Cable Sizing and Voltage Drop Calculation
Cable sizing: Cable size is set by three separate criteria: current capacity, voltage drop and short-circuit withstand.
ContentsCable sizing is not only about carrying the current. A correct size must satisfy three separate criteria at once: current carrying capacity, voltage drop and short-circuit withstand. Whichever criterion demands the largest conductor is the one that governs.
First criterion: current carrying capacity
Catalogue ratings apply to reference conditions: a stated ambient temperature, a single circuit and a defined installation method. Site conditions rarely match exactly, so correction factors are applied. Multiplying the factors gives the real capacity of the cable, and the circuit current must remain below that value.
| Correction factor | Typical range | Note |
|---|---|---|
| Ambient temperature | 0.71 – 1.12 | Relative to a 30 °C reference |
| Grouping (number of circuits) | 0.50 – 1.00 | Circuits side by side in a tray or trunking |
| Installation method | 0.80 – 1.00 | In conduit, on tray, buried in a wall |
| Soil thermal resistivity | 0.86 – 1.10 | For underground routes |
Second criterion: voltage drop
On long runs this is usually the governing criterion. Voltage drop depends both on conductor resistance and on route length; exceeding the limit reduces motor torque, causes flicker in lighting and instability in electronic equipment. The common approach is to stay within 3% on lighting circuits and 5% on power circuits.
Third criterion: short-circuit withstand
During a fault the conductor must carry the fault current until the protective device clears it, without the insulation exceeding its permitted temperature. The longer the clearing time, the larger the conductor required, so coordination settings and cable selection are decided together. See short-circuit withstand for detail.
Neutral and protective conductor sizes
Where harmonic content is high, third-harmonic currents add up in the neutral and it may carry more current than the phases. In that case the neutral is sized equal to or larger than the phase conductor. The protective conductor is derived from the phase size: equal up to 16 mm², 16 mm² up to 35 mm², and half the phase size above that, as a common rule.
A practical calculation order
Following a repeatable order reduces mistakes:
- Establish the design current (load, diversity, starting condition)
- Select the rating of the protective device
- Apply correction factors to find the required size
- Check voltage drop and increase the size if needed
- Verify short-circuit withstand
- Determine neutral and protective conductor sizes
On the busbar side of the assembly, busbar ampacity and derating applies the same logic.
Key takeaways: cable sizing
- First criterion: current carrying capacity — Catalogue ratings apply to reference conditions: a stated ambient temperature, a single circuit and a defined installation method.
- Second criterion: voltage drop — On long runs this is usually the governing criterion.
- Third criterion: short-circuit withstand — During a fault the conductor must carry the fault current until the protective device clears it, without the insulation exceeding its permitted temperature.
- Neutral and protective conductor sizes — Where harmonic content is high, third-harmonic currents add up in the neutral and it may carry more current than the phases.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Can aluminium cable be used?
Yes, and it is cost-effective in large sizes. However roughly one size step larger is needed for the same current, and terminations require the correct lugs and controlled torque.
From which point is voltage drop measured?
Common practice is to assess total drop from the origin of the installation to the most remote load. Drops across intermediate boards are added together.
Is a larger conductor always safe?
Electrically yes, but the lugs, terminals and cable entry openings must suit that size. Otherwise installation problems appear on the panel side.
Related articles
- Busbar Current Carrying Capacity and Derating Calculation
- Busbar Trunking or Cable? Choosing the Right Distribution Method
- Cable Zone and Cable Entry Design: Common Switchboard Mistakes
- All technical articles
Devpan reviews cable sizing and voltage drop together with the design of the cable compartment. Share your route lengths and load schedule and we will verify the calculation with you.
