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I'm working on an existing structure that was fitted to be a grocery store, so a majority of the junction boxes are 3 phase and above 240 volt service or higher. My problem is that there are two 120 volt junction boxes that connect to all the 120 volt outlets throughout the store.

There are already about 5 receptacles tied together in one box using 10 gauge wire. Do I need 10 gauge wire to tie into this pre-existing circuit or can I use 12 gauge cable or will it carry too much load?

  • Depends, you really have have someone perform a voltage drop calculation to get a proper answer. – Retired Master Electrician Sep 29 '17 at 17:04
  • Also what size breaker is feeding this commercial rules are slightly different than residential. – Ed Beal Sep 29 '17 at 17:14
  • If the breaker likely needs to be downgraded if adding smaller gauge wiring to the circuit. – Tyson Sep 29 '17 at 18:34
  • What size breaker is controlling these outlets? Also, what are you trying to do to this structure? – ThreePhaseEel Sep 29 '17 at 22:17
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Feeding common NEMA 5-15 receptacles (plural), the circuit (i.e. the breaker) must be 15A or 20A. You may not put these receptacles on a 30A circuit (breaker)!

You're always allowed to use larger wire. A 20A circuit calls for 12AWG wire minimum, but you may use 10AWG.

So most likely you must change the breaker serving the wires to 120V/20A, then extend with 12AWG wire.

Safety first

Be very wary of strange and dangerous voltages including 136V, 208V, 277V or 480V. These voltages will be happy to kill you. If you don't know what you're doing in commercial panels, get out of them and call an electrician.

Add more wire

Since you are commercial, and therefore in conduit, you always have the option to fish additional wires into the conduit. You can have up to 4 circuits per conduit without complications. Pro tip: You have a choice of like 11 wire colors, so use colors different than those already there.

  • Commercial doesn't necessarily mean conduit. It could be MC armored cable. But you can always pull more of that, too. – Craig Sep 30 '17 at 17:39
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If the #10 AWG wiring is protected with a breaker as required to protect 30 amps, you would need to provide protection for the #12 AWG wiring (and the attached appliances) at is lower rated ampacity.

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