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I have a variac rated for a maximum output of 30A. The problem is it takes 120V on the input and all of my 120V circuits are 15A, as expected. I unfortunately cannot just add an additional 30A circuit since I live in an apartment.

I have a 240V circuit with a 30A double-pole breaker on it (2x 15A breakers connected together). This goes to a NEMA 6-30R outlet but that means there's no neutral for me to get 120V. Since I live in an apartment and this normally powers my A/C, I'd prefer to not rewire it to be a 120V circuit (the A/C unit already sucks enough as-is; I really don't want to see it struggle with 1/4 of the power it was designed for).

I was wondering if it would be possible to use another transformer to step the 240V down to 120V, and if this would properly split the 30A load across both 15A hot legs so as to avoid starting exciting electrical fires in my wall.

If this is possible, I'm having trouble finding a reasonably-priced transformer with the appropriate power rating (3600VA+, not accounting for any core/coil losses) to do this. I guess I'd be stuck winding my own if I go this route and don't want to spend a ton of cash?

The only other thing I could think of was to rectify the 240, step that down, and then pass that to an H-bridge inverter. No idea if that is workable though, or if the variac can even handle a non-sinusoidal input (I assume it can since it's just a transformer, but I didn't see anything in the product description explicitly stating such).

Any ideas or help are appreciated.

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  • \$\begingroup\$ Explain the bigger picture. Are you planning to power an air conditioner from a variac? If so, why? \$\endgroup\$ Nov 16 '20 at 21:48
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    \$\begingroup\$ 2x15A breakers on a 240V circuit is not a 30A double-pole breaker!. A 30A double-pole breaker is 2 x 30A breakers!! \$\endgroup\$
    – brhans
    Nov 16 '20 at 21:51
  • \$\begingroup\$ @ElliotAlderson - no, I'm planning on using the variac for testing/prototyping circuits. I just want to be able to plug my A/C unit back in when the variac is not in use, which will be the majority of the time. \$\endgroup\$
    – flashbang
    Nov 16 '20 at 21:55
  • \$\begingroup\$ @brhans - everything I googled seemed to say that 2x 15A breakers would give me a maximum current draw of 30A - which seems to make sense since I'd be pulling 15A from each phase, no? Maybe "double-pole" was the wrong wording - it looks like two 15A breakers on opposite phases connected together with a bar between them. \$\endgroup\$
    – flashbang
    Nov 16 '20 at 22:09
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    \$\begingroup\$ 2 15A breakers with a bar between them is a 'handle-tied' 15A double-pole breaker. That's 15A on each pole - which will give you 240V at 15A or a multi-wire-branch-circuit of 2 x 120V at 15A each. You don't get to add the 2 15A breakers together, ever, because they're on different poles of your incoming mains supply. \$\endgroup\$
    – brhans
    Nov 16 '20 at 22:33
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Connect your 120 V variac to your 120 V circuit. It will only draw current determined by its load, plus a bit of magnetising current.

The fact that the variac is rated to 30 A means it will stay nice and cool when it's supplying the maximum 15 A at 120 V output that your circuit can deliver.

Your supply/variac combo will then be rated like this

output voltage 0 to 60 V, maximum output current 30 A (variac limited)
output voltage 60 to 120 V, maximum power output 1800 watts (supply limited)

Use it like this for now, to discover how actually use your variable testing / prototyping supply. You might find it's adequate. If later on you really need that top right-hand corner above 60 V filling in all the way to 30 A, then you can invest in a 240 V to 120 V auto-transformer to beef up your supply.

Note that this variac will not be giving you isolation, it's usually isolation that's the number one priority for doing testing / prototyping work. You might want that 240 V to 120 V transformer anyway, and invest a bit more on making it a proper isolating transformer, rather than using a cheaper, non-isolating, autotransformer.

You may find that a 30 A rated variac trips your breakers from time to time when powering up on a 15 A circuit, maybe more often than you would like. All transformers have the capability to draw significantly more than their rated maximum current for a few cycles, if powered on at zero crossing, which is the worst case point in the mains cycle. If it just happens from time to time, then it's called nuisance tripping, and we can live with it. If it happens too often, then there are ways to deal with it. A time-delay breaker is perhaps the easiest solution, one that's specified to ride out a few cycles of overload without tripping. But there are electronic ways to turn on the load at peak mains voltage, which results in no power-on transient, or soft-start it using other methods.

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  • \$\begingroup\$ Thank you for the answer! Good point about 30A being available at 60V - I didn't even think of that. Not sure why autotransformer didn't occur to me when looking for an appropriate step-down transformer but I was able to find some - unfortunately not with the needed power rating and still more expensive than I'd like so I'd probably have to make my own still. Also, the soft-starter/inrush issue is another good point I hadn't considered either. Thanks again! \$\endgroup\$
    – flashbang
    Nov 17 '20 at 15:44

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