GetWattLogic

Solar cable sizing is the least-glamorous part of an off-grid build — and the one most likely to burn something down. Undersized wire doesn't just waste energy as heat; at high enough current it can melt insulation, damage terminals, and in the worst case start a fire. The good news: the math is simple, the tables are standard, and getting it right costs you maybe $20 more than getting it wrong.

Here's how to size solar cable the easy way, without an electrical engineering degree.

The three things cable size has to handle

Every wire in a solar system has to survive three stresses:

1. Current (amps) — the safety limit. Wire has a maximum safe current rating. Exceed it and the wire overheats. This is non-negotiable.
2. Voltage drop — the efficiency limit. Even a safe wire loses some energy as heat over its length. Too much loss and your panels' power never reaches the battery. This is the "performance" concern.
3. Temperature — the derating factor. Wire in a hot attic or engine bay carries less current safely than wire in cool air. Ambient temperature matters.

For most off-grid builds, current and voltage drop are the two you calculate. Temperature derating matters in extreme environments (engine bays, attics in Phoenix).

Step 1 — find your current

The current your array produces depends on how you wire it:

• Parallel: currents add (two 10A panels in parallel = 20A)
• Series: voltage adds, current stays the same (two 10A panels in series = 10A)

Always use the panel's short-circuit current (Isc) — the maximum current it can produce — and multiply by 1.25 for the NEC safety factor. A panel with 10A Isc needs wire rated for at least 12.5A.

Step 2 — pick a wire size that handles the current

Here's the standard ampacity table for copper wire (NEC, 90°C insulation, single-wire in free air):

AWG	Max amps (chassis wiring)	Common use
10 AWG	40A	Small array wiring, panel leads
8 AWG	55A	Mid-size arrays
6 AWG	70A	Larger arrays, short runs
4 AWG	95A	Battery-to-inverter, large arrays
2 AWG	130A	Large inverter runs
1/0 AWG	150A	Big inverter, 2,000W+ at 12V
4/0 AWG	200A	Huge inverter, 3,000W+ at 12V

For most panel-to-controller runs, 10 AWG is the starting point (40A capacity, covers arrays up to ~500W on a 12V system). For controller-to-battery and battery-to-inverter, you'll usually need thicker.

Step 3 — check the voltage drop

This is the step everyone skips and where the real loss happens. Voltage drop depends on current, length (round-trip), and wire size:

Voltage drop (%) = (2 × Length_ft × Current × 0.0001) ÷ Wire_circular_mils

A simpler rule: aim for under 3% voltage drop on solar runs, under 2% on battery-to-inverter runs (battery runs are shorter but carry much higher current, so the loss is proportionally more important).

Quick reference for a 10A run (common 200W panel):

Length (round-trip)	10 AWG drop	8 AWG drop	6 AWG drop
5 ft	0.6%	0.4%	0.2%
10 ft	1.2%	0.8%	0.5%
20 ft	2.5%	1.6%	1.0%
30 ft	3.7%	2.3%	1.4%
50 ft	6.2%	3.9%	2.5%

At a 20-foot run with 10A on 10 AWG, you lose 2.5% — acceptable. At 50 feet, you lose 6.2% — bump to 6 AWG or shorten the run.

The two runs that matter most

1. Battery to inverter (the critical one)

This is where undersized wire causes fires. A 2,000W inverter on a 12V battery pulls 167A at full load. At that current, even a short run needs thick wire:

Inverter watts	12V current	Min wire (3 ft run)	Min wire (6 ft run)
1,000W	83A	4 AWG	2 AWG
2,000W	167A	2 AWG	1/0 AWG
3,000W	250A	1/0 AWG	4/0 AWG

Never undersize this run. A 2,000W inverter on 6 AWG wire will overheat at full load. The cost difference between 4 AWG and 1/0 AWG for a 6-foot run is ~$25 — do not skip it.

2. Panel to charge controller (the efficiency one)

This run is longer (panels on the roof, controller in the cabin) and the current is lower, so voltage drop is the main concern. Use the table above. If your run is over 30 feet at 10A, step up from 10 AWG to 8 AWG.

Pro tip: wiring panels in series raises voltage and lowers current, which lets you use thinner wire over long runs. A 400W array as 2 series × 2 parallel produces 24V at 20A — at a 40-foot run, that needs 6 AWG. The same array in pure series produces 48V at 10A — at 40 feet, 10 AWG is fine. Higher voltage = thinner wire = lower cost. See our 12V vs 24V vs 48V system guide.

The temperature derating factor

If your wire runs through a hot attic or engine bay, the safe current capacity drops. At 40°C ambient, reduce ampacity by ~10%. At 50°C (Phoenix attic in summer), reduce by ~15%. If you're sizing wire in a hot location, go one size larger than the table suggests.

Fuses and breakers

Wire size only protects you if you have overcurrent protection. Every wire segment should have a fuse or breaker sized to the wire's ampacity, not the load's current. A 10 AWG wire rated at 40A needs a 30–40A fuse. If the current exceeds the wire's rating, the fuse blows before the wire melts.

Critical fuse locations:

• Between battery and inverter — sized to inverter max current
• Between charge controller and battery — sized to controller output
• Between solar array and charge controller — sized to array Isc × 1.56
• On every parallel battery string — to protect against one battery shorting into another

The short version

1. Find your current (array Isc × 1.25, or inverter watts ÷ system voltage).
2. Pick wire that handles the current from the ampacity table.
3. Check voltage drop for your run length — bump up a size if it exceeds 3% (solar) or 2% (battery-to-inverter).
4. Add 1 size in hot environments.
5. Fuse every wire segment to the wire's rating.

For a typical van build (400W array, 2,000W inverter, 12V system), the wire list looks like: 10 AWG panel-to-controller, 6 AWG controller-to-battery, 2 AWG battery-to-inverter, 1/0 AWG if the inverter run is over 6 feet. Total wire cost: ~$80–150.

If you want the whole system sized together — panels, battery, inverter, controller, and the wiring math handled automatically — the GetWattLogic calculator does it. For cables, fuses, and the hardware to assemble a safe system, see the Recommended Gear page.