Wire Size Calculator
Size the next conductor for a US NEC job-site run.
Enter volts, amps, and one-way feet. Get the next AWG and what limited it.
How this result was chosen
- Check ampacity for the load, material, insulation, and install method.
- Check voltage drop for the one-way length and your target percent.
- Recommend the smallest standard AWG or kcmil that clears both.
The card tells you which check limited the size. If voltage drop is the limit, a short run may still use a smaller size. If ampacity is the limit, a longer run may still need to go up.
Why ampacity alone is not enough
A conductor can carry the load and still drop too much voltage on a long run.
Manufacturer pages can do the math, but they are slow on a phone.
Generic calculators often start in mm² or Ohm's Law. That is the wrong job.
This page keeps US volts, feet, and AWG, then shows the next size and the reason.
How to size a conductor here
- Enter the circuit voltage, load current, and one-way feet.
- Keep copper or aluminum, then set the voltage-drop target.
- Read Recommended size and What limited this result.
Example: 240 V, 40 A, 180 ft, copper
Task: Size a copper conductor for 240 V, 40 A, 180 ft one-way, 3% voltage-drop target.
Planning math (single-phase, K = 12.9, circular mils from common AWG data):
Allowed drop at 3% of 240 V is 7.2 V.
8 AWG: about 11.3 V / 4.7%. Over the 3% target.
6 AWG: about 7.1 V / 3.0%. Inside the 3% target.
Ampacity still has to be checked in the current NEC table for the insulation and termination temperature. Under common 75 °C copper THHN planning assumptions, 8 AWG often clears 40 A on ampacity. On this run, voltage drop still pushes the recommendation to 6 AWG.
Example result labels: Recommended size 6 AWG. What limited this result: Voltage drop. Voltage drop: about 7.1 V (3.0%). Ampacity check: 40 A load vs the table value you must verify. Next size up: 4 AWG.
These numbers are a planning walk-through, not a stamped design and not a copy of an official table. Confirm circular mils, ampacity, temperature correction, and continuous-load rules in the current code, manufacturer data, and the AHJ.
Related tools
Guides for this tool
Wire size FAQ
Does this calculator include voltage drop?
Yes. The recommended size must clear both the ampacity check and your voltage-drop target.
Is the result NEC approved?
No. It is a planning reference. Verify against the current NEC, manufacturer data, and the AHJ.
Why did it pick a larger AWG than I expected?
Voltage drop on a long run often forces a size-up after ampacity already passed. Read What limited this result.
Do I need an account to calculate?
No. Wire size, voltage drop, and conduit fill stay free without signing in.
Is 3% voltage drop a code requirement?
No. 3% and 5% are common planning targets. Your job, equipment, and AHJ decide the real limit.
Can I use mm² or 230 V defaults here?
No. This page is US NEC job-site units: volts, amps, feet, and AWG or kcmil.