277V Lighting - shared neutral

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cjfreeman

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I have come to work in a plant that uses 277V metal halide lights. The way they are wired is with a common neutral, but nothing is marked as such. I have looked but cannot find in the code the articles i am looking for. I was thinking that there was a voltage limitation to using a shared neutral. Can someone point me in the proper direction to the relevent articles for sharing a neutral in a industrial environment?
 
I don't think there is anything that forbids this. If you are under the 2008 NEC, however, there must be a means of simultaneously disconnecting power from all ungrounded conductors in the same multi-wire branch circuit.

Welcome to the forum.
 
Welcome to the froum, I think you will see what you are looking in multi wire branch circuits in art. 210.4
 
charlie b said:
If you are under the 2008 NEC, however, there must be a means of simultaneously disconnecting power from all ungrounded conductors in the same multi-wire branch circuit.
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Charlie...in a plant situation [maint.] are you implying the existing lighting circuits must comply with a code more modern than what was in place during original build out?
 
So what i am looking at is Art.210.4(C) and this would require using 2 pole breakers rather than single pole breakers because of the Shared Neutral if im not mistaken.
 
celtic said:
Charlie...in a plant situation [maint.] are you implying the existing lighting circuits must comply with a code more modern than what was in place during original build out?
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Not at all. I should have made that clear. Good catch.
 
cjfreeman said:
So what i am looking at is Art.210.4(C) and this would require using 2 pole breakers rather than single pole breakers because of the Shared Neutral if im not mistaken.
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Yes, or in the alternative you can use handle ties, but the requirement applies only if you are under the 2008 NEC. I think it's a good idea anyway.

But as was pointed out to me, you don't have to go back and retrofit the existing system. The new rule would only apply to new installations.
 
cjfreeman said:
So what i am looking at is Art.210.4(C) and this would require using 2 pole breakers rather than single pole breakers because of the Shared Neutral if im not mistaken.
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yes.and a three pole if its a 4 wire circuit. they make some type of handle for to tie single pole breakers together, but i dont have any experience with them.
 
charlie b said:
I think it's a good idea anyway.
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As do I
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The plant manger may not agree with our opinions, tho'
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charlie b said:
I think it's a good idea anyway.
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  • I think it is a bad idea.
  • I think the new rules about this in the 2008 stink.
  • I think ultimately it will cause more hot work, not less.

In a 'plant' the circuit is likely a 3 phase 4 wire circuit, we know it runs metal halide fixtures which have a long re-strike time so even just bumping then off looking for the circuit puts large areas out of light for 10 to 20 minutes.

Now lets say I want to work on one branch of the MWBC, past the point of the common neutral. This happens often and is perfectly safe by just turning off the one phase I want to work on.

Now with the new rules I have to make a choice, shut off all 3 poles putting the lighting out in a much larger area OR I can work hot.

Honestly, which way do you think most guys are going to go?


IMO they really did not think this through with an eye on the real world.
 
iwire said:
  • I think it is a bad idea.
  • I think the new rules about this in the 2008 stink.
  • I think ultimately it will cause more hot work, not less.
In a 'plant' the circuit is likely a 3 phase 4 wire circuit, we know it runs metal halide fixtures which have a long re-strike time so even just bumping then off looking for the circuit puts large areas out of light for 10 to 20 minutes.

Now lets say I want to work on one branch of the MWBC, past the point of the common neutral. This happens often and is perfectly safe by just turning off the one phase I want to work on.

Now with the new rules I have to make a choice, shut off all 3 poles putting the lighting out in a much larger area OR I can work hot.

Honestly, which way do you think most guys are going to go?


IMO they really did not think this through with an eye on the real world.
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I've been saying this too since I read the new requirement. A competent electrician can work on a MWBC when de-energizing one circuit at a time. In commercial office spaces it likely that you can have a large portion of the lighting fed from one 4 wire MWBC. Shutting off the entire floor isn't practical so the electrician will shut of nothing instead.
 
What I am looking at is rewiring a few fixtures that currently stay on to be turned off with others. None of the fixtures are moving, but will need to be rewired, meaning i will have to comply with 2008 code if im not mistaken.
 
cjfreeman said:
What I am looking at is rewiring a few fixtures that currently stay on to be turned off with others. None of the fixtures are moving, but will need to be rewired, meaning i will have to comply with 2008 code if im not mistaken.
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Typically, the requirement to comply with the code is when a new circuit is installed or when an existing circuit is extended. I have the impression that your fixtures are existing and the wiring is existing and you simply plan to reconfigure the conductors to allow a different switching scheme.
I would think this would be allowable without having to adhere to a newer code provision. :smile:
 
benaround said:
cjfreeman said:
What I am looking at is rewiring a few fixtures that currently stay on to be turned off with others. QUOTE]

I wonder why they didn't switch these few fixtures in the first place ?
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probably security lighting 24/7. we have alot of these. with all the financial troubles people are looking to save money. except for dorms we have disconnected and switched to on/off alot of 24/7 lights in non essential buildings. makes since these are unoccupied at night. never understood the reasoning behind 24/7 lighting(execept emergerncy stuff) in buildings after closing.
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iwire said:
  • I think it is a bad idea.
  • I think the new rules about this in the 2008 stink.
  • I think ultimately it will cause more hot work, not less.

In a 'plant' the circuit is likely a 3 phase 4 wire circuit, we know it runs metal halide fixtures which have a long re-strike time so even just bumping then off looking for the circuit puts large areas out of light for 10 to 20 minutes.

Now lets say I want to work on one branch of the MWBC, past the point of the common neutral. This happens often and is perfectly safe by just turning off the one phase I want to work on.

Now with the new rules I have to make a choice, shut off all 3 poles putting the lighting out in a much larger area OR I can work hot.

Honestly, which way do you think most guys are going to go?


IMO they really did not think this through with an eye on the real world.
Click to expand...

I agree with Bob here.
I do think in the real long run, designers just might employ multiple two wire circuits after feedback comes in.
 
iwire said:
Now with the new rules I have to make a choice, shut off all 3 poles putting the lighting out in a much larger area OR I can work hot.
Click to expand...


...or the plant can schedule the task during "off" hours.

Is working hot on 480 really an option?
 
I agree with Bob etc, and will add that I think this was an underhanded way by the CMP to eliminate the use of MWBC's without banning them outright.
 
sandsnow said:
I agree with Bob here.
I do think in the real long run, designers just might employ multiple two wire circuits after feedback comes in.
Click to expand...

I think you said it right there, I'm already back to 2-wire runs so as not to disturb the flow any more than necessary for future maintenance.
 
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