Wire Size Calculator
Find the wire gauge that keeps voltage drop within limit for a circuit, from the current, one-way distance, voltage, conductor metal, and phase. Calculates in your browser.
Recommended wire size
-
Show the math
The smallest gauge whose drop stays within your limit is chosen:
drop = (factor × K × amps × distance) ÷ circular mils
factor = 2 (1-phase) or √3 (3-phase); K = 12.9 Cu, 21.2 Al
Size for voltage drop and ampacity, pick the larger result and follow the NEC and your local code. A common target is 3% on a branch circuit. All math runs locally in your browser.
What this does
A wire size calculator finds the gauge that keeps voltage drop within limit for a circuit, from the current, distance, voltage, conductor metal, and phase.
How to use it
- Enter the current and voltage.
- Enter the one-way distance.
- Choose copper or aluminium.
- Read the recommended gauge.
How it works
The tool steps up the gauge until the drop meets your limit, commonly 3% on a branch circuit. Size for ampacity as well.
Vdrop = 2 × I × R × distance
Understanding your result
This gauge keeps voltage drop within your limit, which matters most on long runs, but you must also size for ampacity, the current the wire can safely carry, and use whichever requirement calls for the heavier wire. The result is a planning guide; the breaker rating and your local electrical code have the final say on what is legal and safe.
Example
20 A over 100 ft of copper at 120 V often needs 10 AWG.
Sources & methodology
Last updated .
Frequently asked questions
How much voltage drop is acceptable?
A common guideline is 3% on a branch circuit and up to 5% total including the feeder. The tool steps up the gauge until your limit is met.
Copper or aluminium?
Aluminium has higher resistance, so it needs a larger gauge for the same drop. The tool uses the right resistivity for each.
Does this replace the ampacity table?
No. Size for both voltage drop and ampacity, pick the larger result, and follow the NEC and your local code.
Do you store my numbers?
No. Everything is calculated locally in your browser.
Related tools
BTU Calculator
Estimate the cooling BTU and tons an air conditioner needs for a room from its size, ceiling height, sun exposure, occupants, and whether it’s a kitchen. Calculates in your browser.
HVAC Load Calculator
Get a rough whole-house heating and cooling load in BTU and tons from the floor area, climate-based factors, window count, and occupants. Calculates in your browser.
Drywall Calculator
Work out the drywall sheets a project needs from the wall and ceiling area and sheet size, plus screws, joint compound, and tape. Calculates in your browser.
Conduit Fill Calculator
Check NEC conduit fill (total conductor area versus the allowed percentage for EMT) from your wire gauges and counts, with a pass or fail. Calculates in your browser.