277V lighting from 3ph/4wire 480
277V lighting from 3ph/4wire 480
(OP)
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.






RE: 277V lighting from 3ph/4wire 480
RE: 277V lighting from 3ph/4wire 480
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.
RE: 277V lighting from 3ph/4wire 480
RE: 277V lighting from 3ph/4wire 480
RE: 277V lighting from 3ph/4wire 480
RE: 277V lighting from 3ph/4wire 480
RE: 277V lighting from 3ph/4wire 480
RE: 277V lighting from 3ph/4wire 480
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
RE: 277V lighting from 3ph/4wire 480
Your own decision whether to connect Y or delta. :)
Don't, for Heavens sake, ground them, though. ;)
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