Wire Size Calculator
Size copper or aluminum wire (AWG) for any load, distance, and voltage. Accounts for NEC 310.16 ampacity and 3% voltage drop, with a full breakdown.
Continuous load in amps.
Panel to load, one way.
NEC recommends 3% for branch circuits, 5% total.
- 14 AWG · 15A
- Lighting, receptacles
- 12 AWG · 20A
- Kitchen, bath, garage receptacles
- 10 AWG · 30A
- Dryer, water heater, A/C
- 8 AWG · 40A
- Range, large A/C
- 6 AWG · 55A
- Subpanel, EV charger (verify)
For estimating only — verify with the NEC.
Provides conductor ampacity sizing per NEC 310.16 and small-conductor overcurrent limits per 240.4(D). It does NOT account for:
- ambient-temperature derating and conduit-fill adjustment (310.15);
- continuous-load 125% sizing factors;
- terminal temperature ratings (110.14(C));
- the 240.4(B)/240.6 “next standard size up” overcurrent rule for conductors 8 AWG and larger;
- motor circuits (Art. 430), A/C equipment (Art. 440), EV charging (625), or solar PV (690).
Results follow the NEC base tables and may differ from the edition and local amendments your jurisdiction has adopted (most US jurisdictions are on the 2023 NEC or earlier as of 2026). All work must be designed and verified by a licensed electrician.
Recommended wire size
copper conductor
6.25 V at this size
- Required circular mils
- 14,333
- Wire ampacity
- 50A
- Limiting factor
- Ampacity
- NEC reference
- NEC 310.16 & 240.4(D)
Uses 75°C copper/aluminum NEC ampacity. Check your installation conditions for derating.
Conduit fill, ambient temperature, and bundling reduce ampacity. Always verify with current NEC and local codes.
Residential quick reference: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A.
How this calculator works
Picking a wire gauge is a balance of two independent limits, and the conductor has to pass both. The first is ampacity — how much current the wire can carry before its insulation overheats. The second is voltage drop — how much voltage the load loses to conductor resistance over the run. This calculator works out each one and recommends the larger of the two wire sizes, so the result is safe for the breaker and still delivers usable voltage at the load.
How the voltage-drop math works
Voltage drop depends on the conductor material, the current, and the round-trip length of wire. The calculator uses the standard formula Vdrop = (2 × K × I × D) / CM for single-phase and (√3 × K × I × D) / CM for three-phase, where K is resistivity (12.9 for copper, 21.2 for aluminum), I is the load in amps, D is the one-way distance in feet, and CM is the conductor's area in circular mils. Rearranged, it solves for the minimum circular mils that keep the drop within your target percentage, then finds the smallest standard AWG that meets it.
Ampacity from NEC 310.16
Separately, the load current is checked against NEC Table 310.16 using the 75°C column — the most common termination rating. The recommended conductor must have an ampacity at or above the load. It must also respect NEC 240.4(D): 14, 12, and 10 AWG conductors have fixed maximum overcurrent-device ceilings (15A, 20A, and 30A for copper) no matter what the ampacity table allows, so the calculator upsizes when a small conductor cannot legally be protected at the load — a 20A circuit needs 12 AWG copper, not 14 AWG. On short runs these current limits usually govern; on long runs the voltage-drop limit pushes the size up. The result panel labels which factor is driving the recommendation so you can see why.
Derating and verification
The ampacities here are baseline 75°C values. Real installations are adjusted down for ambient temperature, for more than three current-carrying conductors bundled together, and for conduit fill. This tool does not apply those derating factors, and it stops at 4/0 — beyond that it flags the need for parallel conductors or larger kcmil sizes. Treat every result as an estimate to be verified by a licensed electrician against the current NEC and local codes, especially on any installation that requires a permit or inspection.
Frequently asked questions
- Why does wire size depend on distance?
- Every conductor has resistance, so voltage is lost along its length — this is voltage drop, and it grows with distance. A 12 AWG wire that is fine for a 30-foot run can drop too much voltage over 150 feet, starving the load and wasting energy as heat. To hold the drop within the target (3% for branch circuits), longer runs need a larger conductor with lower resistance. That is why this calculator asks for one-way distance: it sizes the wire so the load still sees usable voltage at the far end.
- What is the difference between ampacity and voltage-drop sizing?
- They are two separate limits, and the conductor must satisfy both. Ampacity is the current a wire can carry without overheating its insulation — NEC 310.16 sets it (for example, 12 AWG copper is 25A at 75°C, used at 20A for a typical branch circuit). Voltage drop is about performance over distance, not heat. On short runs ampacity governs; on long runs voltage drop usually forces a larger wire than ampacity alone would require. This tool calculates both and recommends the larger of the two.
- Copper or aluminum — when do you use each?
- Copper has lower resistance (K ≈ 12.9 vs 21.2 for aluminum), so it carries more current in a smaller gauge and is standard for branch circuits and most residential wiring. Aluminum is lighter and cheaper per amp, which makes it common for large service-entrance and feeder conductors. Because aluminum is more resistive, it needs a larger gauge for the same load and distance, and it requires terminals and antioxidant compound rated for aluminum. Note that NEC 310.16 does not list aluminum smaller than 12 AWG.
- What is NEC 310.16?
- NEC Table 310.16 is the National Electrical Code ampacity table for insulated conductors rated 0–2000V in a raceway, cable, or earth (up to three current-carrying conductors). It lists the allowable amperage for each wire size by temperature column — 60°C, 75°C, and 90°C. This calculator uses the 75°C column, the most common termination rating for breakers and lugs. The listed ampacities are starting points that must be adjusted for ambient temperature and conductor bundling.
- What is voltage drop and why does it matter?
- Voltage drop is the reduction in voltage between the panel and the load caused by conductor resistance. The NEC recommends (in 215.2(A)(1) Informational Note No. 2 and 210.19) keeping it to 3% on a branch or feeder and 5% total across both. Excess drop makes motors run hot and lose torque, dims lighting, makes electronics misbehave, and wastes energy as heat in the wire. Staying within 3% keeps equipment operating as designed and the conductor running cool.
- Can I use this for solar or EV-charger circuits?
- The voltage-drop and ampacity math is the same, so this is a useful first pass for sizing PV and EV conductors. But those circuits carry extra code rules: EV charging equipment is a continuous load sized at 125% (NEC 625), and PV source and output circuits have their own ampacity and 125% factors (NEC 690.8). Apply the relevant continuous-load multiplier to the current before sizing, and verify against the specific article. Always confirm with a licensed electrician.