Cable Size Calculator
Select cable size from load current, voltage, and allowable voltage drop.
Calculator
No signup required. Results are indicative—verify for your standards.
Load current: 36.0 A
Minimum cable size: 6 mm² copper (ampacity 38 A)
Voltage drop: 9.6 V (2.3% of 415 V)
Formula
Load current I = P / (√3 × V × PF) for 3-phase. Voltage drop ΔV = (√3 × I × L × R) / 1000 for 3-phase, where R is cable resistance (Ω/km). Select cable so ΔV ≤ allowable (typically 3–5% of supply voltage).
Example calculation
22 kW motor, 415 V, PF 0.85, 3-phase: I = 22000/(√3×415×0.85) ≈ 36 A. For 50 m run, ΔV limit 5%: suggested 10 mm² copper cable (R = 1.83 Ω/km, ΔV ≈ 5.8 V = 1.4%). Derate for ambient and grouping to confirm ampacity.
Engineering notes
Two criteria govern cable selection: (1) Ampacity — the cable must carry the load current continuously without overheating. (2) Voltage drop — must not exceed 3–5% of supply voltage (IS/IEC limit). Always check both and select the larger conductor that satisfies both. Also verify short-circuit rating for protection coordination.
When to use this calculator
- MCC and panel design — size feeder and branch cables for motor control centres
- New plant electrical design — size cables for all electrical loads from the load list
- Energy audit — identify undersized cables causing excessive voltage drop and energy loss
- Motor installation — select the correct cable before purchasing and installing a new motor feeder
- Generator connection — size cables from generator to main switchboard for adequate fault current capacity
Frequently asked questions
- What is ampacity and how does installation method affect it?
- Ampacity is the maximum current a cable can carry continuously without exceeding its temperature rating. Ampacity depends on: conductor size and material, insulation temperature rating (70°C PVC, 90°C XLPE), and installation conditions. De-rating factors apply for: high ambient temperature (above 30°C), cables bunched together (grouping factor 0.6–0.9), cables in conduit, and burial depth. Always apply all applicable de-rating factors before comparing with load current.
- What voltage drop is acceptable for industrial installations?
- IS 732 and IEC 60364-5-52 recommend maximum voltage drop of 3% for lighting circuits and 5% for power/motor circuits from the supply origin to the load. For very long runs or sensitive equipment (VFDs, precision instruments), 2–3% is a better target. Excessive voltage drop causes: motor overheating (runs hotter at low voltage), reduced motor torque (torque ∝ V²), tripping on under-voltage protection, and poor power factor.
- Should I use copper or aluminium cables for industrial installations?
- Copper: higher conductivity (better ampacity per mm²), lighter, easier to terminate, more durable at connections, preferred for control cables and smaller power cables (up to 50 mm²). Aluminium: lower cost per ampere (50–60% cheaper than copper), lighter for large conductors, widely used for HT cables, feeders above 50 mm², and underground cables. For Indian industrial installations, copper is standard for motor feeders up to 240 mm², aluminium for large power cables and transformer HV connections.