Extension Cord Size Chart — Ampacity by Gauge & Computed Voltage Drop
Extension cord ampacity by gauge per NEC Table 400.5(A)(1), with computed voltage drop for common cord lengths and loads at 120 V.
Updated August 22, 2026
Extension cords and other flexible cords are not building wire, and the NEC rates them from a different table: Table 400.5(A)(1), not 310.16. The first table below gives those cord ampacities for the common service-cord constructions (SJT and SJTW jacketed cords, SOOW hard-service cord, and the rest of the S/SJ families) in 18 through 10 AWG. The two columns matter: a two-wire cord (hot and neutral carrying current — the ground does not count) uses Column B, while a three-wire cord with three current-carrying conductors, such as a 240 V cord with both hots and a loaded neutral, drops to Column A. A cord’s printed rating (e.g. “13 A” on a 16 AWG cord tag) comes from this table plus the listing.
Ampacity is only half the sizing problem — a cord can be safely within its ampacity and still starve the tool at the far end. The second table transcribes the stranded-copper DC resistance from NEC Chapter 9, Table 8 (extended down to 18 AWG, which no building-wire chart carries) and computes percent voltage drop at 120 V for representative loads and lengths: drop = 2 × current × resistance × length, both directions of the circuit. Motors are the sensitive case — a saw pulling 15 A through 100 ft of 16 AWG loses over 12% of its voltage, which shows up as heat and stalled starts. Move up the table until the computed drop for your load and length sits in the low single digits, then check the run with the voltage drop calculator using your exact numbers.
Flexible Cord Ampacity (NEC Table 400.5(A)(1), Copper)
| Size | 3 current-carrying conductors (A) | 2 current-carrying conductors (A) |
|---|---|---|
| 18 AWG | 7 | 10 |
| 16 AWG | 10 | 13 |
| 14 AWG | 15 | 18 |
| 12 AWG | 20 | 25 |
| 10 AWG | 25 | 30 |
Thermoset and thermoplastic service cords (S, SO, SOOW, SJ, SJO, SJT, SJTW and similar), 30 °C ambient. An equipment grounding conductor is not a current-carrying conductor: a typical 3-prong extension cord uses the 2-conductor column. SO-family cord continues to 8 AWG (35 A / 40 A) and larger; SJ junior-service constructions stop at 10 AWG.
Cord Resistance & Computed Voltage Drop at 120 V
| Size | R, stranded Cu 75 °C (Ω/1,000 ft) | Drop @ 5 A, 50 ft | Drop @ 10 A, 50 ft | Drop @ 10 A, 100 ft | Drop @ 15 A, 100 ft |
|---|---|---|---|---|---|
| 18 AWG | 7.95 | 3.3% | 6.6% | 13.3% | 19.9% |
| 16 AWG | 4.99 | 2.1% | 4.2% | 8.3% | 12.5% |
| 14 AWG | 3.14 | 1.3% | 2.6% | 5.2% | 7.8% |
| 12 AWG | 1.98 | 0.8% | 1.6% | 3.3% | 5.0% |
| 10 AWG | 1.24 | 0.5% | 1.0% | 2.1% | 3.1% |
Resistance per NEC Chapter 9, Table 8 (stranded, uncoated copper, DC at 75 °C — the same basis as the AWG wire gauge chart). Drop computed as 2 × I × R × L at 120 V, both conductors of the circuit; percentages scale linearly with current and length, and halve at 240 V for the same load.
Sources & Further Reading
- NEC (NFPA 70), Table 400.5(A)(1) — Ampacity of Flexible Cords and Cables, digit-verified against two independent published reproductions (Nassau National Cable Article 400 guide; Mike Holt forum reproduction; Industrial Monitor Direct for the 8 AWG SO row)
- NEC (NFPA 70) 2017, Chapter 9, Table 8 — Conductor Properties (licensed text reproduction; stranded uncoated copper at 75 °C), cross-checked against the NEC 2014 reprint — values identical, and identical to the wire-gauge-ampacity chart for 14–10 AWG
Frequently Asked Questions
What gauge extension cord do I need for a 15 A tool at 100 ft?
From the tables: 14 AWG is rated 15 A but drops 7.9% at 15 A over 100 ft — enough to make a motor run hot. 12 AWG (rated 20/25 A) computes to 5.0% and 10 AWG to 3.1%. For a heavily loaded 100 ft run, 10 AWG is the comfortable answer, 12 AWG the minimum; save 14 AWG for 50 ft and shorter, where the same load drops only half as much.
Why is a 16 AWG cord rated 13 A when 16 AWG fixture wire and building charts differ?
Because each rating comes from a different table for a different construction. Flexible cord uses Table 400.5(A)(1) — 13 A for 16 AWG with two current-carrying conductors, 10 A with three. The cord ratings account for the bundled, jacketed construction and mobile service. The 240.5 overcurrent rules then let a 16 AWG cord live on a 20 A branch circuit as part of a listed extension cord set, which is why store-bought 16 AWG cords are legal on kitchen receptacles.
Does the third prong (ground) reduce my cord ampacity?
No. The equipment grounding conductor carries no current in normal operation, so it does not count as a current-carrying conductor. An ordinary hot/neutral/ground extension cord is a 2-conductor cord for ampacity — Column B, the higher rating. The 3-conductor column applies when three conductors genuinely carry load, e.g. both hots plus a loaded neutral on a 120/240 V cord.
Do two cords plugged together double the voltage drop?
Length adds, so drop adds: 50 ft plus 50 ft of the same gauge behaves exactly like the 100 ft column — compare the 10 A rows at 50 and 100 ft and you will see each cell exactly doubles. If the two cords are different gauges, each contributes its own 2 × I × R × L share, and the thinner cord dominates. Chaining also adds two more plug/receptacle contact resistances, which the table does not include.
Try the Calculators
Extension Cord Size Chart — Ampacity by Gauge & Computed Voltage Drop — reuven.tools/reference/extension-cord-ampacity — verified against: NEC (NFPA 70), Table 400.5(A)(1) — Ampacity of Flexible Cords and Cables, digit-verified against two independent published reproductions (Nassau National Cable Article 400 guide; Mike Holt forum reproduction; Industrial Monitor Direct for the 8 AWG SO row); NEC (NFPA 70) 2017, Chapter 9, Table 8 — Conductor Properties (licensed text reproduction; stranded uncoated copper at 75 °C), cross-checked against the NEC 2014 reprint — values identical, and identical to the wire-gauge-ampacity chart for 14–10 AWG