Double Check My Math? is being discussed in the ControlBooth Lighting and Electrics forum; I have a SR48 dimmer rack fed by a 200A disconnect . W=VA W=120*200 W=24,000 Assuming a 750 HPL Source ...

1. ## Double Check My Math?

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?

2. ## Re: Double Check My Math?

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. ## Re: Double Check My Math?

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.

4. ## Re: Double Check My Math?

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. ## Re: Double Check My Math?

Originally Posted by epimetheus
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.
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.

6. ## Re: Double Check My Math?

Originally Posted by ScottT
W=VA
Although we all know what you were trying to say, watch out putting V and A together like that as VA is yet another term.
see:
Watts the difference?

7. ## Re: Double Check My Math?

Originally Posted by rochem
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.

The bad thing is that, the person who held my position before me had a Sensor Portable Pack installed on the catwalks because she assumed (incorrectly, like I did!) that our power was so severely limited. Anyone want to buy an empty Sensor Portable Pack?

Originally Posted by JD
Although we all know what you were trying to say, watch out putting V and A together like that as VA is yet another term.
see:
Watts the difference?
Yeah, yeah, yeah. Right as always JD

Thanks for all the help guys!