# Double Check My Math?

Discussion in 'Lighting and Electrics' started by ScottT, Aug 31, 2011.

1. ### ScottTActive Member

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I have a SR48 dimmer rack fed by a 200A disconnect.

W=VA
W=120*200
W=24,000

Assuming a 750 HPL Source 4:

24,000/750=32 instruments

Is this correct?

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Is the 200A disconnect single-phase or three-phase? It's the difference between 24kW and 72kW. Is the 200A derated or faceplate?

Incidentally, it's P=VI (neglecting power factor)
where:
P = power (W)
V = voltage (V)
I = current (A)
3. ### rochemWell-Known Member

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Not quite. Chances are your 200A disconnect is actually three phase - so without complicating it too much, you actually get 200A * 3 phases = 600A of power to work with. So:

W=120(200*3)
W=120*600
W=72,000

72,000/750=96

Hey, that number looks familiar!

Hope that helps.

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No, there's more to it then that.

First, single phase or 3 phase installation?

If it's 3 phase (208Y/120), then you have 200A per leg, or 200A*208V*sqrt(3)=72kVA. This could also be looked at as 200A*120V*3=72kVA. (208V=120V*sqrt(3)) Assuming W=VA, 72kW/750W=96 fixtures.

If it's single phase (240/120), then you still have 200A per leg, but you only have 2 legs. This would be 200A*120V*2=48kVA. Assuming W=VA, 48kW/750W=64 fixtures.

Dang-nabbit, beat several times over.
5. ### DionysusMember

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Dang-nabbit you beat me to it!
By the way you may notice that things work out to 12, 48, 64, 96, etc. (circuits/fixtures), this is not be accident! Things are worked out to come easily to these easy to work with.

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