Watts to Volts Calculator

Working backwards from real power and current to voltage is the check engineers run on motor and pump nameplates before signing off a new installation. A 15kW three-phase motor labelled at 400V should draw around 27A at full load, and the voltage seen at the motor terminals should sit close to 400V. If the calculated voltage drops below 380V, the cable is too long or too thin, the motor will run hotter than its rated insulation class allows, and the warranty position with the manufacturer is at risk.

Voltage drop is the silent cost of long cable runs in warehouses, factories and large commercial sites. BS 7671 limits voltage drop to 3% on lighting and 5% on other circuits. On a 100-metre run feeding a 22kW load, the difference between 6mm squared and 10mm squared cable is the difference between a compliant install and one that fails the next inspection. The watts to volts calculation gives the design engineer a fast way to validate the proposed cable size against the actual demand, before the cable is in the wall.

Purely Energy advises mid-market clients on the interaction between site electrical design and the supply contract. When a site adds 100kW of motor load on a long cable run, the supply transformer sees a different demand pattern, the power factor on the meter changes, and the next contract renewal needs to reflect that. We make sure the operational change shows up in the supply procurement so the client is not surprised by a tariff that no longer fits the operation it was negotiated for.