I have a DC circuit that runs on 8.1 A × 12 V (97.2 W). Power is supplied from a AC-DC transformer that plugs into a socket.

How many of these can I run in a place that provides a maximum of three-phase 30 kVA?

I cannot make sense of the kVA definition and it is beyond my comprehension. I do not understand real power, apparent power, power factors, resistive load, reactive load, linear load, nor can I calculate these.

Is there a simple answer?

  • 7
    \$\begingroup\$ Don't take this the wrong way, but if the concept of VA is beyond your comprehension, I would suspect you are also not qualified to install anything that deals with that kind of power. \$\endgroup\$
    – Joren Vaes
    Commented Jun 23, 2018 at 12:47
  • \$\begingroup\$ Not taken wrong way. I'm not planning on installing myself. I just need to understand what's available to me before I hire an electrician. I dont want to hire an electrician only to find out that the place I've acquired doesn't provide sufficient energy for my operations. \$\endgroup\$
    – Samid
    Commented Jun 23, 2018 at 13:02
  • \$\begingroup\$ Get the electrician first to make sure the calculations are correct then find the suitable place... \$\endgroup\$
    – Solar Mike
    Commented Jun 23, 2018 at 13:16
  • \$\begingroup\$ In my experience, an electrician would usually come over first and asses the situation, free of charge. Only after this they would provide you with a quote and you can agree on the terms and cost. \$\endgroup\$
    – Joren Vaes
    Commented Jun 23, 2018 at 13:34
  • \$\begingroup\$ See my previous answer here: electronics.stackexchange.com/questions/300296/… \$\endgroup\$
    – winny
    Commented Jun 23, 2018 at 13:38

2 Answers 2


Transformers are rated in kVA which if the load was pure linear resistive heater loads then kVA=kW kilowatts while some converter is going to be also storing some energy called Reactive which adds to the DC load here which is tiny V*I= watts.

Normally Amp breaker ratings may exceed kVA/V=kiloAmps at line V but is chosen for protection of wires but the transformer will be fused for kVA per phase divided by kV voltage to get rated current.


If you ignore the effects of efficiency and non-linear load, you can divide 30 kVA by 97.2 watts, 30,000 / 97.2 = 309, and say that you can connect 309 of the devices in question. You might make a rough estimate that the ignored effects might cut that number in half and limit the allowable number to 150 units. Beyond that, you need a qualified person to look at the specifications of the device and the details of your location.


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