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.
Best Answer
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.