I have a step down 3 phase Delta/Wye transformer. 400 volts on the primary and 208 volts on the secondary. I have about 4 amps per phase on the primary and 13 amps on the secondary.

How is this possible? It should be around 8 amps on the secondary but the amperage has more than tripled. This means that the transformer is generating over 1 KW of power by itself.

Has anyone else seen this?

  • \$\begingroup\$ Can you post a picture of the transformer's rating plate or nameplate? \$\endgroup\$ – Li-aung Yip Sep 1 '15 at 14:48
  • \$\begingroup\$ Did you measure the actual voltage too? \$\endgroup\$ – Olin Lathrop Sep 1 '15 at 14:51
  • \$\begingroup\$ No, but it is a 50 KVA Delta/Wye K-13 xfmr. \$\endgroup\$ – Matt Wells Sep 1 '15 at 14:51
  • \$\begingroup\$ That is the actual voltage. \$\endgroup\$ – Matt Wells Sep 1 '15 at 14:52
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    \$\begingroup\$ Post a schematic as to how exactly it is hooked up. I will guarantee you though that the transformer is not generating any extra power. \$\endgroup\$ – R Drast Sep 1 '15 at 15:00

I think the 3-phase input complicates things so the calculation is not the same as for a "normal" (1 phase) transformer.

There's a calculator and some formulas here

If I fill in:

Three phase P = 4.8 kVA

Line to line voltage = 208 V

then I get the 13.3 A

BUT if I select:

Line to neutral voltage = 208 V

I get 7.7 A

So it depends on how you connect the loads to the transformer, see Three-phase power.

Also this might also help you understand.


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