ECN Forum Forums Code Related Discussion NEC & other Code issues What is it ? Help me out please ...

Here is the skinny ...

We are using a generator and the book states the following but didn't show the amps:

120/208 vac - 60Hz
Load capacity: 60Kw
Power factor: .8

So to find out the amps I used: kWx1000/1.73xExPF
60000/1.73x208x.8
Amps = 208.42596

So my question is what is it? Is it 208 amps per phase or is it 208 split across the 3 phases for 69-70 amps per phase?

The reason that I ask this is because each phase is reading 120-124vac not 208vac.

Any input would be greatly appreciated.

It's an easy oversight… "E" in your formula is intended to be phase-to-phase balanced quantity. Your voltage measurements were from connecting a meter phase-to-neutral... 120V.

The cure is repetition, repetition, repetition.

A sanity check may be looking at the stator leads—thinking, "Are these more like 70-amp leads, or are they closer to 200-amp leads?"

Working with transformers and capacitors use the same calculations, but take time and experience to make sense.

Commit http://www.themeterguy.com/ohms%20law.jpg to memory, then beat into your head the cases where 3ø and square-root-of-3 apply.

Another consideration is, “When should I use 208.42596 amps, as opposed to 208 amps… or 210 amps… or 200 amps?”

After that, there’s ELI the ICE man. And X=1/2pifL… and 2pifC…




[This message has been edited by Bjarney (edited 09-08-2003).]

Hey, I learned that memory aid as well!

Bjarney is correct, but I don't think he answered the question. "Is it 208 amps per phase or is it 208 split across the 3 phases for 69-70 amps per phase?" It is 208 amps per leg, maximum.

It is the same as a 200 amp three phase breaker. Each leg is capable of delivering 200amps, not 200amp divided by 3.

He correctly told you about the voltage issue, where the 208vac is phase to phase voltage, while the 120vac is one phase to neutral. The nameplate shows the two voltages, that is, 120/208 vac, and should also state that this is a 3-phase system.

I am a little concerned that you do not know the difference. Are you really qualified to be using test instruments on live voltages? If not, but have a desire, thats fine, but be careful! One slip could cost you your life.

Rick Miell