About the Fuse / MCB / MCCB Sizing
Fuse, MCB (Miniature Circuit Breaker), and MCCB (Moulded Case Circuit Breaker) sizing for solar systems determines the correct current rating and breaking capacity for DC string protection, AC output protection, and the grid connection — following IS 13947, IS 9224, and IEC 60947 standards.
Formula
String fuse rating: Isc × 1.25 ≤ I_fuse ≤ cable ampacity × 0.8
- Isc
- String short-circuit current (A) from panel datasheet
- 1.25
- Safety factor per IEC 60364-7-712 for string fuse minimum
- cable ampacity × 0.8
- Fuse must not exceed 80% of the cable's rated current capacity
AC MCB rating ≥ Inverter rated AC output current × 1.25
- Inverter rated AC output current
- From inverter datasheet (A)
- 1.25
- Standard safety derating for continuous loads per IS 3043
How to calculate it
- 1For each DC string: find panel Isc, multiply by 1.25 for minimum fuse rating.
- 2Check the selected fuse rating against cable ampacity (must not exceed 80% of cable's continuous rating).
- 3For DC combiner/ACDB: calculate total DC current (parallel strings × Isc) and size busbar and DC MCB/MCCB accordingly.
- 4For AC side: size main AC MCB at ≥ 1.25 × inverter rated AC output current.
- 5Verify breaking capacity (kA): must exceed the prospective short-circuit current at the installation point.
Worked example
10 A Isc panels, 4 strings in parallel, 6 mm² DC cable. String fuse: ≥ 10 × 1.25 = 12.5 A → use 15 A DC fuse (standard value). Cable ampacity at 6 mm²: ~47 A; 80% = 37.6 A — 15 A fuse is well within limit. AC MCB: 5 kW inverter output ≈ 21.7 A at 230 V; MCB ≥ 21.7 × 1.25 = 27.1 A → use 32 A MCB (standard value).
What you can calculate
- gPV string fuses
- MCB B/C/D curve
- DC-rated MCCB
- Interrupt kA
Standards & references
Frequently asked questions
Do I need DC fuses in every string of a solar system?
For systems with 2 or more strings in parallel, DC string fuses (or string diodes) are required to prevent reverse current flowing through a shaded or faulted string — which could cause fire in the DC wiring. A single-string system does not need a string fuse (there is no reverse current path). String fuses are mandatory per IEC 60364-7-712 for parallel-string DC systems.
What is the difference between a DC fuse and an AC MCB?
DC fuses are specifically designed to interrupt the sustained DC arc that forms when a DC circuit is broken — ordinary AC MCBs will NOT reliably break DC faults because AC current naturally passes through zero twice per cycle (which helps extinguish the arc), while DC current is constant. Using an AC MCB in a DC circuit is unsafe and a common installation error.
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