In this guide
A 2.5mm² wire feeding a 5kW motor "works" — until a summer afternoon pushes the panel past 60°C, the insulation softens, and a fault that should have been a tripped MCB becomes a melted terminal block. Cable sizing is not about what carries the current on a good day; it's about what carries the current safely on the worst day, for decades, while letting protection operate.
This guide gives you the practical calculation method used in real installations: current-carrying capacity with derating, voltage drop, and short-circuit/protection coordination — with worked examples you can adapt to your project.
The three checks every cable must pass
A correctly sized cable satisfies all three of these. Students usually do only the first.
- Thermal (current-carrying capacity): the cable's derated current rating Iz must be ≥ the design current Ib, and protection must coordinate: Ib ≤ In ≤ Iz, where In is the protective device rating.
- Voltage drop: ≤ 5% from origin to load (the IEC 60364 / IS 732 guideline for most installations; some utilities require ≤ 3% to the meter). Motors misbehave — high starting current, torque loss — on undervoltage.
- Fault withstand (adiabatic): under short circuit, the cable must survive until the protective device clears: I²t ≤ k²S², where k depends on conductor/insulation (115 for copper/PVC, 143 for copper/XLPE) and S is cross-section in mm².
Warning: Cable sizing calculations assume correct installation — terminations torqued, no damaged insulation, proper glands. A perfectly calculated cable with a loose lug is still a fire. All mains cable work (stripping, terminating, glanding) should be done de-energized and checked under supervision.
Step 1: find the design current Ib
| Load | Formula |
|---|---|
| Single-phase resistive | Ib = P / (V × pf) |
| Single-phase motor | Ib = P / (V × pf × η) |
| Three-phase | Ib = P / (√3 × V_L × pf × η) |
Use the motor's full-load current from its nameplate, not a formula estimate, when it's available. Include efficiency η and power factor pf honestly — a 5kW, 400V, 0.85pf, 88% efficient motor draws Ib = 5000/(1.732 × 400 × 0.85 × 0.88) ≈ 9.65A.
Step 2: apply derating to get Iz
Published current ratings assume ideal conditions (30°C ambient, single circuit in free air or buried per the table's reference method). Reality derates:
Iz = I_table × Ca × Cg × Ci
- Ca — ambient temperature factor. PVC at 45°C ambient: ~0.79; at 50°C: ~0.71. Indian rooftops and plant rooms routinely hit 45–50°C.
- Cg — grouping factor. Four circuits bunched in one tray: ~0.65. Six: ~0.57. This is the factor students forget most — a cable perfect alone overheats in a bundle.
- Ci — thermal insulation factor (cable surrounded by insulation: 0.5 or worse).
Worked example: The 9.65A motor feeder runs in a tray with 5 other circuits (Cg = 0.6) through a 45°C plant room (Ca = 0.79). Required table rating = 9.65/(0.79 × 0.6) ≈ 20.4A. A 2.5mm² copper/PVC cable rated ~24A in the reference method just passes thermally; 4mm² (~32A) gives comfortable margin and is the common practical choice.
Reference current ratings (copper, PVC, 3-core, clipped direct / in conduit — always verify against your cable manufacturer's table to IS 694 / IEC 60364-5-52):
| Size | Approx. rating (A) |
|---|---|
| 1.5mm² | 15–18 |
| 2.5mm² | 21–24 |
| 4mm² | 28–32 |
| 6mm² | 36–41 |
| 10mm² | 50–57 |
| 16mm² | 66–76 |
Note: These are indicative — the exact figure depends on the installation method (conduit, tray, buried, free air), insulation (PVC vs XLPE runs 90°C vs 70°C), and number of loaded cores. Always design from the manufacturer's tabulated data, and document which table you used in your report.
Step 3: check voltage drop
ΔV ≈ √3 × I × L × (R cosφ + X sinφ) (three-phase; drop the √3 for single-phase)
For small cables, resistance dominates; use the tabulated mV/A/m value:
ΔV = (mV/A/m) × Ib × L / 1000
Typical mV/A/m (copper, from standard tables): 1.5mm² ≈ 29, 2.5mm² ≈ 18, 4mm² ≈ 11, 6mm² ≈ 7.3, 10mm² ≈ 4.4.
Worked example: The 9.65A motor, 60m run, 4mm² cable: ΔV = 11 × 9.65 × 60/1000 ≈ 6.37V → 1.6% of 400V. Passes comfortably. But the same motor on 2.5mm² over 120m: 18 × 9.65 × 120/1000 ≈ 20.8V → 5.2% — fails the 5% limit, so 4mm² is required by voltage drop even though 2.5mm² passed thermally. Long runs are voltage-drop-driven; short runs are thermally-driven. Check both, size for the worse.
Motor starting deserves a separate look: a DOL-started motor pulls 6–7× FLC for a second or two. The transient dip should stay within ~10–15% at the motor terminals or the contactor may drop out and the motor may stall — one more reason long motor feeders get upsized.
Step 4: coordinate protection (Ib ≤ In ≤ Iz)
Choose the MCB/MCCB rating In between design current and cable rating: for our 9.65A motor on 4mm² (Iz ≈ 32A derated-real ~15A… wait, recompute: 32 × 0.79 × 0.6 ≈ 15.2A). So we need 9.65 ≤ In ≤ 15.2 → a 10A or 12A device; standard choice 10A with motor-duty (type D or a motor-protection MCCB/overload relay handling the starting transient — an MCB alone is not motor overload protection).
Note the subtlety students miss: the derated Iz (15.2A) is what coordinates with protection, not the 32A table value. Protection coordinates with the cable as installed.
Step 5: fault withstand (the adiabatic check)
t = (k × S / I_fault)² must exceed the device's clearing time, or equivalently check I²t ≤ k²S².
Worked example: 4mm² copper/PVC (k = 115), prospective fault current 1.5kA at the far end, MCB clears in 0.1s. Cable withstand: t_max = (115 × 4/1500)² ≈ 0.094s. That's marginal — 0.094 < 0.1. Options: upsize to 6mm² (t_max ≈ 0.21s ✓), or verify the actual let-through I²t from the MCB's datasheet (current-limiting MCBs often pass this check where the crude calculation fails). Document whichever route you take.
Earth conductor sizing
The protective earth (PE) is sized from the phase conductors per IEC 60364:
| Phase conductor S | Minimum PE |
|---|---|
| S ≤ 16mm² | S (same as phase) |
| 16 < S ≤ 35mm² | 16mm² |
| S > 35mm² | S/2 |
Or calculate adiabatically with the same k²S² formula using the actual fault current and disconnection time. For our 4mm² feeder: 4mm² PE.
Common mistakes
- Sizing from the table rating without derating — the cable is perfect on paper and overloaded in a 48°C tray with five neighbors.
- Checking thermal but not voltage drop — the classic long-run failure; motors that won't start properly 100m from the panel.
- Coordinating protection with the un-derated Iz — the MCB protects a cable that doesn't exist (the one in free air at 30°C).
- Using FLC from a formula instead of the nameplate — nameplate first, always.
- Forgetting starting current on DOL motors — thermal sizing is for running current; starting needs the voltage-dip check and correct protective device characteristic.
- Undersized neutral on feeders with heavy single-phase non-linear loads — see the harmonics guide; triplens add in the neutral.
Quick selection checklist
- Ib from nameplate (or P/(√3·V·pf·η))
- Derating: ambient × grouping × installation method → required table rating
- Pick size: table rating ≥ required
- Voltage drop ≤ 5% (≤ 3% if utility requires to meter)
- Ib ≤ In ≤ Iz(derated); motor starting handled by device characteristic + overload relay
- Adiabatic fault check or MCB let-through I²t verification
- PE sized per table or calculation
- Cable manufacturer's table referenced in the report
Where to go from here
- MCB vs MCCB vs ELCB Selection — choosing the In that coordinates with your Iz.
- Single-Phase vs Three-Phase Motors — the motor FLC values your feeder sizing starts from.
- Three-Phase Wiring Basics — neutral and phase conductor roles in the installation.
- DOL vs Star-Delta Starter — starting current, the transient your voltage-drop check must survive.
- More installation design in the Electrical branch hub.