calculate multi-fam service (help)

[*dmangalo]

I need to know if this is correct.

Given: 20 unit multi fam unit, fed with 120/208 volt single phase feeder. Originating from a 3 phase service. calculate the load.

I used this method: 275,000/208 * square root of 3 = 275,000/360.27 = 763.3 A.

would I regaurd this as a single phase calculation? Or a 3 phase calculation? I used a 3 phase in my calc. but am not sure this is the right way.

Thanks,
dmangalo

[*Wgoodrich]

If you are dealing with a single motor then you would not use the 1.732 [aka square root of three hot conductor of three phase] because the 1.732 is already involved in that amp rating of the motor. If you were to convert an amp rating of a motor from three phase amps to watts or volt amps then you would have to use the 1.732 to covert to watts then again to covert back to amps.

If you are dealing with watt already as you are in a multifamily dwelling then you WOULD have to use the 1.732 in your calculation because you are converting from watts that does not have the 1.732 in its calculation to amps that needs the 1.732 in its calculation.

Now if for some reason you just added up amps then you would not need to use the square root of three [aka 1.732] because it already has been done.

In your case you are adding up watts that does not have the 1.732 of three phase included until it is converted to amps. This is what you are doing therefor the answer to your question is yes you must calculate the watts divided by (volts times 1.732] to get your three phase amps.

20,000 voltamps / (208 volts x 1.732 {sq root of 3}) = 55.5 amps {three phase]

Hope this helps