Neutral(s) for step dimming ballast fed by 2 circuits?

We have designed a large room with 40 light fixtures, each with a step dimming ballast. Two 277V circuits feed this area from a 480/277V, 3 phase, 4 wire panel. Say the circuits are LP-1 and LP-2 (both on phase A). For our purposes, each ballast has 3 wires: a neutral, and 2 hots. (Feeding either hot lights the fixture's lamps at 50% brightness; feeding both hots lights the lamps at 100% brightness.) Each circuit (LP-1 & LP-2) has 2 wires, of course, a hot & a neutral. However, note that while the ballast has 2 hot leads, it only has 1 neutral lead. Neutrals for the 2 circuits were originally sized based on load, and are #10 from the panel to a local J-box. From the J-box to each fixture, #12's were run. The neutral lead in the ballast itself is #16. The neutral home runs were originally sized correctly for the load of the fixtures on that circuit. The electrician has asked: what neutrals does he connect? Currently he's connecting 1 neutral per fixture: the neutral for circuit LP-1. The neutral for LP-2 (again, same phase, same panel) isn't connected or run. Will this suffice? Or must he run the neutral for LP-2 also, in essence creating a parallel neutral feed from the panel, and connect both neutrals to the ballast's neutral lead? Please educate me. What's the right way to wire the neutrals here? Thanks for your time helping on this - you guys make the world safer and smarter.
 

GoldDigger

Moderator
Staff member
As far as I can see the only code compliant way to wire that configuration with one neutral and two hots at a fixture would be to use an MWBC. Which you do not have if both hots are derived from the same hot.
Otherwise using a common neutral wire for both hots at the luminaire would, as you say, require connecting the two branch neutrals together, which is not compliant.
 

iwire

Moderator
Staff member
I agree with GoldDigger, you can't supply these fixtures from two separate panels.
 

david

Senior Member
We have designed a large room with 40 light fixtures, each with a step dimming ballast. Two 277V circuits feed this area from a 480/277V, 3 phase, 4 wire panel. Say the circuits are LP-1 and LP-2 (both on phase A). For our purposes, each ballast has 3 wires: a neutral, and 2 hots. (Feeding either hot lights the fixture's lamps at 50% brightness; feeding both hots lights the lamps at 100% brightness.) Each circuit (LP-1 & LP-2) has 2 wires, of course, a hot & a neutral. However, note that while the ballast has 2 hot leads, it only has 1 neutral lead. Neutrals for the 2 circuits were originally sized based on load, and are #10 from the panel to a local J-box. From the J-box to each fixture, #12's were run. The neutral lead in the ballast itself is #16. The neutral home runs were originally sized correctly for the load of the fixtures on that circuit. The electrician has asked: what neutrals does he connect? Currently he's connecting 1 neutral per fixture: the neutral for circuit LP-1. The neutral for LP-2 (again, same phase, same panel) isn't connected or run. Will this suffice? Or must he run the neutral for LP-2 also, in essence creating a parallel neutral feed from the panel, and connect both neutrals to the ballast's neutral lead? Please educate me. What's the right way to wire the neutrals here? Thanks for your time helping on this - you guys make the world safer and smarter.
From your description I assume 40 ballast are feed from LP-1 and LP-2 and by the way you are switching the ballast the assumption would be two circuits would be needed for the load.
Is there a reason LP-1 could not supply both ballast leads for 20 fixtures and have LP-2 supply both ballast leads for the remaining 20 fixtures. I’m guessing the answer to that to be you would need one additional step dimmer
 

david

Senior Member
As far as I can see the only code compliant way to wire that configuration with one neutral and two hots at a fixture would be to use an MWBC. Which you do not have if both hots are derived from the same hot.
Otherwise using a common neutral wire for both hots at the luminaire would, as you say, require connecting the two branch neutrals together, which is not compliant.
i also believe a multi wire circuit using LP-1 and say LP-3 instead of LP-2 would work for the neutral current since this is a three phase 277 volt panel except you would be supplying the ballast with 480 volts
 

david

Senior Member
From your description I assume 40 ballast are feed from LP-1 and LP-2 and by the way you are switching the ballast the assumption would be two circuits would be needed for the load.
Is there a reason LP-1 could not supply both ballast leads for 20 fixtures and have LP-2 supply both ballast leads for the remaining 20 fixtures. I’m guessing the answer to that to be you would need one additional step dimmer
i should add step dimming by adding additional switching
 
Thanks

Thanks

From your description I assume 40 ballast are feed from LP-1 and LP-2 and by the way you are switching the ballast the assumption would be two circuits would be needed for the load.
Is there a reason LP-1 could not supply both ballast leads for 20 fixtures and have LP-2 supply both ballast leads for the remaining 20 fixtures. I’m guessing the answer to that to be you would need one additional step dimmer
David - I believe this is the answer - to simply feed given fixtures from 1 circuit. I like simple answers; thanks so much for your time & help. I may post a wiring diagram later for any who are interested, once I digest it all.
 
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