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My circuit panel has a 20 amp breaker on the circuit that leads to the garage (I know this only by the little 20 on the head of the breaker switch). The garage has one 15A outlet, a few lights, and two garage doors.

I want to plug in a treadmill into the garage. The treadmill has a NEMA 5-20P plug on it and has a label that says it draws 18 amps at 120 V.

Is it safe to replace the 15A outlet with a 20A (NEMA 5-20R) outlet and plug the treadmill into it? I imagine running the treadmill at full velocity and opening a garage door might trip the breaker, but are there any other downsides?

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Yes, you can do that but it would have to be a 20 Amp GFCI outlet. It's always a good idea to check the wire size to make sure you have #12 AWG wire.

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There are rare circumstances and sometimes local exception that allow a #14 tap to a single receptacle on a #12 20A circuit, so you need to examine the wire feeding the receptacle.

But 18A with a NEMA 5-20? That's odd, NEC 210.21(B)(2) limits cord connected loads to 20A receptacles to 16A, and it is very rare to see a Listed (UL) device violate the electrical code.

Also the instructions included with the appliance are part of the Listing (UL), if they say to use an a dedicated circuit then this is required to satisfy Code and the Listing.

Also as noted by others the code now requires receptacles replaced in garages to be GFCI.

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  • Our treadmill has a standard 15A plug and says 120V 15A on the sticker. Go figure how they come up with those numbers.
    – Olivier
    Commented Jun 26, 2020 at 12:40
  • It might be 18A peak, but some lower value continuous. The 16A limit is for continuous loads; peaks up to 20A are okay.
    – Nate S.
    Commented Jun 26, 2020 at 15:10
  • Thanks. I do not see UL on the label, just ETL.
    – drs
    Commented Jun 26, 2020 at 18:59
  • ETL is a competitor of UL and is functionally equivalent. The parentheses was intended to describe the type of listing. I realize mentioning UL is somewhat confusing, I should specifically say "Listed by a Nationally Recognized Testing Laboratory like UL or ETL", but that can be more confusing, and a mouthful, but I guess I should try.
    – NoSparksPlease
    Commented Jun 26, 2020 at 19:24
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    2017 Code Language: 210.8(A)"Dwelling Units. All 125-volt, single-phase, 15- and 20- ampere receptacles installed in the locations specified in 210.8(A)(1) through (10) shall have ground-fault circuit interrupter protection for personnel.(1) Bathrooms (2) Garages..." The 250 volt is a major change.
    – NoSparksPlease
    Commented Jul 12, 2021 at 23:41
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Yes, what Jack said: GFCI, and make sure the cable from 20A breaker is no less than #12 AWG.

There are extremely limited cases where 20A breaker with #14 wire is permitted, I think all related to dedicated circuits for certain electric motors.

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