Extension Cord Gauge Chart: Amperage and Wattage by Wire Gauge and Length, Voltage Drop, SJTW vs SJEOW vs STW, and OSHA Rules for Job Sites
Sep 10, 2026
Summary: Extension cord capacity is set by conductor gauge and length together. At 120 V, a 16 AWG cord is good for 10 A (1,200 W) to 50 ft and 7 A beyond; 14 AWG carries 15 A (1,800 W) to 50 ft and 13 A to 100 ft; 12 AWG carries 15 A to 100 ft and 20 A to 50 ft; 10 AWG carries 20 A (2,400 W) to 100 ft and 15 A to 150 ft. The limiting factor past 50 ft is voltage drop: a 12 A tool on 100 ft of 16 AWG loses about 9.5 V (8 percent), which overheats motors; on 12 AWG it loses 3.8 V (3 percent), which is the design limit. Jacket letters: S is hard service, J is 300 V junior service, T is thermoplastic, E is elastomer (TPE) for cold flexibility, O is oil resistant (OO is oil-resistant jacket and insulation), W is outdoor and wet rated. OSHA 29 CFR 1926.405(a)(2)(ii)(J) requires cords on construction sites to be hard or extra-hard usage (S, ST, SO, STO, SJ, SJO, SJT, SJTW and similar) and 1926.404(b)(1) requires GFCI protection for every cord-connected 120 V tool.
Capacity by gauge and length, 120 V
| Gauge | 25 ft | 50 ft | 100 ft | 150 ft | 200 ft | Typical loads |
|---|---|---|---|---|---|---|
| 18 AWG | 7 A / 840 W | 5 A / 600 W | Not recommended | No | No | Lamps, phone chargers, holiday lights |
| 16 AWG | 13 A / 1,560 W | 10 A / 1,200 W | 7 A / 840 W | 5 A | No | Drills, sanders, small fans, TVs |
| 14 AWG | 15 A / 1,800 W | 15 A / 1,800 W | 13 A / 1,560 W | 10 A | 7 A | Circular saws, shop vacs, space heaters (25 ft only), refrigerators |
| 12 AWG | 20 A / 2,400 W | 20 A / 2,400 W | 15 A / 1,800 W | 13 A | 10 A | Miter saws, table saws, air compressors, 1,500 W heaters, RV 20 A |
| 10 AWG | 30 A / 3,600 W | 25 A | 20 A / 2,400 W | 15 A | 15 A | Welders to 20 A, large compressors, RV 30 A adapters, temporary power |
| 8 AWG | 40 A | 40 A | 30 A | 25 A | 20 A | RV 50 A pedestals (with 6/3 + 8/1), welders, temporary panels |
Ampacity of the conductor itself is governed by NEC Table 400.5(A)(1): 18 AWG 10 A, 16 AWG 13 A, 14 AWG 18 A, 12 AWG 25 A, 10 AWG 30 A for three-conductor cords. The table above is derated for length and for the plug and receptacle rating; a 15 A plug limits the cord to 15 A regardless of gauge.
Voltage drop, 120 V single phase, copper
| Load | Length | 16 AWG | 14 AWG | 12 AWG | 10 AWG |
|---|---|---|---|---|---|
| 10 A | 50 ft | 4.0 V (3.3%) | 2.5 V (2.1%) | 1.6 V (1.3%) | 1.0 V |
| 10 A | 100 ft | 8.0 V (6.7%) | 5.0 V (4.2%) | 3.2 V (2.7%) | 2.0 V |
| 15 A | 50 ft | 6.0 V (5.0%) | 3.8 V (3.2%) | 2.4 V (2.0%) | 1.5 V |
| 15 A | 100 ft | 12.0 V (10%) | 7.6 V (6.3%) | 4.8 V (4.0%) | 3.0 V (2.5%) |
| 20 A | 100 ft | No | 10.1 V (8.4%) | 6.4 V (5.3%) | 4.0 V (3.3%) |
| 20 A | 200 ft | No | No | 12.8 V (10.7%) | 8.0 V (6.7%) |
Formula: V drop = 2 × I × L × R, with R per foot of 0.00404 ohm (16 AWG), 0.00254 (14), 0.00160 (12), 0.00101 (10), 0.00063 (8). Design to 3 percent (3.6 V at 120 V) for motor loads; universal-motor tools run hot and lose torque past 5 percent, and induction motors (compressors, refrigerators) can fail to start.
Jacket designations
| Letter | Meaning | Notes |
|---|---|---|
| S | Hard service, 600 V | Extra-hard usage; full jacket thickness |
| SJ | Junior hard service, 300 V | Hard usage; the common contractor cord; thinner jacket than S |
| T | Thermoplastic (PVC) | Stiffens below about 0 F |
| E | Elastomer (TPE) | Flexible to −40 F; the cold-weather choice |
| O | Oil-resistant jacket | OO: oil-resistant jacket and conductor insulation |
| W | Weather and water resistant | Outdoor rated; replaces the older W-A |
| P | Parallel (flat) construction | SPT-1, -2, -3: lamp cord; not for tools or job sites |
Common combinations: SJTW is the standard outdoor contractor cord; SJEOW or SJEOOW is the cold-weather, oil-resistant upgrade; STW and SOOW are extra-hard usage for temporary power and generators; SPT-2 is an indoor household cord that fails OSHA's hard-usage rule on a site.
OSHA and NEC rules on job sites
- 1926.405(a)(2)(ii)(J): flexible cords for temporary wiring must be hard or extra-hard usage and marked as such (S, ST, SO, STO, SJ, SJO, SJT, SJTO and their W and E variants). Household SPT cords are not permitted.
- 1926.404(b)(1): GFCI protection for all 120 V, 15 and 20 A receptacles used by cord-connected tools, or an assured equipment grounding conductor program; in practice, a GFCI cord end or a GFCI spider box.
- 1926.405(g)(2)(iv): cords must not be run through doorways, windows or holes in walls where they can be damaged, and not be fastened with staples or bare wire.
- 1926.416(e)(1): worn or frayed cords and cords with a missing ground pin are removed from service; the ground pin cannot be cut off.
- NEC 400.12: cords are not a substitute for fixed wiring, may not run through walls, ceilings or floors, and may not be concealed.
- NEC 590: temporary power installations; cords used for temporary lighting and power on construction sites must be listed and protected from damage.
Choosing a cord
| Use | Cord |
|---|---|
| Cordless-tool charger, radio, work light at 25 to 50 ft | 16/3 SJTW |
| Circular saw, shop vac, drill at 50 to 100 ft | 12/3 SJTW, lighted end |
| Miter saw or compressor at 100 ft | 12/3 SJTW; 10/3 past 100 ft |
| Winter job site, cords stay flexible | 12/3 SJEOW |
| Generator to spider box, temporary power | 10/3 or 10/4 STW or SOOW, twist-lock ends |
| RV at a 30 A pedestal | 10/3 STW, TT-30 ends, 25 to 50 ft |
| RV at a 50 A pedestal | 6/3 + 8/1 STW, 14-50 ends |
| Welder, 240 V 30 A | 10/3 STW, 6-30 or 6-50 ends, 25 to 50 ft |
Quick answers
What gauge extension cord for 100 ft? 12 AWG for any tool up to 15 A; 10 AWG for 20 A loads or anything with a motor over 1.5 HP.
Can I use a 16 gauge cord for a space heater? A 1,500 W heater draws 12.5 A; use 14 AWG at 25 ft or 12 AWG at 50 ft. Never 16 AWG, and never past 50 ft.
What does SJTW mean? Junior hard service (300 V), thermoplastic jacket, weather resistant. The standard outdoor contractor cord.
Are household extension cords allowed on a construction site? No. OSHA 1926.405 requires hard or extra-hard usage cords (S or SJ types); SPT lamp cords are not permitted.
Conversions Tech stocks 12/3 and 14/3 SJTW contractor cords in 25 to 100 ft, lighted ends, tri-taps, RV 30 and 50 A cords, dryer and range cords and IEC and NEMA power cords. Case pricing for crews and fleets.
