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I have a 208/120V 3 phase circuit currently feeding an outlet for a drill press. I would like to change this to 3 separate branch circuits for lighting. Can I keep the shared neutral? I plan on keeping the common internal trip breakers in place

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    You will probably need to up size the neutral (or equipment ground) to the same size as the phase conductors. I almost never run neutrals to 3 phase equipment, but do run equipment grounds. The equipment ground is usually much smaller than the needed neutral. If the wire is sized correctly for the breaker(s) it could be done. – Ed Beal Nov 22 '16 at 20:14
  • What size is the current wiring? – ThreePhaseEel Nov 22 '16 at 23:23
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This only works with genuine 3-phase power. If you are using a "phase converter" to fabricate "3-phase" from a single-phase supply, Do Not Do This.

What you would be creating is called a multi-wire branch circuit (MWBC). There are several articles on here on how to do MWBC's, such as mine here, or you can Google it. You will need to pigtail neutral, mark the wires as grouped wherever they are near other wires, and as Kris points out, use a 3-pole breaker.

The circuit breaker would need to be sized for the thinnest wire including the ground. Sometimes 4-conductor circuits are installed with a smaller neutral than the hots, in that case the smallest wire decides the breaker size.

Also if you are feeding common USA 120V receptacles (i.e. NEMA 5-15 or 5-20), the breaker can be no larger than 20A. There are special rules which permit 30A for lighting circuits.

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  • Good answer. Also required would be a 3-pole 20A breaker to cut off all the ungrounded conductors. Add that little detail and you got my upvote. – Kris Nov 23 '16 at 0:27
  • @Kris -- it looks like the OP has a 3-pole breaker already – ThreePhaseEel Nov 23 '16 at 1:19
  • @ThreePhasrEel yes but maybe not 20A rated – Kris Nov 23 '16 at 3:14
  • @Kris We don't know that 20A would be the correct size breaker to use. Admittedly I'm not in the US and don't know how common it is, but it's not impossible that they've run 14 gauge. – SomeoneSomewhereSupportsMonica Nov 23 '16 at 4:03
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    Right, 15 ... 20 A should be sized accordingly and a 3-pole breaker. Harper already addressed the wire size and that's good, but I wanted to emphasize the importance of still using 3-pole breaker with the MWBC – Kris Nov 23 '16 at 13:20
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The three circuits need to be individually breaker protected. You have to replace the three pole breaker with three single pole ones. Save the old breaker in case of future upgrades.

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    Uh, have you not heard of multiwire branch circuits before? – ThreePhaseEel Nov 23 '16 at 1:26
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    @Trout, You got it wrong. It's the opposite. The 3-pole, or handle-tied, breaker is required for a MWBC. – Speedy Petey Nov 24 '16 at 0:45

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