Common neutrals and relay controlled circuits

BillConnerFASTC

Well-Known Member
Is there any reason to not allow common neutrals for power distribution with only air gap relays/motorized circuit breakers? Hasn't technology changed this?

Reviewing shop drawings with all kinds of notes about dimmers AND THERE ARE NONE.
 
The trend has been to home-run all neutrals. @STEVETERRY can probably clarify if that is even allowed, I don't think so. Even in the case of relays, circuit assignment can still leave the neutral overloaded. When I was growing up, it was common to have a common neutral between two circuits provided the circuits were on opposite legs. About the only time that is done now is when you are feeding a single device (like a dryer or oven in a home.)
 
I think multi wire circuits require the circuit breaker to disconnect all circuits - so two devices can be on separate legs and share a neutral if its a two pole breaker.
Correct.
I don't know the exact code section, but the NEC requires all phases feeding a neutral to be disconnected at the same time (common trip). The official term is "Multi-Wire Branch Circuit".
I believe it's so the neutral is actually dead, not just "mostly dead". While voltage is still zero, current is flowing. And I've personally seen what a not-dead neutral can do; think welder.
(There are reasons to also break the neutral but they're rare for most people).
 
The only down side to multi wire branch circuits is the handle tie requirement. If you overload one leg you can knock out up to 3 sets of outlets. Since a MWBC is normally wired close together and with the low power requirements of LEDs, that could take out a large chunk of lighting. Adding a "no MWBC" spec would result in individual breaker and a neutral per breaker.
 
So it should be one neutral per breaker, not no common neutrals. It really shouldn't have to be mentioned on a project without dimming because it is basic NEC, nothing particular to relay controlled stage lighting circuits.

Multiwire circuits, while common, have a few downsides:

1. Requirement for common means of disconnect per NEC 210.7. Practically speaking this means a 2 or 3 pole common trip breaker. The neutral (grounded) conductor need not be disconnected.
2. Can't be used for emergency lighting circuits per NEC 700.10.
3. May not be suitable for high harmonic content loads like switchmode power supplies in LED luminaires.

ST
 
Last edited:
Multiwire circuits, while common, have a few downsides:

1. Requirement for common means of disconnecto per NEC 210.7. Practically speaking this means a 2 or 3 pole common trip breaker. The neutral (grounded) conductor need not be disconnected.
2. Can't be used for emergency lighting circuits per NEC 700.10.
3. May not be suitable for high harmonic content loads like switchmode power supplies in LED luminaires.

ST

Steve

When we redid our theatre a few years ago, we went with ETC smartpacks. Ie pulling 20 amp 3 phase feeders to various locations. My thought was that we could convert some of our feeders into three 20 amp/ 120 v circuits for LEDs in the future.

You say “May not be suitable”. Can you be more precise. IE should I plan on only getting one 20 amp circuit from each feeder if I am plugging in LED fixtures

If it matters to the discussion, I am not talking about some kind of adapter cable, but something like a lex bento box.

Thoughts?
 
Steve

When we redid our theatre a few years ago, we went with ETC smartpacks. Ie pulling 20 amp 3 phase feeders to various locations. My thought was that we could convert some of our feeders into three 20 amp/ 120 v circuits for LEDs in the future.

You say “May not be suitable”. Can you be more precise. IE should I plan on only getting one 20 amp circuit from each feeder if I am plugging in LED fixtures

If it matters to the discussion, I am not talking about some kind of adapter cable, but something like a lex bento box.

Thoughts?
I think you'll be fine.
Inrush current will likely be your limiting factor as to how many fixtures you can put on each 20A circuit.

ST
 
The trend has been to home-run all neutrals. @STEVETERRY can probably clarify if that is even allowed, I don't think so. Even in the case of relays, circuit assignment can still leave the neutral overloaded. When I was growing up, it was common to have a common neutral between two circuits provided the circuits were on opposite legs. About the only time that is done now is when you are feeding a single device (like a dryer or oven in a home.)
I was rewiring some outlets in my house and found out the hard way that was how it was wired.
 

Users who are viewing this thread

Back