Wire Ampacity Calculator

How it is calculated

The tool uses one multiplication: I = J × A, where A is the conductor's cross-sectional area (mm² for the usual metric wire sizes), J is the current density you choose for the installation (A/mm²) and I is the resulting current in amperes, shown to two decimals. There is no built-in table of materials, insulation ratings or ambient temperatures, so the density you enter carries all of that judgement: use a lower value for aluminium, bundled cables, warm locations or long continuous loads. Both inputs must be greater than zero; a zero or negative value gives no result. Cable standards differ from country to country, so treat the answer as an estimate and check the official ampacity tables for the real installation.

Common mistakes

• Entering the wire diameter instead of the cross-sectional area. A 1.8 mm round conductor is about 2.5 mm² in area, and typing 1.8 would underestimate the current by roughly a third.
• Mixing units, for example area in mm² with a density given per cm² or per circular mil. The tool just multiplies, so a unit mismatch gives a result that is off by a factor of 100 or more.
• Using the same density for every case. Aluminium, bundled conductors, high ambient temperature and enclosed conduit all need a lower density than a single copper wire in free air.
• Reading the result as the breaker rating. The breaker or fuse protects the cable and must be chosen against the cable's tabulated ampacity and the load, not simply set equal to this estimate.
• Ignoring voltage drop on long runs. A wire can be within its current capacity and still lose too much voltage over distance, which this tool does not check.