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I need to power an ungrounded device with live chassis. According to my Google fu I can protect myself using an isolation transformer: https://www.youtube.com/watch?v=3rvEdHyAd2k.

Of course I am not stupid enough to touch two wires running 220 V in order to be electocuted. As I was searching I saw various transformers with various volt ampere ratings.

As far my Google fu goes watts differ from voltamperes. In listings I see transformer srated in voltamperes instead of watts.

I ordered a cheap power meter to measure power consumption, but most of them will either output it in watts or kilowatt-hours. How I can calculate how many voltamperes I need for an isolation transformer?

The device/load uses 220 V, 50 Hz.

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    \$\begingroup\$ you forgot to state the amps required by the load \$\endgroup\$
    – jsotola
    Commented Jun 13 at 20:55
  • \$\begingroup\$ How much current a computer draws??? @jsotola \$\endgroup\$
    – Dimitrios Desyllas
    Commented Jun 14 at 6:45
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    \$\begingroup\$ depends on the computer \$\endgroup\$
    – jsotola
    Commented Jun 14 at 17:22

3 Answers 3

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Watts equal VA for purely resistive load. You need to figure out the power factor for your load to know how much VA it needs when it dissipating a certain amount of real power in Watts. A good power meter would show load in VA or the power factor.

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You can get a VA number by reading the nameplate on the device, which will (or should) tell you the maximum current. Multiply that by the mains voltage. Usually the current number on the label will be conservative (the actual current will be less), though equipment that draws heavy start-up current may draw more for a short time.

For example, here is a piece of NI equipment that happens to be sitting on my desk:

enter image description here

The current is 4A maximum at 100-120V and 2A maximum at 200-240VAC.

So 480VA is worst-case. In reality, I know this device is entirely powered from a switching supply so I would expect the current draw to be no more than 1.6A at 240VAC in. Actual measurement with the plug-ins I have (this is a chassis which accepts modules) is 0.7A at 116VAC in, using a cheap 'Kincrea' power meter. Power is shown as about 80W so the power factor is good, at least according to the meter.

\$VA \ge W\$

For a heater, or other resistive load they are equal.

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  • \$\begingroup\$ Is a computer considered as a resistive load? \$\endgroup\$
    – Dimitrios Desyllas
    Commented Jun 14 at 6:44
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    \$\begingroup\$ @DimitriosDesyllas no, but if it has a well-designed power supply with power factor correction it may be close in reality. \$\endgroup\$
    – Spehro Pefhany
    Commented Jun 14 at 11:42
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You are to obtain the technical data from the name plate of the equipment. this will help understand the required parameters.

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