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Phase To Neutral Calculations

Phase To Neutral Calculations

I am planning to run a series of e-ballasts on phase to neutral power, with an equal number of ballasts on each phase for a balanced load.  The phase to phase difference is 400v do I use that or the 400/SQRT(3)v when calculating watts.  It doesn't seem right that when you calculate watts using

Watts =V*I*pf*SQRT(3)

The answer is greater than the sum of all the ballasts watts.  Am I missing something here?  

Also am I right in assuming that line and phase voltage can be considered the same if your running the same number of identical ballasts on each phase.   

RE: Phase To Neutral Calculations

If the ballasts are connected line to neutral, you need to use the line-to-neutral voltage to compute watts used by each ballast.  Then add up how many ballast you have on each phase.


RE: Phase To Neutral Calculations

Your equation is for a Three Phase Power

3 Phase Watts: SQRT(3)* V(L-L)* I * pf

1 Phase Watt:  V x I x pf

RE: Phase To Neutral Calculations


dpc and zazmat are correct, in addition, base on zazmat formula.

Basic Ohms Law;

P(Watts) = V x I x pf (eq. 1)

V = Voltage, per your post, it is 400Volts
I = Current
pf = power factor, assumed as 1

Since you will buy lighting fixtures, the fixtures is complete with lamp and ballasts.

Let us assume 1 Lamp (40 Watts) per fixture. Sometimes, fixture has more than 1 lamp.

To find the line current (I);

From eq. 1;

I (Current) = P(Watts) / V (Voltage) x pf (Power Factor)

Substitute the given data;

I = 40/ 400 x 1

I = 0.1 Ampere

Your last question is confusing. Let me know the following.

1. Does your panelboard a 3-Phase, 4 wire?

2. What is the rated voltage of your input power supply.

If we follow your phase-phase voltage above, it should be 692.8 Volts.

RE: Phase To Neutral Calculations

If your phase to phase voltage is 400V then your phase to neutral voltage will be 231V.
1> Select your ballasts for either 400 volts or 230 volts.
2> Use the nameplate watts to determine the energy consumption.
3> Use the nameplate amps to determine the circuit loading and KVA.
4> If you use 400V fixtures you must connect them phase to phase and use a 2 pole breaker.
5> If you use 230V fixtures you must connect them phase to neutral and use a 1 pole breaker.
6> If you connect from phase to neutral you can not use a floating neutral. The neutral must be connected back to the supply transformer neutral.
7> There will be harmonic currents on the neutral and the neutral size must not be reduced.

"Why not the best?"
Jimmy Carter

RE: Phase To Neutral Calculations

  If these are fluorescent ballasts, and you are in canada, they have to comply with he following rule:
(4) Each fluorescent luminaire installed on branch circuits with voltages exceeding 150 volts-to-ground shall be:

(a) Provided with a disconnecting means integral with the luminaire that simultaneously opens all circuit conductors between the branch circuit conductors and the conductors supplying the ballast(s); and

(b) Marked in a conspicuous, legible, and permanent manner adjacent to the disconnecting means, identifying the specific purpose

RE: Phase To Neutral Calculations

What do they mean by "disconnecting means intergral with the luminaire"?  This sounds to me that a switch has to be integrated, that is, a part of, the luminaire.  This doesn't sound practical, so I must be interpreting it incorrectly.


RE: Phase To Neutral Calculations

A plug connection will serve.

"Why not the best?"
Jimmy Carter

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