Wire Size Calculator
Find the correct wire gauge for any electrical circuit based on current load, run length, and allowable voltage drop. Use this when wiring outlets, appliances, or sub-panels to avoid overheating and energy loss.
Last updated: September 2026
Minimum Wire Size
10 AWG
Formula below · 2 sources (nfpa.org, Wikipedia) · Updated Sep 2026
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About this calculator
The minimum wire size is the smallest standard copper conductor that passes two separate checks. 1) Ampacity (NEC Table 310.16): the conductor must carry the load current, multiplied by 1.25 when the load is continuous (NEC 210.19(A)(1)). Loads of 100 A or less are checked against the 60 °C column (the NEC 110.14(C)(1)(a) default) and larger loads against the 75 °C column, with the NEC 240.4(D) limits of 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG. 2) Voltage drop: Vdrop = 2 × I × d × R / 1,000, where d is the one-way distance in feet and R is the resistance in ohms per 1,000 ft from NEC Chapter 9 Table 8 (stranded copper, 75 °C). Vdrop must not exceed the selected percentage of the system voltage. Equivalently, the required area is CM = 2 × 12.9 × I × d / Vdrop, where 12.9 ohm-cmil/ft is the copper K-factor. Sizes run from 14 AWG to 750 kcmil; 1/0 AWG and larger are shown as text. Assumes copper conductors, 30 °C ambient, no more than three current-carrying conductors together and a single-phase two-wire circuit (a three-phase run with the same inputs drops less, so the result stays on the safe side). Lower AWG numbers mean thicker wire.
How to use
Example: a 20 A load on a 120 V circuit to a garage 50 feet away, 3% maximum drop. Allowed drop: 120 × 0.03 = 3.6 V. 12 AWG (1.98 ohm/1,000 ft) drops 2 × 20 × 50 × 1.98 / 1,000 = 3.96 V, which is too much; 10 AWG (1.24 ohm/1,000 ft) drops 2.48 V, which passes. Ampacity: 20 × 1.25 = 25 A for a continuous load, which 10 AWG (30 A) also meets. Result: 10 AWG. With 5% allowed drop (6 V) and a non-continuous load, 12 AWG passes both checks. At 240 V and 3%, 12 AWG passes for a non-continuous 20 A load, but a continuous one still needs 10 AWG for ampacity.
Frequently asked questions
What wire gauge should I use for a 30-amp, 240-volt circuit?
For a 30 A, 240 V circuit such as a dryer, 10 AWG copper is the NEC minimum (240.4(D) allows 10 AWG on a 30 A breaker), and it keeps voltage drop under 3% up to about 95 ft at a full 30 A. If the 30 A load is continuous (3+ hours), the conductor must be rated for 37.5 A, which means 8 AWG. Enter your actual one-way distance and load type; the calculator applies both the ampacity and the voltage-drop checks.
How does one-way distance affect wire gauge selection?
Longer wire runs increase total circuit resistance, which causes more voltage to be lost before it reaches the load. The NEC and most electrical codes recommend keeping voltage drop at or below 3% for branch circuits. Doubling the distance roughly doubles the voltage drop, which can require jumping up one or two wire gauges to compensate. That is why a 100-foot run to an outbuilding needs heavier wire than an identical 20-foot run inside the home.
Why does voltage drop matter when choosing wire size?
Excessive voltage drop means the appliance or motor at the end of the circuit receives less voltage than it was designed for. Motors running under-voltage draw higher current, generate extra heat, and wear out prematurely. Lighting may appear dim, and sensitive electronics can malfunction. The NEC recommends (in informational notes to 210.19 and 215.2, not as a mandatory rule) a maximum of 3% on a branch circuit and 5% total including the feeder.
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