Hot Topics: Running a 20 Amp Circuit with 12/2 Wire

electrical wire

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Alaskanstar - Member

Basically, I have a small outbuilding that I want to run electricity to. I have a 20 amp slot open in my panel.

Can I run a 12/2 wire from the panel to the outbuilding? The total circuit would be about 60 -70 feet in length. It would only be supplying about 5 outlets and one light. It would be a light load, running small things, no heaters or med/large equipment.

Thanks for any help.

Pick up 12/2 wire on Amazon.

Bill190 - Member

Here's a voltage drop calculator.

Alaskanstar - Thread Starter

Thanks, that info helped.

From what I can see, it is suggesting a 10 gauge wire to run the length I need.

Could I run 10/2 from the panel to a j box in the building and then branch my circuit off that with 12/2?

Tolyn Ironhand - Group Moderator

With voltage drop, you take into account the amps. You should not necessarily use 20 amps for your calculation because you will be running only a light and some small tools. It is based on the connected load, not the max load. 10 amps would be a more realistic number, in which case #12 would be just fine.

Bill190 - Member

"From what I can see, it is suggesting a 10 gauge wire to run the length I need. Could I run 10/2 from the panel to a j box in the building and then branch my circuit off that with 12/2?"

Very good!

And you would certainly want to do the J box thing and transition to 12 gauge because some outlets only take a maximum size wire. I think some GFCI outlets will only take up to a size 12 wire?

Also, if using the screws on the side of an outlet/switch, it takes a bit of manpower to connect 12 gauge wire and then cram everything back into the electrical box. I can't imagine trying to do this with #10 wire!

In any case, the switch/outlet should say somewhere what the max wire size is. (On-device or in the documentation.)

ibpooks - Forum Topic Moderator

"Can I run a 12/2 wire from the panel to the outbuilding?"

You can use 12/2 UF-B cable, which is not the same as NM-B Romex. Standard Romex is not waterproof and therefore cannot be used outdoors. Burial depth should be 24" unless you use a 20A GFCI breaker, in which case burial depth can be 12". You should use conduit sleeves for protection on the vertical entrance and exit to the trench.

The very first box in the outbuilding should have a heavy-duty toggle switch which is the "building disconnect." It should be wired to cut power for the outbuilding.

"The total circuit would be about 60 -70 feet in length. It would only be supplying about 5 outlets and one light. It would be a light load, running small things, no heaters or med/large equipment."

#12 provides reasonable performance up to about 100' with general loads like you describe. Upsize to #10 if you plan to use any heavy tools like an air compressor. You can switch back to #12 after the building disconnect switch for easier wiring to the devices.

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