1 phase to 3 phase amps

[dave_asdf]

Here's a question about single phase to 3 phase conversion that I'm having a little trouble figuring out on my own. I would like to supply a total of 10 single phase, 120V loads that will pull about 43 amps. These loads will be supplied by buss duct plug in breakers. My question is what kind of amperage should i expect on the breaker supplying the buss duct?

I realize somebody posts this same question every week but I just wanted to make sure i understood.
Thanks guys.

Edited to add:

43A (per rack) X 10 (Racks/row) = 430A

120V X 430A X 1.732 = 89.268 KVA 3-phase.

89.268KVA / 208 / 1.732 = 248A per phase going through the main breaker.

This was my first attempt but it seems a little high... should i have divided the 430A by 3?

 

[charlie b]

Whenever you deal with addition of loads, especially when you deal with single phase and three phase in the same problem, you should always convert to VA, do the additions, then convert back to amps.

In your case, you have 43 amps at 120 volts. That is 5160 VA. Since you have no other loads (e.g., three phase loads) to add to this figure, you can immediately convert to three phase amps. 5160 divided by 120 and divided again by the square root of 3 gives you 24.8 amps.

I know you could have skipped two steps by simply dividing 43 by 1.732. But I wanted to make the point that all adding must be done in terms of VA.

 

[dave_asdf]

Thanks for the fast reply and spelling that out for me Charlie.

I don't care how many times I calculate something, I always second guess my answer when 1.732 is involved.

-dave

 

[steve66]

You added an extra sqrt(3) when you figured the KVA.

You should have 10 * 120V * 43A = 51.6KVA.

51600/208/sqrt(3) = 143 amps.

Since you have 10 loads, one leg will get an additional load. That leg will have a slightly higher current, and the other ltwo legs will have a slightly lower current.

Steve