DC Cable Size Calculator
Enter your current, cable run length, and system voltage. We calculate the required conductor cross-section and show you which standard cable size you need.
Cable Run Details
Enter your system parameters to find the correct cable cross-section.
How voltage drop is calculated
Every metre of cable has resistance. When current flows through it, you lose voltage - and therefore power. The voltage drop formula for DC is:
Voltage drop (V) = 2 x Current (A) x Distance (m) x Resistance (Ω/m)
The factor of 2 accounts for both the positive and negative conductors (round trip). For example, 30A through 10m of 6mm² cable (0.00308 Ω/m) at 48V:
Drop = 2 x 30 x 10 x 0.00308 = 1.85V, which is 3.9% of 48V. That's 55W of power lost as heat in the cable.
For solar PV, aim for under 2% drop from panels to MPPT. For battery to inverter, under 1% is standard because you can't afford voltage sag under heavy loads.
Standard DC Cable Sizes
Copper, single-core, PV-rated reference
| Cross-section (mm²) | Max current (A) | Resistance (Ω/km) | Typical use |
|---|---|---|---|
| 1.5 | 15 | 12.1 | Small panels, short runs |
| 2.5 | 21 | 7.41 | Single 400W panel |
| 4 | 28 | 4.61 | Two panels in series, short runs |
| 6 | 36 | 3.08 | Most common PV cable, up to 3kW |
| 10 | 50 | 1.83 | Battery interconnects, 48V systems |
| 16 | 68 | 1.15 | Inverter DC input, 5kW+ |
| 25 | 89 | 0.727 | Large battery banks, long runs |
| 35 | 110 | 0.524 | Main battery cables, 10kW+ |
| 50 | 135 | 0.387 | Industrial / large arrays |
Ratings are for PVC-insulated single-core copper in free air at 30°C ambient. Derate for conduit, high ambient temperatures, or bundled cables. For PV use, always choose double-insulated (PV1-F or H1Z2Z2-K) cable rated for DC use at 1kV+.
Frequently Asked Questions
What cable type do I need for solar panels?
Use PV1-F or H1Z2Z2-K cable. These are double-insulated, UV-resistant, and rated for 1kV DC or higher. Standard twin-and-earth (used in AC house wiring) is not suitable for DC solar circuits. The outer sheath must be black and marked "PV1-F" or "H1Z2Z2-K".
Can I use the same cable for AC and DC?
No. DC solar cable is different from AC mains cable. Solar DC cable has tinned copper conductors (for corrosion resistance), cross-linked polyethylene insulation (for heat and UV), and is rated for continuous DC voltage. AC twin-and-earth has bare copper, PVC insulation, and is not UV-rated.
What size cable for battery to inverter?
This is the most demanding cable in your system. At 48V, a 5kW inverter draws about 105A at full load. For a 2m run at that current, you need at least 35mm² cable. At 12V, the current is four times higher for the same power - so cable sizing becomes critical. Many 12V van installs use 50mm² for the inverter connection, even over short distances.
Do I need ferrules or lugs on DC cable?
Yes. Bare stranded copper should never go directly into a screw terminal. Use bootlace ferrules for cables up to 16mm² going into MPPT or inverter terminals. Use crimped ring terminals (lugs) for battery connections. A proper crimp tool is essential - loose connections are the most common cause of electrical fires in solar installations.
What is a safe voltage drop percentage?
Aim for under 2% from solar panels to MPPT, and under 1% from battery to inverter. Higher drops waste power as heat, increase cable operating temperature, and can cause MPPT tracking issues or inverter under-voltage shutdowns. The 3% figure is the commonly cited upper limit for non-critical PV-to-MPPT runs.
Recommended Solar Cable & Accessories
Buy quality PV-rated cable, MC4 connectors, and crimping tools from trusted UK suppliers. These are the components that will safely carry your DC current for decades.
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