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Solar Cable Sizing — How to Select the Right Wire for Your PV System
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.
Two Criteria for Cable Sizing
Every solar cable must satisfy both of these:
Ampacity — the cable must carry the maximum current without overheating
Voltage drop — the cable must not drop more than 2-3% of system voltage
DC String Cable (Panel to Inverter)
Step 1: Determine Maximum Current
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
Step 2: Select Cable Size
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.
Step 3: Check Voltage Drop
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%)
AC Output Cable (Inverter to Distribution Board)
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 |
Cable Type Requirements
DC string cables — must be USE-2 or PV Wire rated (UV resistant, double insulation, rated for 90°C, listed for direct burial and outdoor use). Standard THHN is NOT acceptable for DC outdoor runs.
AC output cable — THHN/THWN is acceptable inside conduit. For any exposed outdoor AC run, use THWN-2 (wet location rated).
Cable color coding (PEC standard): DC positive = red, DC negative = black, AC live = black, AC neutral = white, ground = green or bare
The Real-World Cost of Undersizing
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