Commercial Solar DC Cable Sizing in Bangladesh: Voltage Drop, Ampacity and Worked Example

Direct answer: For a commercial rooftop solar plant in Bangladesh, DC cables should be sized by two checks, and the larger result wins: ampacity after temperature and grouping derating, and voltage drop kept below 1% for string circuits and below 2% for the whole DC run from array to inverter. Most commercial strings use 6 mm² copper PV cable, stepping up to 10 mm² or larger for long runs, high-current modules, or combiner-to-inverter feeders. Undersized DC cable is not a small inefficiency — it wastes energy every sunny day for 25 years and, in the worst case, overheats.

Why DC cable sizing deserves its own design step

Panels and inverters get the attention in a commercial tender, but the DC cable is the part that carries every watt the array produces. A cable that is too small does two kinds of damage at once. First, resistance turns electricity into heat, so voltage drops along the run and the inverter receives less power than the array generated. Second, a cable carrying more current than its derated rating allows runs hot, ages its insulation faster, and becomes a fire risk. Both problems are invisible on a commissioning-day snapshot and only show up in monthly generation shortfalls or, worse, in a thermal incident years later.

Bangladesh makes both checks harder than a temperate-climate datasheet suggests. Rooftop air temperatures regularly exceed 35°C, dark roofs and sun-exposed conduit run hotter still, and long factory roofs push inverter rooms far from the array. A cable size copied from a European residential example will usually be wrong here.

Check 1: Ampacity — how much current the cable may safely carry

Ampacity is the maximum current a cable can carry continuously without exceeding its insulation temperature rating. Solar design current starts from the module short-circuit current (Isc) on the datasheet, not the operating current (Imp). Under the NEC 690.8 approach widely used in PV design, the design current is 1.25 × 1.25 × Isc = 1.56 × Isc: one factor for continuous operation and one for the higher irradiance a clear day can deliver above standard test conditions.

That design current must then survive derating. Ampacity tables assume a 30°C ambient and a single cable in free air, neither of which describes a Bangladeshi rooftop. For 90°C-rated PV cable, the NEC temperature correction factor falls to 0.91 at 36–40°C and 0.87 at 41–45°C, and conduit exposed to direct sun can add a further 15–30°C to the effective temperature before the table is even applied. Grouping several loaded conductors in one conduit or tray reduces capacity further — a factor of 0.80 is typical for four current-carrying conductors in one conduit. Multiply the factors: at 45°C with four conductors in a conduit, 0.87 × 0.80 = 0.696, so a cable whose catalogue ampacity is 40 A is only good for about 28 A in that installation. The practical lesson for buyers is simple: ask the designer to show the derated ampacity, not the catalogue number, and insist on 90°C-rated, UV-resistant PV cable (EN 50618 H1Z2Z2-K or IEC 62930) for any outdoor run. Lower-rated insulation has no safe margin left on a hot roof.

Check 2: Voltage drop — how much energy the cable itself eats

Voltage drop for a two-conductor DC circuit is calculated as Vdrop = 2 × L × I × R / 1000, where L is the one-way length in metres, I is the operating current in amps, and R is the conductor resistance in ohms per kilometre. The factor of 2 accounts for the out-and-back path. Industry best practice, reflected in NEC Article 690 guidance and installer guides, is to keep DC voltage drop at or below 2% per segment and 3% total for the complete DC circuit; many commercial designers tighten that to 1% for string circuits and 1% for the main DC feeder, so the total stays near 2%.

A worked example shows why length matters more than most buyers expect. Take a string of 20 modules with an operating voltage of about 800 V and an operating current of 14 A, with the inverter 60 m away. With 6 mm² copper cable (about 3.08 Ω/km), the drop is 2 × 60 × 14 × 3.08 / 1000 = 5.2 V, or 0.65% — comfortably fine. Stretch that same run to 120 m with thinner 4 mm² cable (about 4.61 Ω/km) and the drop becomes 2 × 120 × 14 × 4.61 / 1000 = 15.5 V, or 1.94% on the string alone, before feeder losses are added. On a plant generating hundreds of megawatt-hours a year, a persistent 2% DC loss is a large, entirely avoidable revenue leak. Note also that resistance rises with temperature, so voltage-drop calculations should use resistance corrected to the expected operating temperature, not just the 20°C catalogue value.

Copper vs aluminium, and where each belongs

Commercial strings almost always use copper PV cable: it has lower resistivity, terminates reliably in MC4-style connectors, and is available in the 4–10 mm² sizes strings need. Aluminium appears in larger main DC or AC feeders, where its lower cost per amp justifies the larger cross-section — roughly one to two sizes up for the same resistance — and where terminations are made with proper bi-metallic lugs and torque discipline. Aluminium in small string sizes is a false economy: the savings are small and the termination risk is real. Whatever the metal, every DC connection must be crimped with the correct tool and die; a poor crimp has higher resistance than metres of cable and is a common cause of hot spots found in thermographic surveys.

Routing rules that protect the calculation

A correct size on paper can still fail if the route is careless. Keep DC runs as short and direct as the roof allows, and place string inverters close to their arrays where the design permits — every 10 m of avoided run is avoided loss. Separate DC and AC routes, support cables so they do not rest on sharp metal edges or ponding water, and leave slack for thermal movement and future module replacement. Label both ends of every string: when a fault appears months later, the monitoring data can only be traced to a physical string if the strings are identifiable, and a string design record that lists each string's cable size and length makes that fault-finding fast instead of forensic.

What to demand in a tender or EPC proposal

A credible commercial proposal should include, per string group and feeder: the design current with the 1.56 × Isc basis, the derating factors applied for temperature and grouping, the calculated voltage drop as a percentage, the cable type and standard (EN 50618 or IEC 62930), and the routing. If any of those lines is missing, the cable size is an assertion, not a design. Also ask for a post-installation check: an insulation resistance test, a polarity and open-circuit voltage check per string, and a thermographic scan under load within the first months of operation. Together with a peak shaving strategy for the load side, correct DC sizing is part of the same discipline — design the whole plant to verified numbers, not to catalogue headlines.

FAQs

What size DC cable does a commercial solar string need?

Most commercial strings use 6 mm² copper PV cable, with 4 mm² acceptable only for short runs with moderate current. Long runs, high-current modules above about 14 A, or grouped conduit runs typically need 10 mm². The correct size comes from the two checks in this guide — derated ampacity and voltage drop — not from a rule of thumb.

How much voltage drop is acceptable on the DC side?

Keep each DC segment at or below 2% and the total DC circuit at or below 3%; a stricter commercial target is 1% per segment and about 2% total. A persistent 2% loss on a large commercial plant costs far more over 25 years than the one-time cost of the next cable size up.

Why does a Bangladeshi rooftop need larger cable than a datasheet suggests?

Because derating bites twice: ambient temperatures above 30°C reduce ampacity, and sun-exposed conduit adds further temperature before the correction table is applied. A cable rated 40 A in a catalogue may only be safe for about 28 A on a hot, grouped rooftop run — so the design must use the derated figure.

Is aluminium cable acceptable for solar DC runs?

Aluminium is acceptable for larger feeders where its lower cost justifies a larger cross-section and proper bi-metallic terminations, but copper is the right choice for string circuits. In small sizes the savings are minimal and termination quality becomes the dominant risk.