277V lighting from 3ph/4wire 480

[biofueler]

We are installing 480/277 3phase in our building. I want to install 277V lighting fixtures that use 1.9A @ 277V. Is the following logic correct?:

1) I should try to balance the current in the circuit by running equal numbers of lights on each leg.

2) wiring three fixures on each line to neutral results in 5.7A on each line, and a 3 bus breaker should be sized according to this current. They can share the same neutral wire.

3) the neutral wire needs to be sized accordingly to provide sufficient return path. According to Section 220.22, the neutral demand load shall be “the maximum unbalance computed load between the neutral and any one ungrounded conductor.” So I size that at 5.7A as well?

4) Doubling the unbalanced current is good practice, so it should actually be sized at 12A

Just want to make sure I am not missing something with respect to circuit sizing. I also see 480V ballasts are available and less common. I can see why the 277's are more popular if my assumptions above are correct.

 

[biofueler]

oops, after rereading the post above, in a 3ph unbalanced system, the max neutral current is the sum of the max current of two legs assuming zero current in the third leg. so in this case it would be (5.7A+5.7A) x 2 to provide for harmonics = 23A

 

[rbulsara]

1) Yes.
2)No. You should run three separate single phase circuits (phase and neutral) for each group of lights. Do not use common neutral. ( nor it will be practicle in field, but that is beside the point). Use a 1 pole breaker for each circuit, Do not use a 3 pole breaker.

3)While your code reference is correct, it is slightly misapplied here. You need to do as stated in answere to item (2) above and you do not worry about the common neutral. The neutral of each circuit will be same as the phase conductor size.

4)Doubling the neutral is a good practice only apply to a feeder, that is the feeder the panelboard from which you want to feed this lights. Using 3 seprate branch circuits, eliminates need for upsizing neutral. This again is true if almost all of the load fed by the panel is harmonic rich and the panel is going to be used neart its capacity. In your case dont worry about it.

5) 277 is more common for indoor commercial lights.

6) In reference to your second post, if you have no load in one phase, the net current in neutral will be vector sum of two vectors 5.7A in magnitude, 120 degree apart. Do your math analytically or graphically. Which should turn out to be 5.7.

 

[alehman]

I agree with the above except that in my part of the U.S., shared neutrals are common practice for 277V lighting circuits. The neutral sees only the unbalanced portion of the current, including triplen harmonics. For lighting circuits it is normally sized to match the phase conductors. There is normally no need to increase beyond that if using quality ballasts.

 

[ccjersey]

One thing more. In the same box on the wall you may not have 277V switches on 2 different circuits. This can be worked around by using separate boxes or using some lighting control equipment to remotly switch the light circuits.

 

[WoodrowJWeen]

It is common to install metal box partitions so that multiple 277V switches can be installed under one cover plate.

 

[bacon4life]

I think I remember reading something about needing two electricians for working anything over 300V, or needing to follow different safety precautions, hence a preference for using 277V ballasts. Is this true?

 

[itsmoked]

Two electricians in series?

 

[advidana]

Just remember this- normal neutral current will never be greater than the largest current on one leg plus the sum of the harmonic currents of all legs. In your case (just guesting)about 10 to about 27 percent sum for harmonics added to normal unbalanced three phase neutral system. therefore to be safe just a add a couple of amps for harmonic.The neutral could carry a maximum of about 7.7 amps.

The ballast manufacture what they use for the harmonics currents on light fixtures used on this project. Then these Harmonics values should be added per phase to the neutral to be fully accurate, instead of my best guest of 10 the 25 percent.

good luck

 

[fsmyth]

smoked, probably better to use 3.
Your own decision whether to connect Y or delta.
Don't, for Heavens sake, ground them, though.
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