Best Outdoor Extension Cord for Christmas Lights: Gauge, Length and Picks

It's Thanksgiving weekend, the last plug is 40 feet from the only outdoor outlet, and the cord in the garage says "indoor use only." Leave it there. CPSC says to use an extension cord outdoors only if it's marked for outdoor use, and the W in SJTW is that mark. For most light strands, 16 gauge covers up to 100 feet; longer runs need 14 or 12.

Quick pick: an SJTW cord long enough to reach your first strand in one piece, 12 gauge if inflatables or old C9s share it.

Free math: count strands, watts and circuits first in our Christmas light roofline math. No sign-up.

Which gauge your lights need

Find your amps: total watts on the boxes ÷ 120.

25 or 50 ft: 16 gauge to 13 amps; 14 gauge for 14 to 15.

100 ft: 16 gauge to 10 amps; 14 gauge for 11 to 13; 12 gauge for 14 to 15.

150 ft: 14 gauge to 7 amps; 12 gauge for 8 to 10; 10 gauge for 11 to 15.

LED example: 120 ft of LED minis draws 19.2 watts, 0.16 amps. 16 gauge works to 100 ft.

Old C9 example: the same roofline in C9s draws 875 watts, 7.3 amps. 16 gauge works to 100 ft; at 150 ft, take the thicker row, 12 gauge.

Lower number means thicker wire. Chart: UT Austin safety office. CPSC's floor is 16 AWG, or 18 AWG with fuse protection.

What SJTW means on the tag

W means rated for outdoor use. S is flexible cord, J is 300-volt insulation and T is a vinyl jacket (UT Austin).

Rating: watts and amps are on the tag or package (CPSC). Stay under them.

Lab mark: look for UL, ETL or CSA (CPSC). Before you buy, the product listing should show one. Utah State Extension says to use three-prong outdoor cords.

How to measure your cord run

Walk the path: measure from the outlet to where the first strand plugs in, including the climb to the eave.

Add slack: we add about 8 ft. Say 30 ft to the corner plus 12 ft up: 42 ft, plus slack is 50, so buy 50 ft.

Joined cords: add their lengths and read the chart as one run.

Which cord fits your setup

LED strands, short run: 16 gauge, 25 ft.

LED strands, far outlet: 16 gauge to 100 ft; 14 or 12 past that.

Old C9s or inflatables: 12 gauge, for room on the tag.

Yard stakes and trees: a power stake marked for outdoor use, with three-prong outlets and a UL, ETL or CSA mark.

Not a fit: lights up all year. CPSC says extension cords are temporary, so ask an electrician about an outlet.

Cord safety checklist

☐ Outlet: plug into a GFCI-protected circuit (CPSC).

☐ Connections: keep them above ground, out of snow and water (Utah State). A wet connector can leak current to whoever picks it up (UT Austin).

☐ Doors: don't run the cord through a doorway (CPSC).

☐ Damage: replace it; don't tape it (CPSC).

☐ Hot cord: a warning sign of overload (CPSC).

☐ Bedtime: lights off when you go to bed or leave (CPSC). A timer does it for you.

Same cord rules apply to a corded snow blower.

Outdoor cords for Christmas lights (#ad)

As an Amazon Associate, Curb Calc earns from qualifying purchases.

Iron Forge Cable 12 Gauge Extension Cord, 50 ft: SJTW, 15 A, 1,875 W, UL certified, lighted end, per the maker. See it on Amazon

Conntek 20251-100 12/3 SJTW Extension Cord, 100 ft: for the far corner: 15 A, listed as UL certified, per the maker. See it on Amazon

Outdoor extension cord FAQ

What gauge extension cord do I need for outdoor Christmas lights?

16 gauge up to 100 ft for loads to 10 amps; past that, 14 or 12 (UT Austin).

Can I use an indoor extension cord outside?

No. CPSC says only cords marked for outdoor use.

How many light strands can go on one extension cord?

Stay under the cord's rating and the strand maker's limit. CPSC's old tip capped it at three sets, which predates most LED sets.

Watch Curb Calc on YouTube: youtube.com/@curbcalc

Sources: CPSC Pub. 5032; CPSC holiday tips, 2001; UT Austin EHS; Utah State Extension, 2008.