What rated current should I use for DC MCBs or gPV fuses at string level?

Per IEC 60364-7-712, the string protective device rated current must be ≥ 1.25 × Isc of the string. For a typical 13.5A Isc module, this gives a minimum of 16.9A — select the next standard rating: 20A. Both DC MCBs and gPV fuses are acceptable at string level; gPV fuses are common in combiner boxes with many strings. Devices must be rated for DC voltage equal to the system voltage.

What voltage rating must DC MCBs and MCCBs have for 1000V solar systems?

For 1000V DC systems, all protective devices must be rated ≥ 1000V DC — not 1000V AC. AC ratings are not equivalent; a device rated 230V AC may only be rated 125V DC. Always check the DC voltage rating on the product datasheet. Devices tested to IEC 60947-2 (DC section) or UL 489 (DC) will have a clear DC voltage rating.

How is the DC SPD Ucpv rating calculated for a solar PV system?

Per IEC 60364-7-712, the SPD maximum continuous operating voltage (Ucpv) must exceed 1.2 × Voc_max_string, where Voc_max_string is the temperature-corrected open-circuit voltage of the string at the lowest design site temperature — not the STC module Voc alone. DC SPDs for PV systems must comply with IEC 61643-31, which covers devices rated up to 1500V DC. For example, an 18-module string with module Voc 49.5V and −10°C minimum site temperature gives Voc_max ≈ 983V, requiring Ucpv ≥ 1200V DC.

Preliminary RFQ Sizing Tool

Solar DC Protection
Coordinator

Enter your PV system parameters — get preliminary MCB, MCCB and SPD sizing for RFQ and BOM review, with current-ratio screening per IEC 60364-7-712.

✓ IEC 60364-7-712✓ IEC 61643-31 SPD✓ UL 489 DC / NEC Art. 690✓ 600V / 1000V / 1500V DC

OmniSol sources and coordinates DC MCBs, MCCBs, combiner boxes and SPDs from qualified manufacturers certified to IEC 60947-2 and IEC 61643-31 — supplying 1000V and 1500V PV projects worldwide.

Preliminary sizing for procurement and BOM review — not a substitute for formal engineering design or AHJ-approved protection coordination study.

System Parameters

Enter per-module datasheet values — the tool calculates string Voc with temperature correction.

System DC Voltage

Strings per Combiner / MPPT

Number of Combiners / MPPTs

Module Isc — Short Circuit Current (A)

Per-module value from datasheet

Module Voc at STC (V)

Per-module, STC (25°C) — not string Voc

Modules per String

Modules wired in series

Voc Temp Coefficient (%/°C)

Magnitude only — typical: 0.25–0.40 (check datasheet)

Lowest Site Temperature (°C)

Design minimum — determines Voc_max

Applicable Standard

Related Tools & Guides

DC Protection Selection Guide

DC MCB vs MCCB vs gPV fuse vs isolator — which device for which application in 1000V / 1500V systems

PV Cable Sizing & Voltage Drop Calculator

4mm² vs 6mm² solar cable, H1Z2Z2-K vs PV1-F, voltage drop worked examples

Combiner Box Selection Guide

2-string vs 16-string, 1000V vs 1500V, fused vs unfused combiner boxes

BOS 1500V Solar Selection Guide

Complete BOM guide for 1500V DC system balance-of-system components

Frequently Asked Questions

What is protection coordination in a solar PV system?

Protection coordination (selectivity) checks that when a fault occurs, only the nearest upstream device trips — not the entire system. The string MCB or gPV fuse should clear before the combiner MCCB, which clears before the main DC switch. Review this against IEC 60364-7-712 and local requirements such as AS/NZS 5033, NEC Article 690 and UL 489 DC device ratings.

Can I use a gPV fuse instead of a DC MCB at string level?

Yes. Both DC MCBs and gPV fuses (IEC 60269-6) are acceptable for string-level overcurrent protection. gPV fuses are commonly used in combiner boxes with many strings because they are compact and cost-effective. The rated current rule is the same: In ≥ 1.25 × Isc. A fuse holder rated for the system DC voltage is required alongside the fuse.

Is string-level protection always required?

Per IEC 60364-7-712, string-level overcurrent protection is usually required or should be reviewed when parallel strings are 3 or more. The exact threshold also depends on the module maximum series fuse rating, reverse current capacity, cable ampacity, and local AHJ requirements. With 2 parallel strings the standard permits omission, but this should still be confirmed against cable sizing and local regulations.

What DC voltage rating must MCBs and MCCBs have for 1000V systems?

Devices must be rated ≥ 1000V DC specifically — not 1000V AC. AC and DC ratings are not equivalent. Always verify the DC voltage rating on the datasheet. Look for IEC 60947-2 (DC section) or UL 489 (DC) compliance.

How is the DC SPD Ucpv calculated, and which standard applies?

Per IEC 60364-7-712, Ucpv must exceed 1.2 × Voc_max_string, where Voc_max_string is the temperature-corrected string open-circuit voltage at the lowest design site temperature. DC SPDs for PV systems must comply with IEC 61643-31 (specifically for PV DC SPDs up to 1500V DC). For example, an 18-module string with module Voc 49.5V at −10°C minimum site temperature gives Voc_max ≈ 983V, requiring Ucpv ≥ 1200V DC.