DC Cable Sizing and String Design for a C&I Plant: Losses, Voltage Drop and Safety

DC cable for a C&I solar plant is sized on string current, with voltage drop kept low, insulation rated for string voltage and roof heat, and connectors that stay tight. MNRE's draft rooftop manual asks for DC cable loss below 2 percent. In the worked example, 4 mm² copper passes on a 60 m run and sits at the limit at 120 m.

What limits the length and voltage of a string?

A string is modules in series. More modules raise the voltage, which lowers the current, which lets a smaller cable carry the same power. The limit is the inverter and the module. MNRE's draft manual puts it plainly: the array's open-circuit voltage must not exceed the inverter's maximum input voltage at the lowest temperature. Modules produce more voltage when cold, so the number that matters is the winter morning, not the datasheet's 25°C value.

The worked example uses a 550 Wp-class module with Voc 49.5 V, Vmp 41.5 V, Imp 13.3 A, and a Voc temperature coefficient of −0.27 percent per °C. These are illustrative values; use your module datasheet. Twenty modules in series give 990 V Voc at 25°C and about 1,057 V at 0°C. An inverter with a 1,100 V ceiling has little margin left, and a 1,500 V system has more. The same 20 modules produce 830 V at maximum power, which is the voltage the cable drop is judged against. At high roof temperature the string voltage falls and must stay inside the inverter's MPPT window, so the check runs in both directions.

How is the cable size chosen?

Three tests apply, and the cable must pass all of them.

• Current rating. The cable must carry the string's short-circuit current with a margin set by the design standard, commonly 1.25 times Isc, after derating. Roof cables run in hot conditions, often tight against modules, so the temperature correction and the number of cables grouped together reduce what a nominal rating gives. Our glossary entry on cable derating explains the factors.
• Voltage drop. MNRE's draft manual asks that array interconnections, array to junction box, and junction box to inverter be sized to keep the DC cable loss below 2 percent, and the same limit for AC cables. Think of 2 percent as a ceiling for the whole run, not for each segment.
• Fault level and mechanical strength. Larger plants need to check the combined fault current where strings join, and every cable needs UV resistance and protection from abrasion.

For the voltage drop, use the two-conductor formula: drop equals current times resistance, with resistance taken as the resistivity times two times the one-way length, divided by the cross-section. For copper that is about 0.0172 ohm·mm² per metre at 20°C, rising by roughly 0.4 percent per °C, so use the conductor's hot resistance, not the cold one. You can test your own sizes with our cable size calculator. The note on cable sizing and voltage drop failures shows the same logic on the AC side.

What do losses look like for real cable sizes?

The table applies the formula to the example string: 830 V at 13.3 A, 11.0 kW, with the conductor at 70°C. These are calculated for illustration; your temperatures and lengths will differ.

Two readings matter. First, the 4 mm² cable at 120 m is at the 2 percent limit on this one segment alone, before any array interconnection or combiner-to-inverter run is added. Second, the loss is not free: on a 1 MW plant, which at about 120 units per kW per month in north India produces roughly 120,000 units a month, each 1 percent of DC loss is about 1,200 units a month. Because the loss repeats every sunny hour for the life of the plant, a larger cable on the longest runs can be worth the extra metal, which is why the arithmetic should be done per string group and not once for the plant.

Which cable and connectors should the specification name?

MNRE's draft manual asks for flexible, UV and weather-resistant multi-strand copper on the DC side, and it says multi-core cables shall not be used for DC cabling. For systems rated 1,500 V DC it names IS 17293:2020, the BIS standard for electric cables for photovoltaic systems. Some sources describe a Quality Control Order that makes BIS certification of solar DC cable mandatory, and others show it only as a draft, so we could not confirm its status. Ask the supplier for the BIS licence number and the test certificate regardless, and check the order's current status on the BIS site.

Connectors need equal care. The draft manual specifies MC4 or equivalent connectors, crimped with the proper tool and tightened with tools, not by hand. Mixing connector brands or using a crimp tool that does not suit the contact is a classic source of heat at a joint.

Why does DC arc risk deserve its own check?

Unlike AC, a DC arc does not extinguish itself when the current passes zero, so a loose connection can burn for as long as the string is lit. The MNRE draft manual lists the causes it sees: poor installation, wrong system design, underrated cables, loose connections, poor O&M, faulty equipment, no isolator switches, and DC cables damaged by mechanical stress or vermin. Almost every item is a workmanship or design check, not a product.

• Run strings in tray or conduit with UV-rated supports, not loose on the roof.
• Keep positive and negative of a string together to reduce loop area.
• Check polarity of each string before it is connected, and record open-circuit voltage and short-circuit current per string.
• Test insulation resistance, and ask for thermal imaging of connections and combiners after the first weeks of full load.
• Some inverters offer arc-fault detection. Ask whether it is fitted and what it does when it trips.

How do you check an EPC's string design?

Ask for four documents before award. A string table showing modules per string, Voc at the coldest expected temperature and the MPPT range at the hottest. A cable schedule with size, length, material and calculated drop for each run. The derating assumptions behind the current rating. And the make and standard of cable and connectors, with the BIS licence reference. If the same size is used for every run regardless of length, ask why. Our string versus central inverter entry explains how the choice of inverter changes where the DC runs end.

How Secured Engineers helps

• We design the plant in-house, including string design and a cable schedule with calculated drop per run.
• Fire NOC, CEIG and DISCOM net-metering filings are prepared in-house.
• MEPF and solar can sit under one contract.
• We work from offices in Noida (Delhi NCR) and Ludhiana, with 535+ projects across 18+ states and 89.77% repeat clients.
• If you hold another EPC's design, we review it against the checks above and tell you what to change.

Read how our solar EPC service is run, then message us on WhatsApp, call +91 70099 85817 or ask for a free proposal and send the string table you want reviewed.