GCC Solar Academy

Cable sizing should be treated as a multi-constraint engineering problem. The objective is not simply to find a conductor that carries the current. The selected cable must carry the design current under the actual installation conditions, keep voltage drop within the project criterion, tolerate the expected thermal environment and work with the protective device and termination system.

1. Establish design current

For DC PV circuits, the design current basis should come from the actual module/string/combiner arrangement and the governing design rules. For AC circuits, current can be calculated from power, voltage, phase configuration and power factor. For three-phase systems, I = P/(√3 × V × PF) when P is active power.

2. Apply correction factors

Base ampacity tables assume a particular installation condition. Real installations may require corrections for ambient temperature, grouping, installation method, thermal insulation, buried conditions or other project-specific factors. The corrected ampacity is therefore different from the catalogue base value.

3. Use a design margin deliberately

A design margin should not be used to hide uncertainty. State what it represents: continuous-load treatment, future margin, design practice or another documented requirement. Then apply the project or standard requirement consistently.

4. Check voltage drop

Even when a conductor passes ampacity, it may fail the voltage-drop criterion. This is common on long rooftop cable routes. Run both checks and select a conductor that satisfies the governing criteria together.

5. Copper versus aluminium

Aluminium has higher resistivity than copper for the same cross-sectional area, so the voltage drop and conductor size relationship differs. Aluminium may still be advantageous on larger feeders because of cost and weight, but terminations, lugs, installation practice and compatibility must be verified.

6. Temperature and rooftop PV

High ambient temperature increases conductor resistance and can reduce ampacity. PV designers should avoid blindly copying a cable size from a cooler climate. Use the actual project design temperature and the correction method required by the governing standard.

7. DC PV cable checklist

• Maximum system voltage and string Voc at minimum temperature.
• Design current and continuous current basis.
• Ambient temperature and cable operating temperature.
• Grouping and installation arrangement.
• Voltage-drop criterion.
• PV cable insulation, UV and environmental suitability.
• Connector and termination compatibility.
• Protection and short-circuit considerations.

8. AC cable checklist

• Single-phase or three-phase configuration.
• Active/apparent power and power factor.
• Design current and correction factors.
• Voltage drop and feeder length.
• Conductor and insulation temperature ratings.
• Protective-device coordination.
• Short-circuit withstand and installation method.

9. Practical selection sequence

1. Calculate design current.
2. Determine required corrected ampacity.
3. Select a candidate conductor from the approved table.
4. Check voltage drop at operating temperature.
5. Check protective device and fault withstand.
6. Check termination, cable route and installation environment.
7. Document the final selection and assumptions.