DC cables and voltage drop: sizing, colours, routing, UV

DC cables carry the whole output of the array for 25 years, in full sun, on a roof that reaches 60–70 °C in May. The cable is cheap compared with modules, but a wrong choice shows up as lost units every day, burnt connectors after a few summers, or insulation faults in the first monsoon. This lesson covers how to size DC cable, which cable to buy, and how to lay it.

Two checks: current and voltage drop

Every DC cable must pass two separate checks.

1. Current-carrying capacity (ampacity): the cable must carry the maximum current without overheating. IEC 62548 treats the maximum current of a string as 1.25 × Isc(STC). Then derate the cable's rating for high ambient temperature, conduit and grouping.
2. Voltage drop: the cable resistance wastes energy. A common design target, and the figure in many DISCOM and tender specifications, is a DC drop of 2% or less at the maximum power point. Good designers aim for about 1%.

On most homes the ampacity check is easy (a 4 mm² solar cable is rated far above 17 A), so voltage drop decides the size.

The voltage-drop formula for DC

Current goes out on the positive cable and back on the negative, so the length counts twice:

Vd = 2 × L × I × ρ ÷ A

• L = one-way route length in metres
• I = current in amps (use Imp for normal operation)
• ρ = resistivity of copper: 0.0172 Ω·mm²/m at 20 °C, about 0.021 at 70 °C
• A = conductor cross-section in mm²

Drop % = Vd ÷ string Vmp × 100. Power lost in the cable = I² × R, where R = 2 × L × ρ ÷ A.

Worked example 1: a long string on a small shop

A shop roof has a string of 5 modules (Vmp 41.6 V each, so 208 V) with Imp 12.98 A. The inverter is in the ground-floor store, 40 m of cable route away.

With 4 mm² copper: - R = 2 × 40 × 0.0172 ÷ 4 = 0.344 Ω - Vd = 12.98 × 0.344 = 4.47 V - Drop = 4.47 ÷ 208 = 2.15%: too high - Power loss = 12.98² × 0.344 = 58 W

With 6 mm² copper: - R = 2 × 40 × 0.0172 ÷ 6 = 0.229 Ω - Vd = 2.98 V, drop = 1.43%: acceptable - Power loss = 38.6 W

The upgrade saves about 19 W at full sun. Over a year that is perhaps 15–20 kWh, because losses scale with the square of current and shrink quickly at part sun. At ₹8 a unit that is about ₹120–160 a year, against roughly ₹1,800 more for 80 m of 6 mm² instead of 4 mm² (Sunlectric catalogue, about ₹22 a metre more). That is a slow payback; the real reason to upsize is to bring the drop inside the 2% limit.

The better fix here is design, not copper: a 10-module string at 416 V carries the same current, so the same 4 mm² cable gives only a 1.07% drop. Higher string voltage means lower percentage drop.

Worked example 2: a typical 5.4 kWp home

One string of 10 modules (416 V, 12.98 A), 25 m from the roof to the inverter, 4 mm² copper: - R = 2 × 25 × 0.0172 ÷ 4 = 0.215 Ω - Vd = 2.79 V, drop = 0.67% - At 70 °C cable temperature (ρ ≈ 0.021): 0.82%

4 mm² is fine. Most homes with one or two strings end up at 4 mm², and 6 mm² for longer routes or low-voltage strings.

Which cable to buy

Use proper solar DC cable, not ordinary PVC house wire or welding cable.

Aluminium is not used for string cables on homes: it does not crimp reliably into MC4 contacts. Larger plants use aluminium for main DC cables with proper lugs, which is a separate design job.

Colours and labelling

India does not have one universal colour rule for PV DC on homes, but the common and sensible practice is red for positive and black for negative, using factory-coloured cable. If you only have black cable, mark both ends of every cable with red and black sleeves and a printed tag ("String 1 +"). Never use green or green-yellow for anything except protective earth.

Routing: the details that matter

• Small loop area. Run the positive and negative of each string next to each other, from the array all the way to the DCDB. A large loop acts like an antenna: a nearby lightning strike induces a voltage in it that can kill the inverter input or the SPD.
• UV protection. Even UV-rated cable lasts longer in shade. Run cables under the modules and along the structure, then in UV-stabilised UPVC conduit, a GI conduit or a covered cable tray from the array to the inverter. Never leave cable lying loose on a terrace where people walk, water ponds or tank cleaners drag pipes.
• Support. Clip cables to the structure at short intervals (some DISCOM specs say every 50 cm) with UV-stable ties or stainless clips. Loose cables rub on sharp module frames in the wind and wear through.
• No sagging into the rain. Leave a drip loop before cable enters a box, and use cable glands at the bottom of enclosures, not the top.
• Rodents and monkeys. In many Indian towns monkeys pull at cables. Use conduit for any exposed run and fix it firmly.
• Separation from AC. Keep DC and AC in separate conduits, and label each DC conduit "Solar DC: live in daylight".

Key points

• DC cable must pass both an ampacity check (based on 1.25 × Isc, then derated) and a voltage-drop check.
• DC drop = 2 × L × I × ρ ÷ A; aim for about 1% and never exceed the 2% that most specs allow.
• Higher string voltage gives lower percentage drop for the same cable.
• Use 1.5 kV DC solar cable to EN 50618 or IS 17293, double insulated, tinned copper, UV-rated.
• Run + and − together to keep the loop area small, and protect cables in conduit or under the modules.

Check yourself

1. A 300 V string carries 13 A over a 30 m one-way route on 4 mm² copper. What is the voltage drop %?
2. Why is the length doubled in the DC drop formula?
3. Give two reasons not to leave DC cable lying on an open terrace.