Wire Size Calculator
Enter the load, voltage, one-way distance and allowed voltage drop to get the smallest copper or aluminum wire that meets both NEC Table 310.16 ampacity and your voltage drop limit, with the actual drop and a full conductor table. Calculated privately in your browser.
Conductor table
| Size | Area | 60 °C | 75 °C | 90 °C | Drop (V) | Drop % | Max length at limit | OK? |
|---|
Ampacities from NEC Table 310.16 (not more than three current-carrying conductors in raceway, cable or earth, 30 °C ambient), limited by NEC 240.4(D) for 14–10 AWG (marked *: max 15/20/30 A copper, 15/25 A aluminum). Voltage drop uses K = 12.9 (copper) or 21.2 (aluminum) Ω·cmil/ft. This is an estimate: derating for ambient temperature, conduit fill and terminal ratings is not applied. Follow your local electrical code and consult a licensed electrician.
How to use the Wire Size Calculator
- Choose the conductor (copper or aluminum) and the system: single-phase, three-phase or DC.
- Enter the voltage and load in amps, watts or kilowatts.
- Enter the one-way length from the panel to the load, in feet or meters.
- Set the voltage drop limit and the insulation temperature rating; tick continuous load if the load runs 3 hours or more.
- Press Calculate wire size and check the conductor table to compare neighbouring sizes.
How the wire size is chosen
The calculator picks the smallest conductor that passes two checks:
- Ampacity: the allowable current from NEC (NFPA 70) Table 310.16 in the chosen temperature column must be at least the load current (×1.25 for continuous loads). For 14, 12 and 10 AWG the overcurrent limits of NEC 240.4(D) also apply: 15, 20 and 30 A for copper, 15 and 25 A for 12 and 10 AWG aluminum.
- Voltage drop: VD = 2 × K × I × L ÷ CM (single-phase or DC) or 1.732 × K × I × L ÷ CM (three-phase) must stay within your limit, where K = 12.9 Ω·cmil/ft for copper and 21.2 for aluminum, and CM is the conductor area from NEC Chapter 9, Table 8.
Worked example: 30 A at 240 V single-phase, 100 ft, copper. 10 AWG (10,380 cmil): VD = 2 × 12.9 × 30 × 100 ÷ 10,380 = 7.46 V = 3.11%, just over 3%. 8 AWG (16,510 cmil): 4.69 V = 1.95%, so 8 AWG copper is recommended.
Ampacity of common sizes (NEC Table 310.16)
Not more than three current-carrying conductors in raceway, cable or earth, ambient 30 °C (86 °F). Values as in the 2020 and 2023 editions of the NEC.
| Size | Cu 60 °C | Cu 75 °C | Cu 90 °C | Al 60 °C | Al 75 °C | Al 90 °C |
|---|---|---|---|---|---|---|
| 14 AWG | 15 | 20 | 25 | – | – | – |
| 12 AWG | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 AWG | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 AWG | 40 | 50 | 55 | 35 | 40 | 45 |
| 6 AWG | 55 | 65 | 75 | 40 | 50 | 55 |
| 4 AWG | 70 | 85 | 95 | 55 | 65 | 75 |
| 2 AWG | 95 | 115 | 130 | 75 | 90 | 100 |
| 1/0 AWG | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 AWG | 145 | 175 | 195 | 115 | 135 | 150 |
| 4/0 AWG | 195 | 230 | 260 | 150 | 180 | 205 |
| 250 kcmil | 215 | 255 | 290 | 170 | 205 | 230 |
| 500 kcmil | 320 | 380 | 430 | 260 | 310 | 350 |
Tips and limits
- Use the one-way distance; the formula already accounts for the return conductor.
- Watts are converted with a power factor of 1 (I = P ÷ V, or P ÷ (1.732 × V) for three-phase). For motors use the nameplate or NEC table current instead.
- More than three current-carrying conductors in a raceway, high ambient temperature or rooftop runs require ampacity adjustment that this tool does not apply.
- Low-voltage DC (12/24 V) runs are dominated by voltage drop; raising the system voltage is often cheaper than heavier cable.
For quick current and power conversions, see the Ohm's Law Calculator and the Power Factor Calculator.
Frequently asked questions
What size wire do I need for 30 amps at 100 feet?
At 240 V single-phase with copper, 10 AWG has enough ampacity (35 A at 75 °C) but drops about 7.5 V (3.1%) over 100 ft, so 8 AWG copper (about 1.95% drop) is the better choice if you want to stay under 3%. At 120 V the drop doubles in percentage terms, so you would need 6 AWG.
How is voltage drop calculated?
For single-phase and DC circuits: VD = 2 × K × I × L ÷ CM; for three-phase: VD = 1.732 × K × I × L ÷ CM. K is about 12.9 for copper and 21.2 for aluminum, I is the current in amps, L the one-way length in feet and CM the conductor area in circular mils.
What voltage drop is acceptable?
The NEC recommends (in informational notes, not as a requirement) no more than 3% on a branch circuit and 5% total for feeder plus branch circuit. Sensitive equipment, motors and low-voltage DC systems often use tighter limits.
Which temperature column should I use?
Use the lowest temperature rating of anything in the circuit: wire insulation, breaker and terminals. Equipment rated 100 A or less, or for 14–1 AWG, is often limited to 60 °C unless marked 75 °C; most modern breakers and panels are rated 75 °C. The 90 °C column is mainly used as a starting point for derating.
Why is aluminum wire larger than copper?
Aluminum has about 61% of copper's conductivity, so it needs a larger cross-section for the same current and voltage drop, typically about two AWG sizes larger. It is lighter and cheaper, which is why it is common for services and feeders.
Does this replace an electrician or local code?
No. It does not apply ambient temperature correction, conduit fill adjustment, terminal ratings or special rules for motors, dwellings and services. Use it to plan, then confirm with your local code and a licensed electrician.