Can I run 12/2 wiring from a load center to a junction box, then use 14/2 for branch circuits?

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    Never a good idea to do this. Maybe if you ran out of the one size of wire. – JACK Jan 26 at 15:10
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    Why are you wanting to do this to begin with? – ThreePhaseEel Jan 26 at 15:50
  • The only reason I would consider this is to minimize voltage drop for a long run on a lighting-only circuit. Doing lighting circuits entirely in 12 awg is unnecessary in the age of LED lighting while increasing cost (more expensive wire, larger junction boxes) and 14 awg is much easier to work with at the fixtures and switch boxes. Definitely make sure to leave a note at the panel though. – statueuphemism Jan 26 at 17:02
  • @ statueuphemism Why lighting only? It would seem your LED reasoning reduces the need to increase size to compensate for voltage drop, while receptacles would be prone to overload. – NoSparksPlease Jan 26 at 17:17
  • I seriously doubt you'll have voltage drop problems with #14 AWG. Just how many miles is this run you're talking about? – JACK Jan 26 at 18:37

You can, but only if the breaker is 15 amps, and it creates possible confusion for someone who looks at the breaker box and thinks "huh, that's 12GA, I can change that to a 20A breaker." So you might want to leave that person a note in the box.


You may be miscalculating voltage drop

The first rule of voltage drop is: Calculate voltage drop based on the actual, normal circuit load, not breaker trip. Suppose you have 7A of lighting, but only 3A will be realistically on at one time, on a 15A trip. What number do you punch into the voltage drop calc?

3 amps. The normal load.

The second rule of voltage drop is: 3% is nonsense - nothing in Code even talks about this. 3.000% is super nonsense. Most of the voltage drop calculators online will consider 3.001% voltage drop to be condemning, and will force you into a larger wire size at that point. That is patently absurd. If the calc reveals a 3.5% drop, that is unquestionably fine. Larger drops are a judgment call, but they're your judgment call. Some loads these days are 100-264V voltage-flexible, so you can tolerate a LOT of drop, especially on a 240V circuit, which can be awesome for pole lights on farms. So suppose you are wiring a pole light that takes 100-240V. 120V voltage drop calculates at 7%. Is that OK?


If #12 is used in a 15A circuit, mark, mark, mark.

For instance now that Romex is color coded, bring about 2" of Romex jacket into the box, and overtape the visible yellow jacket with white tape - to make clear it is #14. Otherwise it will be too tempting to put a 20A breaker there. Mark the hot wire with a tag and write #14 wire on it.

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    Why is 3% nonsense? Voltage drop = energy wasted in transmission. A 7% voltage drop on a 12 A resistive load like a space heater is ~100W just being lost to the wires in your walls and not being used to heat the space you want to heat. It makes more sense to me to take a one-time hit on increased cost of wire to minimize recurring costs (and be environmentally friendly if you're into that sort of thing) due to unnecessary voltage drop. – statueuphemism Jan 26 at 22:35
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    @statueuphemism Fair enough, and I changed my example. My point is we ought to make a considered decision ourselves instead of blindly doing what the wire salesman says. The usual case I see is someone spending $600 extra to avert 50 cents a month worth of transmission losses to occasional-use tools in a shed. – Harper - Reinstate Monica Jan 27 at 0:26

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