White Paper: Sizing PV String Protection — NEC 690 in Practice
CT TECHNICAL LIBRARY • WHITE PAPER WP-02
Sizing PV String Protection: NEC 690 in Practice
The complete worked method — fuse, conductor, voltage class and surge protection — with a modern high-current module example. 7-minute read for PV designers and plan reviewers.
1. Why strings get fused at all
A PV string is a current-limited source: its own fault contribution barely exceeds Isc. The hazard is its neighbors. When strings parallel on a combiner bus and one develops a fault, every healthy string backfeeds the faulted one. Two strings in parallel put at most one string's current into a fault — generally inside the module's series-fuse rating. At three or more strings, combined backfeed exceeds what module wiring can carry, and per-string overcurrent protection becomes mandatory. That is the logic compressed into NEC 690.9.
2. The worked sizing method
Take a current 580 W bifacial module: Isc 13.9 A, Voc 51.9 V, max series fuse 30 A, temperature coefficient of Voc −0.25%/°C. Design ambient record low: −15 °C.
| Step | Rule | Calculation | Result |
|---|---|---|---|
| Fuse minimum | ≥ 1.56 × Isc — 690.9(B) | 13.9 × 1.56 = 21.7 A | next gPV size: 25 A |
| Fuse maximum | ≤ module series-fuse rating | 25 ≤ 30 | 25 A passes |
| Conductor | ≥ 1.56 × Isc before derates — 690.8 | ≥ 21.7 A → 4 mm² PV wire (~44 A ref.) | 4 mm² / 12 AWG |
| String Voc, cold | Voc corrected to record low — 690.7 | 51.9 × [1 + 0.0025 × (25+15)] = 57.1 V | 57.1 V per module |
| String length | ≤ system voltage class | 1000 / 57.1 = 17.5 | 17 modules max (1000 V class) |
3. Fuse format follows voltage class
Through 1000 V DC, the 10×38 mm gPV cartridge in a touch-safe, lever-out holder is the standard — the carrier de-energizes the fuse before fingers can reach it. At 1500 V, physics demands more ceramic: 10×65 and 10×85 mm bodies provide the arc-quenching length, and blown-fuse indicators earn their cost on a 24-string combiner the first time O&M walks the row. gPV (IEC 60269-6) is the only correct fuse class for strings — general-purpose gG fuses are neither rated for the DC voltage nor characterized for the low, sustained fault currents PV produces.
4. Surge: the other string protection
Lightning does not need a direct strike to kill an inverter — induced transients on long DC home runs do it retail. Type 2 MOV arresters in a Y network protect (+)–PE, (−)–PE and (+)–(−) modes in every combiner, with a second set at the inverter input when runs exceed roughly 10 m. Buildings with lightning protection systems require Type 1+2 devices at the DC entry, rated for partial lightning current (10/350 µs). Two installation rules dominate performance: Uc at or above maximum system voltage, and total lead length to bus and PE under half a meter — each additional meter of lead adds on the order of a kilovolt to the voltage the inverter actually sees.
5. The combiner as a protection system
Assembled correctly, the combiner is the string-protection system in one enclosure: per-string gPV fusing on both polarities where required, bus, SPD with thermal-disconnect indication, and a DC-rated output breaker or isolator whose series poles match the voltage class — with monitoring CTs where an O&M contract rides on string availability. Factory-assembled combiners move that entire coordination problem from a rooftop to a test bench, which is why they increasingly appear in specifications by default.
Every device class in this paper — gPV fuses and holders in both formats, Type 2 and Type 1+2 DC SPDs, DC MCB/MCCBs to 1500 V, and assembled combiners from 4 to 24 strings — is specified with ratings matrices and ordering codes in Catalog CT-31, with live SKUs online. Verify final designs against the adopted code cycle and the authority having jurisdiction.
Design it once, correctly.
CT-31 carries the full worked reference — and the engineering desk reviews one plan set per account, free.
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