Wire Size Calculator
Estimate the recommended electrical wire gauge (AWG) based on current load, voltage, material, and voltage drop limits.
Calculation Results
Calculated cross-sectional area in circular mils (cmils) required to satisfy voltage drop requirements.
Recommended American Wire Gauge (AWG) size (smaller numbers indicate thicker wire). Code AWG 1 indicates AWG #1/0 or larger is required.
How This Calculator Works
To estimate electrical wire gauge sizing:
- Enter the load current (amperage rating of the breaker or device).
- Choose the system voltage (standard US household is 120V or 240V).
- Specify the total one-way run distance of the wire.
- Select the conductor type (Copper is standard; Aluminum is often used for heavy service mains).
- Specify the allowable voltage drop percentage (NEC recommends a maximum 3% drop for branch circuits).
Formula & Calculations
Wire size is calculated by determining the minimum cross-sectional area (in circular mils) to keep the voltage drop under the allowable limit:
Circular Mils = (2 × K × I × D) / V_drop
Where:
- K: Resistivity of the metal (12.9 ohms-cmil/ft for Copper; 21.2 for Aluminum).
- I: Current load in Amperes.
- D: One-way wire distance in feet.
- V_drop: Maximum allowable voltage drop in Volts (e.g.
System Voltage × Drop % / 100).
The recommended AWG size is mapped to circular mil sizing standards (#14 AWG ≤ 4,110 cmils, #12 ≤ 6,530, #10 ≤ 10,380, #8 ≤ 16,510, #6 ≤ 26,240, #4 ≤ 41,740, #2 ≤ 66,360).
Step-by-Step Calculation Examples
Outbuilding Subpanel Feed (20 Amps, 120V, 100ft)
Calculating the required copper wire gauge for a 100-foot run drawing 20 Amps at 120V with a 3% allowable drop.
- current: 20
- voltage: 120
- distance: 100
- material: 12.9
- allowable_drop: 3
- required_cmil: 14333
- recommended_awg: 8
Frequently Asked Questions
What is voltage drop?
Voltage drop is the loss of electrical pressure (voltage) caused by the resistance of the wires as current flows through them. Wires that are too thin or runs that are too long cause voltage drop, resulting in motor overheating or dimmer lights.
What is the NEC limit for voltage drop?
The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits, and a maximum of 5% combined overall for feeder and branch circuits.
Why does a longer distance require a thicker wire?
Electrical resistance increases linearly with the length of a wire. To offset the increased resistance of a long run and prevent voltage drop, the wire cross-sectional area (thickness) must be increased.
What is the standard wire gauge for a 20 Amp circuit?
For standard household runs (under 50 feet), AWG #12 copper wire is standard for 20 Amp circuits. If the run is longer, a thicker gauge like AWG #10 or #8 may be needed to limit voltage drop.
When should I use aluminum wire instead of copper?
Aluminum wire is cheaper and lighter than copper but has higher resistance. It is commonly used for heavy structural service entry mains or long subpanel runs where cost savings are significant, but it requires larger gauges.