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.

Parameters

Every field this calculator accepts, what it means, the range it accepts, and how it feeds the result. The last four sit under Cable and Environmental but are always used.

Motor rating (kilowatts, 0.1 to 500)

Rated mechanical output at the shaft, as printed on the nameplate. It is not the electrical input power: the efficiency and power factor fields convert one into the other. A motor marked 5.5 kilowatts draws more than 5.5 kilowatts of electrical input.

Voltage (230, 400, 415 or 690 volts)

Only these four values are accepted. For three phase it is the line to line voltage. It divides into the full load current calculation and it is also the denominator for both voltage drop percentages, running and starting.

Phase (single phase or three phase)

Decides whether the square root of 3 appears in the full load current formula. Note that it does not change the voltage drop multiplier, which is fixed at 1 in this engine for both cases. See the limits below.

Efficiency (decimal fraction, form range 0.70 to 0.95, default 0.88)

Motor efficiency at full load as a decimal, not a percentage: enter 0.88, not 88. Use the nameplate or datasheet figure where you have it. Small motors sit around 0.75 to 0.82, and modern premium efficiency motors above 7.5 kilowatts sit at 0.90 to 0.94. Efficiency divides into the current, so a low value gives a higher current.

Power factor (decimal fraction, form range 0.70 to 1.00, default 0.85)

Displacement power factor at full load, again as a decimal. Typical induction motors run 0.80 to 0.90 at full load and much lower when lightly loaded. It divides into the current in the same way as efficiency. It is used only for the current; it does not appear in the voltage drop calculation.

Starting method (direct on line, star delta, soft starter, variable frequency drive)

Sets the starting current multiplier applied to full load current: 6 for direct on line, 2 for star delta, 3 for soft starter, 1.5 for a variable frequency drive. It also selects the breaker curve, D for direct on line and C for everything else. These multipliers are fixed defaults in the engine, not motor data; the underlying module accepts an explicit multiplier but this page always sends the default for the method.

Cable length (metres, 1 to 2,000, one way)

Route length from the starter or distribution board to the motor terminals, one way. Above 100 metres the tool raises a warning to check the drop and consider a drive. It affects the voltage drop figures only; it has no effect on the breaker, contactor or overload selection.

Conductor material (copper or aluminium)

Aluminium multiplies both the current rating and the conductor resistance by 0.78. The rating reduction is right in direction; the resistance reduction is not, since aluminium resists more than copper. Treat aluminium voltage drop from this tool as unreliable and check it against manufacturer data.

Ambient temperature (degrees Celsius, 0 to 50, default 40)

The only cable derating applied. The factor is the square root of 75 minus ambient over 75 minus 40. At 40 degrees it is exactly 1.000, at 45 degrees 0.926, at 50 degrees 0.845. A motor cable in a plant room or on a hot roof should be entered above 40.

Maximum voltage drop (percent, 1 to 10 in the form, default 5)

The running voltage drop limit the selected cable must meet, and one of the two conditions in the cable selection loop. The AS/NZS 3000 figure of 5 percent covers the whole installation, so on a submain fed motor enter only the allowance left for this final run. The starting voltage drop is checked separately against a fixed 15 percent guideline that you cannot change.

Assumptions and limits

This is a first pass sizing tool for a single motor circuit. Here is exactly what it assumes and where it stops.

What it assumes

• Full load current is derived, not read. It is the rating in watts divided by voltage, the phase factor, efficiency and power factor. Where the nameplate gives a current, use the nameplate. Real motors vary from the calculated figure.
• Cable selection ignores starting current. The first standard size whose derated capacity is at or above full load current and whose running voltage drop is inside the limit wins. Starting current is calculated and reported, but it never pushes the cable up a size.
• The voltage drop multiplier is fixed at 1 for both single phase and three phase. The conventional factors are 2 for single phase and the square root of 3 for three phase, so single phase drop reported here is about half of the true figure and three phase drop is about 1.7 times optimistic. Both running and starting percentages are affected. Re check the run in the voltage drop or cable sizing calculator before committing, particularly on a single phase motor.
• Resistance only, no reactance. Values are the engine placeholder direct current resistance at 75 degrees Celsius for copper, not transcribed from a licensed copy of AS/NZS 3008.1.1. Aluminium is scaled by the same 0.78 factor as the rating, which understates its resistance.
• One circuit, clipped direct, ambient derating only. Grouping is assumed to be 1.0. There is no installation method column, no thermal insulation, no burial correction.
• Starting multipliers are typical values, not motor data. Real direct on line locked rotor current runs 5 to 8 times full load current depending on the design code, and the star delta figure depends on the changeover arrangement and the load inertia.
• The breaker versus starting current check is misleading by design. It compares the breaker rating in amperes directly against the starting current, so a direct on line motor will almost always show a fail. That is not how motor protection is coordinated. A curve D device is chosen so that its instantaneous trip band, roughly 10 to 14 times rated current, sits above the inrush, and the time current curve is what decides whether it holds. Read that line as a prompt to look at the device curve, not as a defect in the design.
• Protection and switchgear selections are generic. The breaker is the next standard size at or above full load current from 6 to 160 amperes. The contactor is the next alternating current utilisation category 3 rating at or above full load current. The overload range is 0.8 to 1.15 times full load current. None of these comes from a manufacturer range, and there is no type 1 or type 2 coordination.

What it does not do

• It sizes a single cable run at full line current. A star delta connection uses six leads with each run carrying phase current, which is a different sizing problem the tool does not handle.
• No earth fault loop impedance, no disconnection time, no protective earthing conductor size, no prospective fault current or breaking capacity check.
• No duty cycle, starting time, load inertia or motor thermal withstand, so it cannot tell you whether the motor accelerates before the overload operates.
• No supply fault level and no voltage dip at the point of common coupling, which is what a distributor actually asks about for a large direct on line motor.
• Nothing specific to variable frequency drives: no screened cable requirement, no cable length limit for the drive, no rate of voltage rise at the motor terminals, no harmonic assessment and no input side filtering.

Check every result against the current editions of AS/NZS 3000, AS/NZS 3008.1.1 and AS/NZS 1359, against the motor and device datasheets, and have the design signed off by the person responsible for the installation. Nothing here is validated or certified, and the underlying figures have not been reviewed by a chartered professional engineer.

Worked examples

Three complete runs with every intermediate figure shown, so you can check the tool against your own arithmetic.