Wire Size Calculator
Find the right wire gauge (AWG) for a circuit from its current, one-way run length, supply voltage, and acceptable voltage drop. Uses the NEC resistance and ampacity tables for copper and aluminum, in conduit, free air, or direct burial.
Input
Load
System
In Conduit uses NEC 310.16 ampacity. Free Air uses NEC 310.17 (higher). Direct Burial is derated to 85% of conduit.
NEC recommends ≤3% for branch circuits, ≤5% combined feeder + branch.
Output
| Property | Value |
|---|---|
| No data yet | |
| AWG | mm² | Ampacity (A) | V-Drop (V / %) | Status |
|---|---|---|---|---|
| No data yet | ||||
AWG sizes 14 down to 4/0 with ampacity and the voltage drop your circuit would have at each size. The recommended size is marked.
Guides
The Wire Size Calculator recommends the correct wire gauge (AWG) for an electrical circuit based on how much current it carries, how far the cable runs, the supply voltage, and how much voltage drop you can accept. It checks each standard conductor size against both ampacity (can the wire safely carry the current?) and voltage drop (does too much voltage get lost over the run?), then recommends the smallest wire that satisfies both.
How to use it
- Enter the load current in amps — the current the circuit will draw.
- Enter the supply voltage (e.g. 120 V, 240 V).
- Enter the one-way cable length and pick feet or meters. This is the distance to the load, not the round-trip length — the calculator doubles it internally.
- Choose the voltage system (AC single-phase, AC three-phase, or DC), the conductor material (copper or aluminum), and the installation method.
- Set your maximum acceptable voltage drop as a percentage. The NEC recommends ≤3% for a branch circuit and ≤5% for feeder plus branch combined.
The recommended gauge, its cross-section, ampacity, actual voltage drop, voltage at the load, and heat loss appear instantly, along with a full reference table showing every size from 14 AWG to 4/0.
The voltage-drop formula
Voltage drop is the voltage lost to the resistance of the conductors. Because current flows out and back, both conductors count:
- DC and AC single-phase:
Vdrop = 2 × L × I × R - AC three-phase:
Vdrop = √3 × L × I × R
where L is the one-way length in thousands of feet, I is the current in amps, and R is the conductor resistance in ohms per 1000 ft. Voltage-drop percentage is Vdrop ÷ supply voltage × 100.
For example, a 15 A load on a 50 ft copper run at 120 V single-phase in conduit: 14 AWG (3.07 Ω/1000 ft) would drop 2 × 0.05 × 15 × 3.07 = 4.6 V, or 3.84% — over a 3% limit. Stepping up to 12 AWG (1.93 Ω/1000 ft) drops 2 × 0.05 × 15 × 1.93 = 2.9 V, or 2.41% — within limit and rated for 25 A. So 12 AWG is recommended.
AWG reference (copper, 75°C)
| AWG | mm² | Ω/1000 ft | Ampacity (conduit) |
|---|---|---|---|
| 14 | 2.08 | 3.07 | 20 A |
| 12 | 3.31 | 1.93 | 25 A |
| 10 | 5.26 | 1.21 | 35 A |
| 8 | 8.37 | 0.778 | 50 A |
| 6 | 13.3 | 0.491 | 65 A |
| 4 | 21.2 | 0.308 | 85 A |
| 2 | 33.6 | 0.194 | 115 A |
| 1/0 | 53.5 | 0.122 | 150 A |
| 4/0 | 107.2 | 0.0608 | 230 A |
Resistance values are from NEC Chapter 9, Table 8 (uncoated stranded); ampacities from NEC 310.16 (in conduit) and 310.17 (free air). Free-air ratings are higher; direct burial is derated to 85% of the conduit value. Aluminum conductors have higher resistance and lower ampacity, and 14 AWG is not a standard aluminum size.
Is this a substitute for an electrician?
No. This tool is a planning aid using standard 75°C table values. Real installations must account for ambient temperature, conductor bundling, termination ratings, and local code — always confirm with a licensed electrician and the current edition of the NEC or your local electrical code.
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