Silly and simple question for all the electrical gurus (I'm mechanical newbie)

[downtownfool]

We have a single phase 480V to 240/120V transformer being fed from a Motor Control Center by two phases/legs (phase A and C)...if the only load on the transformer is 240V (say 40 Amps) do I then say that phase A has 40A and phase C has 40A? What if there is 120V load? Can this load be on only Phase A? Only on Phase C? Or a combination thereof?

 

[dpc]

The 240 V load is as you say. The 120 V loads will be on either Phase A or Phase C depending on which side of the panelboard the breaker is plugged into. Normally, we try to balance the 120 V load between the two legs. In terms of the load on the transformer, your concern would be the higher of the two legs (A or C) - the sum of the 240 V and 120 V load combined. You could, in theory, put all of the 120 V load on one or the other, but there is no reason to do that and good reasons not to.

 

[downtownfool]

Thank you. So to clarify, if there is only 40A of load at 240V will the 480V side of the transformer (fed by phases A and C) see 20A on each phase or will it be split between the two (A = 10A and C = 10A)?

 

[dpc]

20 A on each leg at 480 V

 

[waross]

480:240/120 Volts.
One half of any 240 Volt load will be seen on both primary phases, A and C.

One quarter of any 120 Volt load will be seen on both primary phases, A and C.

Bill
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"Why not the best?"
Jimmy Carter