NYC. We have 208V "Wye" 3-phase.

I have two devices - a dryer and a water heater. Each takes 208V single-phase. I am also considering an air conditioning unit that will also take 208V. All are 23A draw for a 30A breaker.

But the conduit to their location already has 6 wires I cannot change, and only 3 wires remaining before we'd have to pull out all the wires and upsize for a 310.15(b)(3)(a) derate -- and the conduit won't physically fit that.

My question is: Can I run a single 3-phase circuit (3 wires, neutral not needed)... and wire the dryer from pole L1 to L2, and the water heater from L2 to L3?

I know how to do that with a subpanel... my question is how to do it without a subpanel. Can I put those 2-3 loads on one circuit? How would I size the breakers?

  • Do these appliances require specific breaker sizing in their instructions? – ThreePhaseEel Jul 6 '19 at 4:18

I can't comment on whether your code has anything specific to say about this but looking at this from a generic safety/functionality point of view.

The currents in each shared live conductor will partially but not completely cancel.

Assuming the loads are resistive, the currents to the loads are 60 degress out of phase, so assuming the loads are equal the total current in each shared live conductor is.

23A * 2 * cos(30°) = 39.8A

If the loads are non-resistive then the calculation gets more complex but 40A is probably still in the right ballpark. AIUI with the way American breakers are rated, if these loads count as "continuous" then that would require a 50A breaker.

The main question IMO is are the appliances suitable for connection to a 50A breaker? or do they require a 30A breaker to operate safely? In the latter case it would seem you need a subpanel.

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  • If all the conductors are confined to one conduit then Kirchkoff's 1st law indicates that the total current flowing in one direction inside the conduit at any instant is completely cancelled by the total current flowing in the other direction. – A. I. Breveleri Jul 6 '19 at 6:12
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    @A.I.Breveleri Yes for the whole conduit, but not for the shared single conductor. – SomeoneSomewhereSupportsMonica Jul 7 '19 at 4:40

The short answer is no, no, no. Pull it out and do it right. This is wrong in too many ways to count.

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  • Eh? Can you give justification as to why this is wrong? – ThreePhaseEel Jul 7 '19 at 11:23

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