Motor Calculator

Motor Installation Guide for AS/NZS 3000:2018 and AS/NZS 1359

Designing a motor circuit involves more than just connecting the motor to a supply. The electrician must calculate the full load current, select an appropriate cable that handles both running current and voltage drop, choose a protective device that rides through starting inrush without nuisance tripping, and specify the correct starting method for the application. This calculator handles all of those steps from motor nameplate data. It is used whenever a new motor is being installed, an existing motor is being replaced with a different rating, or a circuit needs to be verified during commissioning or periodic inspection. The results comply with AS/NZS 3000:2018 for wiring rules and AS/NZS 1359 for motor ratings and performance characteristics.

Key concepts

• Full load current (FLC). The current the motor draws at rated power, voltage, power factor, and efficiency. For three-phase motors: FLC = (kW x 1000) / (1.732 x V x pf x eff). This value sets the baseline for cable sizing and protection. Nameplate values should be used for power factor and efficiency where available; otherwise, 0.85 and 0.90 are reasonable defaults for general-purpose induction motors.
• Starting current and protection coordination. During a direct-on-line start, the motor draws 5 to 7 times FLC for several seconds. The protective device must allow this transient without tripping, while still providing short-circuit and overload protection during normal running. Curve D MCBs or dedicated motor circuit breakers are designed for this characteristic. Undersized protection will nuisance-trip on every start; oversized protection may not clear an overload fast enough.
• Cable sizing for motor circuits. The cable must be rated for the motor FLC (not starting current) per AS/NZS 3008.1.1 current carrying capacity tables. Voltage drop at FLC must stay within 5% of nominal supply from the point of supply to the motor terminals. For long cable runs, voltage drop often governs and forces a larger cable than current rating alone would require.
• Supply authority coordination. Most Australian DNSPs set a maximum voltage dip at the point of common coupling, typically 4% for residential and 3% for commercial. Motors that cause voltage dip above this threshold during starting require supply authority approval and may need a reduced-voltage starting method even if the site electrical system can handle DOL starting.

Common scenarios

• Installing a 5.5 kW air conditioning compressor motor. The electrician enters the motor kW, voltage (typically 400 V three-phase), and starting method (usually DOL for this size). The calculator outputs the FLC (approximately 11 A), recommends a cable size based on the run length, and suggests a curve D MCB rating. If the cable run is long (over 30 m), the calculator may upsize the cable from the current-rated minimum to meet voltage drop limits.
• Replacing a pump motor with a larger unit on an existing circuit. Before swapping a 3.7 kW pump motor for a 5.5 kW unit, the electrician checks whether the existing cable and protective device can handle the higher FLC. The calculator determines the new FLC, checks the existing cable current rating and voltage drop, and flags whether the breaker needs to be upsized. This avoids the costly mistake of overloading an existing circuit.
• Specifying a star-delta starter for a 15 kW motor on a light commercial supply. A 15 kW motor started DOL would draw approximately 65 to 80 A of starting current, which may exceed the DNSP voltage dip limits on a 100 A supply. The electrician enters star-delta as the starting method, and the calculator shows the reduced starting current (approximately 22 A) alongside the required contactor and overload relay ratings for the star-delta switching arrangement.

Motor starting methods

• Direct-on-line (DOL): Highest starting current (5 to 7 times FLC). Suits small motors on robust supplies. Requires curve D breaker.
• Star-delta: Reduces starting current to approximately 2 times FLC. Requires switching equipment; reduced torque during start.
• Soft-starter: Electronic control allows gradual acceleration; approximately 3 times FLC. No torque loss.
• VFD: Soft acceleration, lowest starting current (approximately 1.5 times FLC). Most efficient; speed control included.