DC Breaker for Solar Panels: How to Size DC Breakers, Isolators and Fuses for Strings, Arrays and Batteries

The short answer

• Use DC-rated devices only. AC breakers and switches may not break a DC arc and can catch fire.
• Voltage: rate every DC device for the string’s open-circuit voltage (Voc) on the coldest morning, not at 25 °C.
• Current: rate a PV breaker or isolator for at least 1.25 × the short-circuit current (Isc) of its strings (IEC approach); the US NEC 690 uses 1.56 × for breakers and fuses.
• String fuses are usually not needed for one or two strings in parallel, and must never exceed the module’s maximum series fuse rating.
• Battery: fit a DC fuse or breaker near the battery, rated above the inverter’s maximum battery current and able to break thousands of amps.

Why AC breakers fail on the DC side

Alternating current passes through zero 100 times a second at 50 Hz, and an AC breaker relies on those zeros to put out the arc as its contacts open. Direct current has no zero, so a DC arc can keep burning even at low current. DC breakers use larger arc chutes, magnets or contacts in series; an AC breaker may fail to clear the arc and start a fire.

Some AC MCBs list a DC rating, but only for low voltages such as 60 V per pole.

• Polarised DC MCBs use a magnet to drive the arc into the arc chute, so wire them to the + and − marks; wired backwards, they may not clear a fault and can catch fire.
• Non-polarised DC MCBs break current either way; prefer them where current reverses, such as battery circuits.
• Poles in series set the DC voltage rating on many ranges (some need four poles for 1,000 V); follow the maker’s diagram.
• Heat: in a sun-baked combiner box, check the maker’s derating table.

Voltage rating: size for the coldest morning

Module voltage rises as temperature falls, peaking at sunrise on a cold, clear winter morning. IEC 62548-1 and the US NEC 690 both correct Voc to the lowest expected site temperature, not the 25 °C of standard test conditions (STC).

Cold string Voc = Voc(STC) × (1 + β × (Tmin − 25)) × modules in series. β is the datasheet Voc temperature coefficient as a fraction (−0.27 %/°C is −0.0027) and Tmin the lowest expected cell temperature in °C. Modern modules are typically about −0.25 to −0.30 %/°C.

The result must stay below the inverter’s maximum DC input voltage and the module’s maximum system voltage. Some UK designers (for example, IET Wiring Matters guidance) use a flat 1.2 × Voc(STC) instead, which is more conservative.

Current rating: breakers, isolators and string fuses

A PV string cannot push much more than its Isc into a fault, only a little above its working current, so a breaker rarely trips on a fault within one string. PV overcurrent devices mainly stop parallel strings driving reverse current into a faulty one.

Follow your code and the inverter manual, as the approaches differ. For bifacial modules, use the datasheet current with rear-side gain (IEC 62548-1 adds a factor).

When string fuses are needed

Count the strings in parallel on each MPPT tracker, including connector pairs joined inside the inverter; separate trackers are not in parallel. With one or two, a faulty string is fed by one other string at most, normally below the module’s maximum series fuse rating, so no fuses are needed, provided nothing else can feed the array (some inverters and DC-coupled batteries can: check the inverter manual) and your code does not require them. With three or more, apply your code’s rule.

Use gPV fuse-links to IEC 60269-6 (made to break the low fault currents of PV strings, up to 1,500 V DC), with a voltage rating of at least the cold Voc. Fuses derate in heat: check the maker’s curve at the box temperature, shade the box, and confirm the derated rating still exceeds 1.5 × Isc without passing the module’s maximum series fuse rating. A fuse holder is not a switch: open the isolator before removing a fuse.

DC isolators

A DC switch-disconnector (often called a DC isolator) is normally chosen to switch the array under load as well as isolate it, but it never trips. Choose one to IEC 60947-3, rated for the cold Voc in the maker’s pole wiring and for at least 1.25 × the Isc of the strings it switches, in a category that suits its job:

• DC-PV1: single strings, where significant overcurrent cannot occur.
• DC-PV2: parallel strings, where significant overcurrent and current in either direction can occur.
• DC-PV0: disconnector only: open it with no current flowing, never as the on-load switch.
• DC-21B or DC-22B: general DC categories (B means infrequent operation); confirm PV use with the maker.

Worked example: 550 W modules on a 500 V hybrid inverter

Assumed values, typical of a 550 W module: Voc 49.8 V, Isc 14.0 A, Voc coefficient −0.27 %/°C, maximum series fuse 25 A. Inverter: 500 V DC maximum PV input, two strings in parallel on one input. Design minimum: −5 °C, below Lahore’s record low; use your own site’s figure.

1. Cold factor: 1 + (−0.0027) × (−5 − 25) = 1.081.
2. Cold module Voc: 49.8 × 1.081 = 53.83 V.
3. Modules in series: ten give 498.0 V at 25 °C but 10 × 53.83 = 538.3 V when cold, over 500 V. Nine give 9 × 53.83 = 484.5 V: use nine. (A flat 1.2 factor allows only eight. In the colder north, where lows can reach −15 °C or −20 °C, nine modules give 496.6 V or 502.7 V, so eight may be the limit.)
4. Voltage rating: all DC devices 500 V DC or more (above 484.5 V), in the maker’s pole wiring.
5. PV main breaker or isolator: array Isc = 2 × 14.0 = 28.0 A. IEC: 1.25 × 28.0 = 35.0 A, so 40 A. NEC: 1.56 × 28.0 = 43.7 A, so 50 A, with cable rated to match.
6. String fuses: the other string can feed a fault with at most about 1.25 × 14.0 = 17.5 A, below 25 A, so none are needed. A third string could make it 35.0 A; then fit 25 A gPV fuses (IEC: over 1.5 × 14.0 = 21.0 A; NEC: 1.56 × 14.0 = 21.8 A or more).
7. Inverter: check that its input accepts 28.0 A of short-circuit current.

Battery, AC output, surge protection and cable

Battery to inverter

Lithium and lead-acid batteries can deliver thousands of amps into a short circuit, so this is the most dangerous DC circuit. Fit a DC fuse or breaker as close to the battery as practical, unless the battery manual says its own breaker does this job.

• Current: above the inverter’s maximum battery current, commonly 1.25 × it, then the next size: for a 140 A maximum, 1.25 × 140 = 175 A, so 200 A, with cable rated to match.
• Breaking capacity: above the battery’s short-circuit current (datasheet). Car-style fuses are often rated only 32 V DC, unsuitable for 48 V.
• Direction: current flows both ways; use a fuse or a non-polarised breaker.

Inverter AC output

Use an ordinary AC MCB of the size in the inverter manual, with breaking capacity above the board’s fault level (see MCB vs MCCB vs RCCB). If none is given, a common rule is the next size above 1.25 × the maximum output current (1.25 × 27.3 A = 34.1 A, so 40 A). A hybrid’s backup output needs its own breaker; use the RCD type the inverter maker states (often Type A, sometimes Type B).

Surge protection and PV cable

Fit a PV-rated DC surge protective device (SPD), usually Type 2, to IEC 61643-31, with its PV voltage rating (UCPV) at least the cold Voc: one near the inverter, another near the array if the DC cable exceeds about 10 m. Protect the AC side too; a building with lightning protection may need Type 1.

Use solar cable such as H1Z2Z2-K (EN 50618 or IEC 62930), rated 1.5 kV DC and UV-resistant; 4 mm² and 6 mm² are common string sizes. Use our cable size guide for the method, but take the current rating from the solar cable maker’s datasheet, and mate connectors only with the same type from the same maker.

Installation checklist

1. Collect the datasheets: module Voc, Isc, Voc coefficient and maximum series fuse; inverter voltage, current and battery limits; battery short-circuit current.
2. Work out the cold string Voc and modules in series; check the summer voltage against the MPPT range.
3. Count the parallel strings on each MPPT tracker and decide on string fuses.
4. Choose DC-rated breakers, isolators, gPV fuses and SPDs; check polarity marks and pole wiring.
5. Size the battery fuse or breaker, the AC breaker and RCD, and the cables they protect.
6. Before switching on, test polarity, string Voc and insulation resistance (IEC 62446-1 covers commissioning tests).

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