Correct cable sizing is essential for safe, efficient and dependable electrical installations. A properly selected cable helps limit voltage drop, avoid overheating, withstand fault conditions and reduce unnecessary energy loss.
Cable sizing is not a single calculation. It is a set of checks that need to be considered together, including load current, installation method, ambient conditions, voltage drop, short-circuit withstand and the applicable design standard.
Main cable sizing checks
Current-carrying capacity
Current-carrying capacity is the maximum current a cable can carry without exceeding its thermal limits. The selected cable needs to carry the design load current after allowing for installation conditions, grouping, ambient temperature and any required design margin.
Voltage drop
As current flows through a cable, voltage is lost due to conductor impedance. Voltage drop checks help confirm that the receiving-end voltage remains suitable for the connected equipment. Cable length, conductor material, conductor size, load current and power factor all influence the result.
Fault withstand
Fault withstand is the ability of the cable to tolerate the thermal and mechanical effects of short-circuit or earth-fault current until the protective device operates. The cable and protective device need to be coordinated so the cable is not damaged during fault clearance.
Key variables in cable sizing
- Conductor size and material: conductor area and material affect current rating, voltage drop, fault rating, cost and physical installation.
- Ambient temperature: high ambient temperatures reduce heat dissipation and can reduce current-carrying capacity.
- Installation conditions: ducts, conduits, tray, ladder, direct burial, grouping and thermal backfill all influence cable temperature.
- Cable insulation: insulation material determines the permissible operating temperature and affects current rating and fault withstand.
- Route length: longer routes usually make voltage drop and cable pulling more significant.
- Protection settings: overload and short-circuit protection need to coordinate with the selected cable.
- Standard cable sizes to IEC 60228 – the metric conductor size sequence used for IEC-based cable selection.
- BS – a practical worked example using BS 7671 tabulated voltage drop values.
- Cable – complex impedance example for phase and earth fault current at the load end.
- AC – temperature, voltage drop, power loss and indicative life calculations.
Effects of incorrect cable sizing
- Excessive voltage drop: poor equipment performance, reduced efficiency or nuisance operation.
- Overheating: insulation damage, reduced cable life and increased fire risk.
- Energy loss: unnecessary operational cost and reduced system efficiency.
- Equipment malfunction: unstable supply voltage can cause premature failure or unreliable operation.
- Insufficient fault withstand: cable damage during short-circuit or earth-fault conditions.
Related cable sizing topics
The original knowledge base includes several more detailed cable sizing topics. These are good candidates for migration as standalone technical posts, with this article acting as the category overview.
- Cable derating factors
- Cable sizing input data checklist
- Cable sizing standards
- Fictitious dimensions
- IEC 60287 current capacity of cables
- Dielectric loss in cables
- Sheath induced voltage and circulating current
- Economic optimisation of cable size
- Single-core armoured cable buried sizing
- Solar radiation effects
- Photovoltaic PV design
- Fault current calculations
- Circuit protective conductor (CPC)
- Earth fault loop impedance
- IEC 60909 fault calculations
- Network fault level
- Transformer secondary fault level
- Thermal withstand
- Voltage drop
- Complex power
- Cable impedance
- Cable inductance
- Cable reactance
- Conductor resistance
- Metric AWG conversion
- Electrical resistivity
- Electrical resistance
- Cable power loss
- Cable thermal analysis
- 8 steps to low voltage cable selection and sizing
- Parallel cables