Wire Size Calculator

How Does the Formula Work?

The wire size calculator determines the correct cable cross-section based on current load, system voltage, cable length, conductor material, and maximum allowed voltage drop. Undersized wiring causes excessive voltage drop leading to equipment malfunction, overheating, energy waste, and fire hazards. This calculator uses the standard electrical engineering formula to find the minimum cross-section and recommends the next standard size up from the IEC 60228 series. It supports both copper and aluminum conductors and single-phase and three-phase systems.

Understanding Voltage Drop

Every electrical conductor has resistance that causes voltage loss as current flows through it. This voltage drop is proportional to current, wire length, and resistivity, and inversely proportional to cross-sectional area. The National Electrical Code (NEC) recommends a maximum of 3 percent voltage drop for branch circuits and 5 percent total including feeders. Excessive voltage drop causes motors to run hotter and less efficiently, lights to dim, electronic equipment to malfunction, and increased energy consumption. For sensitive equipment like computers and medical devices, keeping voltage drop below 2 percent is advisable. This calculator shows the actual voltage drop with the recommended wire size so you can verify it meets your requirements.

Copper vs Aluminum

Copper has a resistivity of 0.0175 ohm-mm²/m making it the standard choice for most residential and commercial wiring. Aluminum has a resistivity of 0.0283, requiring approximately 1.6 times the cross-section for equivalent performance. However, aluminum costs significantly less per meter, making it preferred for large feeders and utility lines. Aluminum connections require special anti-oxidation compound and compatible connectors because aluminum oxidizes and can create high-resistance connections that overheat. Modern aluminum alloy conductors (AA-8000 series) offer improved properties over older pure aluminum wiring. For residential branch circuits, copper remains the standard recommendation.

Common Residential Circuits

Typical residential wiring uses 1.5 mm² for lighting circuits (10A), 2.5 mm² for general outlets (16-20A), 4 mm² for kitchen and high-power outlets (25-32A), and 6-10 mm² for electric ovens, water heaters, and air conditioning units. Main service entrance cables range from 16 to 35 mm² depending on total household load. These standard sizes assume short cable runs under 25 meters; for longer distances, this calculator may recommend upsizing to maintain acceptable voltage drop. Always verify wire size selections with local electrical codes and a licensed electrician.

Three-Phase Systems

Three-phase power uses a factor of square root of 3 (1.732) instead of 2 in the voltage drop formula because current flows through a single conductor rather than making a round trip. Three-phase systems are standard in commercial and industrial settings, operating at 400V (line-to-line) in most countries. The higher voltage means lower current for the same power, resulting in smaller wire sizes. A 20 kW load at 230V single-phase draws 87A, while the same load at 400V three-phase draws only 29A per phase; dramatically reducing wire size requirements and cost. This calculator handles both configurations automatically.

Safety Considerations

Wire sizing for voltage drop is only one aspect of cable selection. Current-carrying capacity (ampacity) based on insulation temperature rating, installation method (conduit, tray, buried), ambient temperature, and grouping factors must also be considered. Circuit breaker sizing must match wire ampacity to provide overcurrent protection. Always follow local electrical codes; NEC in the US, IEC in Europe, and national standards elsewhere. This calculator provides voltage drop-based sizing as a starting point. For critical or complex installations, consult a licensed electrical engineer. Enter your current, voltage, and cable length for an instant recommendation based on proven electrical engineering principles.

Wire Size Reference Table

Standard wire sizes follow the IEC 60228 series in metric countries and the AWG (American Wire Gauge) system in North America. Common cross-reference points: 1.5 mm² is approximately AWG 16, 2.5 mm² is AWG 14, 4 mm² is AWG 12, 6 mm² is AWG 10, 10 mm² is AWG 8, and 16 mm² is AWG 6. The steps are not doublings, which is a common misconception: each size is about 1.25 to 1.7 times the previous one, and the current rating climbs more slowly still. In the IEC 60364-5-52 method A1 table this calculator uses for PVC-insulated copper with two loaded conductors, 1.5 mm² is rated 14.5 A, 2.5 mm² 19.5 A, 4 mm² 26 A, 6 mm² 34 A, 10 mm² 46 A, and 16 mm² 61 A. Ratings change with installation method, ambient temperature and grouping, so read these as this calculator's basis rather than a universal table. The calculator always selects from the standard series, so you get a commercially available size rather than a theoretical minimum.

Energy Efficiency Impact

Voltage drop is energy converted to heat in the conductor, and the wasted power is simply current times the drop. On a 230 V circuit carrying 20 A, holding the drop to 2 percent (4.6 V) instead of 5 percent (11.5 V) cuts the loss from 230 W to 92 W. Running 8 hours a day for a year, that difference is about 403 kWh; scale it by your own duty cycle, because few circuits draw their full current for that long. Over a 20-year cable life the saving often exceeds the extra cost of the larger cable, which is why long runs, high-current circuits and 24/7 loads such as refrigeration, servers and pumps are the ones worth upsizing. The calculator reports both the voltage drop and the resistance, so you can run this arithmetic for any size.