Cable Size Selection Guide: How to Select Cable Size in mm² for Any Load
Selecting the wrong cable size is one of the most common — and costly — mistakes in industrial electrical installations. This guide walks through the exact two-step method (ampacity + voltage drop) used by electrical engineers to select cable size in mm² for any load.
Cable selection is governed by two independent criteria: the cable must be able to carry the load current continuously without overheating (ampacity), and the voltage drop along the cable run must not exceed the allowable limit (typically 5% for motor circuits per IS 732). Both checks must be passed — the larger cable from either check governs the selection.
This guide uses IS 694 (PVC insulated cables up to 1.1 kV), IS 732 (code of practice for electrical wiring), and IEC 60364-5-52 as the reference standards. The method is applicable to any industrial installation: motor feeders, MCC incomer cables, lighting circuits, and power distribution panels.
Step 1: Calculate Load Current
For a 3-phase motor load: I = P / (√3 × V × PF × η)
Where: • P = motor shaft power in watts (W) • V = line-to-line supply voltage (415 V for standard Indian industrial supply) • PF = power factor (use 0.85 if unknown) • η = motor efficiency (use 0.90 for IE2 motors if datasheet unavailable)
For a 3-phase resistive load (heaters, furnaces): I = P / (√3 × V)
For a single-phase load: I = P / (V × PF)
Example: 22 kW (30 HP) motor, 415 V, PF 0.85, η 0.91: I = 22,000 / (1.732 × 415 × 0.85 × 0.91) ≈ 40 A
Step 2: Apply Derating Factors
IS 732 / IEC 60364-5-52 specify derating factors for installation conditions. The rated ampacity in cable tables assumes a standard reference condition (single cable in free air, 30°C ambient). Apply all applicable derating factors:
| Condition | Derating Factor | Notes |
|---|---|---|
| Ambient temperature 35°C | 0.94 | PVC insulation, 70°C rating |
| Ambient temperature 40°C | 0.87 | Common in Indian industrial environments |
| Ambient temperature 45°C | 0.79 | Engine rooms, outdoor in summer |
| 2 circuits touching (grouping) | 0.80 | Reduces to 0.70 for 3, 0.65 for 4 |
| In conduit (single circuit) | 0.77 | Restricted airflow |
| In conduit (2 circuits) | 0.65 | Combined grouping + conduit factor |
| XLPE insulation (90°C) | 1.00 to 1.25 | Higher rating than PVC — check IS 7098 |
Step 3: Select Cable from Ampacity Table
Select the smallest cable whose derated ampacity ≥ load current. The table below gives single-core PVC copper cable ampacity in free air at 30°C (IS 694 / IEC 60364-5-52, Method E):
| Cable Size (mm²) | Ampacity in Air (A) | Ampacity in Conduit (A) | Resistance (Ω/km) |
|---|---|---|---|
| 2.5 | 27 | 20 | 7.41 |
| 4 | 36 | 27 | 4.61 |
| 6 | 46 | 35 | 3.08 |
| 10 | 63 | 49 | 1.83 |
| 16 | 85 | 66 | 1.15 |
| 25 | 110 | 86 | 0.727 |
| 35 | 133 | 104 | 0.524 |
| 50 | 159 | 125 | 0.387 |
| 70 | 200 | 158 | 0.268 |
| 95 | 241 | 191 | 0.193 |
| 120 | 278 | 220 | 0.153 |
| 150 | 318 | 253 | 0.124 |
| 185 | 362 | 289 | 0.0991 |
| 240 | 424 | 338 | 0.0754 |
Step 4: Check Voltage Drop
For 3-phase circuits, voltage drop: ΔV = √3 × I × L × R / 1000 (Volts)
Where L is one-way cable length in metres and R is cable resistance in Ω/km from the table above.
Allowable voltage drop: • Lighting circuits: 3% of supply voltage (415 V × 3% = 12.45 V max) • Power/motor circuits: 5% of supply voltage (415 V × 5% = 20.75 V max) • VFD and sensitive equipment: 2–3%
Example for 22 kW motor: I = 40 A, selected 10 mm² cable, L = 50 m, R = 1.83 Ω/km: ΔV = 1.732 × 40 × 50 × 1.83 / 1000 = 6.34 V = 1.5% ✓ (well within 5%)
If the calculated ΔV exceeds the limit, upgrade to the next cable size and recheck.
Step 5: Verify Short-Circuit Rating
For final selection, confirm the cable can withstand the prospective short-circuit current for the clearing time of the upstream protection device:
Minimum cross-sectional area for fault withstand: A (mm²) = I_fault (kA) × √t / k
Where t is fault clearing time (seconds) and k = 115 for PVC copper, k = 143 for XLPE copper.
For most LT industrial panels, this check is satisfied if the cable is sized correctly for ampacity and the upstream MCB/MCCB has a fault clearing time below 0.1–0.5 seconds.
Common Cable Sizing Mistakes to Avoid
The most frequent errors seen in industrial electrical installations:
- Using nameplate kW without applying motor efficiency — actual input current is 5–10% higher than calculated from output power alone
- Ignoring derating for ambient temperature — a cable in a 45°C engine room has only 79% of its rated ampacity
- Not checking voltage drop for long runs — a cable that passes the ampacity check on a 100 m run may fail the voltage drop check
- Using the same cable size for a motor as for the overload relay setting — cable must be sized for full-load current, not the relay trip current
- Mixing copper and aluminium terminations without anti-oxidant compound — causes high-resistance connections that overheat
- Selecting cable for average load instead of maximum continuous load — the cable sees the peak current, not the average
Frequently Asked Questions
How do I select cable size for a motor in India?
Step 1: Calculate full-load current: I = P/(√3 × V × PF × η). Step 2: Apply derating factors (ambient temperature, grouping, installation method). Step 3: Select cable where derated ampacity ≥ load current. Step 4: Check voltage drop ΔV = √3 × I × L × R/1000 ≤ 5% of 415 V = 20.75 V. Step 5: Verify short-circuit rating. For a 22 kW motor at 415 V: FLC ≈ 40 A, typical cable 10 mm² copper XLPE for runs up to 80 m.
What is the minimum cable size for a 3-phase connection in India?
The minimum practical cable size for 3-phase power circuits in Indian industrial installations is 2.5 mm² copper, which handles up to about 20 A (derated for typical installation). For motors, 2.5 mm² is typically used for loads up to about 3.7 kW (5 HP) at 415 V. Smaller cables (1.5 mm²) are used for 230 V single-phase circuits and control wiring. IS 694 specifies PVC insulated cables from 1 mm² upward.
What is IS 694 and how does it relate to cable sizing in India?
IS 694 is the Bureau of Indian Standards (BIS) specification for PVC insulated cables rated up to 1.1 kV. It specifies conductor dimensions, insulation thickness, sheath thickness, and current-carrying capacities for standard conductor sizes. IS 732 is the code of practice for electrical wiring installations that specifies installation methods, voltage drop limits, and derating factors. Both are mandatory references for cable selection in Indian industrial electrical installations.
What size cable for a 415V 3-phase 7.5 kW (10 HP) motor?
For a 7.5 kW motor at 415 V, PF 0.85, η 0.89: FLC = 7500/(1.732 × 415 × 0.85 × 0.89) ≈ 13.8 A. In free air at 30°C, 2.5 mm² copper (27 A rated) is adequate for ampacity. For runs above 40 m at this current, check voltage drop: 50 m run, 2.5 mm² (R = 7.41 Ω/km): ΔV = 1.732 × 13.8 × 50 × 7.41/1000 = 8.86 V = 2.1% ✓. However, for outdoor/conduit installation with derating, upgrade to 4 mm² for margin.
How does voltage drop affect cable size selection?
For short cable runs (under 20 m), ampacity usually governs cable size. For long runs (50 m and above), voltage drop often requires a larger cable than ampacity alone. The voltage drop limit is 5% of supply voltage for motor circuits (20.75 V on a 415 V supply). To reduce voltage drop, you must use a larger conductor (lower resistance) or reduce current by improving power factor. As a rule of thumb, for runs above 50 m, voltage drop typically requires the cable to be 1–2 sizes larger than the ampacity check alone.