Wire Gauge Calculator
Find the right conductor size for your circuit. Enter load current, circuit length and system voltage. The calculator checks both voltage drop and current-carrying capacity (ampacity), then recommends the minimum wire size in mm² and the nearest AWG equivalent.
Last updated: May 2026
Enter current, circuit length and voltage to find the recommended wire gauge.
Recommended size = larger of (voltage-drop minimum, ampacity minimum). Single-phase: 2 x rho x L x I / (V x drop%). Three-phase: sqrt(3) x rho x L x I / (V x drop%). IEC 60364 ampacity at 30 °C.
How wire gauge is determined
Selecting the right conductor size involves two independent constraints that must both be satisfied. The first is voltage drop: the conductor must be large enough that the resistive loss along the circuit stays within the chosen limit (typically 3%). The second is ampacity: the conductor must be able to carry the load current continuously without overheating. The correct gauge is the one that satisfies both; whichever gives the larger cross-section wins.
For continuous loads (motors, EV chargers, solar inverters), NEC and IEC both require sizing at 125% of the actual running current before looking up ampacity. This is already applied automatically when you select those application types above.
Standard conductor sizes, ampacity and AWG equivalents
| mm² | AWG equiv. | Max current in conduit (Cu) | Max current free air (Cu) | Typical use | Resistance (Ω/km) |
|---|---|---|---|---|---|
| 1.5 | AWG 15 | 16 A | 19 A | Lighting circuits | 11.5 |
| 2.5 | AWG 13 | 25 A | 27 A | Power outlets, 20 A circuits | 6.92 |
| 4 | AWG 11 | 32 A | 37 A | Cookers, shower circuits (10 kW at 230 V) | 4.61 |
| 6 | AWG 10 | 40 A | 50 A | EV charger 7.4 kW, large appliances | 3.08 |
| 10 | AWG 7 | 54 A | 70 A | EV charger 11 kW, sub-panels | 1.83 |
| 16 | AWG 5 | 73 A | 96 A | 3-phase loads, main supply | 1.15 |
| 25 | AWG 3 | 95 A | 130 A | Main supply, generator feed | 0.727 |
Ampacity values approximate IEC 60364-5-52 for PVC-insulated copper conductors at 30 °C ambient. De-rate for higher temperatures or grouped cables.
Worked example: 7.4 kW home EV charger
A 7.4 kW single-phase EV charger at 230 V draws 7400 / 230 = 32.2 A. Because EV charging is a continuous load, both NEC and IEC require sizing at 125%: 32.2 × 1.25 = 40.3 A. From the ampacity table, 6 mm² (AWG 10) carries 40 A in conduit, exactly at the limit, so go to 10 mm² if the run is long or the conduit is warm. For a 10 m run at 3% drop limit: minimum for voltage drop = 2 × 0.0172 × 10 × 40.3 / (230 × 0.03) = 1.98 mm², well below 6 mm². Ampacity drives the selection here.
Where wire gauge sits in wiring a circuit
Sizing the conductor is the first decision in a new circuit, because every step after it assumes the cable is already correct. Get the gauge right and the protection, the conduit and the supply all follow cleanly; get it wrong and no breaker or fuse downstream can make the run safe. The order a circuit comes together in:
- You are here: size the conductor. Load current, run length and your allowable drop give the minimum cross-section. The calculator above checks ampacity and voltage drop together and hands you the thicker of the two.
- Confirm the drop on the actual run. A gauge that is fine for current can still lose too much voltage over distance. The voltage drop calculator checks the run against the 3% IEC limit.
- Protect the cable, not the load. The fuse or breaker exists to stop the wire overheating, so it is rated to the conductor's ampacity. The fuse calculator sizes it and flags the minimum wire it allows.
- Fit it in the conduit. Bundled cables derate and a packed conduit fails inspection. The conduit fill calculator checks the cross-section against fill limits.
- Size the supply behind it. Add up the connected load and the power supply calculator gives the rating to buy; on motor and reactive loads the power factor calculator corrects W to the VA a generator or UPS must deliver.
- Know what it costs to run. Once the circuit feeds real appliances, the appliance running cost and electricity cost calculators turn the load into a monthly figure.
A note from my own wiring: I sized the cable for a 20 A workshop circuit and for a 12 V camper system the same way, current and run length first, and both taught the same lesson from opposite ends. On the 230 V workshop run the ampacity sets the gauge and voltage drop is an afterthought; on the 12 V camper run it is the drop that forces a much thicker cable than the current alone suggests, because a couple of volts lost is a rounding error at 230 V but a sixth of the whole supply at 12 V. I check both limits here every time and take the thicker result.
Frequently Asked Questions
What wire gauge do I need for a 20-amp circuit?
For a 20-amp continuous load, size at 125%: 20 × 1.25 = 25 A minimum ampacity. That requires 4 mm² (AWG 12) copper in conduit at 30 °C ambient. For intermittent 20-amp loads, 2.5 mm² (AWG 14) may be code-compliant depending on your jurisdiction, but the continuous-load rule applies to most appliance and EV circuits. Always verify against NEC Article 210 or IEC 60364-5-52 for your installation.
What is AWG and how does it relate to mm²?
AWG (American Wire Gauge) uses a lower number for thicker wire: AWG 10 is thicker than AWG 14. European and IEC wiring standards use cross-sectional area in mm² instead. Common equivalents: 1.5 mm² = AWG 16, 2.5 mm² = AWG 14, 4 mm² = AWG 12, 6 mm² = AWG 10, 10 mm² = AWG 8. The calculator shows both units for every recommended gauge.
What is the maximum allowed voltage drop for a circuit?
IEC 60364-7-722 and most national wiring codes recommend a maximum of 3% voltage drop for final circuits, and 5% total from the supply origin. For a 230 V circuit, 3% equals 6.9 V. Exceeding this causes motors to run hot, chargers to throttle output and dimmers to flicker. Enter your load, cable run length and voltage above to check your installation against the limit.
Is the calculator result the minimum, or can I use a larger gauge?
It is the minimum. Going one standard size larger is always safe and often worthwhile: it reduces heat build-up in conduit, lowers resistive losses over the life of the installation, gives headroom for future load growth, and keeps voltage drop well inside the guideline. The cost difference between 4 mm² and 6 mm² cable is small compared to the labour of re-pulling wire later.
Does ambient temperature affect the wire gauge I need?
Yes. Published ampacity figures assume 30 °C ambient (IEC) or 60 °C / 75 °C conductor temperature ratings (NEC). At higher ambient temperatures, such as cables in a hot attic, roof space or direct sunlight, ampacity de-rates significantly. A rule of thumb: above 30 °C, go one standard size larger per 10 °C of additional ambient temperature. In conduit with multiple cables grouped together, apply a further de-rating factor (typically 0.7 to 0.8 for three current-carrying conductors).
What gauge do I need for a 7.4 kW EV home charger?
A 7.4 kW charger at 230 V draws about 32 A. NEC and IEC treat EV charging as a continuous load, so size at 125%: 32 × 1.25 = 40 A. The minimum conductor for 40 A ampacity in conduit is 6 mm² (AWG 10). For a typical 10 m run at 3% drop, the voltage-drop minimum is under 2 mm², so ampacity governs. Use 6 mm² for runs up to roughly 25 m; switch to 10 mm² for longer runs or if the cable passes through a warm roof space.
Can I use aluminium cable for indoor wiring?
Aluminium is permitted by both NEC and IEC for fixed wiring, but it is generally restricted to conductors 35 mm² and larger (service entrance, main feeds). For branch circuits, copper is strongly preferred: aluminium requires anti-oxidant compound at every termination, special aluminium-rated connectors, and careful torque on terminals to prevent the cold-flow that causes loose connections over time. If you do use aluminium, size it one or two standard sizes larger than the copper equivalent; aluminium carries about 60% of the current of the same cross-section copper.
Methodology and sources
This tool sizes a conductor against two independent constraints, voltage drop and current-carrying capacity (ampacity), and recommends the larger of the two minimum cross-sections in mm² with the nearest AWG equivalent.
- Method: Voltage-drop minimum cross-section uses A = 2 × rho × L × I / (V × drop%) for single-phase and A = sqrt(3) × rho × L × I / (V × drop%) for three-phase, where rho is conductor resistivity (copper 0.0172, aluminium 0.0282 Ω·mm² / m), L is the one-way run length and I the rated current. The ampacity minimum is read from a standard mm² ampacity table. Continuous loads (motor, EV charger, solar) are rated at 125% of the entered current before sizing.
- Standards and sources: ampacity values approximate IEC 60364-5-52 (PVC-insulated copper, in conduit, 30 °C ambient), cross-checkable against NEC Table 310.15; the 3% / 5% voltage-drop guidance follows IEC 60364-7-722 and common national wiring codes; the 125% continuous-load rule follows NEC Article 210 / 625.41 and equivalent IEC practice. The cross-section formula itself is standard conductor physics.
- Assumptions and limits: values assume copper or aluminium conductors at 30 °C ambient with no grouping de-rate; raise one standard size per 10 °C above 30 °C and apply a grouping factor (about 0.7 to 0.8 for three bundled conductors). The result is a minimum, not a final design; long runs, warm spaces and future load growth all argue for the next size up.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This calculator is a planning aid, not a substitute for a qualified electrician or your local wiring and building code; always verify the final conductor size and protection against the code in force for your installation.
Next step in this workflow
Now verify the voltage drop across your cable run stays within the 3% limit.