Yet another MWBC question.....pushing the limits........

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dionysius

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WA
I need to pack as many 650kVA 277V LED fixtures as possible using the minimum copper in 1/2" metal conduit.

Has anyone got a better solution than the following.......

Use......

3 pole 277V 20A common trip breaker
MWBC using one shared neutral
1/2" metallic conduit with 4 #12 AWG wires (3 phases A,B,C and neutral; assume neutral is not a CCC so no derating????)
Metal conduit is the ground conductor
8 650 kVA/277VAC fixtures per phase (650/277 = 2.35A and 20/2.35 = 8.5)​

So one can pack 3 x 8 = 24 fixtures on to each 1/2" conduit leaving the panelboard.......

Do you see any code issues??????

 
As Smart stated the neutral is a current carrying conductor however it should be fine since #12 @90 C is rated 30 amps. 30 x .8= 24 amps

310.12(B)(5) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit shall not be
required to be counted when applying the provisions of
310.15(B)(3)(a).
(b) In a 3-wire circuit consisting of two phase conductors
and the neutral conductor of a 4-wire, 3-phase, wyeconnected
system, a common conductor carries approximately
the same current as the line-to-neutral load currents
of the other conductors and shall be counted when applying
the provisions of 310.15(B)(3)(a).
(c) On a 4-wire, 3-phase wye circuit where the major
portion of the load consists of nonlinear loads, harmonic
currents are present in the neutral conductor; the neutral
conductor shall therefore be considered a current-carrying
conductor.
Click to expand...
 
If he does, run 'em as a 3Ø circuit. As 2-pole circuits, he'll use double the breaker spaces and substantially more wire, perhaps larger wire, but less than double the length.
 
Smart $ said:
If he does, run 'em as a 3Ø circuit. As 2-pole circuits, he'll use double the breaker spaces and substantially more wire, perhaps larger wire, but less than double the length.
Click to expand...

ofc. He may be limited by the fixtures being 277V, or the 1/2" conduit being existing. or both. I dont see how going to 480V would require larger wire than 277V tho.

eta: no code issues, but I wonder the application. Having repaired a LOT of conduit in a warehouse last week that got snagged by forklifts/loads, using the conduit as the EGC there would have been bad. Fortunately there was a ground pulled in there.

If 480V is an option, you can pull 3 3ph (9 #12s) in one pipe and still be good on wire fill and derating. or two boats like mentioned above.
 
Last edited:
JFletcher said:
I dont see how going to 480V would require larger wire than 277V tho.
Click to expand...
It won't for same VA, 277 just requires the neutral on top of the other three.

If equally split across the three phases it will work with floating wye point - but not a great idea because once something happens to throw the balance off you don't really know what series of failures will happen next throwing the balance off even more.
 
JFletcher said:
... I dont see how going to 480V would require larger wire than 277V tho.
Click to expand...
I was referring to running two wire circuits and getting the same amount of power to the load with more conductors in one raceway needing derating kind of thing. :blink:
 
JFletcher said:
Gotcha!

Are 277/480V breakers rated 100%? iow, can you (NEC wise) put a 20A continuous load on a 20A breaker?
Click to expand...

Your typical breaker you plug into a panelboard is not rated 100%. I don't know how likely it is to run into one that is 100% rated, I think 100% breakers are more common as units that are mounted individually, with lugs on both ends.
 
kwired said:
It won't for same VA, 277 just requires the neutral on top of the other three.

If equally split across the three phases it will work with floating wye point - but not a great idea because once something happens to throw the balance off you don't really know what series of failures will happen next throwing the balance off even more.
Click to expand...

I am not understanding what problems might occur voltage wise here, tho I can see that a short across 480V would take out everything on that 3p breaker whereas a problem with 277V would do the same, just 1/3 as many lights. Yes, 277V requires a neutral. 480V doesnt.

OP hasnt chimed in if 480V is even possible. He just asked about pushing the limits. 480V is taking it as far as they can go.

24 fixtures... 1st, 4th, 7th, 10th... on AB, 2nd, 5th, 8th, 11th.. on BC, 3rd, 6th, 9th, 12th on AC. No neutral, EMT as EGC. 75% as much wire (3 vs 4), less VD. or am I way off?
 
JFletcher said:
I am not understanding what problems might occur voltage wise here, tho I can see that a short across 480V would take out everything on that 3p breaker whereas a problem with 277V would do the same, just 1/3 as many lights. Yes, 277V requires a neutral. 480V doesnt.

OP hasnt chimed in if 480V is even possible. He just asked about pushing the limits. 480V is taking it as far as they can go.

24 fixtures... 1st, 4th, 7th, 10th... on AB, 2nd, 5th, 8th, 11th.. on BC, 3rd, 6th, 9th, 12th on AC. No neutral, EMT as EGC. 75% as much wire (3 vs 4), less VD. or am I way off?
Click to expand...
All I was saying is if you have 3000 VA total and you connected 480 volt loads you would have same current per line as you would have with 3000 VA total connected @ 277 volts across all three phases, but is highly recommended you run a neutral to carry any minor imbalance. If it were heating elements you get away with slight imbalance if you just floated the neutral point of the loads, with LED drivers, you may burn them up much easier. Then later on when some LED's of the luminaire begin to fail - you throw the balance off even further, a neutral will still stabilize voltage - a floating neutral point will just shift the neutral voltage as loads change.
 
JFletcher said:
I am not understanding what problems might occur voltage wise here, tho I can see that a short across 480V would take out everything on that 3p breaker whereas a problem with 277V would do the same, just 1/3 as many lights. Yes, 277V requires a neutral. 480V doesnt.

OP hasnt chimed in if 480V is even possible. He just asked about pushing the limits. 480V is taking it as far as they can go.

24 fixtures... 1st, 4th, 7th, 10th... on AB, 2nd, 5th, 8th, 11th.. on BC, 3rd, 6th, 9th, 12th on AC. No neutral, EMT as EGC. 75% as much wire (3 vs 4), less VD. or am I way off?
Click to expand...

480V is not possible here.....only 277V version is available.
 
kwired said:
Your typical breaker you plug into a panelboard is not rated 100%. I don't know how likely it is to run into one that is 100% rated, I think 100% breakers are more common as units that are mounted individually, with lugs on both ends.
Click to expand...

well, I might see a problem with putting 20A worth of commercial/industrial (continuous load) lights on a 20A 277/480V breaker. 210.20(A).

dionysius said:
480V is not possible here.....only 277V version is available.
Click to expand...

You stuck with the 1/2" EMT as well? and see above for a potential code problem.
 
I've kind of been overlooking some of the more specific questions in the OP.

Since the lighting is continuous loads and there likely isn't 100% breakers being used - we can only get 6 fixtures (per phase) on a 20 amp circuit.

If we are stuck with 1/2 EMT we can still get two "full houses" using 12 AWG in there, may not be a fun pull if long runs but is within the 40% fill capacity for THHN/THWN, and even if you count the neutral as a CCC ampacity adjustment on 8 CCC's still will allow 12 AWG.
 
Smart $ said:
LED drivers (aka power supplies, or ballasts as some are now calling them) are non-linear loads, thus the neutral is a CCC.
Click to expand...

A quibble, but IMHO a significant one:

The neutral of a full boat, even with only linear loads, is still a CCC if the load is not perfectly balanced. It is just not counted as a CCC for derating purposes. :)
 
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