Full Load Current Calculator

Full Load Current Guide

Full Load Current (FLC) is the steady-state current drawn by an electrical load when operating at its rated output. It is the starting point for almost every downstream design decision: cable sizing, circuit breaker selection, switchboard busbar rating, and maximum demand calculations. An incorrect FLC value cascades errors through the entire design, potentially resulting in undersized cables, nuisance-tripping, or overheating. This calculator determines FLC for motors, transformers, heaters, lighting circuits, and general loads based on the rated power, supply voltage, power factor, and efficiency.

The formulas follow standard electrical engineering practice as applied in AS/NZS 3000:2018 and AS/NZS 3008.1.1. For motors, the calculator accounts for both power factor and efficiency in the denominator, which is a common source of error when calculating by hand. For transformers, it uses the apparent power (kVA) rating directly, since transformer losses are already factored into the nameplate data.

Key concepts

• Three-phase FLC formula. For three-phase loads: FLC = P / (1.732 x V x PF x efficiency). The 1.732 factor (square root of 3) accounts for the phase relationship in a balanced three-phase system. For resistive loads like heaters, PF is 1.0 and efficiency is 1.0, simplifying the calculation.
• Power factor (PF). The ratio of real power (kW) to apparent power (kVA). Induction motors typically operate at 0.80 to 0.90 PF at full load. A lower power factor means the motor draws more current for the same mechanical output. PF varies with load; it drops significantly at light loads, which is why oversized motors waste energy.
• Efficiency. The percentage of electrical input power converted to useful mechanical output. Modern IE3 motors range from 85% (small, 0.75 kW) to 96% (large, 200 kW). Efficiency appears in the denominator, so a less efficient motor draws more current for the same output power.
• Nameplate current vs calculated FLC. Motor nameplates list the rated current at full load conditions. This should match the calculated FLC closely. If the nameplate is available, always use it for cable and protection sizing. The calculator is most useful when the nameplate is not available (design stage), when comparing equipment options, or when verifying nameplate data.

Common scenarios

1. Sizing a motor circuit at design stage. A mechanical engineer specifies a 15 kW pump motor at 400 V three-phase, but the motor has not been purchased yet and no nameplate data is available. Using default values of 0.85 PF and 0.90 efficiency, the calculator returns an FLC of approximately 28.4 A. The electrical designer uses this to select a 32 A circuit breaker and 6 mm squared cable (with derating checks via the Cable Sizing calculator).
2. Checking transformer secondary current. A 500 kVA three-phase transformer with a 400 V secondary needs busbar and cable sizing on the LV side. FLC = 500,000 / (1.732 x 400) = 722 A. This current determines the minimum busbar cross-section, the main switch rating, and the cable size from the transformer to the main switchboard.
3. Maximum demand calculation for a switchboard.An electrician needs the FLC of each circuit to build a maximum demand schedule for a commercial fit-out. The loads include a 15 kW three-phase oven (resistive, FLC = 21.7 A), a 7.5 kW exhaust fan motor (FLC = 14.2 A at 0.85 PF, 0.90 efficiency), and 4.8 kW of LED lighting (single-phase, FLC = 20.9 A per phase). The calculator provides each FLC value for the demand schedule.