480v Lighting

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darrin

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I have 480v low bay lighting fixtures that I am putting on a three phase, 20 amp circuit breaker. Naturally I will be wiring the fixtures in a delta fashion. One on Phase A-B, One on Phase B-C, and One on Phase A-C. What is the TOTAL amount of fixtures that I can put on one said three pole breaker. Each fixture is drawing approximately 1 amp RLA. Refresh my memory on the calculations.

[ August 13, 2004, 10:19 AM: Message edited by: darrin ]
 
Re: 480v Lighting

If we assume the load is continuous-

20 x .8 = 16 amps per line
16 / 1.732 = 9.2 amps per phase
9 fixtures per phase x 3 = 27 fixtures

Ed
 
Re: 480v Lighting

Ed,
Why did you include the 1.732 factor? We are not converting between line-line and line-neutral loads. A three pole breaker is able to carry full rated current in each of it's poles, so you only have to use the 80% factor for an effective loading of 16A per phase. In a balanced system this would allow 16 1A fixures on AB, BC, and CA for a total of 48 fixtures.
 
Re: 480v Lighting

I'm with Ed here. Each breaker pole carries current from 2 phases. Since the currents are 120? out of phase, 9.2A+9.2A=16A. Lets assume each fixture is a 480watt load. (the current is 1A/fixture). (27 fixtures*480V)/(480V*1.732)=15.58A
 
Re: 480v Lighting

If I remember correctly (and yes that is a big if), the square root of three is not used in delta calculations.

The vectorial formula for the addition of the B and C phase currents is:
It=(.5Ib+j.866Ib)+(.5Ic-j.866Ic)=(.5Ib+.5Ic)+(j.866Ib-j.866Ic)
If both phase currents are equal, the irrational numbers cancel and the formula becomes:
It=(.5Ib+.5Ic)=Ib=Ic+16A
 
Re: 480v Lighting

The three pole breaker confuses me and you're worried about 1.73

Oh I see what he is doing now we always used just two pole breakers. As usual I didn't read the post good enough. But I don't see where 1.73 would come into play either.

Ronald :confused:

[ August 13, 2004, 05:11 PM: Message edited by: ronaldrc ]
 
Re: 480v Lighting

Why did you include the 1.732 factor?
Click to expand...
The 480 volt fixtures are connected "line-to-line".
In effect, the groups of fixtures are connected delta, in relation to each other. (sketch)

In a delta connection, I line = I phase x 1.732.

Ed

3ph8.gif
 
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