ElectricalToolbox

What Do the Numbers on an Extension Cord Mean?

A stamp like 16/3 SJTW tells you almost everything you need before you plug in a Halloween display. The first number is the wire gauge, and smaller means thicker and stronger: a 12-gauge cord carries far more than a 16-gauge one. The second number is how many wires are inside, and for anything outdoors you want three, so there is a ground. The letters spell out how the cord is built, and the one that matters most outside is the W, which means it is rated for weather and wet locations. Read those parts and you can tell in a moment whether a cord belongs on the lawn or should stay in the closet. Here is what each part means.

The gauge sets how much load the cord can carry and how far it can run before the voltage sags. Put your cord's gauge, length, and load into the Extension Cord Voltage Drop Calculator to see whether the far end of the run still gets full voltage, which is what usually limits a long cord before its amp rating does.

The first number: wire gauge

The first number is the American Wire Gauge (AWG) of the copper inside. The scale runs backward from what you would expect: the larger the number, the thinner the wire, and the less current it can carry before it heats up. A 16-gauge cord is thin and light. A 12-gauge cord is noticeably thicker and heavier, and it carries much more. On common outdoor cords you will see 16, 14, 12, and sometimes 10.

The code puts a current rating on each size in its flexible-cord table. A normal 120-volt extension cord carries current on two of its wires, the hot and the neutral, while the ground wire carries none, so the two-conductor column is the one that applies. For that case the listed values are:

Gauge (AWG) Listed amps Rough watts at 120 V
16131,560
14182,160
12253,000
10303,600

Those are the code ceilings. The number the listing agency actually stamps on the jacket is often lower, because it also accounts for the cord's length and how it is used: a 16-gauge cord is very commonly printed 10 amps, or about 1,200 watts, rather than the table's 13. When the stamp and the table disagree, follow the stamp. And whatever the stamp says, a load left running all evening should sit at about 80 percent of it, so an 11-amp draw does not belong on a cord rated 10.

The second number: how many wires

The number after the slash is the count of conductors in the cord. A 16/2 cord has two wires, a hot and a neutral, and no ground. A 16/3 cord has three: hot, neutral, and a ground. For any outdoor decoration, and for anything with a three-prong plug, you want the three-wire cord. The ground wire is the path that carries a fault safely away instead of through you, and it is the reason a two-prong cord does not belong on a wet lawn. The ground pin on the plug is useless if the cord has no ground wire behind it.

The letters: what the cord is built for

After the numbers comes a short letter code that describes the jacket. Each letter is a fact about how the cord is built and where it is allowed to work:

  • S: a flexible service cord, the general-purpose portable type.
  • J: junior, a lighter 300-volt insulation. No J means a heavier 600-volt build.
  • T: a thermoplastic (vinyl) jacket. An E in its place means a rubber-like TPE jacket.
  • O: oil resistant, if present.
  • W: rated for weather and wet locations. This is the outdoor letter.

So SJTW reads as a flexible, junior-duty, thermoplastic, weather-rated cord, which is a normal outdoor extension cord. The single letter to look for before you use a cord outside is the W. A cord with no W, such as an indoor SPT-marked lamp cord, is not built to keep water out and does not belong on a Halloween display, no matter how long it is. Look for a UL or other listing mark on the jacket too; an unmarked cord has not been tested by anyone.

Worked example: reading a cord for a display

Say the cord in your hand reads 16/3 SJTW and the jacket also stamps 13 A. You now know it is 16-gauge, three-wire with a ground, and weather-rated, and good for about 13 amps, though a common longer version of the same cord is stamped 10. That is plenty for an all-LED yard. But watch the distance. Using the resistance the code lists for 16-gauge copper, an 8-amp load on a 100-foot run of that cord loses:

Drop = 2 × amps × resistance × (length ÷ 1000)

Drop = 2 × 8 A × 4.89 Ω × (100 ÷ 1000)

Drop ≈ 7.8 V, about 6.5% of 120 V

That is past the 5 percent most equipment is happy with, so at 100 feet the thin cord is maxed out on voltage drop before it is anywhere near its amp limit. Step up to a 12-gauge cord for the same run and load and the same math gives about 3.1 volts, roughly 2.6 percent, which is fine. The higher the load and the longer the run, the more the gauge earns its weight.

The short version

  • First number is gauge. Smaller is thicker and stronger. 12-gauge carries far more than 16-gauge.
  • Second number is wire count. Use a three-wire cord outdoors so there is a ground.
  • Look for the W. It is the letter that means the cord is rated for weather and wet locations.
  • Follow the amp stamp, and stay near 80 percent of it for a load that runs all evening.
  • Length can max out a cord before amps do. Around 100 feet a 16-gauge cord is already losing more than 5 percent.

NEC reference

NEC 2020. Flexible-cord current ratings are Table 400.5(A)(1). A normal 120-volt cord has two current-carrying conductors, the hot and neutral, since the equipment grounding conductor is not counted as current-carrying: for that two-conductor case 16 AWG is listed at 13 amps, 14 AWG at 18, 12 AWG at 25, and 10 AWG at 30. Where all three conductors carry current those same sizes drop to 10, 15, 20, and 25. The cord-type letter codes (S, J, T, O, W and others) are defined in Table 400.4. The 4.89 ohms per 1,000 feet used in the voltage-drop example is the direct-current resistance for solid uncoated copper in Chapter 9, Table 8, and 1.93 ohms is the same value for 12 AWG. The 80 percent figure for a load running three hours or more mirrors the continuous-load rule in 210.19(A) and 210.20(A). Watts equal volts times amps is the power relationship, not a code value. A cord's printed rating reflects its UL listing, which can be more restrictive than the table, and it governs.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

For the whole season

The Holiday Electrical Safety Handbook

This guide answers one question. The Handbook covers Halloween through New Year's: how many amps a circuit can really carry (a 15 amp breaker means 12 amps, a 20 means 16), extension cords, GFCI outlets, outdoor displays in the weather, the Thanksgiving kitchen, tree lights, space heaters, and what to do when something goes wrong. Every figure is pre-converted to amps, so you add instead of calculate.

Read on Kindle Paperback on Amazon Free circuit map and load worksheet

Get the next seasonal safety check by email

Optional. Short emails before each holiday, written for homeowners.

Related