The tag on the cord or its packaging gives the most it can carry, in amps, watts or both. A 15 ft. Southwire 16/3 cord’s tag reads 13 amps, 125 volts, 1625 watts.
Watts by gauge and length
A thicker cord, with a lower gauge number, carries more, and a long cord carries less. Southwire rates its own extension cords by gauge and length.
Southwire’s amp ratings for its cords, with the watts at 120 volts
Gauge
25 ft
50 ft
100 ft
16 gauge
13 A (1,560 W)
13 A (1,560 W)
10 A (1,200 W)
14 gauge
15 A (1,800 W)
15 A (1,800 W)
13 A (1,560 W)
12 gauge
15 A (1,800 W)
15 A (1,800 W)
15 A (1,800 W)
10 gauge
20 A (2,400 W)
20 A (2,400 W)
20 A (2,400 W)
Watts are the amps times 120. Southwire’s tags work the watts out at 125 volts, so they print a little more: 13 A × 125 V = 1,625 W.
Other makers’ cords can be rated differently, so the tag on yours is the number to use.
A heavy cord can be rated lower
Southwire’s catalog rates its 10 gauge cords at 20 amps, but its 10/3 SJTW cord with a standard NEMA 5-15 plug is rated 15 amps, 1875 watts on its product page. So read the tag, even on a heavy cord.
Adding up what’s plugged in
Add the watts on the nameplate of everything that will share the cord, and keep the total under the cord’s rating. The watts to amps calculator takes a sum such as 600 + 300 + 60 and turns it into amps.
A cord that runs warm or hot, which means it may be overloaded: see hot extension cords.
An extension cord is meant to be temporary. For something that stays plugged in, an electrician can add an outlet where you need it.
Common questions
Is a 12 gauge extension cord good for 20 amps?
Not by Southwire's ratings, which give its 12 gauge cords 15 amps at 25, 50 and 100 feet. Southwire rates only its 10 gauge cords at 20 amps. Check the tag on your cord.
Can a 16 gauge cord run a 1,500 watt appliance?
1,500 watts is 12.5 amps at 120 volts. Southwire rates its 16 gauge cords at 13 amps up to 50 feet but only 10 amps at 100 feet, so a long 16 gauge cord falls short. Go by the tag on your cord, and never use one for a space heater.
The rating on the tag and the advice on adding up a load come from the CPSC, and the ratings by gauge from Southwire’s cord catalog and product pages. Checked against published data from CPSC and Southwire.
See all 5 sources
US Consumer Product Safety Commission, “Household Extension Cords Can Cause Fires” (Pub 5032): the rating on the tag, hot cords, at least 16 AWG, temporary use, cords marked for outdoor use, not through doorways
Southwire, Cord Solutions Catalog: decoding jacket types, amp ratings of its 16, 14, 12 and 10 gauge extension cords at 25, 50 and 100 ft, and generator cables and voltage drop
US Consumer Product Safety Commission, Safety Alert 5123, “Portable Generator Hazards” (2021): carbon monoxide, extension cords, backfeeding, CO alarms
US Consumer Product Safety Commission, ballot vote sheet and staff package, extension cord proposed rule (January 2015): household current at nominal 120 volts AC, and UL 817's jacketed cord for outdoor extension cords
Southwire product pages: 16/3 SJTW 15 ft (2306SW8808) and 16/3 SJOW 10 ft (2211SW8808) outdoor extension cords, 13 A, 125 V, 1625 W; 10/3 SJTW 50 ft with a NEMA 5-15P plug (2688SW0002), 15 A, 125 V, 1875 W
General guidance from manufacturer and standards data. Products vary, so check the label and the maker's instructions.