Solar installation fuses and protections: what goes where

A well-protected solar system has at least 5 protection points. Skip one and you can ruin thousands of dollars of gear or burn the house down. Here's each one with the rationale.

1. String fuse (array to controller)

Only needed with 3+ strings in parallel. gPV type, 10-15 A based on panel Isc. Without it, a shorted panel becomes a sink for the other strings and ignites.

2. PV DC disconnect (controller-panels)

Lets you isolate panels from controller before touching anything. 600 V DC minimum, rated for 2× array Isc. Must be DC-rated: an AC breaker welds shut on a DC arc.

3. Fuse/breaker between controller and battery

MIDI or MEGA type at 1.25 × controller rated current. 60 A MPPT → 80 A fuse. If the controller shorts out, this fuse saves the battery from a 2000 A dump.

4. Class T fuse between battery and inverter

The critical one. Large LiFePO4 packs can discharge 2000-4000 A on short. Only Class T (20 kA breaking capacity) interrupts that without exploding. Mount within 12 inches of the bank.

5. 30 mA GFCI in AC panel

What stops you dying if you touch a bare wire. Press test monthly. Without GFCI: floating inverter output = lethal hazard.

6. Branch breakers per circuit

One per AC circuit: lighting 15 A, outlets 20 A, kitchen 30 A. Protects against shorts and overload. Curve C in EU, standard UL in US.

7. Surge protection device (SPD)

Type II on PV line and another on AC line. Diverts nearby lightning surges to ground before they kill the controller. $35-60 each, saves thousands. Mandatory in storm-prone areas.

Size everything with the wire gauge tables and review the most expensive install mistakes.