Multi Wire Circuits, 3 hots 1 neut. 1 ground 3 phase

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bigjohn67

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Ok. I need someone or everyone's opinion on this so I can print this out to show some people at work. Here is the case: We are dealing with pvc conduit.

I have a 3/4" pvc conduit in a slab. Thru this conduit I need 2- 3/0 multiwire circuits. (208v)20amp using #12 wire. Load on each circuit will be 9 amps.
It will be supplying 6- 120v lighting circuits.
Thus 2 sets of: 3 hots, 1 neutral and 1 ground
total of 10 wires.

Some of the guys say I only need to pull one #12 ground conductor to satify both sets. I say a #12 ground needs to accompany each set (2 ground conductors)

Any Help would be appriciated.
Also, would this change is it were emt? since the emt is metal and can be used as a ground?
 
Re: Multi Wire Circuits, 3 hots 1 neut. 1 ground 3 phase

John, you only need one EGC in this conduit.

Think of it this way, if only one conductor (120 volts in this case) faulted the #12 EGC would accomodate the fault.

If two conductors faulted to each other (240 volts bolted) they would clear themselves without the EGC's assistance.

Roger
 
Re: Multi Wire Circuits, 3 hots 1 neut. 1 ground 3 phase

If I'm getting it right. You are running 2 4-wire 3 phase multi-wire circuits, as such per each circuit= 3-Hots 1 neutral Then you only need 1 grounding conductor per race-way sized per the largest circuit which would be #12
You do have to have a neutral per each set of hots though so you will end up with 6-Hots, 2-Neutrals, 1-Grounding conductor.
And yes the EMT can be used as the EGC, and no grounding conductor would have to be pulled.

Just a note when doing multi-wire circuits make sure the neutral connections are done real good as loosing a neutral on these circuits can cause alot of damage and or a fire.

[ January 18, 2004, 09:52 PM: Message edited by: hurk27 ]
 
Re: Multi Wire Circuits, 3 hots 1 neut. 1 ground 3 phase

I'd say the EGC would have to be size to the largest over current device (20amp). So one # 12 should do for the ground.

The EMT would be a problem if you are below grade, it just does'nt stand up to good. 358.10(B)
 
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