as 5 wires depending on the purpose of the 240 circuit. But, doesn't hurt to run and share a ground as long as its rated for the largest circuit, which in this case it is.

 

[vlocci]

Sberry

Thanks for confirming what I though I knew. <GRIN>

The 240 volt circuit does need a neutral (it is a VFD that requires it)

No excuses now for not getting started pulling wires.

Vin

 

[walrus]

If you're using EMT you don't need to pull a ground

 

[Aceman]

If you're using EMT you don't need to pull a ground

I would recommend it though.

 

[Charles (in GA)]

I plan on running some 1/2 EMT to power two 120 volt circuits, and 1 240 volt circuit.

Each circuit is 20Amps (12 gauge wire) I believe I'm limited to 9 total (12 gauge) conductors before reducing capacity of the wiring when using 1/2 EMT.

Not correct. Nine conductors (12 AWG THHN) is the MOST that you can run in the conduit. This includes ALL wires run in the conduit, as the limiting factor here is FILL of the conduit. (greater than two conductors you calculate at a max fill of 40% of inside area.

Your concern with de-rating comes after THREE current carrying conductors, where you de-rate the wire to 80% of capacity and after 6 total conductors, you de-rate to 70% of capacity. This may or may not be significant as THHN, 90 degree C conductors, CU, 12 AWG, 90C are rated at 30 amps to begin with.

Personally, I'd either run two different conduits, one for the 240 and one for the 120, or I'd use 3/4 EMT.

I agree with Aceman, run the ground, you never know when a set screw will loosen and the conduit separates, and you lose your ground. The wire insures its grounded.

Charles

 

[Gooch]

Not correct. Nine conductors (12 AWG THHN) is the MOST that you can run in the conduit. This includes ALL wires run in the conduit, as the limiting factor here is FILL of the conduit. (greater than two conductors you calculate at a max fill of 40% of inside area.

Your concern with de-rating comes after THREE current carrying conductors, where you de-rate the wire to 80% of capacity and after 6 total conductors, you de-rate to 70% of capacity. This may or may not be significant as THHN, 90 degree C conductors, CU, 12 AWG, 90C are rated at 30 amps to begin with.

Personally, I'd either run two different conduits, one for the 240 and one for the 120, or I'd use 3/4 EMT.

I agree with Aceman, run the ground, you never know when a set screw will loosen and the conduit separates, and you lose your ground. The wire insures its grounded.

Charles

when using 14, 12, or 10 AWG wire you don't need to deal with derating of branch circuits as long as your OCPD is rated 15, 20, or 30A respectivly.

 

[Aceman]

when using 14, 12, or 10 AWG wire you don't need to deal with derating of branch circuits as long as your OCPD is rated 15, 20, or 30A respectivly.

You're mistaken.

If what you said was true, Table 310.15(B)(2)a wouldn't exist.

 

[comedyman809]

why would you use a 2 pole breaker for 2 separate 120v circuits???

 

[Aceman]

why would you use a 2 pole breaker for 2 separate 120v circuits???

That's why:

The separate circuits would share a neutral

 

[Charles (in GA)]

why would you use a 2 pole breaker for 2 separate 120v circuits???

i would install a double pole breaker with a tie bar. The separate circuits would share a neutral (and ground).

NEC 210.4(B)

Charles