DC cabling
DC string cable voltage drop
- where
- conductor resistance at operating temperature (Ω/km)
- conductor resistance at 20 °C (Ω/km)
- conductor operating temperature (°C)
- voltage drop (V)
- cable route length, one way (m)
- string operating current (A)
- voltage drop as a percentage (%)
- string voltage at maximum power (V)
- power lost in the cable (W)
Explain this equation
Works out the voltage drop and power loss in the DC cable between a PV string and the inverter. Use it to choose a cable size that keeps losses low.
Copper resistance rises with temperature by about 0.393% per degree, which corrects the 20 °C table value to the working temperature. Current goes out and back, so the length is doubled. Resistance is per kilometre, so the 1000 converts to metres. Drop is current times resistance; as a percentage it is compared with the string voltage. Drop times current gives the watts turned into heat.
Ohm's law on a two-wire run, with the usual temperature correction for copper.
Sense check: 20 m of 4 mm² at 13 A and 70 °C drops about 3.2 V, under 1% of a 400 V string, losing about 41 W.
Watch out: use the resistance for the actual conductor (tinned, class 5 PV cable is slightly higher than plain copper) and the one-way route length.
Voltage drop and power loss in the positive and negative cables of a PV string between the array and the inverter.
Enter your values to see the result.
Good to know
The 2 accounts for the go and return conductors. PV cable uses fine-stranded tinned copper (class 5), whose resistance is higher than plain solid copper: about 8.21, 5.09, 3.39 and 1.95 Ω/km at 20 °C for 2.5, 4, 6 and 10 mm². Copper resistance rises by 0.393% per °C. Use Imp and Vmp at STC for the design check. MIS 3002 says 1% is practicable for domestic AC cabling; aim for the same on DC.
Common mistake
Using the one-way length without the factor of 2, or the 20 °C resistance for cable that runs hot behind modules.
Limits
Resistive drop only; DC has no reactance. Ignores connector resistance. Use the cable maker's declared resistance where available.
Sources
- IEC 60228:2004 Conductors of insulated cables (Table 3, class 5 flexible tinned copper, maximum resistance at 20 °C: 2.5 mm² 8.21, 4 mm² 5.09, 6 mm² 3.39, 10 mm² 1.95 Ω/km; temperature coefficient 0.00393 /K) – as used for BS EN 50618 PV cables
- MCS MIS 3002:2025 Solar PV installation standard, Issue 1.0 (3.6.8: 1% voltage drop practicable for domestic AC cabling, up to 3% for larger systems; 3.9.6: no modules within 400 mm of any edge of a domestic roof unless extra measures are taken; BS 7671 takes precedence)
- IET Code of Practice for Grid-connected Solar Photovoltaic Systems, 2nd edition (2022)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.