Balance of system: DCDB, ACDB, cables, MC4, SPD, MCB, isolators

"Balance of system" (BOS) means everything except the modules and inverter: boxes, breakers, cables, connectors and surge protection. BOS is cheap compared with modules, which is exactly why it gets skimped. Most rooftop fires and failures start in BOS: a loose MC4, a fake breaker or an undersized cable. This lesson covers what goes where and how to size it. Detailed protection design is in module m06.

The single-line path

DC side

Solar DC cable

• Use purpose-made PV cable: tinned, fine-stranded copper with double insulation (XLPO, electron-beam cross-linked), UV and ozone resistant, rated 1.5 kV DC (or 1.0/1.8 kV), to EN 50618 or IEC 62930.
• Common sizes: 4 mm² for single-string home runs, 6 mm² for long runs or paralleled strings.
• Red for positive, black for negative, or mark clearly at both ends.
• Never use ordinary PVC house wire on the roof. It cracks in Indian sun within a few years.

MC4 connectors

• Use connectors to IEC 62852, of the same make and type on both halves. Cross-mating different brands is the most common cause of hot, melted joints.
• Crimp with the correct crimping tool and die, not pliers. Tug-test every crimp.
• Keep connectors off the roof surface where water ponds, and support them so they do not hang by the cable.

DCDB (DC distribution box)

• DC isolator: a DC-rated switch-disconnector (IEC 60947-3, DC-PV2 utilisation category), rated above the string's maximum voltage and current. It lets you isolate the array for inverter work.
• DC surge protective device (SPD), Type II to IEC 61643-31, with continuous voltage Ucpv above the maximum string Voc (cold morning), and a short, straight earth connection.
• String fuses (gPV type, IEC 60269-6) only when three or more strings are paralleled on one input, because then a faulty string can receive reverse current from the others. Fuse rating (IEC 62548): between 1.5 × and 2.4 × Isc, and never above the module's maximum series fuse rating (module m06, lesson 3).
• Enclosure: UV-resistant, IP65, with glands at every entry.

Many home inverters have built-in DC switches and SPDs. Even so, many DISCOM and scheme specifications ask for an external DCDB; follow the applicable specification.

AC side

ACDB (AC distribution box)

• MCB (IS/IEC 60898-1), C-curve, rated at least 1.25 × the inverter's maximum AC output current (from the datasheet), next standard size, and no larger than the cable can carry.
• RCCB (IS 12640): type and sensitivity as the inverter manual states. Type A is usual; use Type B only if the manual requires it (transformerless inverters without internal residual-current monitoring). 30 mA is common on home circuits, but many manuals allow 100 mA on the inverter feeder to avoid nuisance trips from array leakage on wet mornings (module m06).
• AC SPD, Type II to IEC 61643-11.
• Isolator at the meter end, lockable, so DISCOM staff can isolate solar.
• Optional: voltmeter, ammeter, energy meter.

AC cable

Copper for small runs; for 16 mm² and above, armoured aluminium XLPE is often cheaper and widely accepted (with bimetallic lugs at copper terminals). Size for current and voltage drop.

Worked example: sizing BOS for a 5 kW single-phase system in Pune

System: 10 × 550 Wp in one string (5.5 kWp), 5 kW single-phase inverter, DC run 18 m one way, AC run 20 m from inverter to meter.

DC side

• Max Voc (5 °C): 10 × 52.6 = 526 V → DC isolator and SPD rated 600 V DC or higher (Ucpv ≥ 526 V; 1,000 V devices are common)
• One string per MPPT → no fuses needed
• Voltage drop on 4 mm² copper at Imp 13.11 A, hot conductor (resistivity about 0.0225 Ω·mm²/m, a deliberately conservative figure; module m06 uses 0.0172 at 20 °C and 0.021 at 70 °C): ΔV = 2 × 18 × 13.11 × 0.0225 ÷ 4 = 2.65 V String Vmp in summer ≈ 354 V, so drop = 2.65 ÷ 354 = 0.75%, well under the 1.5% target

AC side

• Inverter current: 5,000 ÷ 230 = 21.7 A (use the datasheet's maximum output current if it is higher)
• MCB: 1.25 × 21.7 = 27.2 A → 32 A, C-curve, double pole
• RCCB: 40 A, Type A, sensitivity per the inverter manual (30 or 100 mA), rated at or above the MCB
• Cable: 6 mm² copper (derated ampacity about 34 A, above the 32 A MCB)
• Voltage drop: ΔV = 2 × 20 × 21.7 × 0.0225 ÷ 6 = 3.26 V → 3.26 ÷ 230 = 1.4%, just inside 1.5%. With a longer run, go to 10 mm².

Cable routing and support

• Run DC cables in UV-stabilised conduit or covered trays; tie with UV-resistant ties, never ordinary nylon ties that crack in a year.
• Keep positive and negative of a string together to minimise loop area (lightning-induced surges).
• Seal wall and slab penetrations against rain.
• Label everything: string numbers at both ends, phase and neutral, and earth conductors, so the boxes match the single-line diagram.

Key points

• BOS failures (loose MC4s, fake breakers, undersized cables) cause most rooftop fires; do not cut corners.
• Use proper PV cable (4–6 mm², 1.5 kV DC, UV-rated) and same-make MC4 pairs crimped with the right tool.
• DCDB: DC isolator and Type II DC SPD rated above cold Voc; fuses only with three or more parallel strings.
• ACDB: MCB at 1.25 × the inverter's maximum output current (next size up), an RCCB of the type and sensitivity the inverter manual specifies, Type II AC SPD and a lockable isolator.
• Keep DC and AC voltage drop around 1–1.5%, and label everything to match the single-line diagram.

Check yourself

1. A 3 kW single-phase inverter: what MCB rating would you choose?
2. Two strings go into two separate MPPT inputs. Do you need string fuses?
3. Why must both halves of an MC4 joint be the same make?