Motor Cable Size Calculator
Calculate motor cable size, running voltage drop, starting voltage drop, derating and feeder current for single-phase and three-phase motor installations.
⚠️ Practical note: This page is focused on motor feeder cable sizing. For general AWG/mm² conversion and wire reference, use the already built Wire Gauge AWG Calculator. For protection coordination, also use Breaker Size Calculator, Voltage Drop Calculator, Motor Current Calculator and Overload Relay Setting Calculator.
⚙️ Motor feeder: source → protection → cable → motor
Phase
Motor Power
Voltage
Power Factor
Efficiency
One-way Cable Length
Conductor Material
Installation Type
Total Derating Factor
Running VD Limit
Starter Type
Starting VD Limit
This mode estimates FLA from motor kW/HP, then selects the smallest standard cable that passes ampacity, running voltage drop and starting voltage drop checks.
Presets:
Phase
Motor FLA / Design Current
Voltage
One-way Length
Material
Cable Insulation
Total Derating Factor
VD Limit
Starting Multiplier
Starting VD Limit
Use this mode when you already know the motor nameplate FLA or the design current from another calculation.
Current presets:
Phase
Cable Size
Motor Current
Voltage
One-way Length
Material
Power Factor
Starting Multiplier
Running VD Limit
Use this mode to check an existing motor cable size for running and starting voltage drop.
Cable presets:
📐 Formula Reference
Three-phase Motor Current
I = P ÷ (√3 × V × PF × η)
Single-phase Motor Current
I = P ÷ (V × PF × η)
Three-phase Voltage Drop
ΔV ≈ √3 × I × R × L
Derated Ampacity
Allowed current = table ampacity × derating factor
📋 Quick Reference
Running Voltage Drop
Excellent≤2%
Common target≤3%
Review>5%
Starting Current
DOL5–7× FLA
Star-delta2–3× FLA
VFD1–1.5× FLA
Derating Causes
High ambientreduce
Grouped cablesreduce
Tray/conduitcheck
📚 Design Notes
Different from AWG referenceThis motor page checks ampacity, derating and motor starting voltage drop. For AWG conversion, use the Wire Gauge AWG Calculator.
Check both conditionsA cable can pass ampacity but fail voltage drop, especially for long motor feeders and high starting current.
Starting voltage mattersHigh starting voltage drop can reduce motor torque and cause long acceleration, nuisance trips or starter faults.
Final design needs local codeUse this as an engineering estimate. Final cable selection must follow local standards, installation method, temperature, grouping and protection coordination.
What is a Motor Cable Size Calculator?
A motor cable size calculator helps select a suitable cable size for motor feeders by checking full-load current, cable current capacity, voltage drop, installation derating and motor starting condition.
How to calculate cable size for a 3 phase motor
First calculate motor current from kW, voltage, power factor and efficiency. Then choose a cable whose derated ampacity is above the motor current and whose voltage drop is within the allowed limit over the cable length.
Why motor cable sizing is different from normal wire sizing
Motor loads have high starting current. A cable that is acceptable for running current may still cause excessive voltage drop during starting, especially with DOL starting and long cable runs.
Motor cable size and AWG wire size
This calculator recommends cable in mm² because many motor installations use metric cable sizes. For AWG comparison, open the Wire Gauge AWG Calculator.
❓ Frequently Asked Questions
Calculate the motor full-load current, apply installation derating to the cable ampacity, then check running voltage drop and starting voltage drop. The selected cable should pass all checks.
A common design target is around 3% during running. Starting voltage drop may be temporarily higher, often checked around 10–15%, but the acceptable value depends on standards, motor torque needs and equipment tolerance.
Nameplate full-load current is better whenever available. kW-based current is an estimate and depends on efficiency and power factor assumptions.
DOL starting current can be 5 to 7 times the full-load current. On long cable runs this can create high voltage drop, reducing starting torque and causing trips.
A VFD usually reduces starting current, so starting voltage drop is less severe. However, cable ampacity, shielding, EMC, motor lead length and VFD manufacturer guidance must still be followed.
No. Breaker size protects against short circuit and may be larger than motor FLA for starting. Cable sizing must check ampacity, derating, voltage drop and protection requirements.
Derating factor reduces cable current capacity due to ambient temperature, cable grouping, conduit/tray installation, soil condition or other installation factors.
Copper has lower resistance and usually needs a smaller cross-section. Aluminium is lighter and cheaper but usually requires a larger size and proper termination practice.
It depends on voltage, current, distance, installation and voltage drop limit. For a 415 V three-phase 15 kW motor, current is typically around 28–32 A, but the final cable may change with length and derating.
Yes, protective earth conductor sizing should follow local electrical standards and fault current requirements. Use a dedicated earthing conductor or fault-loop check where required.