Cable sizing is one of the most commonly cut-corner areas in Philippine solar installations. Installers use undersized cables to reduce material cost — the result is overheating, voltage drop, reduced efficiency, and in worst cases, electrical fires.
Every solar cable must satisfy both of these:
The maximum DC current per string = panel Isc × 1.25 (safety factor per PEC)
Example: JA Solar 550W panel, Isc = 13.83A
Design current = 13.83 × 1.25 = 17.3A
| Cross-section | Ampacity (outdoor, 40°C) | Common Application |
|---|---|---|
| 2.5 mm² | ~20A | Short runs (<10m), low-current strings |
| 4 mm² | ~27A | Standard residential DC string cable |
| 6 mm² | ~34A | Long runs (>20m) or parallel strings |
| 10 mm² | ~46A | Main DC trunk cable in commercial systems |
For most 5-10kW Philippine residential systems, 4 mm² solar DC cable is standard.
Voltage Drop = (2 × Length × Current × Resistivity) ÷ Cross-section
Where resistivity of copper = 0.0172 Ω·mm²/m
Example: 4mm² cable, 15m run, 13.83A current:
VD = (2 × 15 × 13.83 × 0.0172) ÷ 4 = 1.78V
As percentage of 345V string voltage: 1.78 ÷ 345 = 0.52% — acceptable (under 2%)
| Inverter Size | AC Output Current | Recommended Cable |
|---|---|---|
| 3 kW | ~13.6A @ 220V | 3.5mm² THHN |
| 5 kW | ~22.7A @ 220V | 5.5mm² THHN |
| 8 kW | ~36.4A @ 220V | 8mm² THHN |
| 10 kW | ~45.5A @ 220V | 14mm² THHN |
A 2.5mm² cable instead of 4mm² on a 15m string run in a 5kW system causes approximately 2.1% extra voltage drop. Over a 25-year system life, that's ₱15,000–₱25,000 in lost production for a one-time material saving of about ₱800. It's never worth it.
Engr. Jason Morales — Founder, SolarEnergyPH