Wire size calculator.
Enter your load and run. Get the minimum conductor size by NEC 310.16 ampacity — copper or aluminum, 60°C or 75°C — then a voltage-drop check that upsizes the wire if the run is too long, plus a breaker note.
| What we computed | Value | Reference |
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How the math works
Wire sizing is a two-step gate — ampacity first, then voltage drop. Both use standard published NEC values:
- Design current — load amps, multiplied by 1.25 if you flag it continuous (NEC 210.19 / 215.2).
- Ampacity size — the smallest conductor whose NEC Table 310.16 ampacity (in your material and termination column) is ≥ the design current.
- Voltage drop — single phase: VD = 2 × K × I × L ÷ CM; three phase: VD = 1.732 × K × I × L ÷ CM. K = 12.9 (copper) or 21.2 (aluminum); CM = circular mils from NEC Chapter 9, Table 8; L = one-way length in feet; I = load current.
- Upsize check — if VD% exceeds your target (3% default per the NEC informational note), the tool steps up to the next size until the run is within limit.
- Final recommendation — the larger of the ampacity size and the voltage-drop size, with a breaker note (overcurrent device sized to the design current).
Worked example: 40 A copper, 75°C, 240 V single phase, 100 ft, 3% max. Ampacity needs 8 AWG (50 A ≥ 40 A). Voltage drop on 8 AWG: 2 × 12.9 × 40 × 100 ÷ 16,510 = 6.25 V = 2.60% — within 3%, so 8 AWG holds.
This is the easy 20%. The other 80% is on your drawings.
Sizing one feeder is quick. What eats an electrical bid is doing it for every homerun, feeder and branch on the set — then counting the conduit and wire footage that goes with each, across a panel schedule and a one-line that live on different sheets. That’s the part that leaks money — and it’s exactly what Pilars AI takeoff does from your actual plans: it reads the panel and feeder schedules, follows the runs, and returns wire and conduit by size and type. $100 per trade, per plan.
Questions electricians actually ask
How do I calculate the right wire size for a load?
Size for ampacity first: multiply continuous loads by 1.25, then pick the smallest conductor whose Table 310.16 ampacity (right material and termination column) meets the current. Then check voltage drop over the run and upsize if the drop exceeds your target.
Why does continuous load multiply by 1.25?
The NEC requires conductors and overcurrent devices serving a continuous load (three hours or more) to be sized at 125% of the load. The tool applies 1.25 when you toggle the continuous option.
Should I use the 60°C or 75°C column?
Per 110.14(C), circuits 100 A or less often default to 60°C unless equipment and conductors are both listed 75°C — most modern gear and building wire is 75°C. Choose the column that matches your terminations.
Does this include voltage drop?
Yes — after ampacity it computes VD = 2 × K × I × L ÷ CM (single phase; 1.732 × … for three phase) using circular mils from Chapter 9 Table 8, and upsizes the conductor until the run is within your limit.
Is this NEC compliant for my project?
It uses standard published NEC values and the 1.25 and 3% conventions. It’s a reference tool — it does not apply ambient or conduit-fill derating, so verify against your adopted code edition and conditions.
Let Pilars take off your whole set.
Upload your plans. Pilars reads the panel and feeder schedules, follows every run, and returns wire and conduit by size and type. $100 per trade.