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Solar cable section
Choosing the correct cable section is a safety decision as much as a performance decision. An undersized cable creates voltage drop, heat buildup, wasted energy, and premature insulation aging.
SolarDim Pro sizes cables in the Python solardim-calc engine. The calculation uses current, one-way length, voltage, allowed drop, conductor material, and ambient temperature.
Voltage drop is the voltage lost in the cable because every conductor has resistance. If a 12 V battery or charge controller only receives 11.5 V because of cable losses, performance drops quickly. In high-current systems, even 0.5 V can become significant heat loss.
| Circuit | Target drop | Note |
|---|---|---|
| PV DC | 2% | Isc current with safety margin |
| Battery DC | 1% | High current, short but critical cable |
| AC output | 3% | Minimum section 2.5 mm2 |
| Pump DC | 2% | Motor startup margin |
| Lighting / small circuits | 3-5% | Acceptable only for low-current runs |
S = (2 x L x I x rho) / dVS in mm2, L one-way length, I current, rho resistivity, dV allowed voltage drop. The factor 2 covers the positive and negative round trip.
For copper, SolarDim Pro uses a resistivity close to 0.0175 ohm.mm2/m. For aluminium, the required section is larger: use roughly 1.6 times the copper section to carry the same current with a similar drop.
The result is rounded up to the next standard section and checked against temperature-derated ampacity.
A cable can satisfy voltage drop and still be unsafe if its ampacity is too low at 40C or 45C. SolarDim Pro increases the section until both checks pass.
| AWG | Approx. section | Typical ampacity |
|---|---|---|
| 10 AWG | 5.26 mm2 | 30-35 A |
| 8 AWG | 8.37 mm2 | 40-55 A |
| 6 AWG | 13.3 mm2 | 60-80 A |
| 4 AWG | 21.2 mm2 | 85-100 A |
| 2 AWG | 33.6 mm2 | 110-150 A |
The entered length is one-way length. SolarDim Pro already applies the return path in the formula, so measure the physical distance between the two devices, not the round trip total.
Higher system voltage reduces current. For the same power, moving from 12 V to 48 V divides current by 4 and often allows a much smaller cable section.
Keep high-current runs short
Place batteries, inverter, and charge controller close together. The battery-to-inverter cable is often the most critical run.
Check real installation conditions
Hot roofs, conduits, bundled cables, or direct sun exposure require derating. Use the manufacturer datasheet whenever available.
Use proper terminations
Use suitable lugs, clean crimping, heat shrink, and mechanical protection. A poor connection can overheat even when the cable section is correct.
Use parallel cables carefully
For very high current, identical cables may be paralleled. They must have the same section, length, and route so current is shared correctly.
Common mistakes
- Measuring round-trip length and entering it as-is (the formula already doubles it)
- Checking only voltage drop without checking temperature-derated ampacity
- Mixing copper and aluminium sections without applying the 1.6× factor