220 Volt Wire Size Calculator

Find the minimum copper or aluminum wire gauge for a 220-240V circuit, sized by NEC ampacity rules and checked against voltage drop over your wire run.

Quick Facts

Ampacity rule
Continuous loads need 125% of the running current
Per NEC 210.19(A); conductor and breaker are both sized for that inflated value.
Voltage drop
VD = (2 x K x I x D) / CM
K = 12.9 for copper, 21.2 for aluminum (ohm-cmil/ft); most branch circuits target 3% or less.

Your Results

Calculated
Recommended wire size
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Smallest gauge meeting ampacity and voltage-drop limits
Required ampacity
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Load current after the continuous-load factor
Voltage drop
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Percent and volts lost over the run
Voltage at the load
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Circuit voltage minus the drop

Ready

Enter the load current, wire run length, and circuit details, then press Calculate.

About the 220 Volt Wire Size Calculator

This tool sizes the conductor for a 220-240V circuit — the kind used for dryers, ranges, water heaters, window and central air conditioners, welders, and EV chargers — using the two checks an electrician runs by hand: the National Electrical Code (NEC) ampacity table, and the voltage-drop formula for the actual length of the run.

Step 1: required ampacity

The conductor must be rated for at least the load current. If the load is continuous (expected to run 3 hours or more, such as an AC compressor or EV charger), NEC 210.19(A) requires sizing for 125% of the load current: Required ampacity = Load current x 1.25. Non-continuous loads (a range, a dryer cycling on and off) only need to be sized for the actual current.

Step 2: minimum gauge from the ampacity table

The calculator compares that required ampacity against NEC Table 310.16 (75°C column, the typical rating for THHN/THWN-2 residential branch-circuit wire) to find the smallest copper or aluminum gauge whose listed ampacity meets or exceeds it — for example, 30A calls for 10 AWG copper, while 50A calls for 8 AWG copper.

Step 3: check voltage drop over the run

Ampacity alone ignores wire length. Every conductor has resistance, so current flowing through it drops some voltage before reaching the load. The standard formula, using circular-mil wire area, is:

VD = (2 x K x I x D) / CM

where K is a resistivity constant (12.9 for copper, 21.2 for aluminum, in ohm-cmil/ft), I is the load current in amps, D is the one-way distance in feet, and CM is the conductor's cross-sectional area in circular mils. The factor of 2 accounts for the round trip through both the hot and neutral/return conductors. Dividing VD by the circuit voltage gives the percentage drop. If that percentage exceeds your target (commonly 3% for a branch circuit), the calculator steps up to the next larger gauge and rechecks, repeating until the drop is within limit or the table is exhausted.

Copper vs. aluminum

Aluminum has roughly 61% of copper's conductivity, so it needs a larger cross-section for the same current and voltage-drop target — commonly about two AWG sizes bigger than the equivalent copper run — and NEC does not list aluminum below 12 AWG. This calculator applies the correct ampacity table and K constant for whichever material you select.

Limits of this calculator

This is a planning estimate, not a code inspection. It does not account for ambient-temperature derating, conduit fill and conductor bundling adjustments, terminal temperature ratings, or local amendments to the NEC. Always verify the final wire size and breaker against your local electrical code, and have the installation inspected by a licensed electrician.

Frequently Asked Questions

What size wire do I need for a 220V circuit?
It depends on the load current and the length of the run. As a rough guide for copper at typical residential distances, 20 amps calls for 12 AWG, 30 amps for 10 AWG, 40-50 amps for 8 AWG, and 50-65 amps for 6 AWG. Longer runs need a heavier gauge than the ampacity table alone suggests, because voltage drop grows with distance. Always confirm against NEC Table 310.16 and local code.
What is the 125% rule for continuous loads?
NEC 210.19(A) requires branch-circuit conductors supplying a continuous load (running 3 hours or more) to be sized for 125% of that load's current. A 30-amp continuous load therefore needs a conductor and breaker rated for at least 37.5 amps. Non-continuous loads only need to be sized for the actual current.
Why does wire length affect the wire gauge?
Every conductor has resistance, and current flowing through that resistance causes a voltage drop proportional to length. A short 20-foot run may lose under 1% of the voltage, while a 150-foot run at the same current and gauge can lose well over 3%. Increasing the wire's cross-sectional area (a lower AWG number) reduces resistance and keeps the drop within an acceptable limit, typically 3% for branch circuits.
Does aluminum wire need a different size than copper?
Yes. Aluminum has roughly 61% the conductivity of copper, so for the same current and voltage-drop limit, aluminum conductors must be larger — commonly about two AWG sizes bigger than the equivalent copper conductor. Aluminum also has lower NEC ampacity ratings at each gauge and is not used below 12 AWG.

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