Find the minimum conductor size for your circuit — checked against both ampacity and voltage drop, so the gauge you get actually passes on both counts, not just one.
What Size Wire Do I Need?
Picking a conductor comes down to two questions, and a correct answer has to satisfy both: can the wire carry the current without overheating (ampacity), and will the voltage at the far end still be high enough (voltage drop)? Short runs are usually limited by ampacity; long runs are usually limited by voltage drop. This calculator sizes for whichever governs and tells you which one it was.
The Two Checks
Ampacity is the current a conductor can carry continuously, set by its size, material and insulation temperature rating. This tool uses the 75°C copper and aluminum values in the style of NEC Table 310.16.
Voltage drop grows with distance and current. A common target is 3 percent on a branch circuit (5 percent total including the feeder). For a one-way run of length L:
where R is the conductor resistance per 1000 ft, and k is 2 for single-phase and DC circuits or √3 for three-phase. The percentage is Vdrop divided by system voltage.
Worked Example
A 40 A load at 240 V, single-phase, on a 150 ft one-way copper run: 8 AWG carries 50 A (passes ampacity) but its voltage drop would be near 3.7 percent — over a 3 percent target. Stepping up to 6 AWG brings the drop under 3 percent, so voltage drop is the deciding factor and 6 AWG is the answer.
Copper vs Aluminum
Aluminum is cheaper and lighter but has higher resistance, so for the same load it needs roughly one to two sizes larger than copper. Aluminum also requires rated terminations and anti-oxidant practices. The calculator switches the ampacity and resistance tables when you change the conductor material.
Frequently Asked Questions
Is this the breaker size or the wire size?
It is the wire (conductor) size. The breaker is chosen to protect that conductor and the load, often at 125 percent of a continuous load current. Size the wire first, then the overcurrent device.
One-way or round-trip length?
Enter the one-way distance from the source to the load. The k factor (2 or √3) already accounts for the return path.
Why did my long run jump several sizes?
Voltage drop scales directly with length. Doubling the distance doubles the drop, which can push the required size up by two or more gauges even though the ampacity never changed.
Does this include temperature and conduit-fill derating?
No — it uses base 75°C ampacity. In hot spaces or crowded conduit you must apply the code derating factors, which lower the usable ampacity.
Related calculators
- Voltage Drop Calculator — drill into the drop on a specific run and size.
- Three-Phase Power Calculator — turn a kW load into the line current you size for.
- Ohm’s Law Calculator — voltage, current, resistance and power basics.
- More electrical & engineering tools on the Engineering hub.