Solar Fuses and Breakers: DC and AC Sizing Guide

Every solar fire investigation report tells a similar story: a string of panels feeding back into a fault, a connector arcing for hours, and no protective device to interrupt the current. Solar Energy UK reported 38 photovoltaic fires across 2023–2024 in domestic systems, with roughly half traced to DC-side faults that should have tripped a fuse but didn't.

Fuses and breakers are not optional accessories. They are the one component between a fault current and a roof fire. Choosing the right type, sizing it correctly, and installing it on the correct side of the system (DC or AC) is how a small fault stays small instead of becoming a structural problem.

The same wire carries current in both directions on AC and in only one direction on DC. That difference changes everything about how a protective device works. A standard household fuse will happily extinguish a 230 V AC arc 100 times per second (because the current crosses zero each half-cycle), but the same fuse on a 600 V DC string can hold an arc indefinitely. The arc keeps burning until something melts.

DC string fuses protect each parallel string from receiving fault current from the other strings. A single string with no other parallel strings cannot backfeed itself, so it doesn't need a string fuse. Once you have three or more strings in parallel — or two strings whose combined Isc exceeds the panel's Maximum Series Fuse rating — fuses become mandatory.

IEC 62548:2023 gives you both a lower and an upper bound for the fuse rating. The lower bound stops nuisance trips at hot Isc; the upper bound stops the panel from being damaged before the fuse opens.

Imod_max_OCPR is the panel datasheet field labelled "Maximum Series Fuse" or "Maximum Overcurrent Protection Rating" — it is the largest fuse the cell strings inside the panel can survive. Typical values are 20 A, 25 A, or 30 A. NEC 690.9 in the US uses a single 1.56 × Isc rule (1.25 for continuous duty × 1.25 for outdoor conditions), which always lands inside the IEC band.

On the AC side, you are not protecting against backfeed — you are isolating the inverter from the grid during a fault and stopping the wires between the inverter and the consumer unit from overheating. The continuous current the breaker sees is the inverter's rated AC output divided by the grid voltage.

Round up to the next standard breaker size (10, 13, 16, 20, 25, 32, 40, 50, 63 A in the IEC range). A type B or C MCB is the standard choice; type B trips faster on low-magnitude short circuits, type C tolerates the inverter's brief start-up surge better.

For 230 V single-phase systems used in most of Europe, the UK, and Ukraine, divide the inverter watts by 230. For US 240 V split-phase systems, divide by 240. For 3-phase 400 V systems, divide by (√3 × 400) ≈ 693 to get the per-phase current.

Breaking capacity (sometimes written as Icu or Icn, measured in kA) tells you the largest fault current the device can interrupt without exploding. It is a separate spec from the current rating — a 32 A breaker with 3 kA breaking capacity will trip fine on a 100 A overload but may rupture violently if a true short circuit pushes 8 kA through it.

Residential breakers typically come in 3 kA, 6 kA, or 10 kA breaking capacity. The prospective short-circuit current (PSCC) at your consumer unit is set by the grid impedance and is usually 1.5–6 kA for a domestic supply — your distributor can confirm. As a safe default, choose 6 kA Icu for typical UK/EU residential installations and 10 kA Icu for commercial or anywhere the grid is stiff.

On the DC side, gPV fuses are rated by interrupting capacity in kA at the device's DC voltage rating. A typical 1000 V DC gPV fuse is rated at 10 kA, far above any current a residential PV array can produce, so DC breaking capacity is rarely the binding constraint — voltage rating is.

Three parallel strings of 12 × 450 W TOPCon panels (Isc 13.9 A, Imod_max_OCPR 25 A), feeding a 6 kW single-phase string inverter at 230 V grid. Two protection devices to size: DC string fuses and AC output breaker.

You install three 20 A gPV fuses inside the DC combiner (one per string, on the positive conductor) and one 32 A type C MCB on the inverter AC output before it joins the consumer unit. A 30 mA Type A RCD sits between the MCB and the rest of the household circuit, unless the inverter datasheet mandates Type B.

Three regulatory frameworks dominate residential PV installations worldwide. The numbers move slightly between them, but the safety logic is identical.

Australia and New Zealand keep the strictest DC isolation requirement (a disconnect at both the array and the inverter). The US enforces the tightest electrical compliance via UL listing, while IEC is the most flexible on system topology. Whichever applies to you, the underlying physics of fuse sizing barely moves.

Two Solar Stack tools and one companion article cover the rest of the protection-design workflow.

1. Using a gG fuse on the DC side

   gG fuses are made for AC distribution boards and physically cannot extinguish a DC arc above ~24 V. The fuse opens, the arc holds, and the holder melts. Always specify gPV (IEC 60269-6) for any DC protection.

2. Sizing the breaker to the inverter wattage, not the AC current

   A "6 kW" inverter doesn't need a 6 A breaker — it needs a breaker sized to its AC current, which is 26 A at 230 V. Always divide watts by grid voltage first, then multiply by 1.25.

3. Skipping fuses on three or more parallel strings

   Two strings cannot backfeed each other to a dangerous level, but three can. The combined fault current from two healthy strings flowing into a shorted third string will exceed the Imod_max_OCPR of the failing panel within seconds.

4. Ignoring the panel's Imod_max_OCPR limit

   If the calculated fuse range upper bound (2.4 × Isc) is larger than the panel datasheet's Maximum Series Fuse rating, you must use the panel's rating as the ceiling. Otherwise the panel internals fail before the fuse trips.

5. Forgetting breaking capacity (Icu/Ics)

   A 32 A breaker with 3 kA breaking capacity is not the same as a 32 A breaker with 10 kA breaking capacity. Match Icu to the prospective short-circuit current at your installation point — your DNO or utility can supply this number.