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I have a large potentiometer ( that we also call a variac) that is rated for 220V at input and a 0-250 V 8A ouput.

My mains run at 230 V and 50 Hz at 16A I assume.

Is the difference in current the cause of the heating ? Or is it that I have bad connection with the load ?

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    \$\begingroup\$ A variac and a potentiometer are two very different things. \$\endgroup\$
    – JRE
    Dec 1 '19 at 21:02
  • \$\begingroup\$ Add a schematic and photo of your setup. Otherwise we're all guessing. 16 A is the maximum your mains can supply. The load will draw what it requires. \$\endgroup\$
    – Transistor
    Dec 1 '19 at 21:22
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Variacs are variable (auto)transformers, not potentiometers.

The limitation on the variac current is the output current. If it is rated at 8A you can draw 8A at 1V (8VA) or 8A at 200V (1600VA).

You can expect them to get warm during operation due to losses, like any transformer. Probably good quality ones will get less warm than less good ones, due to less copper and/or poorer electrical steel being used in the construction of the latter.

It's not uncommon for variacs to have taps for the primary, if you connect more voltage to the primary than they are designed for (or operate them at a lower frequency than they are designed for) you can get excessive heating even with no load at all.

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The 8 amperes is, according to you, the output rating so that is the maximum current your load should draw. However, 8 amperes at 250 volts is 2 kilowatts of power (you didn't state the current drawn by your load so I will assume it is the maximum allowed). Efficiencies of variacs are high but even assuming an efficiency of 98%, your variac will be dissipating 40 watts which will heat it up. A lower effciency will mean even more heat. Thus it is expected that your variac will get warm. The rating of your mains supply is irrelevant as long as it is greater than your load current.

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