The Cord Gauge Ampacity Index
A cord's printed gauge is not one fixed safe amperage — it changes with run length, because voltage drop becomes the binding limit on long runs, not just heat. This is the normalized index: every common AWG gauge (18 down to 6), the standard length bands buyers actually use (25/50/100/150ft), the maximum safe continuous amperage at that combination, the resulting voltage-drop percentage, and which factor (thermal cap or voltage drop) is limiting. It is a reference dataset, not a substitute for the NEC, your local code, or a licensed electrician.
⚠ Safety notice: This is a general reference, NOT an electrical code determination. Local code always governs. Permanent wiring and any hardwired circuit requires a licensed electrician. Cords must be rated for their actual environment — indoor-only vs outdoor/wet-rated (SJTW vs SJOOW). We round toward the safer (lower) amperage wherever sources disagree, and we have not tested any physical cord ourselves.
License: CC BY 4.0 — free to use with attribution.
The voltage-drop formula and its assumptions
Voltage-drop figures in this index are calculated from Ohm's law: Vdrop = 2 × L × I × R, where
L is the one-way cord length in feet (doubled because current travels down the hot conductor
and back on the neutral), I is the current in amps, and R is the copper
conductor's resistance in ohms per foot at 20°C for that AWG size. We target the commonly-cited 3% voltage
drop recommended for branch circuits (NEC 210.19(A), informational note) on a 120V circuit — a 3.6V drop
ceiling. At each gauge/length combination we compute the maximum amperage that stays at or under both (a)
this 3.6V ceiling and (b) the flexible-cord thermal ampacity cap, and report the LOWER of the two, rounded
down to the nearest whole amp. Where the thermal cap is the binding limit, we label it "thermal"; where
voltage drop is binding (all longer runs), we label it "voltage-drop (derived)" so the number is never
mistaken for a directly-sourced code figure.
The index, by gauge
18 AWG — Light-duty indoor only (lamps, small electronics, holiday lights) — not for tools
| Length | Max safe continuous amps | Voltage drop at that amperage | Limiting factor |
|---|---|---|---|
| 25 ft | 10A | 2.66% | Thermal (NEC flexible-cord cap) |
| 50 ft | 5A | 2.66% | Voltage-drop (derived, 3% target) |
| 100 ft | 2A | 2.13% | Voltage-drop (derived, 3% target) |
| 150 ft | 1A | 1.6% | Voltage-drop (derived, 3% target) |
16 AWG — Light-duty (13A) — small appliances, holiday lights, short-run indoor use
| Length | Max safe continuous amps | Voltage drop at that amperage | Limiting factor |
|---|---|---|---|
| 25 ft | 13A | 2.18% | Thermal (NEC flexible-cord cap) |
| 50 ft | 8A | 2.68% | Voltage-drop (derived, 3% target) |
| 100 ft | 4A | 2.68% | Voltage-drop (derived, 3% target) |
| 150 ft | 2A | 2.01% | Voltage-drop (derived, 3% target) |
14 AWG — 15A general-purpose household extension cord
| Length | Max safe continuous amps | Voltage drop at that amperage | Limiting factor |
|---|---|---|---|
| 25 ft | 15A | 1.58% | Thermal (NEC flexible-cord cap) |
| 50 ft | 14A | 2.95% | Voltage-drop (derived, 3% target) |
| 100 ft | 7A | 2.95% | Voltage-drop (derived, 3% target) |
| 150 ft | 4A | 2.52% | Voltage-drop (derived, 3% target) |
12 AWG — 20A power tools (drills, circular saws, shop vacs) — medium duty
| Length | Max safe continuous amps | Voltage drop at that amperage | Limiting factor |
|---|---|---|---|
| 25 ft | 20A | 1.32% | Thermal (NEC flexible-cord cap) |
| 50 ft | 20A | 2.65% | Thermal (NEC flexible-cord cap) |
| 100 ft | 11A | 2.91% | Voltage-drop (derived, 3% target) |
| 150 ft | 7A | 2.78% | Voltage-drop (derived, 3% target) |
10 AWG — 30A generator feeds, RV shore power, air compressors, sub-panel taps
| Length | Max safe continuous amps | Voltage drop at that amperage | Limiting factor |
|---|---|---|---|
| 25 ft | 25A | 1.04% | Thermal (NEC flexible-cord cap) |
| 50 ft | 25A | 2.08% | Thermal (NEC flexible-cord cap) |
| 100 ft | 18A | 3% ⚠ at 3% recommended limit | Voltage-drop (derived, 3% target) |
| 150 ft | 12A | 3% ⚠ at 3% recommended limit | Voltage-drop (derived, 3% target) |
8 AWG — Welder circuits, generator feeder cables, heavy shop equipment
| Length | Max safe continuous amps | Voltage drop at that amperage | Limiting factor |
|---|---|---|---|
| 25 ft | 40A | 1.05% | Thermal (NEC flexible-cord cap) |
| 50 ft | 40A | 2.09% | Thermal (NEC flexible-cord cap) |
| 100 ft | 28A | 2.93% | Voltage-drop (derived, 3% target) |
| 150 ft | 19A | 2.98% | Voltage-drop (derived, 3% target) |
6 AWG — Heavy welder circuits, large generator feeders, sub-panel feeds
| Length | Max safe continuous amps | Voltage drop at that amperage | Limiting factor |
|---|---|---|---|
| 25 ft | 55A | 0.91% | Thermal (NEC flexible-cord cap) |
| 50 ft | 55A | 1.81% | Thermal (NEC flexible-cord cap) |
| 100 ft | 45A | 2.96% | Voltage-drop (derived, 3% target) |
| 150 ft | 30A | 2.96% | Voltage-drop (derived, 3% target) |
Caveats — read before citing a number
- Thermal caps are a simplified reference figure consistent with the widely-published flexible-cord ampacity table (NEC Table 400.5(A), 2-conductor cords, common ambient temperature), not a verbatim reproduction of the full temperature-corrected NEC table. Cords used in higher ambient heat or bundled together need further derating per the actual code table.
- Voltage-drop figures are derived, not directly sourced. They assume copper conductors at 20°C, a 120V circuit, and the commonly-cited 3% target — real-world resistance rises with temperature, so treat these as a conservative planning figure, not a lab measurement.
- We rounded every value DOWN to the nearest whole amp and took the lower of the two limits at every cell, per our conservative-rounding editorial policy on this safety-critical topic.
- This is general reference, not a code determination for your specific circuit, cord, or jurisdiction. Local code governs; hire a licensed electrician for permanent or hardwired work.
- Cord jacket/environment rating is a separate axis from gauge — see Indoor vs Outdoor Extension Cord Rating.
How to cite
Cord Gauge Decoded, "Cord Gauge Ampacity Index" v1.0 (2026). https://cordgauge-decoded.pages.dev/cord-gauge-ampacity-index/. Licensed CC BY 4.0.
Methodology & versioning
Version 1.0 — published July 30, 2026. Thermal ampacity figures are drawn from the widely-published flexible-cord ampacity chart consistent with NEC Table 400.5(A) for 2-conductor cords. Voltage-drop figures are derived in-house via the Ohm's-law formula shown above, targeting the commonly-cited 3% voltage-drop guideline for branch circuits. We do not test physical cords ourselves — this is a calculation-and-reference page, not a lab result. Corrections: if a cited figure changes or we find an error, this page and both CSVs are updated in place and the version/date bumped — see How We Evaluate for our full editorial policy.
Machine-readable download: the full table is available as CSV at /cord-gauge-ampacity-index.csv (always latest) and the version-pinned /cord-gauge-ampacity-index-v1.0.csv, and a condensed digest is maintained in /llms-full.txt.
Frequently Asked Questions
A versioned reference dataset mapping AWG wire gauge (18 through 6), cord length (25/50/100/150ft), the maximum safe continuous amperage at that gauge and length, the resulting voltage-drop percentage, and the typical application for that rating. It exists because a cord's printed gauge alone does not tell you the safe amperage at YOUR run length.
Each gauge has two limits: a thermal ampacity cap (the widely-published flexible-cord amperage figures consistent with NEC Table 400.5(A) for 2-conductor cords) that does not change with length, and a voltage-drop-derived limit that gets tighter as length increases (calculated from Ohm's law targeting the NEC-recommended 3% voltage drop on a 120V branch circuit, copper conductor). We report whichever number is LOWER at each length, and label which one is limiting.
Because voltage drop, not heat, becomes the binding constraint on long cord runs. We round every value DOWN to the nearest whole amp and always take the safer (lower) figure when our two source calculations disagree, consistent with our editorial policy of conservative rounding on a safety-critical topic.
No. This is a general reference dataset, not a code determination. Local code always governs, permanent/hardwired circuits require a licensed electrician, and cords must be rated for their actual use environment (indoor vs outdoor/wet — SJTW vs SJOOW). We have not tested any physical cord ourselves.
Yes — it is released under CC BY 4.0. See the How to Cite block below for the citation string, and download the versioned CSV for machine use.