SettingsSettings for this calculationUS
The total length of extension cord you plan to use.
Use the full length, including any cords daisy-chained together.
The rated amp draw of what you're plugging in.
Check the tool's nameplate — it's usually printed near the model number.
The nominal outlet voltage.
120V for standard household outlets.
Recommended cord gauge
14 AWG
- Voltage drop at recommended gauge
- 2.56 %
They open the calculator with your figures already in it
Extension Cord Gauge Calculator: 14 AWG — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Standard single-phase voltage drop formula: Vdrop = (2 x K x length in ft x amps) / circular mils, K=12.9 for copper — the same physics used to size branch-circuit wiring, applied here to cord length
- 3% voltage drop is a widely used comfort threshold for extension cords to avoid overheating and underpowered tools
- NEC Table 400.5(A)(1), allowable ampacity for flexible cords with 2 current-carrying conductors: 16 AWG 13 A, 14 AWG 18 A, 12 AWG 25 A, 10 AWG 30 A — the safety limit, checked before voltage drop
Inputs used
- Cord Length
- 49 ft
- Tool/Device Amp Draw
- 10
- Voltage
- 120 V
Intermediate steps
- Voltage drop at recommended gauge
- 2.56 %
What this calculation does not cover
- The ampacity table is a fixed free-air rating for copper cord with two current-carrying conductors. Nothing here derates for a cord left coiled or wound on a reel, bundled with other cords, run under insulation or a rug, or used in high ambient heat — all of which cut how much current the cord can carry before it overheats.
- This sizes a cord; it is not a branch-circuit load calculation and not a code sign-off. It says nothing about whether the circuit behind the outlet can supply the current, whether the breaker and receptacle are rated for it, whether the load counts as continuous, or whether GFCI protection is required for the location.
- Only a single continuous run of copper in 16, 14, 12 or 10 AWG is modelled. Aluminium and copper-clad conductors, 18 AWG cords, the resistance of every plug and coupler where cords are joined end to end, and the voltage already lost in the building wiring upstream of the outlet are all excluded — so the real drop at the tool is worse than the figure shown, and more so the more couplers are in the run.
- The amp figure is treated as a steady running current. Motor inrush at startup is several times nameplate and is not modelled, so a cord that clears the 3% check can still leave a compressor, pump or large saw bogging down or tripping on start.
- The 3% drop figure is a working rule of thumb for cords, not a code limit, and the ampacity data is North American. On a 230/240 V supply outside NEC territory, cords are sized in mm² under local rules and the AWG answer here does not carry across. Cord construction is also outside the model — jacket type, wet and outdoor rating, temperature rating and listing for the use are not checked, and two cords of the same gauge are not interchangeable on those grounds.
Add the equipment this sizes
This result is a specification — 14 AWG — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations3
- Standard single-phase voltage drop formula: Vdrop = (2 x K x length in ft x amps) / circular mils, K=12.9 for copper — the same physics used to size branch-circuit wiring, applied here to cord length
- 3% voltage drop is a widely used comfort threshold for extension cords to avoid overheating and underpowered tools
- NEC Table 400.5(A)(1), allowable ampacity for flexible cords with 2 current-carrying conductors: 16 AWG 13 A, 14 AWG 18 A, 12 AWG 25 A, 10 AWG 30 A — the safety limit, checked before voltage drop
Which documents these citations point at
- National Electrical Code (NFPA 70) — Table 400.5(A)(1) (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
Cite this page
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