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Which Cord Gauge? 16/3, 14/3, 12/3, 10/3
NEC 400.5(A)(1) rates 16/3 at 13 A and 12/3 at 25 A. Match gauge to tool load and cord run length, or voltage drop starves the motor at the far end.
An extension cord that trips a breaker is annoying. One that quietly starves a motor is worse: the tool runs, the windings cook, and the failure gets blamed on the tool. Both come from the same two sizing mistakes - choosing the gauge from the plug instead of the load, and ignoring what the run length does to the voltage. The numbers below come from NEC Table 400.5(A)(1), which rates flexible cord separately from building wire, plus the voltage-drop arithmetic that decides whether the equipment at the far end actually gets its supply.
Two tables, two answers
The first thing to unlearn is that cord is rated like the wire in a wall. Building wire is sized from NEC Table 310.16; flexible cord is sized from Table 400.5(A)(1), and the values are lower for the same gauge because the conductors are bundled inside a jacket with less ability to shed heat. That table carries two columns - one for two current-carrying conductors and one for three - and choosing the right column is the first trap.
On an ordinary three-prong cord the equipment grounding conductor does not carry current in normal operation, so a 16/3, 14/3, 12/3 or 10/3 cord is rated from the two-conductor column. The three-conductor column applies where all three conductors are actually loaded, as on a 240 V cord with two hots and a loaded neutral.
Cord size | 2 current-carrying (A) | 3 current-carrying (A) |
18 AWG | 10 | 7 |
16 AWG (16/3) | 13 | 10 |
14 AWG (14/3) | 18 | 15 |
12 AWG (12/3) | 25 | 20 |
10 AWG (10/3) | 30 | 25 |
Those figures are the cord's own capacity at 30 C ambient. They are not necessarily the figure printed on the tag.
Why the tag says 15 A
A cord set is an assembly, and under UL 817 the assembly takes the rating of its weakest part. Fit a NEMA 5-15 plug to a 14/3 cord and the whole assembly is 15 A, because that is what the plug is. This is why 14/3 sells as 15 A and 12/3 as 20 A even though the conductor table would allow more: the plug and the connector set the ceiling.
Then derate for duty. For loads that run three hours or more the usual practice is to hold to about 80% of the rating, so a 15 A assembly is treated as a 12 A continuous circuit. Heat pushes the same way - a cord left coiled on a reel does not shed heat the way the same cord does laid out straight, and high ambient temperature reduces it further.
Length is what actually bites
Ampacity is a safety limit, not a performance one. A cord can sit comfortably inside its rating and still deliver a useless voltage at the far end. For a single-phase circuit the drop is arithmetic: voltage drop = 2 x current x resistance x length, counting both directions of the run. NEC guidance is 3% on a branch circuit, and motors are the first thing to suffer when you exceed it - low voltage means more current, more heat and hard starts.
Cord size | 10 A at 50 ft | 10 A at 100 ft | 15 A at 100 ft |
16/3 | 4.2% | 8.3% | 12.5% |
14/3 | 2.6% | 5.2% | 7.8% |
12/3 | 1.6% | 3.3% | 5.0% |
10/3 | 1.0% | 2.1% | 3.1% |
Read the last column. A 15 A tool at the end of 100 ft is outside the 3% guidance on every gauge except 10/3. That is the case that produces warranty claims: the cord was correctly rated for 15 A, the breaker never tripped, and the motor still burned out.
Drop scales directly with current and with length, so there are only two levers - shorten the run, or go up one gauge. Going up a gauge adds a few percent to unit cost; a returned pallet costs rather more.
Jacket: what the letters buy you
Gauge answers how much current; the jacket answers where the cord may live. In the S-family, SJT is a 300 V thermoplastic junior-service jacket rated 60 C for indoor use. SJTW adds the W marking for weather and water resistance and is the workhorse for outdoor extension cords. SOOW is a 600 V oil-resistant rubber jacket rated 90 C, intended for hard service where cords are dragged through oil and abrasion. Specifying SJT for an outdoor line is a common error on the electrical side - the gauge is right and the jacket is wrong.
The cable and the finished cord set are covered by different standards: UL 62 for the flexible cord itself, UL 817 for the assembled cord set. When you ask a factory for a quotation, ask which listing the finished set carries, not only what the cable is made of.
What to put on the enquiry
Question | What it fixes |
Load current and duty cycle? | Sets the gauge and whether the 80% rule applies |
Run length? | Drives voltage drop - often the reason to go up a gauge |
Cable only, or finished cord set? | Finished sets need the assembly listing (UL 817) |
Indoor or outdoor? | Jacket: SJT, SJTW or SOOW |
Plug and connector type? | The lowest-rated part sets the assembly rating |
Ambient temperature, coiled or laid out? | Both require derating below the table value |
Give those six and a supplier can quote a part number. Give '14/3, 50 foot' and you will receive exactly that - which may or may not be what the job needs.
Where this leaves the catalogue
Most distributors can cover the market with three gauges if they stock the right lengths: 14/3 for 15 A general-purpose runs out to roughly 50 ft, 12/3 for 20 A tools and 100 ft runs, and 10/3 for compressors and anything long. 16/3 belongs in the light-duty and indoor range, and it should never be the answer to a question about power tools.
Next step
Send us the load current, the run length, the plug type and whether the cord is for indoor or outdoor use, and we will confirm the gauge, jacket and listing before you commit to a production order. If the run is long, send the length first - that is usually the number that changes the answer. Reach our team at /contact-us/ or message us on WhatsApp for a faster reply.