Full Load Current Calculator

Parameters

Six fields drive the calculation. All six are always sent, which matters more than it sounds: the equipment type has built in default values for power factor and efficiency, but because the form always supplies its own numbers for those two fields, the defaults never take effect. Whatever you leave in the power factor and efficiency boxes is what gets used.

Equipment type

Motor, transformer, heater, lighting or general load. Unitless selection. It changes the calculation in two ways only. Choosing motor adds a starting current estimate and an extra compliance check. Choosing transformer makes the apparent power output echo your rating figure straight through as kilovolt-amperes rather than deriving it. Gotcha. The type does not change the full load current formula, and it does not override the power factor and efficiency you have typed in. Selecting heater does not silently set power factor to 1.0 for you.

Rating

The nameplate rating of the equipment, in kilowatts. Form range 0.1 to 10000, default 10. For a motor this is the mechanical shaft output, not the electrical input, which is why efficiency appears in the formula. Gotcha. The box is labelled kW but it is the only rating field on the form, so a transformer rated in kilovolt-amperes goes in here too. When you do that, set power factor to 1.0 and efficiency to 100 so the figure is not divided down a second time. See worked example 3.

Voltage

Supply voltage in volts. Form range 100 to 50000, default 400. For a three-phase selection this is the line-to-line voltage, normally 400 V nominal or 415 V as measured. For a single-phase selection it is the phase-to-neutral voltage, normally 230 V. Gotcha. The form does not change this value when you switch the phase type, so selecting single phase while 400 V is still in the box will calculate a 400 V single-phase circuit and quietly give you a current about 42 per cent too low.

Phase type

Single phase or three phase. Unitless selection. Single phase divides by voltage alone. Three phase divides by voltage multiplied by 1.7320508, the square root of three. Gotcha. The dropdown labels mention 230 V and 400 V for convenience, but the labels are cosmetic. The voltage actually used is whatever is typed in the voltage field.

Power factor

Displacement power factor at rated load. Dimensionless, form range 0.7 to 1.0, default 0.9. Typical values are 0.80 to 0.90 for a loaded induction motor, 1.0 for resistive heating, and 0.90 to 0.95 for modern LED lighting with a corrected driver. Sits in the denominator, so a lower power factor gives a higher current. Gotcha. Use the nameplate value where one exists. Motor power factor collapses at part load, so a motor running at 40 per cent of rating may sit near 0.6 rather than the 0.85 on its plate.

Efficiency

Ratio of mechanical output to electrical input, as a percentage. Form range 50 to 100, default 90. Modern IE3 motors run from about 85 per cent at 0.75 kW to about 96 per cent at 200 kW. Also sits in the denominator, so a less efficient machine draws more current for the same shaft output. Gotcha. Efficiency only belongs in the formula when the rating is a mechanical output. For heaters, lighting circuits, general loads quoted as electrical input, and transformers quoted in kilovolt-amperes, set this to 100 or the current will be overstated.

Assumptions and limits

What the tool assumes

• A balanced, sinusoidal supply. Three-phase results assume equal current on all three lines at fundamental frequency. Harmonic current from drives, rectifiers and switching supplies is not modelled, so the real conductor loading on a distorted supply will be higher than the figure shown.
• Steady state at rated output. The result is the current at the rating you typed in. It is not a demand figure, it carries no diversity, and it says nothing about duty cycle.
• Motor starting current is a flat six times full load current. That is a direct on line rule of thumb. Real locked rotor current varies roughly from four to eight times depending on the motor design code, and star delta, soft starter and variable speed drive starting all give very different figures. The starting current shown is an estimate only and the tool has no input for starting method.
• The apparent power output is not derived from the current. For non transformer types it is calculated as the rating divided by power factor and then multiplied by efficiency. That is not the same as the input kilovolt-amperes implied by the full load current result, and the two will not agree. Treat the full load current as the primary output and use the apparent power figure as a rough indication only.
• Breaker selection is a lookup, not a design. The tool returns the next standard rating at or above the full load current from the series 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500 and 630 A. It applies no motor starting allowance, no curve selection, no discrimination study and no short circuit rating check.
• The cable size is an indicative lookup, not a transcription of the standard. It is a simplified table built for copper conductors at a 75 degree Celsius insulation rating in a typical installation. It carries no derating for ambient temperature, grouping, installation method, insulation type or thermal insulation, and it does no voltage drop check at all, because the tool never asks for a route length. It must not be used as a cable size.

What the compliance checks actually check

Two checks always run, plus a third for motors. The first flags a full load current above 630 A, which is a practical ceiling built into this tool rather than a limit from AS/NZS 3000. The second confirms the selected breaker rating is at least the full load current, which is arithmetic on the tool's own output rather than an independent test. The third, for motors only, asks whether the selected breaker rating is at least 1.1 times the estimated starting current. Because starting current is fixed at six times full load current, that check fails for essentially every motor. It is flagging that a breaker sized on running current alone is not a motor starting study, which is true and worth knowing, but it is not evidence that the design is wrong.

What this must not be used for

• Final cable selection. Use AS/NZS 3008.1.1 with the actual installation method, ambient temperature, grouping and route length, or use the Cable Sizing calculator which asks for all of those.
• Motor protection design, starter selection, or overload relay settings.
• Maximum demand. Full load current is a per circuit figure with no diversity applied. Use the Maximum Demand calculator for a switchboard.
• Transformer inrush, through fault, or protection grading work.
• Any load with significant harmonic content, where conductor and neutral loading need a harmonic study.
• Evidence of compliance. This calculator is not validated or certified. Confirm every result against the equipment nameplate and the current editions of AS/NZS 3000 and AS/NZS 3008.1.1, and have it signed off by the person responsible for the installation.

Worked examples

Three examples covering a three-phase motor, a single-phase resistive load and a transformer. Every figure comes from the formulas the calculator runs, so you can reproduce them on the form above.

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.