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I used to have a 10/2 220 line that went from my panel to my hot water heater.

I have since taken the electric hot water heater out and replaced it with a oil fired unit.
The installers unhooked the wire from the breakers, but left the 10/2 in the finished ceiling.

The old electric water heater was near my workshop. I would like to use the 10/2 wire that is already run (with a new 20A breaker, of course) to get a new 20A circuit to my shop.

However, once the 10/2 line gets to the first plug, can I just run 12/2 to a couple other outlets? I think it is OK because the 10/2 is big enough to carry the 20A and the 12/2 is also big enough. I'm just not sure if mixing 10/2 and 12/2 is to code or not?

So it would be Panel -> 20A breaker -> 10/2 -> outlet -> 12/2 -> outlet -> 12/2 -> outlet.

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  • I'm posting this as a comment not an answer bc I'm not sure about your specific question, however I do think you are correct. But Have you considered putting a small sub-panel in your shop? It would have to be strictly 120v unless you ran another wire for the ground and of course isolated the neutral from the ground in the sub-panel. The rationale for this is you already have the 10/2 available, Let's use its full capacity and connect your outlets to a small sub-panel with a couple of 20 amp breakers. Of course follow code for connections (permanently accessible) on the 10/2 if needed. – George Anderson Mar 21 '20 at 21:15
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    @GeorgeAnderson You can't use the ground wire in 10/2 as a neutral. Not even if the legacy installation misused it as a neutral for a dryer or range. (the only bare neutral for dryers/ranges ever allowed was SE cable, but SE cable is for Service Entrances, so that actually is neutral in that case. #10SE went extinct when the minimum service ampacity was raised to 60A. – Harper - Reinstate Monica Mar 21 '20 at 21:39
  • @GeorgeAnderson: I did not think about that. The "workshop", is really 1/4 of my basement, so not truly a shop. I have one 15A circuit in there already, but occasionally if I have a couple things running and I kick on my Mitre saw, it pops the 15A breaker. So I figured have a 20A circuit for the tools and another for the other "stuff" (chargers, light, radio, etc.) – Weekend Electrician Mar 21 '20 at 22:20
  • Harper, are you saying the minimum ampcity for a small sub-panel is 60 amps? I'm only suggesting to the OP a 120 volt sub-panel with the two hots jumpered, white in the 10/2 w/ground as the neutral and the bare conductor as the ground, properly isolated in the small sub-panel. If there is a minimum capacity for a sub-panel, that's news to me. – George Anderson Mar 21 '20 at 22:56
  • @GeorgeAnderson -- no, the minimum ampacity for all but a single or two circuit service from the utility is 60A, generally. (single family houses, though, require a 100A service, minimum) – ThreePhaseEel Mar 21 '20 at 22:59
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You can do that. Where the 10/2 ends you'll need a junction box to splice to 12/2 or install your first outlet.If this is an unfinished basement, you'll need the outlets to be GFCI protected. So the first outlet you install could be a GFCI outlet and hook the others to the load terminals of that outlet. After that, for the additional outlets, pigtail the hot and neutral to two 8" pieces of wire and connect the hot to the brass screw and the neutral to the silver screw. Don't use the backstabs.

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There is nothing in the code that restricts mixing wire sizes, as long as the smallest wire meets the minimum allowed by ratings of terminations selected from 310.16 and minimum sizes in 240.4.

If fact in commercial work where circuits feed areas that are sometimes 100' or more from an electrical panel it is standard procedure to run those "homeruns" with the next larger size wire to reduce the voltage loss created by resistance of the wire, then the distribution to device locations is done with minimum allowed size.

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