DC wiring for solar panels is not sized the same way as standard electrical wiring. Solar panels behave as current sources — under certain fault or irradiance conditions, they can produce current continuously at or above their rated short-circuit current (Isc). This is why solar DC wire sizing uses a higher safety multiplier than typical load wiring.
| Standard | Multiplier | Reasoning |
|---|---|---|
| NEC 690.8(A) — wire ampacity | Isc × 1.25 × 1.25 = ×1.56 | 125% for continuous duty + 125% for temperature/conduit derating |
| AS 4509.2 — overcurrent device | Isc × 1.25 | Minimum fuse/breaker rating for string protection |
| PEC 2017 (aligned with NEC) | Isc × 1.56 | Same as NEC for PV source circuits |
A 550W mono panel typically has Isc = 13.8A. For a single string:
| Step | Calculation | Value |
|---|---|---|
| Panel Isc | From datasheet | 13.8A |
| Minimum wire rating (NEC 690.8) | 13.8A × 1.56 | 21.5A minimum |
| Standard PV cable selected | 6mm² PV1-F (rated 44A) | ✅ 44A > 21.5A |
| For 2 parallel strings | (13.8A × 2) × 1.56 | 43.1A minimum → 6mm² just meets, use 10mm² |
| Cable Type | Temperature Rating | UV Resistance | Use Case |
|---|---|---|---|
| PV1-F (TÜV certified) | 90°C | ✅ Yes — outdoor rated | Rooftop string wiring |
| THHN (standard PH wire) | 75°C | ❌ No — requires conduit | Indoor conduit runs only |
| USE-2 (US standard) | 90°C | ✅ Yes | Underground DC runs |
Always use PV1-F or equivalent solar-rated cable for exposed rooftop DC wiring. Standard THHN will degrade and crack within 3–5 years under Philippine UV and heat conditions.
Size your PV wires quickly using the free PV Wire Sizing calculator at solarenergyph.shop.
Engr. Jason Morales — Founder, SolarEnergyPH