How to calculate Watts for 3 phase condensing unit

[lumbee]

I am doing a panel shedule for a small commercial building. I am trying to find out the watts of a condensing unit so that I can put it in each of the 3 phases. The unit is 208 Volts/ 3 phase. The minimum branch circuit ampacity according to the manufacturer is 24 amps. The compressor running load amps is 17 and the fan running load amps is 1.5 amps. How do I calculate the (volts x amps)?

 

[EEJaime]

Your load is the amps x volts as you said, so you have 17 plus the 1.5a = 18.5 x 208 x(sqrt3)[because it is 3 ph] which gives you 6660 volt-amps, (va). Normally we do our panel schedules in va not watts, but if you want to convert va to watts you multiply it by the power factor, which many use 0.8 as an average to give you about 5328 watts or 1776 per phase.

The minimum circuit amps, (MCA) is the minimum size breaker which can protect this circuit and is equal to 1.25 the largest motor plus the sum of any other motors or 1.25 x 17 = 21.25 + 1.5 = 22.75 amps. We would put this on a 25 A. circuit breaker with #10awg wire.

Another important figure used is to find out the maximum sized overcurrent protection,(MOCP), which is 2.25 x the largest motor + sum of the rest of the motors. Which in your case is 22.75 + 17 = 39.75 amps. So code would allow up to the next standard size or 40 Amp C/B, (with #8 awg wire).

This is also true for units which have multiple motors, air-cooled condensers, etc...

Hope that helps.

 

[lumbee]

EEJaime,

On my panel schedule, would the 5328 watts be in the column under all 3 phases or would I divde by 3 and put that number under each phase. I plan on using the phase with the highest watts and using that to calculate the maximum amperage.

Lumbee