Wire Size Calculator
Find the minimum wire cross-sectional area in circular mils needed to keep voltage drop within code limits for any DC or single-phase AC circuit. Use this when wiring long runs to panels, subpanels, motors, or outdoor equipment.
About this calculator
Voltage drop over a conductor depends on the conductor's resistance, which increases with length and decreases with wire cross-sectional area. The NEC recommends keeping voltage drop to 3% or less for branch circuits and 5% or less for feeder plus branch combined. The required circular mil area (CM) is calculated as: CM = (2 × current × distance × 12.9) / (voltage × voltageDropPercent / 100), where 12.9 is the resistivity constant for copper in circular mil-ohms per foot, the factor 2 accounts for the round-trip wire length (hot and neutral), and distance is the one-way run length in feet. The result in circular mils is then matched to the nearest standard AWG size that meets or exceeds that area. Aluminum conductors use a resistivity constant of approximately 21.2 instead of 12.9.
How to use
Example: You need to wire a 20 A, 120 V circuit to an outbuilding 75 ft away, with a maximum 3% voltage drop. Step 1 — Enter Current: 20 A. Step 2 — Enter Distance: 75 ft. Step 3 — Enter Voltage: 120 V. Step 4 — Enter Max Voltage Drop: 3 (%). Step 5 — Calculation: CM = round((2 × 20 × 75 × 12.9) / (120 × (3/100)) × 1000) / 1000 = round((38,700) / (3.6) × 1000) / 1000 = 10,750 circular mils. Step 6 — 10 AWG copper = 10,380 CM is slightly under; select 8 AWG (16,510 CM) to meet or exceed the requirement. The calculator returns the exact CM value so you can look up the correct AWG from an NEC table.
Frequently asked questions
What percentage of voltage drop is acceptable for residential wiring circuits?
The NEC recommends, but does not mandate, a maximum of 3% voltage drop for individual branch circuits and 5% total for the combined feeder and branch circuit. Exceeding 3% on a branch circuit causes lights to dim noticeably, motors to run hotter and less efficiently, and sensitive electronics to malfunction. For critical loads like medical equipment or data centers, designers often target 1–2% to provide additional margin. Voltage drop limits are among the most commonly overlooked NEC recommendations in DIY and light-commercial work.
How does wire run distance affect the required wire gauge?
Wire resistance increases linearly with length, so doubling the run distance doubles the voltage drop for the same wire size. To maintain the same percentage voltage drop over twice the distance, you must reduce resistance by half, which means approximately doubling the wire's cross-sectional area — roughly two AWG sizes larger. This is why a 14 AWG wire is perfectly adequate for a 15 A outlet 10 ft from the panel but completely inadequate for the same outlet 150 ft away. Always calculate voltage drop for long runs rather than relying on ampacity tables alone.
Why does the wire size formula use a factor of 2 for the distance?
The factor of 2 accounts for the fact that current travels the full length of the circuit twice — once through the hot (line) conductor to the load, and once back through the neutral (return) conductor to the source. Both conductors contribute resistance, so the effective resistive length is twice the one-way physical distance. In three-phase circuits, the factor changes because the return path is shared among three phases, which is why three-phase voltage drop formulas use √3 instead of 2.