Wire Size Calculator
Size American Wire Gauge (AWG) electrical copper conductors according to National Electrical Code (NEC) ampacity tables and long-distance voltage drop standards. Ensures subpanels, EV chargers, well pumps, and branch circuits operate coolly and safely without risking electrical fire.
Circuit breaker or continuous load amperage rating
Nominal circuit supply voltage
Linear distance from the electrical panel to the load outlet
NEC Section 210.19(A) recommends maximum 3% drop for branch circuits
Safe copper wire size (AWG) satisfying both ampacity and voltage drop
How to Use This Calculator
- Select your circuit breaker amperage rating (e.g. 20A or 50A).
- Choose your line voltage (120V for standard wall outlets, 240V for EV chargers/ovens).
- Enter the one-way distance in feet from your electrical panel to the device.
- View the recommended AWG wire gauge and expected voltage drop.
Formula & Calculation Methodology
Conductor gauge selection requires two checks: first, matching the baseline ampacity from NEC Table 310.16; second, upsizing to a larger cross-sectional area (circular mils) if wire length creates over 3% resistive voltage drop.
Worked Example
20 Amp 120V branch circuit at 50 feet distance
50 Amp 240V Level-2 EV Charger run 100 feet to detached garage
Frequently Asked Questions
Why do long wire runs require thicker wire gauge?↓
All electrical conductors possess internal electrical resistance proportional to length. Over long distances, current flowing through wire resistance causes voltage drop (dimming lights, overheating motors) and generates wasted heat in the wall.
What is the 80% rule in electrical circuit sizing?↓
The National Electrical Code (NEC) specifies that continuous loads (appliances running for 3 hours or more, such as electric vehicle chargers, space heaters, or air conditioners) must not exceed 80% of the breaker and wire ampacity (e.g., a 20A circuit max continuous load is 16A).
Can aluminum wire be substituted for copper wire?↓
Aluminum is cheaper and lighter for large service entrance cables, but has higher resistivity than copper. An aluminum conductor must typically be sized one to two AWG gauges larger than copper for equivalent current capacity (e.g., 2 AWG aluminum vs 4 AWG copper).
What is a limitation of basic wire sizing calculators?↓
Calculations assume normal ambient temperatures (under 86°F / 30°C) and no more than three current-carrying conductors in a single raceway/conduit; hot attics and packed conduits require NEC derating factors.
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Source Citation & Disclaimer
Informational tool only; not licensed tax, legal, or financial advice.