Two separate checks — don't skip either
A cable-and-connector run has two current limits, and both must clear:
- The wire's ampacity — how much current the copper and insulation can carry continuously at temperature (per NEC ampacity tables and your local code).
- The connector's rating — e.g. 30 A on 4 mm² contacts, 39 A on 6 mm² for original MC4 (see the basics guide).
A 39 A connector does not make a 20 A wire safe. Size for the weaker link.
Step 1: design current — the 1.25 × 1.25 rule
Standard practice (NEC 690.8 convention) is to double-derate:
Design current = panel short-circuit current (Isc) × 1.25 × 1.25 ≈ 1.56 × Isc
The first 1.25 covers continuous operation in full sun; the second covers real-world irradiance above standard test conditions. Example: a panel with Isc = 10 A → design current ≈ 15.6 A.
Step 2: pick a starting gauge
| Wire | Cross-section | Common use in small off-grid systems |
|---|---|---|
| 14 AWG | 2.5 mm² | Short runs, small panels, design currents up to ~20 A |
| 12 AWG | 4 mm² | The default choice for most DIY strings |
| 10 AWG | 6 mm² | Longer runs and higher currents; matches the MC4 max contact size |
This table is a starting point, not an ampacity table — always verify against NEC chapter 9 / table 310 (or your local equivalent) and the cable's temperature rating. Use proper PV wire or USE-2, not household THHN, on anything that lives outdoors.
Step 3: check voltage drop (the one people forget)
Even when ampacity is fine, long thin cable wastes power as heat. Aim for under 2% loss on the PV side. The fix that costs the least is almost always higher string voltage, not thicker copper.
Worked example (illustration): one 300 W panel, Vmp ≈ 32 V, Isc ≈ 10 A, operating ≈ 9.4 A. Run: 15 m one-way (30 m loop).
- 4 mm² copper ≈ 4.61 Ω/km → loop resistance ≈ 0.138 Ω → drop ≈ 9.4 A × 0.138 Ω ≈ 1.3 V ≈ 4% of 32 V. Too high.
- Fix A — 6 mm² (≈ 3.08 Ω/km): drop ≈ 0.87 V ≈ 2.7%. Better.
- Fix B — put two panels in series (≈ 64 V string): same 4 mm² now loses ≈ 2%, and you bought no new copper.
Raising voltage wins because drop scales with current, not voltage. Use any online PV voltage-drop calculator with your exact numbers.
Parallel strings: when you need fuses
In series, current is the same everywhere. In parallel, a faulted string gets fed by its healthy neighbors. Common practice: two parallel strings usually don't need string fuses; three or more do — each string gets an inline fuse sized above its Isc. Verify against your charge controller manual and local code. The hardware exists for exactly this: Y-branch MC4 connectors plus MC4 inline fuse holders.
Pre-crimped vs field-crimped
If you don't own a proper MC4 crimper yet, buy pre-crimped extension pairs in standard lengths (2 m / 5 m / 10 m are the common sizes) — factory crimps are consistent and cheap at small volumes. Learn to crimp when you start making custom lengths; the crimping guide shows the full procedure.
Sizing checklist
- Design current = Isc × 1.56 ✓
- Gauge clears ampacity per code table ✓
- Connector rating (30 A / 39 A) clears design current ✓
- Voltage drop < 2% — or raise string voltage ✓
- 3+ parallel strings → inline fuses ✓