How to Calculate Wire Size for a 220V Circuit
Selecting the correct wire size for a 220 volt circuit comes down to two things: the wire must carry the load current without overheating, and the resistance of the cable must not cause more voltage drop than your equipment can tolerate. This calculator applies both checks. It first requires a conductor whose typical current rating meets your load, then sizes up further if the voltage drop would otherwise exceed your limit, and recommends the larger of the two. Voltage drop is usually the binding factor on branch circuit runs of 10 metres or more, while the current rating is the deciding factor on shorter runs.
The voltage drop across a two-conductor cable is calculated as:
Voltage Drop = 2 x L x I x (rho / A)
Where L is the one-way cable length in metres, I is the current in amps, rho is the conductor resistivity (0.01724 ohm·mm²/m for copper, 0.02826 for aluminium), and A is the conductor cross-sectional area in mm². The factor of 2 accounts for the current travelling both ways through the circuit (outward on the live conductor and back through the neutral or return conductor). For split-phase 220V (two live conductors), the same formula applies. For three-phase, the factor changes to the square root of 3 (approximately 1.732).
Rearranging to find the minimum cross-section required to stay within a given voltage drop limit:
A (min) = 2 x L x I x rho / V_drop_max
where V_drop_max = (drop% / 100) x supply voltage.
Standard Wire Sizes: AWG vs Metric (mm²)
| AWG | Area (mm²) | Nearest metric (mm²) | Resistance (copper, ohm/m) | Typical maximum current |
|---|
| 14 | 2.08 | 2.5 mm² | 0.00829 | 15 A |
| 12 | 3.31 | 4 mm² | 0.00521 | 20 A |
| 10 | 5.26 | 6 mm² | 0.00328 | 30 A |
| 8 | 8.37 | 10 mm² | 0.00206 | 40 A |
| 6 | 13.3 | 16 mm² | 0.00130 | 55 A |
| 4 | 21.2 | 25 mm² | 0.000814 | 70 A |
| 2 | 33.6 | 35 mm² | 0.000513 | 95 A |
| 1/0 | 53.5 | 50 mm² | 0.000322 | 125 A |
| 2/0 | 67.4 | 70 mm² | 0.000256 | 145 A |
Worked Example
Default inputs: 220V supply, 20 A load, 15 m one-way cable run, single-phase, 3% voltage drop limit, copper conductors.
Maximum allowable voltage drop = 3% x 220 V = 6.6 V.
Required minimum cross-section for voltage drop = (2 x 15 x 20 x 0.01724) / 6.6 = 10.344 / 6.6 = 1.567 mm².
By voltage drop alone, the next standard AWG size above 1.567 mm² would be 14 AWG (2.08 mm²). But 14 AWG has a typical maximum current of only 15 A, below the 20 A load, so it cannot safely carry the current. The calculator therefore steps up to the smallest size rated for at least 20 A, which is 12 AWG (3.31 mm², nearest metric 4 mm²).
Actual voltage drop with 12 AWG = 2 x 15 x 20 x (0.01724 / 3.31) = 10.344 / 3.31 = 3.13 V, which is 1.42% of 220 V, comfortably within the 3% limit.
This matches what the calculator shows for the default settings.
Voltage Drop Limits
Most electrical codes recommend keeping voltage drop to 3% or less on branch circuits, and 5% or less over the combined feeder and branch circuit. For 220V, 3% equals 6.6V and 5% equals 11V. Sensitive equipment such as computers, motors, and variable speed drives typically performs best with voltage drop below 2%. New Zealand's AS/NZS 3000 Wiring Rules do not specify a single percentage limit but note that voltage drop should not adversely affect equipment operation; in practice, 5% is widely used as the design maximum.
Safety Note
This calculator provides sizing guidance based on voltage drop plus a basic current-rating check against the typical maximum current for each gauge. It does not apply the full ampacity derating required in practice. Actual wire selection must also consider the detailed current-carrying capacity (ampacity) of the conductor, ambient temperature, number of conductors bundled together, conduit fill, and any applicable local wiring code requirements. In New Zealand, all fixed 220V/230V/240V wiring must be installed by a licensed electrical worker (LEW) in accordance with AS/NZS 3000. Never attempt fixed wiring yourself unless you hold the appropriate licence.
Related Calculators
Sources and method: Conductor resistivity values from IEC 60228 (Conductors of insulated cables). Wire size cross-reference from ASTM B258 (AWG) and IEC 60228 standard metric sizes. Voltage drop formula per AS/NZS 3000 Wiring Rules guidance. AWG cross-sections from NIST Handbook of Mathematical Functions. Current-carrying capacity guidance from AS/NZS 3008.1.1.
This calculator is for guidance only and covers voltage drop plus a basic current-rating check for copper or aluminium conductors. It does not substitute for a full electrical design. Actual cable selection must account for full ampacity derating, installation method, ambient temperature, and compliance with applicable wiring codes. Always consult a licensed electrical worker for any fixed wiring installation.
