Extension Cord Gauge: Which Wire Size for Which Load

Gauge runs backwards

In the United States the size of the copper wire in a cord is given in American Wire Gauge, AWG. An OSHA-funded construction safety workbook puts the key fact plainly: the higher the AWG number, the smaller the wire. Cords in common use range from #18 to #10, and normally only even sizes are made.

What each gauge is for

The CPSC adds a floor for household cords: extension cords should be at least 16 AWG, unless they are 18 AWG with fuse protection. Its home electrical safety checklist says a 16 AWG cord handles 1,375 watts and to use 14 or 12 AWG for heavier loads. The OSHA workbook calls #16 the most commonly used general-purpose gauge, fine for work lights and electric trimmers but not for high-power loads, and names #14 as the minimum size recommended for construction work.

These are rough guides. The rating that counts is the one on the cord itself: the CPSC says you can check a cord's capacity on its tag or packaging.

Reading the marking

Cords larger than lamp cord are printed along their length. The CPSC notes that the wire size is imprinted on the cord's surface, and the OSHA workbook explains the format. A marking such as 14/3 means three conductors of #14 wire. The letters after it describe the construction:

• S: hard service. SJ: junior hard service, a lighter-duty, less rugged version of S.
• T: thermoplastic insulation instead of a rubber-type material.
• E: thermoplastic elastomer insulation, more rubber-like.
• O: oil resistant.
• W: moisture and sunlight resistant.

So 12/3 SJTW reads as three #12 conductors in a junior hard service cord with thermoplastic insulation, rated for moisture and sunlight. For outdoor work the CPSC says to use an extension cord outdoors only if it is marked for outdoor use. On job sites, OSHA's flexible cords page says extension cords must be the 3-wire type so tools can be grounded, and the construction standard requires hard or extra-hard usage ratings, examples being S, ST, SO and STO for hard service and SJ, SJO, SJT and SJTO for junior hard service.

Check the load: watts and amps

Find the watts or amps on the appliance's label. Watts are volts times amps, so at 120 volts, amps = watts ÷ 120. The guide to volts, amps and watts explains the units, and the Ohm's law calculator converts between them.

Add up everything on the cord, not just the largest item. As an example, a 500 W work light, a 700 W drill, a 900 W shop vacuum running at once on one cord draw 2,100 W together, 17.5 amps, which is over the 1,375 W the CPSC checklist gives for 16 AWG. The CPSC names overloading, plugging in appliances that consume more watts than the cord can handle, as the usual cause of overheating. To see what a load costs to run rather than what it draws, use the electricity cost calculator.

What should not go on an extension cord

NFPA's 2026 educational messages are direct about a few appliances:

• Major appliances such as refrigerators, stoves, washers, electric dryers and microwave ovens: plug directly into a wall outlet; an extension cord can easily overheat and start a fire.
• Cooking appliances: never on an extension cord, because it can overload the circuit.
• Space heaters: directly into the wall, not into an extension cord or a power strip.
• Window air conditioners: into the wall; many manufacturers prohibit extension cords, and if the instructions allow one, follow them for the type.

Choosing a cord in four steps

1. Add up the load. Total the watts of everything that will run on the cord at the same time and divide by 120 for amps.
2. Compare with the cord's rating. Read the tag or package. A cord whose rating is below your total is the wrong cord, whatever its length or price.
3. Pick the gauge. Light loads within the 16 AWG figure can use 16 AWG; heavier loads need 14 or 12 AWG, and job sites call for at least #14 according to the OSHA workbook.
4. Match the place. Outdoors, the cord must be marked for outdoor use; on a construction site it must be a 3-wire hard or extra-hard usage type.

Power strips and warning signs

A power strip is not a fix for a cord that is too small. NFPA says to use only surge protectors or power strips that have internal overload protection and are listed by a nationally recognized testing laboratory, and to follow the manufacturer's instructions. Space heaters go into neither.

NFPA also lists signs that call for a licensed electrician rather than another cord: fuses that blow or breakers that trip again and again, a tingling feeling when you touch an appliance, metal sink or plumbing fixture, discolored or warm wall outlets or switches, a burning or rubbery smell from an appliance, flickering lights, sparks from an outlet, and cracked or broken outlets. If a breaker trips often, find out why before resetting it.

Using cords safely

• Extension cords are for temporary use. If you need one permanently, NFPA suggests having a licensed electrician decide whether more circuits or outlets are needed.
• Do not run cords under carpets or rugs, through doorways or under furniture (CPSC and NFPA).
• If any part of the cord is hot in use, treat it as overloaded (CPSC).
• Replace cracked or damaged cords; do not tape them or try to repair them (CPSC and NFPA). The OSHA workbook warns that replacing a cord's ends voids its approval, for example a W cord is no longer watertight.
• Never remove the ground pin or use an adapter that makes a three-prong cord fit a two-prong outlet (OSHA workbook).
• Buy cords listed by a recognized testing laboratory such as UL, Intertek (ETL) or CSA (CPSC).
• Unplug cords when not in use, and keep young children away from them (CPSC).

Frequently asked questions

Sources

• OSHA-funded Electrical Hazards in Construction student workbook (AWG sizes; higher number means smaller wire; cords from #18 to #10; #16 general purpose, #14 minimum for construction, #12 large power tools, #10 large loads; the 14/3 marking and the letters S, SJ, T, E, O, W; do not replace cord ends or remove the ground pin)
• OSHA: Flexible cords (extension cords must be 3-wire type; hard or extra-hard usage ratings marked along the cord; damaged conductors cause shocks, burns or fire)
• CPSC publication 5032: Household extension cords can cause fires (overloading; rating on the tag or packaging; at least 16 AWG unless 18 AWG with fuse protection; wire size imprinted on the cord; not under carpets; outdoor marking; temporary use; testing laboratory listing)
• CPSC publication 513: Home electrical safety checklist (16 AWG handles 1,375 W, use 14 or 12 AWG for heavier loads; a warm cord may be overloaded; extension cords not for permanent use)
• NFPA Educational Messages Desk Reference, 2026 edition (major and cooking appliances and space heaters directly into wall outlets; window air conditioners per the manufacturer; replace damaged cords; extension cords for temporary use only; avoid cords under rugs; power strips with internal overload protection; signs that call for a licensed electrician)

Written by Tobias. Updated October 30, 2026.