Cable is the least interesting part of a solar system and one of the few that can start a fire. Undersized wire does two things at once: it drops voltage, so you lose energy you already paid to generate, and it turns that lost energy into heat inside the insulation. Both get worse as current rises, which is why low-voltage systems need much heavier cable than people expect.
The two rules
Ampacity — the wire must safely carry the current, with derating for temperature and for conduit fill. This is a safety limit and it is not negotiable.
Voltage drop — the wire should lose no more than about 3% of voltage over its length. This is an efficiency target, and code treats it as a recommendation rather than a hard requirement, but ignoring it wastes real energy for twenty-five years.
Size for ampacity first, then check voltage drop. Voltage drop very often demands a larger wire than ampacity alone would.
Why voltage matters more than distance
Voltage drop depends on current, and current for a given power falls as voltage rises. The same 1,000W over the same 40-foot run:
| System voltage | Current | Wire for 3% drop over 40 ft |
|---|---|---|
| 12V | 83 A | 2 AWG — very heavy, expensive |
| 24V | 42 A | 6 AWG |
| 48V | 21 A | 10 AWG |
| 150V (PV string) | 6.7 A | 14 AWG — thin and cheap |
That table is the single strongest argument for higher system voltage, and for MPPT controllers that let you run panels in series at high voltage rather than in parallel at high current. The copper saved routinely exceeds the controller premium — see MPPT versus PWM.
The calculation
Voltage drop (V) = 2 × length × current × resistance per foot
The 2 accounts for the current travelling out and back — always measure the round-trip conductor length, not the one-way distance, which is the most common arithmetic error here. Then divide by system voltage for the percentage.
Example: 20A over a 30-foot one-way run on a 48V system, using 10 AWG at roughly 0.00099 Ω/ft: 2 × 30 × 20 × 0.00099 = 1.19V, which is 2.5% of 48V. Inside the target.
Where each run sits
- Panel to combiner or controller — usually high voltage and low current, so thin wire is fine. Must be sunlight-resistant PV wire rated for outdoor UV and temperature
- Controller to battery — the shortest, fattest run in the system. High current at low voltage, and this is where undersizing does the most damage
- Battery to inverter — the highest current anywhere, often requiring 4/0 cable on large systems. Keep it as short as physically possible
- Inverter to loads — standard AC wiring, sized by normal circuit rules
The mistakes that recur
- Measuring one-way instead of round-trip. Halves your calculated drop and undersizes the wire.
- Using indoor wire outdoors. Standard THHN degrades under UV; exterior DC runs need PV wire or conduit.
- Ignoring temperature derating. Ampacity tables assume a reference ambient; a hot attic or a full conduit reduces what a given wire may carry.
- Undersizing the battery-to-inverter cable, which carries the highest current in the system and is the run most likely to get hot.
- Poor terminations. A loose or badly crimped lug creates resistance, and resistance at high current creates heat. More fires start at connections than in the middle of a cable.
When in doubt, go one size up. The extra copper is cheap against twenty-five years of losses, and it is the cheapest safety margin in the whole build. Grounding conductors follow separate rules — see grounding a DIY solar system.