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Here is my setup:

I have a 120V 20A welder that I am fairly certain uses a transformer to drop the voltage which increases the amperage. I have a 120V 20A variac that I want to put in series on the secondary side of that transformer.

Is it safe to push more current through the variac because it's at lower voltage? I believe it should be safe as long as the wattage is the same. The variac is rated for 2000VA (which is the same as Watts).

I know the other way around, higher voltage at lower amperage is NOT necessarily safe. I know components are rated in both WATTS and VOLTAGE, because if you push the volts too high, eventually the resistance of the air between the wires will be lower than the resistance of the circuit and it will arc (short-circuit).

So is higher current at lower voltage safe if the watts are the same?

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    \$\begingroup\$ Not necessarily. You have to consider the losses in the device (Variac) itself. Designing it for 20A will determine the thickness (and resistance) of its wires, and it will handle 20A with a safe temperature rise. Double that current to 40A and you'll see 4x the power loss inside it, 4x the temperature rise, possibly melting components or starting a fire. \$\endgroup\$ – user_1818839 Apr 21 '17 at 22:08
  • \$\begingroup\$ From this answer i see that I might be wrong about this. Higher current generally implies a thicker conductor: electronics.stackexchange.com/a/21038/147013 \$\endgroup\$ – Shorin Apr 21 '17 at 22:26
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The current rating of the variac is based on the size of the conductors in the windings and the size, shape and material used in the wiper. Therefore, using it at a lower voltage will not increase its current capability.

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  • \$\begingroup\$ So what I probably want to do is limit the voltage on the PRIMARY side of the transformer since that is 120V 20A \$\endgroup\$ – Shorin Apr 21 '17 at 22:29

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