Direct answer: Size every DC cable in a commercial rooftop solar plant with two independent checks — ampacity (current-carrying capacity after heat and grouping derating) and voltage drop (IEC 62548 caps total DC loss at 3%) — then buy the larger of the two results. Use 1500 V DC solar cable to EN 50618 (tinned copper, XLPE insulation, 90°C conductor rating), design for conduit temperatures near 55°C on a Bangladeshi rooftop, and always run the voltage-drop check on homeruns longer than 40 m, where most plants silently lose 1–3% of annual yield.

Why a thin cable taxes you every single day

A DC cable is a resistor you install on purpose. Power lost as heat follows I²R, and it is lost for the full 25-year life of the plant. Take a 100 kW factory rooftop in Bangladesh producing roughly 4.2 kWh per kWp per day: about 153,000 kWh a year. A 3% DC voltage drop quietly burns around 4,600 kWh of that every year — enough electricity to run a small factory office for months — while the overheated conductor ages its insulation years ahead of schedule. The hybrid inverter sizing guide treats the DC/AC ratio; this guide treats the copper that carries the DC side.

Undersized DC cable causes three distinct harms: (1) daily energy loss you can never recover, (2) conductor temperatures that cook insulation and void the 25-year design life, and (3) hot terminations at MC4 connectors and combiner boxes — the most common ignition point in rooftop PV fires. All three are prevented at the design desk for the price difference between two cable sizes.

The two checks every DC circuit must pass

Cable sizing is two separate calculations. A cable can pass one and fail the other; you run both and buy the larger size.

Check 1 — Ampacity. The continuous current a conductor can carry without exceeding its temperature rating. Start from the design current: for PV source circuits, take 1.25 × the string short-circuit current Isc (the NEC 690.8 approach, widely used in IEC-land EPC practice), then derate for the real installation conditions — ambient temperature and how many loaded conductors share the conduit.

Check 2 — Voltage drop. IEC 62548 requires the total DC voltage drop from array to inverter to stay within 3%. Good C&I practice splits the budget: about 1% for the string cables (module to combiner) and 2% for the homerun (combiner to inverter). The formula for copper:

S = (2 × L × I × 0.0175) / ΔV

where S is cross-section in mm², L one-way length in metres, I design current in amps, and ΔV the allowed drop in volts. The factor 2 accounts for the positive and negative conductors, and 0.0175 Ω·mm²/m is copper resistivity at operating temperature.

Short string runs (under ~20 m) are almost always governed by ampacity; long homeruns (over ~40 m) are almost always governed by voltage drop. Know which fight you are in before you calculate.

What a proper solar DC cable is

Demand cable manufactured to EN 50618 (H1Z2Z2-K) or IEC 62930, and check the marking on the sheath:

Conductor: tinned copper, class 5 flexible — tinning matters in Bangladesh's humidity, where bare copper oxidises and contact resistance creeps up.
Insulation and sheath: cross-linked (XLPE), halogen-free, UV- and weather-resistant.
Voltage rating: 1.5 kV DC — comfortably above modern 1000–1100 V string voltages.
Temperature rating: 90°C continuous conductor temperature (120°C allowed for a limited 20,000 hours total — not a design target).
Design life: 25 years outdoors.

Never substitute ordinary PVC house wire on the DC side. PVC insulation is typically rated 70°C, goes brittle in direct UV within about a year, and its ampacity collapses exactly when a Bangladeshi rooftop is hottest. The maintenance guide's quarterly inspection checklist exists partly because of cables that should never have been installed.

Bangladesh heat: the derating step nobody does

Nameplate ampacity assumes civilised conditions. A commercial rooftop in Rajshahi or Chattogram in May is not civilised:

Design ambient: 40°C is the honest starting point for Bangladesh.
Rooftop adder: conduit strapped to a dark roof surface runs 10–17°C hotter than the air (the NEC 310.15 rooftop adder). Design for ~55°C effective ambient inside that conduit.
Temperature correction (90°C insulation column): 40°C → 0.91, 45°C → 0.87, 50°C → 0.82, 55°C ≈ 0.76, 60°C → 0.71.
Grouping: 4–6 current-carrying conductors in one conduit → 0.80; 7–9 → 0.70.

Multiply them: 0.76 × 0.70 ≈ 0.53. More than half the free-air rating can vanish before you have carried a single amp. This is the step that separates a cable that survives 25 summers from one that fails in its third.

Worked example: 100 kW factory rooftop

A garment factory rooftop, 100 kW DC across string inverters. One combiner box gathers 4 strings; each string is 20 × 550 Wp modules (Vmp 41.6 V → 832 V string voltage, Imp 13.2 A, Isc 13.9 A). The combiner-to-inverter homerun is 60 m one-way, in a shared conduit on the roof.

Step 1 — design current. 4 strings × 13.9 A × 1.25 = 69.5 A → 70 A.

Step 2 — voltage-drop sizing. Strings already consume ~1%, so the homerun gets 2%: ΔV = 0.02 × 832 = 16.6 V.

S = (2 × 60 × 70 × 0.0175) / 16.6 = 8.9 mm² → next standard size 10 mm². Voltage drop alone says 10 mm² is fine.

Step 3 — ampacity with real derating. 10 mm² solar cable is rated ~98 A in free air. Effective ambient ~55°C (factor 0.76); 8 current-carrying conductors in the conduit (factor 0.70).

Adjusted ampacity = 98 × 0.76 × 0.70 = 52 A < 70 A → fails.

Step 4 — step up. 16 mm² (~132 A free air): 132 × 0.76 × 0.70 = 70.2 A — passes, but with no margin for a heatwave. 25 mm² (~176 A free air): 176 × 0.76 × 0.70 = 93.6 A — passes with margin, and its voltage drop is only (2 × 60 × 70 × 0.0175)/25 = 5.9 V, i.e. 0.7%.

Verdict: 25 mm² copper per polarity. The voltage-drop check said 10 mm² was enough; the heat check overruled it. That is the whole lesson — always take the larger of the two.

For the string cables themselves (module to combiner, typically under 20 m at 13–14 A), 4–6 mm² is standard: ampacity governs on short runs and the drop is negligible.

Copper vs aluminium for DC trunks

Aluminium's resistivity (0.028 Ω·mm²/m) is ~1.6× copper's, so an aluminium trunk needs roughly 1.6× the cross-section for the same loss. It is cheaper per metre and common for long DC trunks on ground-mount plants — but it demands bi-metallic lugs, antioxidant joint compound, larger glands, and disciplined torque. For rooftop plants under ~200 kW, copper's smaller bend radius, simpler termination, and lower connection-failure risk usually win on total installed cost. Whatever you choose, never land aluminium directly on copper terminals without a bi-metallic transition.

Five installation mistakes that burn plants

Hand-tightened or mixed-brand MC4 connectors. A loose MC4 adds series resistance exactly where current is highest; it arcs, melts, and is the classic rooftop PV fire starter. Crimp with the calibrated tool, from one connector system.
PVC cable ties in sunlight. They embrittle within a year and drop cables onto hot sheet metal. Use UV-stabilised ties or stainless clips.
DC and AC sharing one conduit without separation — fault propagation and inspection headaches.
No drip loops at combiner and inverter entries — rainwater tracks down the cable straight into the terminals.
SPD only at the inverter. The combiner box needs its own surge protection on long DC runs; see the lightning and surge protection guide for placement.

Procurement checklist for EPC buyers

Sheath marked EN 50618 / IEC 62930, 1.5 kV DC, with a TÜV or equivalent certificate you can actually read.
Tinned copper — reject bare-copper "solar cable" for humid climates.
Verify the copper: measure DC resistance per km against the datasheet. Undersized "4 mm²" that is really 3.2 mm² is a classic market cheat and it fails both checks above.
UV-stabilised, halogen-free (LSZH) sheath.
Crimp with a calibrated MC4 tool — never pliers — and torque to the connector maker's spec.

FAQs

Is 4 mm² cable enough for each string?

For typical 13–14 A strings with runs under 20 m, yes — ampacity governs on short runs and 4–6 mm² is the industry standard. Run the voltage-drop formula if your string run is long or the combiner is far from the array.

Can I use aluminium DC cable on a rooftop?

Technically yes, with bi-metallic lugs and antioxidant compound at every termination. But for rooftop plants under ~200 kW, copper's smaller bend radius and far simpler, more reliable terminations usually win on total installed cost and failure risk.

Does voltage drop really matter at 800–1000 V string voltage?

Yes — 3% is 3% of your energy regardless of voltage. High string voltage just makes a given percentage cheaper to achieve, because current (and therefore I²R loss) is lower. The 3% IEC 62548 limit applies at any voltage.

Where should surge protection sit relative to the DC cable runs?

At both ends of long DC runs: a Type 2 SPD in the combiner box and another at the inverter DC input, with the shortest possible earth leads. Full placement rules are in the lightning and surge protection guide.

Sources: IEC 62548 (PV array design, ≤3% DC voltage drop); EN 50618 / IEC 62930 (H1Z2Z2-K solar cable, 90°C continuous conductor temperature, 1.5 kV DC); NEC 690.8 / 310.15 (1.25× Isc design current, temperature correction and grouping factors, rooftop temperature adder); LAPP and RR Kabel H1Z2Z2-K datasheets (ampacity and thermal parameters).

DC Cable Sizing for Commercial Rooftop Solar in Bangladesh: Voltage Drop, Ampacity & Worked Example