The one rule the rest of this hub hangs on
Every source of fault current gets its own protection, sized to the wire it feeds. In a solar system there are usually three sources: the array, the battery, and (on grid-interactive gear) the grid. Each one can dump energy into a short faster than you can react, and each needs an overcurrent device that clears before the wire becomes the failure point. Everything else on this page — gauge ladders, fuse-versus-breaker choices, series-versus-parallel strings — is downstream of that sentence.
Wiring mistakes don’t announce themselves. A slightly warm lug today is a melted terminal block next month; an undersized battery cable reads as “inverter shuts off under load” long before it reads as fire. Treat heat, sag, and mystery shutdowns as wiring data, not bad luck — our output troubleshooting and inverter shutdown checklists start from that idea.
Quick navigation (which page answers which question)
| Your question | The page that answers it |
|---|---|
| What gauge wire for this run? | Solar wire size (+ calculator) |
| Wire for the high-current battery-to-inverter run? | Battery cable size for inverters · Inverter cable size chart |
| Fuse or breaker, and where? | Fuses vs breakers |
| What amp rating for each circuit? | Fuse and breaker sizing (+ calculator) |
| How do I wire the array strings? | Series vs parallel solar panels |
| Do I need a combiner box / disconnect? | Combiner boxes and disconnects |
| Bigger system wiring (48V bank)? | 48V off-grid wiring guide |
| Grounding, bonding, surge protection? | Grounding and lightning protection |
| Arc-flash and terminal-fuse safety? | DC arc-flash safety |
The planning ladder (one table, entire site convention)
These are the planning ampacity values used across every calculator and guide on this site — deliberately conservative, short-run, copper:
| Gauge | Planning amp limit | Typical solar job |
|---|---|---|
| 10 AWG | 30 A | Branch circuits, one string to controller |
| 8 AWG | 45 A | Small array runs, 30 A controller output |
| 6 AWG | 65 A | Mid arrays, 60 A class controllers |
| 4 AWG | 85 A | Small-inverter battery runs at 12 V |
| 2 AWG | 110 A | 1–1.5 kW inverters at 12 V |
| 1/0 AWG | 125 A | 2 kW class at 12 V, or 48 V inverter inputs |
| 2/0 AWG | 145 A | 2–2.5 kW at 12 V; common LFP bank bus |
| 4/0 AWG | 175 A | 3 kW+ at 12 V; big-bank standard |
Two universal adjustments: runs over ~5 ft one-way step up one gauge, and anything voltage-drop-sensitive (controller-to-battery especially) targets under 3% drop — the wire size page runs both numbers. This ladder is planning guidance, not a code table: for permitted work, size from NEC 310.16 with your insulation’s temperature column plus 310.15 derating, meet 690.8(B)’s 125% rule on PV circuits, and your AHJ wins.
The protection map (where each device goes)
A typical off-grid system needs protection at four boundaries — each one guards a different source-against-wire pairing:
- Array → string fuses (in a combiner or inline holders): sized to Isc × 1.56, and only required when parallel strings can backfeed a faulted one — the sizing guide has the decision rule.
- Controller → battery fuse/breaker: rated to the controller’s manual, sized above the controller’s max output.
- Battery → inverter fuse: this is the Class T / MRBF territory — fast, DC-rated, near the terminal. See DC arc-flash safety for why the terminal distance matters.
- DC → loads: breakers or fused busbar, sized to each branch wire.
Above roughly 150 A of fault-current potential, marine-practice terminal fusing (Class T, MRBF) is the honest answer — automotive-style fuses don’t clear DC arcs reliably. The fuses-vs-breakers page compares the device classes honestly.
Start with the system goal (not the parts)
- If you’re still sizing: start at daily energy use and peak loads so wiring decisions match reality — how to size a solar system.
- If you already have hardware: verify controller and inverter limits first, then design wiring inside those boundaries.
- If you’re troubleshooting: treat hot wires and voltage sag as signals, not mysteries — start at low output.
- If you’re planning the whole 48 V build: the 48V wiring guide puts all of the above in one walkthrough.
The budget view
For what all these small parts cost as a line item: solar wiring and protection cost.
Next logical reads
12V vs 24V vs 48V solar systems · How to size an inverter · How to size a solar system · Solar system cost breakdown · MPPT vs PWM controllers · Solar components explained · Installation safety guide