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Installing a new heat pump with a contractor and he said he can convert a 120v outlet to 240v outlet on site without bringing new wires from electrical panel, heat pump specs is attached and it has a 5 amps circuit and 15 amps overcurrent protection. Contractor says as long as wire gauge is enough for the amperage, it should be fine. I chatted with other contractors but none of them have brought up this option before. Want to know if this is a safe practice?

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IF the outlet is the ONLY outlet on the circuit, it's perfectly fine to change it to a 240V outlet (or disconnect, given it's for a heat pump) and change the breaker to a two-pole 240V 15A breaker - all your 120V wiring is already rated for 250V if not 600V. Since 15A/14Ga is the minimum wire size for house wiring, the wire size is already adequate by default for a 15A circuit.

If there are other outlets on the circuit, then there's going to be a problem, or more changes / rewiring will be required.

The white wire should be remarked red (preferable, IMHO, more obviously not just tape for tape's sake) or black (or any other "hot color" so not gray, white, or green) at both ends.

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  • thank you for your answer! how do i find out if there are other outlets on the circuit? That outlet is located in the outdoor closet since it is used to power furnace/coil, and it seems like it's pretty isolated from other outlets inside the house. – Dude from SF Sep 10 '20 at 19:23
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    Figure out which breaker it is, shut it off, and see if anything else switches off. Make sure that whatever appears to have switched off switches on when you turn it back on (to avoid issues with things that just were not working that you only noticed when checking it.) Be sure to check lights, etc (I like to keep those circuits separate form any other uses, most don't bother to.) Connections are not always obviously related ("we need to put a light in here and there's this furnace circuit handy!") But you might get lucky as a furnace circuit is often isolated. – Ecnerwal Sep 10 '20 at 19:28
  • And in some rare cases you have two different phase circuits that share a neutral (called a multiwire branch circuit) and if the two hot wires split off at the 120V outlet or farther away, you can switch it to a 240V outlet and not change anything else on the circuit. Or if you have one room on one phase and the room on the other side is on the other phase, you could just run a short wire through the wall to the other side to access both phases and get 240V that way. Somebody will probably mention the NEC (which you have to pay to read) and its restrictions but I'm just saying it would work. – Alex Cannon Sep 11 '20 at 4:18
  • This is brilliant! 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 at 0:03
  • Within the details of the answer, yes, because it meets electrical code. Just make sure you have identified that the furnace is the only thing on the circuit. – Ecnerwal Mar 31 at 0:58

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