Amps between 2 panels and xfmr

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Kuzu

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I have a 400A 480Y/277/4W panel and a 225A 208Y/120/4W panel. As of now I have the 480 panel feeding a 75kVA xfmr which then leads to the 208 panel. For the breaker protecting the xfmr I used a 90A 3 pole breaker (75kVA / [480 * sqrt(3)]). Now my real question is how do you calculate the amperage on the 480 side?

225A panel has phase A = 178A, phase B = 189A, and phase C = 182A. When I show the amps used on my 480 panel do I use these values or is there a similar transformation taking place (like how I found the breaker size).

Thanks for the help!
 
Re: Amps between 2 panels and xfmr

The easy way is to convert the 208 volt amperage to KVA. Then use that KVA to convert to amps on the 480 volt side of the transformer. Why not take a clip on reading on the 480 volt side?

[ June 14, 2005, 11:32 AM: Message edited by: bob ]
 
Re: Amps between 2 panels and xfmr

KVA = volts x amps x 1000. KVA is load. Load is the same at all voltages. Higher volts means less amps.

You have about 69 kVA of load on the transformer. (190 amps x 120 V x 3 phases). Primary current will be about 83 amps at 480. If you really want to know the currents in each primary phase, you need to know the phase angle of each of the phase currents at 120 V and do the vector math. Since your loads are probably guesses anyway, it is not worth the effort.

IMHO you should put down 90 amps on your panel schedule or just list it as KVA.

With a 90 A full load on the transformer, you should look at a 100A or 125A CB size minimum. The 90 amp will nuisance trip and is undersized if the loads are continuous. Size the cables to match the CB size.
 
Re: Amps between 2 panels and xfmr

Thanks for the help... I'm a little confused at where you are getting the 83A primary current from..? If xfmr is 69kVA and u solve for amps, isn't that going to be 144A on the primary?

[ June 14, 2005, 12:07 PM: Message edited by: Kuzu ]
 
Re: Amps between 2 panels and xfmr

I confused you with my general statement about volts amps and KVA. For a 3-phase transformer, I= (KVA x1000)/(Volts x 1.732), where volts = line-line volts. Just like you calculated.

I rounded your load to 69 kVA three phase. At 480V, 69000/(480x1.732) = 83 amps.

Your 208/120 panel had phase currents of about 190 amps. At 120 V, 190 amps is 22.8 kVA, x three phases is 68.4 kVA, about 69 kVA.
 
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