Motor circuit calculator
Cable, breaker, contactor and overload for one motor.
Reads AS/NZS 3008.1.2:2017 (NZ), AS/NZS 3000:2018 and AS/NZS 3008.1.1:2025 (AU). Tables checked 1 Oct 2026.
Try a job 4
Each job fills in example values. Replace them with your values.
- Kit out a pump motor circuitCable, breaker, contactor and overload for a 7.5 kW pump 40 m away.
- Kit a star-delta starter for a fanThe three contactors and the relay for a 37 kW fan.
- Size a soft starter circuitA 22 kW conveyor motor on a soft starter limited to 3 × FLC.
- Kit a single-phase motor circuitA 2.2 kW compressor at 230 V, 25 m from the board.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Shaft power
- 15 kW
- Phases
- Three phase
- Voltage
- 400 V
- Power factor
- 0.85
- Efficiency
- 0.9
- Locked-rotor current
- 0 × FLC
- Starter
- Direct on line
- Circuit-breaker type
- Curve D
- Circuit-breaker rating
- 0 A
- Standard
- NZ · AS/NZS 3008.1.2
- Route length
- 30 m
- Installation method
- Enclosed in air
- Conductor
- Copper
- Insulation
- PVC 75 °C
- Ambient temperature
- 30 °C
- Circuits grouped
- 1
- Ze at the board
- 0.35 Ω
- Runs unattended
- No
- Fire-protection service
- No
Cable and circuit-breaker
16 mm² on 32 A type D
- Full load current
- 28.3A
- Starting current
- 226.4A
- Main contactor, AC-3
- 32A
- Overload relay setting
- 28.3A
- Overload relay range
- 23 to 32 A
- Isolating switch, AC-23, at least
- 28.3A
This result is a design aid. Verify it against the current standard.
Calculation steps
- Full load current28.3A
- Locked-rotor current226.4A
AS/NZS 3000:2018 Cl 4.13.1.2(i)
- Starting current, direct on line226.4A
- Circuit-breaker, type D32A
- Low end of the instantaneous band (generic)320A
- Main contactor, AC-3, at least28.3A
- Overload relay setting28.3A
- Isolating switch rating, at least28.3A
AS/NZS 3000:2018 Cl 4.13.1.2
- Overload protectionRequired
AS/NZS 3000:2018 Cl 4.13.2
- Overtemperature protectionRequired
AS/NZS 3000:2018 Cl 4.13.3.1(b)
- Ambient derating factor1x
AS/NZS 3008.1.2:2017 Tables 27(1), 27(2) p.88
- Grouping derating factor1x
One circuit, no grouping factor
- Soil derating factor1x
AS/NZS 3008.1.2:2017 Table 29 p.90
- Burial depth derating factor1x
AS/NZS 3008.1.2:2017 Tables 28(1), 28(2) p.89
- Total derating factor1x
- Required tabulated current28.3A
AS/NZS 3000:2018 Cl 3.4
- Selected conductor size16mm²
AS/NZS 3008.1.2:2017 Table 13 p.62–64
- Derated current capacity66A
AS/NZS 3000:2018 Cl 3.4
- Voltage drop0.45%
- Device rating In32A
AS/NZS 3000:2018 Cl 2.5.3.1
- Earth conductor size6mm²
AS/NZS 3000:2018 Table 5.1
- Earth fault loop impedance at 230 V phase to earth0.5Ω
AS/NZS 3000:2018 Table 8.1
About this calculator
How this is calculated
The tool sizes the circuit for one motor: the cable, the circuit-breaker, the contactor, the overload relay and the isolating switch. It reads AS/NZS 3000:2018 Cl 4.13 and sizes the cable with the cable size tool.
- Full load current. It divides the shaft power by the supply factor, the power factor and the efficiency. This is the same figure as the motor tool.
- Locked-rotor current. It takes the maker's multiple of full load current. With none, it takes 8 times full load current (Cl 4.13.1.2(i)).
- Starting current. Direct on line, the start equals the locked-rotor current. Star-delta gives a third of it. A soft starter gives the limit you enter. A drive gives about 1.5 times full load current (generic).
- Circuit-breaker. It picks the smallest standard rating at or above full load current. The low end of its instantaneous band must clear the start. The bands are generic: type B 3, type C 5 and type D 10 times the rating.
- Cable. The cable size tool sizes it with that breaker. The cable current capacity must be at least the breaker rating. It also checks the voltage drop and the earth loop with the Ze you type.
- Contactor and overload relay. It picks an AC-3 frame and a relay range from generic lists. Direct on line and soft start use full load current. Star-delta uses full load current divided by √3 for the main contactor, the delta contactor and the relay. The star contactor uses full load current divided by 3.
- Isolating switch. It must carry full load current and break the locked-rotor current. An AC-23 switch at or above full load current does both (Cl 4.13.1.2).
- Motor protection. Overload protection applies above 370 W (Cl 4.13.2). Overtemperature protection applies above 2250 W, or above 240 VA for a motor that runs unattended (Cl 4.13.3.1). A motor on a fire-protection service has neither (Cl 4.13.3.2).
| Step | Clause or source | What it decides |
|---|---|---|
| Full load current | Motor tool | The running current |
| Locked-rotor current | Cl 4.13.1.2(i) | The current at standstill |
| Starting current | Starter type, generic | The current the device must ride through |
| Circuit-breaker | Generic bands | The rating and the type |
| Cable | Cable size tool | The conductor size |
| Contactor and relay | Generic frames and ranges | The AC-3 frames and the relay setting |
| Isolating switch | Cl 4.13.1.2 | The AC-23 rating |
| Motor protection | Cl 4.13.2 and Cl 4.13.3 | Overload and overtemperature protection |
Worked example
- 15 kW, 400 V, three-phase pump, direct on line, 30 m, type D. The tool gives a full load current of 28.3 A and a locked-rotor current of 226.4 A. The result is a 16 mm² cable on a 32 A type D circuit-breaker. The contactor frame is 32 A. The relay setting is 28.3 A in the 23 to 32 A range. The isolating switch must be 28.3 A or more at AC-23.
Limits
- The contactor frames, relay ranges, breaker bands and the soft starter and drive starting currents are generic. They are not from a standard. Check the maker's catalogue and coordination tables.
- The locked-rotor figure of 8 times full load current is a default. Use the maker's figure when you have it.
- The cable is sized to the breaker rating. Where the overload relay protects the cable, a smaller cable can be allowed.
- Star-delta needs six cables to the motor. Each carries full load current divided by √3.
- A drive can draw a different input current. Size the supply from the drive data sheet.
- The tool does not check the prospective fault current or the breaker breaking capacity.
See also: Motor circuit for a 15 kW pump: DOL or star-delta, Motor starters, contactors and overload relays, Motor circuits and starting, the motor calculator and the cable size calculator.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
What does the motor circuit tool give me?
One result for one motor: the full load and locked-rotor current, the starting current, the cable, the circuit-breaker, the contactors, the overload relay setting and range, and the isolating switch rating.
Why does it pick a type D circuit-breaker?
A direct-on-line start draws several times full load for a few seconds. A type D breaker trips instantly only well above its rating, so it rides through the start. The tool raises a C type rating until it clears the start.
How is a star-delta starter sized?
The main and delta contactors and the overload relay carry the phase current, full load divided by √3. The star contactor carries a third of full load. The six cables to the motor each carry the phase current too.
Where does the locked-rotor current come from?
Enter it from the maker or the code letter. Without it, the tool takes eight times full load, the figure AS/NZS 3000 Cl 4.13.1.2 gives for an a.c. motor. The isolating switch must break it.
Are the contactor and relay sizes from a standard?
No. The frame sizes, relay ranges and breaker bands are generic figures. They show the class of part to look for. Check the maker’s catalogue and its coordination tables before you buy.
Clauses and tables this tool reads
- AS/NZS 3000:2018 Cl 4.13.1.2Rating of a motor switch
- AS/NZS 3000:2018 Cl 4.13.2Overload protection of a motor
- AS/NZS 3000:2018 Cl 4.13.3.1Overtemperature protection of a motor
- AS/NZS 3000:2018 Cl 4.13Motor switching, overload and overtemperature figures
- AS/NZS 3000:2018 Table 8.1Maximum earth fault loop impedance multipliers by MCB curve
- AS/NZS 3008.1.2:2017 Table 13Current ratings, multicore cable with three or four cores, thermoplastic 75 °C
- AS/NZS 3008.1.1:2025 Table 3.18Current ratings, multicore cable with three or four cores, thermoplastic 75 °C
Every clause and table, with how each was verified
Tables last checked on 1 Oct 2026.
Related tools
Was this useful?
A question, or a value that looks wrong? Contact us.