DC cable sizing and routing
Solar Pv · Lesson 27 · Advanced
Purpose
Size and route PV DC cable for credible current, voltage, environment, loss and fault conditions while preserving connector compatibility.
Learning objectives
- Identify the governing boundary
- Trace energy and measurement paths
- Apply evidence in the required sequence
- Recognise unsafe assumptions
PV conductors can remain live whenever modules are illuminated. Cable selection must combine insulation/system-voltage rating, current capacity, temperature, UV/weather exposure, mechanical protection and the array’s fault behavior.
Core theory
Establish maximum circuit current including parallel/reverse-current possibilities and correction factors for grouping, ambient/roof temperature and installation method.
Verify voltage drop against the project energy/design allocation; route positive and negative conductors together, minimise loop area, protect edges and segregate/support them for the environment.
Use exact compatible connector pairs and manufacturer tools. Mixed ‘MC4-compatible’ brands are not proven compatible; do not unplug under load. Check polarity and labels before connection.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| PV cable | Cable with declared DC voltage, environmental and installation suitability | Not applicable | Not applicable |
| Reverse current | Current driven into a faulted string by parallel sources | Not applicable | Not applicable |
| Loop area | Area enclosed by outgoing and return conductors | Not applicable | Not applicable |
Assumptions: Values are supplied design inputs, not universal limits; Current DNO and product evidence will be checked.
- Loop resistance: Rloop = 2 × 25 × 0.005 = 0.25 Ω.
- Drop: ΔV = IR = 11 × 0.25 = 2.75 V.
- Loss: Ploss = I²R = 11² × 0.25 = 30.25 W; compare with the allocated budget and all ratings.
Illustrative drop is 2.75 V and conductor loss is 30.25 W at the stated point.
Reasonableness check: The conclusion follows the connection point, aggregate plant and declared equipment rather than a marketing label.
- Treating 3.68 kW as a timeless universal threshold
- Assessing each inverter separately instead of aggregate behavior at the connection point
- Confusing a portal reading with settlement-grade metering
Where this appears in practice
A complete PV installation coordinates mechanical support, DC design, network permission, export control, metering, inspection, testing and handover.
Knowledge check
Can differently branded look-alike PV connectors be treated as compatible?
No. Use the exact approved connector pairing, tooling and assembly instructions.
Answer: No. Use the exact approved connector pairing, tooling and assembly instructions.
Practical exercise
Prepare a DC cable schedule covering current, voltage, temperature, route, drop, fault current, connectors and labels.
Summary
- Use the current rule and exact product record
- Assess the whole installation at the connection point
- Commissioning evidence is broader than successful generation
Sources and review
- Engineering Recommendation G98: Energy Networks Association; Current issue and amendment must be confirmed; Great Britain
- Engineering Recommendations G99 and G100 / Type Test Register guidance: Energy Networks Association; Current issue and Connect Direct status must be confirmed; Great Britain
- IEC 62446-1:2016: PV systems: documentation, commissioning tests and inspection: IEC; 2016 + AMD1:2018; International
- Health and safety in roof work (HSG33): HSE; Fifth edition; Great Britain
Editorial review date: 2026-08-22. Professional electrical review is pending.