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I have a very basic question about three phase power. We have a three phase power supply rates as 32A/415V. The line to line voltage has been measured to be 415 V (using a multimeter) and the line to neutral voltage is 240 V. Does the current rating of 32A refer to the maximum allowable line to neutral current if the load consists of three heating elements connected in wye configuration?

Thanks! Abhishek

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Usually such a current rating for a 3 phase load/generation refers to the rms current per-phase.

The VA rating of a balanced load/generation is given by \$3 * V_{phase} * I_{phase} \$

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  • \$\begingroup\$ So in my case where the heater elements are connected as wye, it refers to the maximum allowable rms current between line to neutral, right? \$\endgroup\$ – Abhishek Sep 24 '15 at 18:25
  • \$\begingroup\$ That is correct. There should be an overall power rating states to confirm \$\endgroup\$ – JonRB Sep 24 '15 at 19:48
  • \$\begingroup\$ Thanks! But what do you mean by "There should be an overall power rating states to confirm" ? .. also..as a thumb rule how much mismatch between the phase currents is generally considered acceptable? I mean suppose my three phase currents are 18A, 15A, 18A is it okay? \$\endgroup\$ – Abhishek Sep 25 '15 at 6:03
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32A is total allowable current for given power supply: momentary RMS I1+I2+I3, including active and apparent currents.

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  • \$\begingroup\$ What is an active current? What is an apparent current? I am not familiar with these terms. Also, when you say "momentary" do you mean instantaneous? Because I am sure the 32 Amps must be RMS, right? Nobody would rate a 3-phase supply in terms of instantaneous current, would they? \$\endgroup\$ – mkeith Sep 24 '15 at 18:21
  • \$\begingroup\$ My bad, i mean not instantaneous, it is RMS. Under active current i mean current for pure resistive load R , apparent - currents for capacitive and inductive loads. Usually heating elements shall have pure resistive load. \$\endgroup\$ – Flanker Sep 24 '15 at 18:49
  • \$\begingroup\$ BTW, what is your power supply, is it genset? Then it shall have rated power in VA or Watts. \$\endgroup\$ – Flanker Sep 24 '15 at 18:57
  • \$\begingroup\$ This is wrong - it's the current in each phase conductor. So a 32A 3ph supply can deliver 3 lots of 32A. If you look at a 32A 3ph MCB, it's basically three single phase 32A breakers stck side-by-side. \$\endgroup\$ – user1844 Sep 24 '15 at 20:43
  • \$\begingroup\$ Flankers: Is not a genset, its the AC mains, I have a 5 pin wall socket \$\endgroup\$ – Abhishek Sep 25 '15 at 6:00
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Does the current rating of 32A refer to the maximum allowable line to neutral current if the load consists of three heating elements connected in wye configuration?

The nature of the load (Y or delta) is immaterial to what current the source can supply - it doesn't know whether you take 32A thru a delta or Y load providing the 32A is not exceeded.

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  • \$\begingroup\$ Thanks! Although I have not been yet able to get my head around the delta configuration yet :-) I mean its a bit messy with the centre tapped neutral and all. I am a mechanical engineer by trade, so this is somewhat outside my domain, but would love to learn more. \$\endgroup\$ – Abhishek Sep 25 '15 at 6:10

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