Selecting the correct power cable ensures electrical safety and system efficiency. A properly sized cable must carry continuous load current, prevent excessive voltage drop, and withstand short-circuit faults. To size a power cable correctly, use the following six-step methodology based on international standards such as IEC 60364-5-52 or BS 7671.
1. Calculate the Design Current (Ib)
Determine the continuous full-load current in Amperes.
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(Where P is power in Watts, V is system voltage (e.g., 230V or 415V), and cos(phi) is the power factor).
2. Determine Protective Device Rating (In)
Select a standard Overcurrent Protective Device (OCPD)—such as a fuse or circuit breaker—rated higher than or equal to the design current (In > Ib). For continuous loads, the protection device is typically rated at 125% of the design load.
3. Apply Derating (Correction) Factors
Environmental and installation conditions affect a cable’s ability to dissipate heat. The cable's required base current capacity (Iz) must account for these.
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- Ca: Ambient Temperature (derating if operating above standard temperatures, e.g., 30° C).
- Cg: Grouping (derating when multiple cables are bunched together in a tray or conduit).
- Cs: Soil Thermal Resistivity (applicable for underground cables).
- Ci: Thermal Insulation (derating if the cable is enclosed in building insulation).
4. Check for Voltage Drop Limitation
Electrical resistance over long cable runs causes a voltage drop. It is critical to ensure the voltage drop at the load does not exceed permissible limits (usually 2.5% - 3% for final sub-circuits and 4% - 5% for main feeders/distribution boards).
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(or using tabular millivolt per Ampere-meter (mV/A-m) ratings).
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5. Verify Short-Circuit Thermal Withstand
A cable must be able to withstand prospective fault currents during a short circuit without exceeding its maximum temperature rating.
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(Where Smin is minimum cross-section in mm², Isc is short-circuit current in Amps, t is fault clearing time in seconds, and k is a constant based on the conductor and insulation type; e.g., k=115 for PVC, k=143 for XLPE).
6. Select the Final Cable Size
Select the standard cable cross-sectional area (mm²) that satisfies all constraints (Current Capacity ≥ Derated Capacity, permissible Voltage Drop, and Short-Circuit Capability).
Standard Cable Size Reference Chart (Copper, PVC/XLPE)
Approximate values; always consult the manufacturer’s specific current-carrying capacity charts for your region.
| Cable Size (mm²) | Max Current Capacity (In Air, ≈ 30°C) | Typical Application |
|---|---|---|
| 1.5 | 15 A – 17.5 A | Lighting circuits, ceiling fans |
| 2.5 | 20 A – 24 A | General power outlets, small appliances |
| 4.0 | 25 A – 32 A | Heavy appliances (water heaters, AC units up to 1.5 HP) |
| 6.0 | 35 A – 41 A | Larger AC units, electric cookers, ovens |
| 10.0 | 50 A – 57 A | Main distribution board sub-feeds, electric showers |
| 16.0+ | 68 A+ | Whole-house main supply, commercial, and industrial loads |
- The power load in kW or Amps
- The cable run length in meters
- The installation method (e.g., in a conduit, buried, or in free air)
GEASB can help you estimate the exact conductor cross-section and check for voltage drop.
