Wire Size Calculator
Use this wire size calculator to estimate the minimum copper or aluminum wire size for a circuit. Enter the current, voltage, one-way length and the voltage drop you allow to get the smallest AWG that meets the drop, alongside a typical NEC ampacity check. It is an estimate, not a substitute for the NEC or an electrician.
Math runs in your browser. Nothing is uploaded.
Estimate only
This tool gives a rough estimate. It is not a substitute for the National Electrical Code (NEC), your local code, or a licensed electrician. It ignores temperature and conduit-fill derating, terminal ratings, continuous-load factors, conductor insulation type and many other conditions. Have the wiring designed and inspected by a qualified professional.
Minimum size for voltage drop
—
Minimum size for ampacity (typical)
—
Suggested minimum size (larger of the two)
—
Typical ampacity at that size (A)
—
Result
—
Voltage drop (V)
—
Voltage drop (%)
—
Voltage drop = 2 × I × R × L ÷ 1000 for DC and single-phase, or √3 × I × R × L ÷ 1000 for three-phase, with L the one-way length in feet and R in ohms per 1000 ft. The suggested size is the larger of the size for voltage drop and the typical ampacity size. Common guidance is about 3% drop for a branch circuit and 5% overall; these are recommendations, not requirements. Continuous loads are typically sized at 125% of the current, so enter that value if it applies.
Reference: resistance and typical ampacity
| AWG / kcmil | Cu A | Al A | Cu Ω/kft | Al Ω/kft |
|---|---|---|---|---|
| 14 AWG | 15 (20) | — | 3.14 | — |
| 12 AWG | 20 (25) | 15 (20) | 1.98 | 3.25 |
| 10 AWG | 30 (35) | 25 (30) | 1.24 | 2.04 |
| 8 AWG | 50 | 40 | 0.778 | 1.28 |
| 6 AWG | 65 | 50 | 0.491 | 0.808 |
| 4 AWG | 85 | 65 | 0.308 | 0.508 |
| 3 AWG | 100 | 75 | 0.245 | 0.403 |
| 2 AWG | 115 | 90 | 0.194 | 0.319 |
| 1 AWG | 130 | 100 | 0.154 | 0.253 |
| 1/0 AWG | 150 | 120 | 0.122 | 0.201 |
| 2/0 AWG | 175 | 135 | 0.0967 | 0.159 |
| 3/0 AWG | 200 | 155 | 0.0766 | 0.126 |
| 4/0 AWG | 230 | 180 | 0.0608 | 0.1 |
| 250 kcmil | 255 | 205 | 0.0515 | 0.0847 |
| 300 kcmil | 285 | 230 | 0.0429 | 0.0707 |
| 350 kcmil | 310 | 250 | 0.0367 | 0.0605 |
| 400 kcmil | 335 | 270 | 0.0321 | 0.0529 |
| 500 kcmil | 380 | 310 | 0.0258 | 0.0424 |
Resistance is DC at 75 °C from NEC Chapter 9, Table 8. Ampacity is the typical NEC Table 310.16 75 °C column for copper and aluminum (up to three conductors, 30 °C ambient). Where a size shows two numbers, the first is the limit under NEC 240.4(D) for 14, 12 and 10 AWG copper (15, 20 and 30 A) and 12 and 10 AWG aluminum (15 and 25 A), and the one in parentheses is the table value. Confirm with the current code.
Voltage Drop Calculator · Ohms Law Calculator · Amp Calculator
More in this group
How it works
- Which wire size do I need for my current?
- Choose the smallest wire whose ampacity is at least the current. In the typical NEC 75 °C column, 12 AWG copper is 25 A in the table but is limited to 20 A, 10 AWG is 35 A (limited to 30 A) and 8 AWG is 50 A. A long run may need a larger size to keep voltage drop down, which is why this tool shows both sizes.
- How is voltage drop used to pick the wire?
- Drop = 2 × I × R × L ÷ 1000 for a single-phase or DC circuit. At 20 A, 120 V and 100 ft one way, 12 AWG copper (1.98 Ω per 1000 ft) drops 2 × 20 × 1.98 × 100 ÷ 1000 = 7.92 V, which is 6.6%. 8 AWG (0.778 Ω) drops 2.59%, so it is the smallest size that stays under 3%.
- How does aluminum differ from copper?
- Aluminum has higher resistance and lower ampacity. For 12 AWG the table lists 3.25 Ω per 1000 ft for aluminum against 1.98 for copper, so aluminum generally needs a larger size, often about two sizes up, for the same drop and current.
- What voltage drop is acceptable?
- Common guidance is about 3% on a branch circuit and 5% for feeder and branch combined. These are recommendations rather than hard rules, and the best limit depends on the equipment and the project. Set the maximum drop field to the value you want and check your local requirements.
- Can I use this to wire a building?
- No. It is a planning estimate. It ignores temperature and conduit-fill derating, terminal temperature ratings, insulation type, continuous-load factors and local amendments. Have an electrician or your inspector confirm the conductor size and the overcurrent protection.