Formula Used
Single phase current: I = P ÷ (V × PF)
Three phase current: I = P ÷ (√3 × V × PF)
Horsepower input watts: W = HP × 746 ÷ efficiency
Design current: Design I = Load I × 1.25 for continuous loads
Effective ampacity: Ampacity = Table ampacity × ambient factor × conductor count factor
Single phase voltage drop: VD = 2 × I × R × L ÷ 1000
Three phase voltage drop: VD = √3 × I × R × L ÷ 1000
Voltage drop percent: VD% = VD ÷ system voltage × 100
The selected conductor must pass both ampacity and voltage drop checks.
How to Use This Calculator
Enter the known load value first. Choose amps, watts, kilowatts, volt amps, kilovolt amps, or horsepower.
Select the system voltage and phase. Add power factor for real power loads. Add efficiency for horsepower loads.
Choose copper or aluminum. Select the temperature rating that matches the allowed ampacity column.
Enter ambient temperature, current carrying conductor count, one way run length, and allowed voltage drop.
Press calculate. The result appears above the form. Use CSV or PDF buttons to save the report.
AC Wire Size Planning Guide
Correct wire size helps an AC circuit carry current safely. It also keeps voltage drop useful. A small wire can heat up. A long run can lose voltage. Both issues can cause poor performance and unsafe operation.
Why AC Wire Size Matters
AC loads often start with higher current. Motors, compressors, pumps, and tools may draw more current. A planning calculator cannot replace local rules. It can still help you compare conductor sizes early. The load current, conductor material, route length, phase, temperature rating, and adjustment factors all matter.
Ampacity and Voltage Drop
Ampacity is the current a conductor can carry under stated conditions. It changes when cables are bundled. It also changes in high surroundings. Voltage drop is the voltage lost along the conductor. Single phase circuits use a two conductor path. Three phase circuits use a square root of three factor. This calculator uses resistance values for each size. It then checks the drop against your chosen limit.
Using the Results
Start with the load value. Select amps, watts, kilowatts, volt amps, or horsepower. Enter voltage and phase. Use power factor for real power loads. Use efficiency for motor style entries. Mark continuous load when the load can run for three hours or more. The tool increases that current for planning. Then choose copper or aluminum. Copper usually needs a smaller size. Aluminum is lighter, but it has higher resistance.
Practical Design Notes
The suggested size should be treated as a planning result. Final work needs code review and field conditions. Terminal temperature ratings may limit the usable ampacity column. Local rules may require a larger equipment grounding conductor, a different insulation type, or special protection. Wet areas, high heat, rooftops, and conduit fill may change the result. For critical circuits, keep voltage drop low. For feeders and branch circuits, many designers target three percent or less. Always verify the breaker, cable type, raceway, and installation method before buying material.
Better Inputs Give Better Estimates
Use one way length, not round trip length. The calculator applies the proper path factor. Keep units consistent. Review the candidate table for margin. A size with extra margin often performs better when future load changes are possible.
FAQs
What is AC wire size?
AC wire size is the conductor size needed to carry alternating current safely. It depends on load current, material, insulation rating, temperature, run length, and voltage drop.
Why does voltage drop matter?
Voltage drop reduces voltage at the load. Too much drop can cause dim lights, hot motors, poor equipment performance, and wasted energy.
Should I use copper or aluminum?
Copper has lower resistance and often uses a smaller size. Aluminum can cost less, but it needs larger conductors and correct terminations.
What does continuous load mean?
A continuous load usually runs for three hours or more. This calculator can apply a 125 percent planning factor for that condition.
Is one way length correct?
Yes. Enter the one way distance from source to load. The calculator applies the return path factor for single phase circuits.
Can this replace an electrician?
No. It is a planning tool. Final conductor size must match local code, terminal ratings, cable type, installation method, and inspection rules.
Why are conductor count factors included?
Bundled current carrying conductors trap heat. Adjustment factors reduce usable ampacity when several conductors share a raceway or cable.
What voltage drop limit should I use?
Many designers use three percent for branch circuits. Some loads may need lower limits. Long feeders may require larger conductors.