Cable Size Calculator: How to Calculate Cable Size for Electrical Installations
Selecting the correct cable size is critical for electrical safety and efficiency. Undersized cables overheat and can cause fires; oversized cables waste material cost. This guide explains the two-step calculation method — current capacity check plus voltage drop check — with worked examples for motors, panels, and distribution circuits.
Calculating the correct cable size is one of the most common tasks in electrical engineering and facility management. Every motor connection, distribution feeder, and lighting circuit needs a cable that is large enough to carry the current without overheating — and short enough in voltage drop to keep the equipment running correctly.
In India, cable sizing is governed by IS 694 (PVC insulated cables), IS 1554 (armoured cables), and the National Electrical Code (NBC 2016). Internationally, IEC 60364 and BS 7671 apply. The principles are the same regardless of standard: cable size must satisfy both current capacity and voltage drop criteria — and whichever gives the larger cable wins.
Two Checks Every Cable Size Must Pass
Every cable size selection must satisfy two independent criteria:
Check 1 — Current Carrying Capacity (Ampacity): The cable must carry the full load current continuously without exceeding the maximum conductor temperature (70°C for PVC, 90°C for XLPE). Standard ampacity tables give the rating for a reference condition (e.g. 3 cables in open air at 30°C ambient). Derating factors are applied for higher temperatures, underground burial, grouping, and conduit installation.
Check 2 — Voltage Drop: The cable must not allow the supply voltage to drop by more than the permitted amount from the source to the load. Excessive voltage drop causes motors to overheat, lights to dim, and electronic equipment to malfunction. IS/IEC standards allow maximum 5% voltage drop from the point of supply to the final circuit (typically 2.5% to the distribution board + 2.5% to the equipment).
Select the cable size that passes BOTH checks. Use whichever size is larger.
Step 1 — Calculate the Load Current
Before you can size the cable, calculate the full load current of the equipment:
For single-phase AC loads: I = P ÷ (V × PF)
For three-phase AC loads: I = P ÷ (√3 × V_L × PF × η)
Where: I = Full load current (Amperes) P = Load power (Watts) V = Supply voltage — 230 V for single-phase, 415 V line-to-line for three-phase PF = Power factor (use 0.85 for motors if nameplate not available) η = Motor efficiency (use 0.90 if not known)
Example: 15 kW, three-phase motor, PF = 0.86, η = 0.91: I = 15,000 ÷ (1.732 × 415 × 0.86 × 0.91) = 15,000 ÷ 562.4 = 26.7 A
Step 2 — Check Current Carrying Capacity (Ampacity)
Use the cable ampacity table (from IS 694 or manufacturer datasheet) to find the base current rating for your chosen cable size. Then apply derating factors.
Derated capacity = Base ampacity × k₁ × k₂ × k₃
Where: k₁ = Ambient temperature derating factor k₂ = Grouping derating factor (multiple cables together) k₃ = Installation method derating factor
| Cable Size (mm²) | PVC 3-core ampacity in air (30°C, IS 694) | XLPE 3-core ampacity in air (40°C) |
|---|---|---|
| 1.5 mm² | 15 A | 18 A |
| 2.5 mm² | 20 A | 25 A |
| 4 mm² | 27 A | 33 A |
| 6 mm² | 35 A | 43 A |
| 10 mm² | 48 A | 59 A |
| 16 mm² | 63 A | 77 A |
| 25 mm² | 84 A | 101 A |
| 35 mm² | 103 A | 125 A |
| 50 mm² | 124 A | 151 A |
| 70 mm² | 158 A | 192 A |
| 95 mm² | 191 A | 232 A |
| 120 mm² | 220 A | 267 A |
| 150 mm² | 253 A | 307 A |
| 185 mm² | 289 A | 351 A |
| 240 mm² | 341 A | 415 A |
Ambient Temperature Derating (k₁)
Indian ambient temperatures are often higher than the 30°C reference temperature used in IS 694 tables. Apply these derating factors for PVC cables:
| Ambient Temperature | PVC Cable k₁ | XLPE Cable k₁ |
|---|---|---|
| 30°C (reference) | 1.00 | 1.00 |
| 35°C | 0.94 | 0.96 |
| 40°C | 0.87 | 0.91 |
| 45°C | 0.79 | 0.87 |
| 50°C | 0.71 | 0.82 |
| 55°C | 0.61 | 0.76 |
Grouping Derating (k₂)
When multiple cables are installed together in a conduit, tray, or bundle, they cannot dissipate heat as effectively. Apply the grouping derating factor:
| Number of Cables Grouped | Derating Factor k₂ |
|---|---|
| 1 (reference) | 1.00 |
| 2 cables | 0.80 |
| 3 cables | 0.70 |
| 4 cables | 0.65 |
| 5 cables | 0.60 |
| 6 cables | 0.57 |
| 7–9 cables | 0.52 |
| 10–12 cables | 0.48 |
Step 3 — Voltage Drop Calculation
Once you have a candidate cable size from the ampacity check, verify the voltage drop:
Voltage drop (V) = I × L × R_cable × 2 ÷ 1000
Where: I = Load current (A) L = Cable length (metres — one way) R_cable = Cable resistance (mΩ/m at operating temperature, from datasheet) 2 = Multiply by 2 for the return conductor (phase + neutral or phase + phase)
Voltage drop percentage = (Voltage drop ÷ Supply voltage) × 100
Maximum permitted voltage drop per IS/IEC: - Lighting circuits: 3% from distribution board to luminaire - Power/motor circuits: 5% from point of supply to motor terminals
R_cable values (approximate, at 75°C operating temperature, copper conductor): 1.5 mm²: 14.5 mΩ/m | 2.5 mm²: 8.71 mΩ/m | 4 mm²: 5.45 mΩ/m | 6 mm²: 3.63 mΩ/m 10 mm²: 2.18 mΩ/m | 16 mm²: 1.36 mΩ/m | 25 mm²: 0.87 mΩ/m | 35 mm²: 0.627 mΩ/m 50 mm²: 0.463 mΩ/m | 70 mm²: 0.321 mΩ/m | 95 mm²: 0.232 mΩ/m | 120 mm²: 0.183 mΩ/m
Complete Worked Example: Motor Feeder Cable
Size the cable for a 22 kW motor: Supply: 415 V, 3-phase | Cable run: 80 m | PF: 0.87 | Efficiency: 0.92 Installation: 4 cables on cable tray | Ambient temperature: 45°C | Cable type: PVC 3-core copper
Step 1 — Full load current: I = 22,000 ÷ (1.732 × 415 × 0.87 × 0.92) = 22,000 ÷ 576.7 = 38.2 A
Add 25% motor service factor: Design current = 38.2 × 1.25 = 47.7 A (use 48 A)
Step 2 — Ampacity check: k₁ (45°C) = 0.79 | k₂ (4 cables grouped) = 0.65 Try 16 mm²: Base ampacity = 63 A Derated: 63 × 0.79 × 0.65 = 32.3 A — TOO LOW
Try 25 mm²: Base ampacity = 84 A Derated: 84 × 0.79 × 0.65 = 43.2 A — TOO LOW
Try 35 mm²: Base ampacity = 103 A Derated: 103 × 0.79 × 0.65 = 52.9 A ✓ (≥ 48 A)
Step 3 — Voltage drop check for 35 mm²: R = 0.627 mΩ/m at 75°C Voltage drop = 38.2 × 80 × 0.627 × 2 ÷ 1000 = 3.84 V As percentage of 415 V: 3.84 ÷ 415 × 100 = 0.93% ✓ (well within 5%)
Result: Select 35 mm² PVC 3-core armoured copper cable.
Cable Size Quick Reference Table
This table shows the maximum load (kW) a copper PVC cable can handle at 415 V 3-phase, with derating for 45°C ambient and 3 cables grouped. Cable run up to 30 m (voltage drop not critical at short lengths).
| Cable Size (mm²) | Derated Ampacity (45°C, 3 cables) | Max kW at 0.85 PF (415 V, 3-phase) |
|---|---|---|
| 4 mm² | 15 A | ~9 kW |
| 6 mm² | 19 A | ~12 kW |
| 10 mm² | 26 A | ~16 kW |
| 16 mm² | 34 A | ~21 kW |
| 25 mm² | 46 A | ~28 kW |
| 35 mm² | 56 A | ~35 kW |
| 50 mm² | 67 A | ~42 kW |
| 70 mm² | 86 A | ~53 kW |
| 95 mm² | 103 A | ~64 kW |
| 120 mm² | 119 A | ~74 kW |
Short Circuit Current Rating
In addition to continuous current and voltage drop, cables feeding switchboards and large distribution panels must be checked for short circuit current withstand. The cable must survive the maximum prospective short circuit current until the upstream protective device clears the fault.
Minimum cable cross-section for short circuit = I_sc × √t ÷ k
Where I_sc is the prospective short circuit current (kA), t is the fault clearance time (seconds), and k is a material factor (115 for PVC/copper, 143 for XLPE/copper).
For most final distribution circuits in Indian industrial installations, the upstream fuse or MCCB clears faults within 0.1–0.4 seconds. Verify with a prospective fault level study for large installations or when the utility supply point is close.
Use the Free Cable Size Calculator
Our free Cable Size Calculator automates both the current capacity check and the voltage drop check. Enter your load kW (or Amperes), supply voltage, cable length, installation method, and ambient temperature — and get the minimum required cable size in mm² instantly.
The calculator follows IS 694 / IEC 60364 derating principles and is suitable for motor feeders, distribution circuits, and general wiring design.