Size the wire for amps and distance.
Enter the load, the run length and your voltage-drop target. The calculator finds the smallest conductor that carries the load under NEC Table 310.16, then checks voltage drop and upsizes if the run is long. You see which one governs.
- Copper & aluminum
- 60 / 75 / 90 °C
- 240.4(D) limits
- Voltage drop
Load
Run & conditions
Nearby sizes
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Uses NEC Table 310.16 (2023), 240.4(D) and Chapter 9 Table 8 (uncoated copper, K = 12.9; aluminum K = 21.2). It does not replace the engineer of record or the code edition your AHJ enforces. Overcurrent device sizing, terminations and conduit fill are separate checks.
Copper ampacity, 75 °C column.
NEC Table 310.16: not more than three current-carrying conductors in a raceway, 30 °C ambient. 240.4(D) limits the overcurrent device for #14–#10.
| Size | 75 °C Cu (A) | 75 °C Al (A) | Max OCPD (240.4(D)) |
|---|---|---|---|
| 14 AWG | 20 | — | 15 A Cu |
| 12 AWG | 25 | 20 | 20 A Cu / 15 A Al |
| 10 AWG | 35 | 30 | 30 A Cu / 25 A Al |
| 8 AWG | 50 | 40 | — |
| 6 AWG | 65 | 50 | — |
| 4 AWG | 85 | 65 | — |
| 2 AWG | 115 | 90 | — |
| 1/0 | 150 | 120 | — |
| 4/0 | 230 | 180 | — |
| 250 kcmil | 255 | 205 | — |
| 500 kcmil | 380 | 310 | — |
Two checks, pick the larger.
Ampacity
Smallest size whose 310.16 ampacity, at the termination rating and after correction, carries the load (×1.25 if continuous).
Small-conductor cap
240.4(D) caps #14, #12 and #10 at 15, 20 and 30 A copper.
Voltage drop
VD = 2·K·I·L ÷ CM for single-phase, √3·K·I·L ÷ CM for three-phase, with circular mils from Chapter 9 Table 8.
Recommend
Whichever check needs the larger conductor governs.
One feeder here. Every feeder in the bid.
Feeder schedules, panel schedules and one-lines drive most of an electrical estimate. RenderDraw reads them from the drawings with the source sheet on every row, so your estimator can approve wire, conduit and gear quantities before pricing.
- Source on every row. Each proposed quantity keeps the sheet and location it came from.
- Estimator approval before pricing. Your estimator accepts, corrects or rejects proposals; nothing is priced until it is approved.
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Wire size calculator, answered.
What size wire do I need for 40 amps?
With copper and 75 °C terminations, 8 AWG (50 A) is the smallest that works, as long as voltage drop on the run is acceptable. Long runs may need a larger size.
What size wire for a 100 ft, 20 amp, 120 V circuit?
12 AWG copper carries 20 A, but at 100 ft one-way and a full 20 A the drop is about 7.9 V, or 6.6%. Holding 3% takes 8 AWG copper (about 3.1 V, 2.6%). Many designs check the drop at the expected load rather than the breaker rating, so use the real load.
How do you calculate voltage drop?
Single-phase: VD = 2 × K × I × L ÷ CM. Three-phase: VD = 1.732 × K × I × L ÷ CM. K is about 12.9 for copper and 21.2 for aluminum, L is the one-way length in feet and CM is the conductor's circular mils.
What voltage drop is acceptable?
The NEC's informational notes suggest about 3% for a branch circuit or feeder and 5% total to the farthest outlet. These are recommendations, not requirements, unless your spec or local code makes them mandatory.
Why use the 75 °C column?
Conductor ampacity is limited by the lowest temperature rating of the terminations. Most equipment is rated 75 °C. The 90 °C column is used as the starting point for derating, not for final sizing.
Send one plan set. Judge the result yourself.
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