Getting the Amps from Watts, Amps and the voltage of a line

Question

What amperage capacity should the supplied wired be rated for a refrigerated fixture which has a 208V power supply and the following loads?

• 4 Evaporator fan motor rated at 9W each
• 4 Evaporator fan motor rated at 9W each
• 6 Florescent lights rated at 40W each
• 1 strip Mullion heater rated at 3 A

What I tried was: (4*9W) + (4*9W) + (6*40W)+(3A*208V)= 936W Then I divided by 208V to get that to AMPS 936W/208V this answer is 4.5 but the expect answer is 20A

Can someone please should how you would calculate this. Thanks for the help!

Answer 1

Your answer is correct given the data we have. The motors will presumably have a higher starting current than running current, so in real life you need to allow for that. 20A is substantially higher than needed, however.

Answer 2

i just spent an hour agonizing over that too....eventually found that question is missing some info. This guide has the correctly worded question... www.aes.gov.nl.ca/app/exams/pdf/StudyGuide_RefrigAirCondMech.pdf‎

there are 2 defrost heaters not listed which add another 12 amps.