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Guide · Jobsite power · Extension cords, portable power unit cords and power blocks

What gauge extension cord do you need, and how long can it be? 10, 12, 14 and 16 gauge, 12/3 and 10/3, watts and amps by gauge and length, voltage drop at 100 feet, outdoor cords and GFCI, what never goes on a cord, and 30 and 50 amp twist-lock cords

For jobsite tools the answer is a 12 gauge, three-conductor cord (12/3) up to 100 feet; it carries 15 amps with little voltage drop, which is what a circular saw, a roofing nailer compressor or a grinder draws. 14 gauge is fine to 50 feet for drills and lights; 16 gauge is a lamp cord and 10 gauge is for 20 to 30 amp loads and runs past 100 feet. The gauge number gets smaller as the wire gets bigger. A cord is rated in amps (NEC Table 400.5(A)(1): 16 AWG 13 A, 14 AWG 18 A, 12 AWG 25 A, 10 AWG 30 A), and the practical limit on a long cord is voltage drop, which starves a motor; that is why a 100 ft cord is 12 gauge and a 150 ft run is 10. Nothing with a heating element or a compressor that runs for hours (space heater, refrigerator, air conditioner) belongs on a light-duty household cord.

Bayshore Safety. Industrial safety, roofing tools and jobsite supplies since 2017. · Current as of November 2026 · the C&R cord product data on this site (12/3 single-tap and triple-tap cords in 50 and 100 ft; 10/3 100 ft cord with L6-30 locking plugs; 12 gauge 3-way power blocks and surge block); NEC Table 400.5(A)(1) ampacity of flexible cords (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 SJTW / SOOW jacket letters as defined in UL 62; 29 CFR 1926.405(a)(2)(ii)(J) on flexible cords on construction sites
Cords and power blocks we stockAll roofing tools
C&R 508-E Extension Cord, 50 ft and 100 ft, 12/3 gauge, Single TapC&R 508-E Extension Cord, 50 ft and 100 ft, 12/3 gauge, Single TapThe product data lists the 508-E and 509-E as 12/3 single-tap extension cords in 50 and 100 ft
$241.10See the product

Reading a cord. The jacket is printed with its gauge and conductor count (12/3 is three 12 AWG conductors: hot, neutral, ground), its jacket type (SJTW: junior hard service, thermoplastic, weather resistant, the outdoor jobsite cord; SOOW: hard service, oil-resistant, the heaviest), its amp rating and a listing mark. From those three facts every question below is answered: how many watts a cord carries, which gauge for portable power or a refrigerator, how long it can be, whether it is an outdoor cord, why a 25 ft cord is or is not enough, and what should never be plugged into one.

Extension cords, question by question

Ampacities are NEC Table 400.5(A)(1) values for three-conductor cords; watts are amps times 120 volts. Jacket letters are the UL 62 designations printed on the cord.

What is the difference between a 12 and 14 gauge extension cord, and are 12 gauge cords good?Wire size. 12 AWG has about 60 percent more copper cross-section than 14 AWG, so it carries more current (25 A vs 18 A in a three-conductor cord by the NEC table, usually marked 15 A and 13 A on the plug) and loses less voltage over the same length. For tools that draw 12 to 15 amps on a 100 ft run, 12 gauge is the trade standard; 14 gauge works to about 50 feet on the same tool and all day on lights and drills. Is a 12-3 cord heavy duty? In the retail labels, 12 gauge is "heavy duty", 14 is "medium duty", 16 is "light duty"; the jacket letters (SJTW) say more than the adjective.
How many watts can a 14 gauge 100 ft cord handle, and what is a 10/3 cord rated for?Watts are amps times volts. A 14 gauge cord rated 15 A carries 1,800 watts at 120 V on paper, but at 100 feet it drops about 5 percent of the voltage at 15 A, so the usable load is nearer 1,500 watts; a 12 gauge cord at 100 feet carries 15 A (1,800 W) with about 3 percent drop; a 10 gauge cord carries 20 to 30 A (2,400 to 3,600 W at 120 V, or 7,200 W at 240 V on a 30 amp twist-lock power cord). A 10/3 cord is three 10 AWG conductors, rated 30 A, which is the L6-30 and L14-30 twist-lock cord size. A 16 gauge cord is 13 A by the table and 10 A as usually marked: lights, radios, battery chargers.
How long can an extension cord be, and is a 25 foot cord enough?As long as the voltage drop allows for the load: roughly 50 ft of 14 gauge, 100 ft of 12 gauge or 150 ft of 10 gauge for a 15 A tool keeps the drop near 3 percent, which motors tolerate. Beyond that, the tool runs hot and slow and the cord warms. Running two cords in series is the same as one long cord of the thinner gauge. A 25 ft cord is enough when the outlet is within 25 feet; it is the right length for a shop bench or a portable power unit sitting next to the work, and the drop is negligible in any gauge. The longest cords sold off the shelf are 100 ft; 150 and 200 ft are made in 10 and 12 gauge for site work.
How much voltage drop is there across a 100 foot extension cord?Copper resistance per 1,000 feet is about 1.6 ohms for 12 AWG, 2.5 for 14, 4.0 for 16 and 1.0 for 10; a 100 ft cord is 200 feet of conductor out and back. At 15 amps that is 15 x 0.32 = 4.8 volts (4 percent) on 12 gauge, 7.5 V (6 percent) on 14 gauge, 12 V (10 percent) on 16 gauge and 3 V (2.5 percent) on 10 gauge. A universal motor slows and a compressor motor may fail to start below about 108 volts, which is why the 100 ft jobsite cord is 12 gauge.
What appliances should never be plugged into an extension cord?The appliance manufacturers’ manuals and the fire-safety agencies say the same list: space heaters, portable air conditioners, refrigerators and freezers, microwaves, toasters and ovens, washers and dryers, and anything else with a heating element or a compressor that runs for hours. The reasons are heat (a 1,500 W heater at the end of a 16 gauge cord runs the cord at its limit all night), the plug connection loosening over time, and cords under rugs or through doors. On a jobsite, a 1,500 watt heater or a refrigerator on a short 12 gauge SJTW cord is within the cord’s rating; the hazard is the thin household cord, not the extension cord as such. The manual for the appliance governs.
Is there an extension cord for a refrigerator, and will a 12 gauge cord run one?A household refrigerator draws 3 to 6 amps running and 10 to 15 amps for a second at compressor start. A 12 gauge, three-prong, grounded cord as short as possible carries it with no voltage drop to speak of; a 14 gauge cord of 25 feet or less does too. The refrigerator manual usually says not to use an extension cord at all because of the plug connection and the risk of a thin cord; if one is used, it is 12 or 14 gauge, grounded, short, and not a 16 gauge lamp cord. The same figures apply to running a refrigerator from portable power: a 12 gauge cord to the unit’s 120 V outlet.
Can I run a 1500 watt heater or an air conditioner on an extension cord?A 1,500 W heater draws 12.5 A; on a 12 gauge SJTW cord of 50 feet or less it is inside the rating, on a 16 gauge household cord it is at the limit and the plug warms. The heater manual almost always says no cord; a jobsite heater on a 12 gauge cord is a different case than a bedroom heater on a lamp cord. A window air conditioner draws 5 to 12 A running and more at start; the same rule applies: 12 gauge, short, grounded, and the unit’s manual has the last word.
What gauge extension cord for portable jobsite power, and 30 amp or 50 amp?From the portable power unit’s 120 V duplex outlets to tools: 12 gauge cords as above. From the power unit’s twist-lock outlet to a transfer switch or a power distribution box: a twist-lock power cord matched to that outlet. L5-30 is 30 A 120 V; L6-30 is 30 A 240 V two-pole (our 10/3 100 ft cord carries L6-30 plugs); L14-30 is 30 A 120/240 V four-wire, the common home transfer-switch cord; 14-50 and SS2-50 are 50 A 120/240 V. 30 vs 50 amp is decided by the power unit’s outlet and the inlet on the house or panel: a 50 A outlet supplies up to 12,000 watts, a 30 A outlet up to 7,200. The cord gauge follows: 10 gauge for 30 A, 6 gauge for 50 A. Are all 30 amp twist-lock power cords the same? No: L5-30, L6-30 and L14-30 are different plugs for different voltages and do not interchange.
How long can a 50 amp power cord be?At 50 A the drop over a 6 gauge cord is about 1.3 volts per 25 feet out and back (0.4 ohm per 1,000 ft), so 50 feet holds the drop under 3 percent at 240 V and 100 feet is about 5 percent at full load; most 50 A twist-lock power cords are sold at 25 to 50 feet for that reason and because a 6 gauge, four-conductor cord is heavy. A 30 A cord in 10 gauge runs 100 feet (our 10/3 is 100 ft) with about 4 percent drop at full load.
Which cords are for outdoor use, and can an outdoor cord be used inside?The W in the jacket code (SJTW, SJEOW, SOOW) means weather resistant: the jacket is rated for sunlight and moisture. Cords without the W (SJT, SPT) are indoor cords. Every cord we stock for the jobsite is a W-rated jacket. An outdoor cord is fine indoors; the reverse is not. Cords left plugged in outside in the rain need weather-resistant connectors kept off the ground, a cord-connector cover or a drip loop, and the circuit should be GFCI protected; on construction sites 29 CFR 1926.405 requires GFCI protection or an assured grounding program for receptacles that are not part of the permanent wiring. To waterproof a connection, use a weatherproof cord connector housing or an outdoor connection box; tape and bags are not a rating.
What are the different types of extension cords?By gauge (16, 14, 12, 10). By conductor count (two-wire ungrounded, three-wire grounded; 3 and 4 conductor for 240 V). By jacket: SPT lamp cord, SJT indoor, SJTW outdoor jobsite, SJEOW cold-weather elastomer, SOOW heavy oil-resistant. By end: single tap, triple tap (three outlets in one head, our 3W-418), lighted ends, locking (twist-lock L5 and L6 series). By use: twist-lock power cords (L14-30, 14-50), RV cords (TT-30), welder cords (6-50). A cord with prongs on both ends does not exist as a listed product and is a shock hazard; power distribution to a building goes through an inlet and a transfer switch.
What is a power block or 3-way plug, and how thick is a cord?A power block is a short, heavy cord (ours is 12 gauge, 2 feet) with three outlets on a rubber block, used at the end of a long cord so three tools can be plugged in; a 3-way plug does the same without the lead, and a surge block adds surge protection for electronics. The loads add up: three tools on one 12 gauge cord share its 15 amps. Cord diameter follows gauge and jacket: a 16 gauge SJTW cord is about 0.3 inch across, 12 gauge about 0.45 inch, 10 gauge about 0.55 inch; a cord that is thick but light is likely a thick jacket on thin wire, so read the printing.

Extension cords in plain words

  • Smaller gauge number, bigger wire: 16 light duty, 14 medium, 12 heavy duty, 10 for 20 to 30 amp loads.
  • 12/3 means three 12 AWG conductors (hot, neutral, ground); it is the 100 ft jobsite cord.
  • Watts are amps times volts: 15 A at 120 V is 1,800 W; voltage drop over a long thin cord takes some of it.
  • Rule of thumb for a 15 amp tool: 50 ft of 14 gauge, 100 ft of 12 gauge, 150 ft of 10 gauge.
  • W in the jacket code (SJTW, SOOW) means outdoor rated; no W means indoor only.
  • Heaters, air conditioners, refrigerators and anything with a heating element stay off light household cords; the appliance manual governs.
  • Twist-lock power cords are matched to the outlet: L5-30 (120 V), L6-30 (240 V), L14-30 (120/240 V) at 30 A on 10 gauge; 14-50 at 50 A on 6 gauge.
  • On construction sites receptacles on cords need GFCI protection or an assured grounding program (29 CFR 1926.405).

Questions people ask

What appliances should never be plugged into an extension cord?

Space heaters, air conditioners, refrigerators and freezers, microwaves, toasters and ovens, washers and dryers: anything with a heating element or a compressor that runs for hours, per the appliance manuals and the fire-safety agencies.

Which is better for my home, a 50 amp or a 30 amp power plug?

Whichever matches the power unit’s outlet and the house inlet: 50 A supplies up to 12,000 W on 6 gauge cord, 30 A up to 7,200 W on 10 gauge.

How long can a 50 amp power cord be?

Most are sold at 25 to 50 feet; 100 feet of 6 gauge runs about 5 percent drop at full load.

What gauge extension cord should you use for a portable power unit?

12 gauge from the 120 V outlets to tools; 10 gauge for a 30 A twist-lock cord; 6 gauge for a 50 A cord.

Is a 25 foot extension cord enough?

When the outlet is within 25 feet, yes, in any gauge the tool needs; voltage drop at 25 feet is negligible.

Is there an extension cord that you can use for a refrigerator?

A short, grounded 12 or 14 gauge cord carries a refrigerator; the manual usually says to avoid any cord, and a 16 gauge lamp cord is the one to avoid.

What is the longest extension cord I can get?

100 feet off the shelf; 150 and 200 feet are made in 10 and 12 gauge for site work.

Is a 12-3 gauge extension cord heavy duty?

Yes; 12 gauge is the heavy-duty class in retail labeling and the 100 ft jobsite standard.

What size extension cord to run a refrigerator on a portable power unit?

12 gauge, grounded, as short as practical, from the power unit’s 120 V outlet.

What is the extension cord for outside use?

One with a W in its jacket code (SJTW, SJEOW, SOOW), 12 gauge for tools, with GFCI protection on the circuit.

Do I need a 30 amp or 50 amp twist-lock power cord?

The one that matches the power unit’s outlet and the inlet it feeds; they are not interchangeable.

Will a 12 gauge extension cord run a refrigerator?

Yes, with capacity to spare; a refrigerator draws 3 to 6 amps running.

What are the different types of extension cords?

By gauge, conductor count, jacket (SPT, SJT, SJTW, SOOW), end type (single, triple tap, locking) and use (portable power unit, RV, welder).

Can you use an outdoor extension cord inside the house?

Yes; the W jacket is rated for both. Indoor cords cannot go outside.

Are all extension cords for outdoor use?

No; only cords with a W in the jacket code are weather rated.

How to waterproof an extension cord connection?

A weatherproof cord connector housing or outdoor connection box, kept off the ground with a drip loop; tape and bags are not a rating.

How thick is an average extension cord?

About 0.3 inch for 16 gauge SJTW, 0.45 inch for 12 gauge, 0.55 inch for 10 gauge; read the printing, not the thickness.

Are all 30 amp twist-lock power cords the same?

No: L5-30 is 120 V, L6-30 is 240 V two-pole and L14-30 is 120/240 V four-wire; the plugs do not interchange.

Can I run a 1500 watt heater on an extension cord?

It draws 12.5 A; a short 12 gauge SJTW cord is inside its rating, a 16 gauge household cord is not, and the heater manual usually says no cord at all.

Is there such a thing as an extension cord with prongs on both ends?

Not as a listed product; it is a shock hazard. Feed a building through an inlet and transfer switch.

What is the difference between a cord and a cable?

In the code, a flexible cord is the portable stranded assembly with a plug (SJTW); a cable is fixed building wiring (NM, MC). Cords are not substitutes for cable.

Can you leave an extension cord plugged in outside in the rain?

A W-rated cord with weather-resistant connectors kept off the ground on a GFCI circuit, yes; a connection lying in a puddle, no.

How many watts can a 14 gauge 100 ft extension cord handle?

About 1,500 watts usable at 100 feet after voltage drop; 1,800 W on paper at 15 A.

What is the difference between a 12 and 14 gauge extension cord?

12 gauge has about 60 percent more copper, carries more current and loses less voltage over the same length.

Can a 10 gauge extension cord handle 30 amps?

Yes; 10 AWG three-conductor cord is rated 30 A, which is the 30 A twist-lock cord size.

What is a 10/3 extension cord rated for?

30 amps; it is the L5-30, L6-30 and L14-30 twist-lock cord size.

What is a 10 gauge cord’s diameter?

About 0.55 inch over the jacket for SJTW; the conductor itself is about 0.1 inch.

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The ratings, test results and standards shown here are the manufacturers' published values, reproduced as published. Nothing on this page is a recommendation by Bayshore Safety that any product is suitable for a particular task, hazard or workplace. Selecting safety products and protective equipment is the responsibility of the end user and the employer, based on their own hazard assessment and the regulations that apply to their work. Confirm every value on the manufacturer's own documentation before use.

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