Wiring & Safety

6 min read Last updated March 1, 2026

Previous: String Design | Batteries

This is the section nobody gets excited about, and it’s the section that keeps your house from catching fire. Every connection, every wire gauge decision, every fuse and disconnect is here for a reason. Don’t skip it, don’t cheap out on it, and don’t guess.

Wire Gauge

Wire gauge is determined by two things: the current it needs to carry (ampacity) and the distance it needs to travel.

Voltage drop is the loss of electrical pressure over distance. For PV runs (panels to inverter), aim for 2-3% voltage drop or less. For battery runs (battery to inverter), aim for 1-2% or less — batteries operate at lower voltage, so the same resistance causes a proportionally bigger loss.

Temperature derating also matters. Wire ampacity ratings assume a standard ambient temperature. On a hot roof in summer, the actual capacity of your wire is lower than the label says. PV wire is rated for higher temperatures than household wire, which is one reason it’s required for outdoor solar runs.

There are voltage drop calculators online — plug in your current, your distance, and your wire gauge, and they’ll tell you the percentage drop. Use them. This is not a place for guesswork.

Rule of thumb: Shorter wire runs are always better. When you’re deciding where to put your inverter and batteries, proximity to the panels matters. A 20-foot run from a garage roof to an inverter below is straightforward. A 100-foot run from a detached shed needs careful attention to wire sizing and conduit routing.

PV Wire vs. THWN-2 vs. NM-B

Wire that runs from your panels to your equipment needs to be rated for outdoor use. PV wire (also called solar cable or USE-2/PV wire) is UV-resistant, weather-resistant, and rated for the voltage and temperature conditions on a roof.

Don’t run regular household Romex (NM-B) between your panels and your inverter. It’s not designed for that environment and it’s not safe. PV wire costs a bit more, but it’s engineered for exactly this job — exposed to sun, rain, heat, and cold for decades.

Where each wire type goes:

  • PV wire (USE-2) — Outdoor, panel-to-equipment runs. UV and weather rated. The only acceptable choice for exposed solar runs.
  • THWN-2 — Inside conduit for runs between equipment. Moisture and heat rated. Good for conduit runs in protected locations.
  • NM-B (Romex) — Interior household wiring only. Never outdoors, never in wet conduit, never between panels and inverter.

Conduit

Wire runs from the roof to the equipment should be in conduit — a protective tube that shields the wire from weather, physical damage, UV, and animals. Common choices:

  • EMT (metal) for exposed runs along walls and ceilings
  • PVC (plastic) for underground runs or where flexibility is needed

Size the conduit for the number and gauge of wires you’re running, with room for future additions if you plan to expand. Running a slightly larger conduit now is cheap insurance against needing to pull additional wires later.

Transfer Switch

The transfer switch is how your solar/battery system connects to your home’s electrical panel. It lets you select which circuits run off solar and which stay on grid. When the grid goes down, it isolates your solar circuits from the grid — this is critical for safety. You don’t want to backfeed a dead grid line that a lineworker thinks is safe to touch.

Manual transfer switches require you to physically flip switches to change circuits between grid and solar. Simple, cheap, reliable. You choose the transfer state. Most DIY builds start here.

Automatic transfer switches (ATS) detect grid loss and switch automatically — usually fast enough that lights don’t even flicker. More convenient, more expensive, more complex. Worth it if you have medical equipment or need guaranteed uninterrupted power.

Overcurrent Protection

Fuses and circuit breakers protect your equipment and wiring from fault conditions — short circuits, ground faults, overcurrent events. You need overcurrent protection at every major junction in your system:

  • Between the panel array and the inverter — DC fuses or breaker, sized for the string’s maximum short-circuit current
  • Between the battery bank and the inverter — DC fuse or breaker, sized for the battery’s maximum discharge current
  • On the AC output side — breakers in your transfer switch or subpanel

Use the right fuse or breaker ratings for each location. Oversized protection doesn’t protect — a 60A fuse on a 20A circuit won’t trip before the wire overheats. Undersized protection nuisance-trips and drives you crazy. The specs for each location come from your component datasheets and wire gauge ratings. Match them.

Disconnects

A disconnect is a switch that physically breaks the circuit, allowing you to de-energize a section of your system for maintenance or in an emergency.

You need DC disconnects between your panels and inverter (so you can safely work on wiring without live solar voltage) and AC disconnects on the output side. Some inverters have built-in disconnects; others require external ones.

Grounding

Every metal component in your system — panel frames, mounting rails, inverter chassis, battery rack — needs to be bonded to a common ground and connected to a grounding electrode (ground rod). This provides a safe path for fault current and protects against electrical shock.

Grounding isn’t complicated, but it needs to be thorough. Every piece of metal that could become energized during a fault needs a path to ground. Use proper grounding lugs, copper ground wire, and ground rod clamps. Don’t improvise here — use the hardware designed for the job.

Equipment grounding bonds all metal enclosures to a common ground conductor. Grounding electrode system connects the equipment ground to earth via a ground rod (or rods). Both are required.

The System Map

Here’s the full component chain from panels to house, with protection at every junction:

Every arrow represents a wire run. Every bold component has a specific job:

  • Combiner box brings multiple panel strings together
  • DC disconnect lets you isolate the inverter from solar
  • Battery OCPD protects the inverter and wiring from battery fault current
  • AC disconnect isolates the inverter output
  • Transfer switch selects which circuits get solar power

This is the skeleton of your system. When you’re planning your build, sketch this out with your actual components and wire runs. It becomes your wiring blueprint.

Label Everything

A label maker is one of the best $30 investments in your entire build. Label both ends of every wire run, label every breaker and fuse with its rating and what it protects, and label every disconnect with what it isolates. Labels are also a safety essential — anyone working on the system needs to know which wires are potentially live and what each disconnect controls.

What’s Next

You’ve designed every component: panels, inverter, strings, batteries, and all the wiring and protection between them. Time to see how it all comes together in a real system.

Next: Worked Examples →

See also: String Design | Batteries | Worked Examples

DATA SOURCED FROM: National Electrical Code (NEC) wiring and overcurrent protection standards, UL listing requirements for PV wire and disconnect hardware, manufacturer installation specifications, 2026.