Electrical – Calculating power with three phase

three phase

I can't seem to find the answer I'm looking for, or perhaps I'm just really not understanding what I've read. I'm just confused.

So, I'm a college student in a Theatre Production program and an assignment we've been given in our lighting course asks the following:

"You will be staging a production in a small theatre, using only 120-volt conventional fixtures. Power is available from a disconnect dedicated exclusively to stage lighting. It provides 100 Amps, 120/208 Volts 3-Phase. What is the maximum amount of power at 120 Volts available for your lighting gear?"

Is this as simple as calculating the watts with the 100 Amps and listed 120 Volts? If so, is the 208 even really relevant here, or just something to make me overthink (with the whole 0.8PF and the square root of 3 and stuff)?

Many thanks in advance!

Best Answer

The 100 amp, 120/208 volts, 3-phase rating tells you that the 120 volts is available from each of three 120 volt line to-neutral circuits. Each of those three circuits provide 100 x 120 = 12,000 watts for 36,000 watts total.

You mentioned 0.8pf, but conventional incandescent fixtures would have 1.0 pf. If you have been told that the fixtures are LED fixtures with 0.8 pf, the power available would be 0.8 x 36,000 watts.

If 100 amp, 120/208 volts, 3-phase is available from a single disconnect it would be a 3-phase disconnect supplying the three circuits described.

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