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I'm encountering a situation where I'm installing a Electric Water Heater in basement and there's a existing unused out for 240 (30 Amp) available right in room above.

Unit specifications are to use 12/2 guage with 20 Amp, independent circuit.

Running a new line is bit of work until is must. Connecting 12 guage to 10 guage with 30 amp is not safe either. Any suggestions would be awesome.

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    Can you ask a Question? I think you might be able to do something with a minor change , but I don’t see a question that we can answer. – Ed Beal Nov 15 '19 at 4:43
  • What make and model is this heater? What make and model is your breaker panel for that matter? – ThreePhaseEel Nov 15 '19 at 5:21
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You are always allowed to use larger wire than is required.

The breaker must be appropriate for the load. If you're upsizing wire, don't upsize the breaker too!

NEC 110.3b requires you install the equipment according to instructions, so if a 20A breaker is called out, a 20A breaker it is!

The instructions call out 12 AWG wire. Upsizing wire is always OK, so #10 wire is just fine on any part of the wiring run.

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The only place instructions or labeling can deter a wire size bump is if the terminals are not rated for the larger wire. Then you simply pigtail a short bit of the specified size wire (e.g. #12) and then splice it to the larger wire (e.g. #10) right there.

There are rules on junction box fill (number of wires) but pigtails are "free" in these rules.

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    I have seen suggestions when using larger wire (#10 in this case) for some of a run to attach a note near the breaker box that the run is continued in smaller (#12) wire elsewhere. This discourages somebody from thinking later "Oh, this is #10, we can upgrade the breaker to 30A" – Martin Bonner supports Monica Nov 15 '19 at 13:33
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    Can I use 12 gauge two wire from 240 wire outlet and replace 30 Amp fuse to 20 Amp in circuit box. – Tech Professional Nov 15 '19 at 15:41
  • Yes, "use 12 gauge two wire from 240 wire outlet and replace 30 Amp". – NoSparksPlease Nov 15 '19 at 15:47
  • Thanks guys for your input. Cant fish wire through basement beams, if need to run expose 2 feet wire out the outlet to reach corner basement opening, what you guys suggest as conduit, MC wire, PVC or other. Trying to keep it clean as possible. Thanks – Tech Professional Nov 16 '19 at 1:21

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