Undersized DC cables in solar PV systems are one of the leading causes of residential solar fires in Southeast Asia. The Philippine Electrical Code (PEC) 2017 — aligned with NEC Article 690 — has specific requirements for PV wiring. Most homeowners never check if their installer followed them.
Why DC Cable Sizing Is Different from AC
DC circuits carry continuous current — unlike AC circuits that have natural zero-crossings. PEC 2017 Section 690.8 requires PV source circuit conductors to be sized at 125% of the module's short-circuit current (Isc). Then apply a second 125% factor for continuous load. Combined: cables must handle 156% of rated Isc.
Formula: Cable ampacity ≥ Isc × 1.25 × 1.25 = Isc × 1.5625
Philippine Heat Derating
Cable ratings are based on 30°C ambient temperature. Philippine rooftop cable runs regularly reach 50-60°C, which significantly reduces ampacity.
Ambient Temp | XLPE Cable Derating Factor | Effect on 6mm² cable (47A rated) |
|---|---|---|
30°C (base) | 1.00 | 47A |
40°C | 0.91 | 42.8A |
45°C | 0.82 | 38.5A |
50°C | 0.71 | 33.4A |
Minimum Cable Sizes for Philippine Rooftop Solar
For a typical 5kW system with 550W panels (Isc ≈ 13.9A per string):
String cables (panel to combiner): Minimum 4mm² — 6mm² recommended for runs over 10m
DC main cable (combiner to inverter): Minimum 6mm² — 10mm² for runs over 15m
AC output cable (inverter to panel): Sized per load — minimum 6mm² for 5kW
Earthing conductor: Minimum 4mm² bare copper or green/yellow insulated
What to Ask Your Installer
"What cable size did you use for the DC strings?" — anything below 4mm² on a residential system is a red flag
"Is the cable rated for outdoor/UV exposure?" — USE-2 or PV Wire rated cable only
"Did you apply derating for ambient temperature?" — correct answer: yes
Use the SolarEnergyPH BOM tool to generate a proper Bill of Materials for your installation — cable sizes are automatically calculated based on your system specification.
Engr. Jason Morales — Founder, SolarEnergyPH