Neutral Current, 277Vac

[dasarmin]

How do you calculate the Neutral Current in a 4Wire 3Phase 277Vac circuit.
For example, 15 parking lot pole lamps @ 400Watts each and pf of 0.88 will be powered from a 3phase 277Vac Lighting Panel.
I believe the total current would 6000 / 277 * 0.88 = 25Amps. Each phase wouild be balanced to carrier less current than the total current, say 5 lamps per phase for approximately 8.2Amps per phase.
But the Neutral should be sizes (wire size) to carry 25Amps.
Is this thought process correct?

FYI, each lamp is rated at 1.7Amps continuous.

 

[bluesmoke...oops]

The way you describe it you have 3 277v single phase circuits at about 8.2 amps each, not a 480V three phase circuit. So you would have to provide a neutral for each circuit.

 

[hurk27]

The max imbalance that any one phase could possibly see is 5 lamps, or 1.7x5 =8.5 amps, this is also the max the neutral should ever see. if all lamps are functioning like new, the neutral current would be 0 in a perfect world?

Or am I missing something?

 

[bluesmoke...oops]

it's not a 3 phase circuit. it's 3 single phase circuits.

 

[stew]

with a balanced load I agree that the neutral current should be at approximatly 0.

 

[hurk27]

it's not a 3 phase circuit. it's 3 single phase circuits.

but its connected in a 3-phase Y configuration as a multi-wire circuit,, so it figures the same as any 3-phase Y circuit, how do you figure a 3-phase Y service with single phase loads?

I have done hundreds of 277 volt multi-wire circuit just like this, and its one of the best ways to avoid voltage drop issues.

 

[bluesmoke...oops]

how do you figure a 3-phase Y service with single phase loads?

I have done hundreds of 277 volt multi-wire circuit just like this, and its one of the best ways to avoid voltage drop issues.

you sum up your load on each and balance them as best you can. and i'm all about not having to deal with voltage drop.

I don't do a lot of designs with a lighting control panel. For site lighting like this generally my plans would use a contactor that is operated with a photocell and time clock. Only time I used a lighting control panel is if it is specifically requested by the client.

 

[dasarmin]

Neutral Current, 480/277Vac

Neutral Current, 480/277Vac

I should have mentioned it's a 480/277Vac Panelboard feeding the 277Vac lamps. So it a 3 phase, 480v phase to phase or 277V phase to Neutral.
The more I thought about it I realized that if all three phases are balanced, say 5 lamps a phase, then there would not be any Neutral current. The only Neutrual currents will be created by unbalanced phase loads.

So for a balanced 5 lamp per phase, each of the 3 phases would draw 8.5Amps and Neutral current would be zero.
But for unbalanced load, say 5, 5, 6 lamps per phase, you would measure 1.7 Amps in the Neutral. Correct?

 

[david luchini]

So for a balanced 5 lamp per phase, each of the 3 phases would draw 8.5Amps and Neutral current would be zero.
But for unbalanced load, say 5, 5, 6 lamps per phase, you would measure 1.7 Amps in the Neutral. Correct?

Yes, that is correct. Or if you had say 5, 6, 6 lamps per phase, you would also measure 1.7Amps in the neutral.

 

[broadgage]

In practice the neutral current may be greater than suggested above, presuming discharge lamps as is probable.
Most discharge lamp ballasts draw a current at 3 times line frequency, known as third harmonic current.
Third harmonic currents add up in the neutral rather than cancelling out. In extreme cases this can result in a neutral current greater than the phase currents on a balanced load.
A more typical result is a neutral current of from 25% to 50% of the phase current.