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Most receptacles in my house only provides 110V. I would like to install a 2500 W 25000 BTU window AC that requires 230V. It draws a current of up to 15 A.

Can I plug a step-up transformer to the receptacle, then plug the AC into the transformer? I understand that the current draw on the receptacle would probably be 35-40A? Accounting for the power loss in the transformer. I am not sure if the circuit breaker and the wire are rated for that much current. Is it safe to assume that the circuit breakers installed are the largest current that the wires can safely handle? i.e., it is unsafe to install a circuit breaker with larger current without upgrading the wires.

Thanks,

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  • Note that 110 volt AC service is uncommon since about 1965. In North America (where you seem to be), 120 and 240 volts are the nominal service voltage since about 1990.
    – wallyk
    Jul 16, 2019 at 22:03
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    @wallyk - that's a distinction without a difference, since people in the USA colloquially refer to their home mains voltage as 110 or 120, and it's not really relevant to his question where the current draw is going to be the bigger factor.
    – Johnny
    Jul 16, 2019 at 23:12
  • $70 gets you half that with zero shenanigans. The only part of this whole thing that was free was you getting it out of their basement.
    – Mazura
    Jul 17, 2019 at 3:23

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Suppose you did. The unit is labeled for 2500 watts, which means that is how much power you must provision.

Let's allow 3% for transformer losses; now we are at 2575 watts. An air conditioner is treated as a continuous load, so like a lot of things, you must provision power for 125% of the actual load. So 2575 watts x 125% = 3219 watts.

If you've heard of an 80% derate, that's the very same thing we just did (inverted) so we don't need to derate further.

3219 watts is 26.8 amps. That's too much for a 20A or 25A breaker. A 30A breaker is required. For that, your circuit requires 10 AWG copper or 8 AWG aluminum wire. You will need to run this as a new circuit because I guarantee you won't find this in your existing wall. And don't even think about overloading a 12 AWG or 14 AWG circuit at 26.8 amps!!!

Or, change the whole circuit to 240V

Turn the breaker in question off and then stick a lamp in every receptacle and see what lost power. Two questions.

  • Imagine you couldn't plug 120V appliances in there anymore. Could you live with that?
  • You have to have a 120V receptacle within 6' of every point along a wall. (i.e. within 6' of the a threshold, then every 12' after that). If these receptacles disappeared, would you still have that?

If both answers are "yes", then you can convert the entire circuit to 240V, and change Every Single Plug to NEMA 6-15. Obviously 120V things won't plug in there anymore, but that's a good thing! That prevents you from setting your 120V appliances on fire.

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A few appliances are capable of working on multiple voltages. Look at their power supply rating to see if they say something like "100-240V" or "90-264V" (that's 100-240V +-10%). For those appliances, you'll need to change the plug to NEMA 6-15P. Are you daring to think the thought? Don't do it! DON'T leave a NEMA 5 receptacle on a 240V circuit merely so you can plug in dual-voltage devices. That's how you burn houses down. The right way is install the proper NEMA 6 in the wall, then change the appliance cord. Worst case, use a cheater cable. Because then, it's obvious that it's a cheater cable.

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    Or better yet, if your multi-voltage appliances use a detachable power cord, just get a cord that's NEMA 6 to the appropriate IEC appliance coupler ;)
    – ThreePhaseEel
    Jul 17, 2019 at 2:33
  • Thank you for the clever fix! I'm considering this for a furnace (110V) to air handler (230V) replacement. Do you know if this can easily pass building permit inspection in most jurisdictions?
    – jiehanzheng
    Mar 31, 2021 at 0:11
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No, you can't do that in this case UNLESS...

Note that the AC requires 240V @ 15A. Assuming perfect conversion with the transformer (not reality but it doesn't matter here) that means you need a 120V @ 30A outlet. While these are possible, chances are you don't have this. It's most likely a 15A or 20A circuit either of which will be inadequate for the AC load.

Instead, consider a new 240V circuit for your A/C. (Contact an electrician if you are not up to speed doing electrical work.)

Just to elaborate on transformer theory. These are PASSIVE devices. All they do is convert one voltage to another. The POWER, which is VOLTS * AMPS remains constant. So if you double the voltage you halve the current and vice-versa.

In the case of stepping up from 120V to 240V you are doubling the voltage and to keep the power constant the current goes down to 1/2 of its original value. But to put it in the context of your problem, you must supply 100% of the needed POWER at 120V to power the AC at 240V. 240V * 15A = 3600 Watts but the problem is that to get 3600 W at 120V you need 3600/120 = 30A. It's actually a bit more than that since the transformer wastes some of the power. So in reality you likely need 35A at 120V to make this work.

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  • Thanks. How do I know if my existing wire is suitable for 30-35A?
    – Shuheng Zheng
    Jul 16, 2019 at 22:27
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    The odds of having wiring capable of 30+ amps on a standard 15 or 20 amp circuit are extremely low, due to the extra cost. There is just no reason a builder would spend the addition cost. If you must check, remove the outlet from the box (turn off the breaker first) and check the marking on the wires. You’ll very likely find AWG 12 or 14. If you find AWG 10, it’ll be aluminum wire. None of this is suitable for 30 amps.
    – DoxyLover
    Jul 16, 2019 at 22:59
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    @DoxyLover I think emphasis is on extremely unlikely. 30 amp wires is not easy to work with. There's no reason an installer will both spend more money and make his work a lot more difficult to install completely unnecessary wires. I can see it for some one-time final pigtail in very specific conditions, but I would not expect to see a 30 amp circuit in the walls.
    – Nelson
    Jul 17, 2019 at 2:04
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  1. Attempting to run this large 2T AC from existing household wiring will probably burn the house down.
  2. Running a proper 240v circuit just for this model AC is the best way to go but not cheap.
  3. Another possibility is to reduce the thermal load of the room and use a 1T mini-split instead of the window AC. The CEER is around 15 vs 8 for the window AC, meaning it will cool the room at 1/2 the cost and pay for itself quickly. Many 1T mini splits will run on a standard 120V circuit. Daily energy savings plus eliminating the cost of running a 240v circuit are a win-win.
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As others have detailed, powering a 240v AC unit in your room isn't practical. Possible but at high expense or losing regular outlets in that room.

Stick with a 120v AC unit. It may not be as powerful as you want, but it'll be more cost effective.

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