Cable sizing for a home circuit is relatively straightforward: look up the current, pick a wire size from a table, and you are done. Industrial cable sizing is a different discipline entirely. Multiple cables run in the same conduit or tray, ambient temperatures in factories often exceed 40°C, motors draw 5–7 times their running current at startup, and fault levels are high enough to cause serious damage if cables are undersized.
This guide covers the complete process for sizing cables for industrial three-phase loads — from calculating full load current through derating factors, voltage drop, and protection coordination.
For three-phase motors and other three-phase loads, full load current (FLC) is calculated from the rated power, supply voltage, power factor, and efficiency:
FLC (A) = P ÷ (√3 × V × pf × η)
P = rated power in watts
V = line voltage (typically 415V in India)
pf = power factor (use 0.85 if not on nameplate)
η = motor efficiency (use 0.90 for standard motors if not specified)
Example: 7.5kW three-phase motor, 415V, pf = 0.85, η = 0.90
FLC = 7500 ÷ (1.732 × 415 × 0.85 × 0.90)
FLC = 7500 ÷ 549.7 = 13.6A
Always read the actual nameplate FLC and use it in preference to calculation
For non-motor loads (heaters, lighting panels, UPS feeds), simply use the actual measured or specified load current.
Cable current ratings published in IS 3961 (Part V for PVC insulated cables) are based on a reference ambient temperature of 30°C and cables installed in free air without adjacent cables. Industrial conditions almost always deviate from these assumptions, requiring derating.
Temperature correction factors (for PVC insulated cable rated at 70°C):
30°C ambient: factor = 1.00 (reference — no derating needed)
35°C ambient: factor = 0.94
40°C ambient: factor = 0.87
45°C ambient: factor = 0.79
50°C ambient: factor = 0.71
55°C ambient: factor = 0.61
Grouping factors (for cables in the same conduit, tray, or duct):
1 circuit (reference): factor = 1.00
2 circuits: factor = 0.80
3 circuits: factor = 0.70
4–5 circuits: factor = 0.65
6–7 circuits: factor = 0.60
8–9 circuits: factor = 0.52
Apply all relevant factors by multiplying them together. The required cable current rating in free air is then: Required rating = FLC ÷ Combined derating factor.
Scenario: 7.5kW three-phase motor in a factory with 45°C ambient temperature. The cable runs in a conduit with two other motor cables (3 circuits total in conduit). PVC insulated copper cable.
Step 1 — FLC = 13.6A (from calculation above)
Step 2 — Temperature derating at 45°C: 0.79
Step 2 — Grouping derating for 3 circuits in conduit: 0.70
Step 2 — Combined derating: 0.79 × 0.70 = 0.553
Step 3 — Required free-air rating: 13.6 ÷ 0.553 = 24.6A
Step 4 — IS 3961 ratings (PVC copper, free air): 2.5mm² = 21A, 4mm² = 28A
Select 4mm² copper: derated rating = 28 × 0.553 = 15.5A > 13.6A ✓
Verify 6mm² not needed: 4mm² gives 15.5A derated, FLC is 13.6A — margin = 14% ✓
Result: 4mm² PVC copper cable is correct for this installation. Note that selecting by free-air rating alone (2.5mm² = 21A > 13.6A) would have resulted in a 2.5mm² cable that is actually only capable of carrying 11.6A derated — less than the motor FLC of 13.6A. This is a thermal overload risk.
Standard induction motors draw 5–7 times their full load current during direct-on-line (DOL) starting. A 13.6A motor may draw 68–95A for 2–8 seconds at startup. This does not require the cable to be upsized for this current — the thermal mass of the cable is sufficient to handle this brief surge — but it does have two important implications.
Protection device must not trip on starting current — use motor-rated MCBs (Type D or motor protection switches), not standard Type B or Type C MCBs
Voltage drop during starting must be checked — excessive starting voltage drop prevents the motor from reaching running speed (see voltage drop calculation guide)
Soft starters and variable frequency drives (VFDs) limit starting current — if used, DOL starting current rules do not apply
Cable thermal withstand for starting is not a concern for cables sized for running current — the short duration of starting current is within cable thermal limits
Once the cable size is selected from thermal considerations, verify that the voltage drop is within IS 732 limits (5% for power circuits). Use the formula from the voltage drop calculation guide. If the run is long (typically over 50 metres for smaller cables), voltage drop often governs the cable size rather than current carrying capacity — and the cable may need to be upsized further.
The selected cable must be properly protected by the upstream device. In India, industrial circuits are typically protected by:
Motor protection switch (MPS): adjustable current setting, built-in overload and short-circuit protection — preferred for motors
MCCB (Moulded Case Circuit Breaker) with thermal-magnetic release: adjustable overload, fixed short-circuit
Fuse + overload relay combination: older design, still widely used in Indian industry
Key coordination rules:
Overload protection rating ≤ cable current carrying capacity (derated): protects cable from sustained overload
Short-circuit protection must interrupt any fault current before the cable's I²t withstand is exceeded (see short-circuit rating article)
Motor overload relay should be set to 1.05–1.10× motor FLC — not 1.25× (overcurrent, not overload)
For long cable runs, check that the minimum fault current at the cable end is still high enough to trip the protection device
For armoured cables installed underground, additional derating factors apply based on soil thermal resistivity, burial depth, and soil temperature. The method is set out in IS 3961 Part IV. As a conservative rule of thumb, apply an additional 0.80–0.85 derating factor for direct buried armoured cables compared to free-air ratings — but always verify with the IS 3961 tables for your specific conditions.
1. Calculate full load current (FLC) from rated power, voltage, pf, and efficiency
2. Determine derating factors: ambient temperature + grouping (× both together)
3. Divide FLC by combined derating factor → required free-air cable rating
4. Select cable from IS 3961 tables with free-air rating ≥ required rating
5. Verify voltage drop (IS 732 ≤ 5%) and short-circuit withstand — upsize if either fails
Always document your calculation assumptions (ambient temperature, grouping method, derating factors used) — this is required for IS 732 compliance documentation and for future maintenance reference.
