Understanding disconnect switch sizing
Why disconnects exist as a distinct safety requirement
A disconnect switch gives utility workers, firefighters, and installers a clear, accessible point to fully de-energize a solar system for maintenance or in an emergency. Unlike a circuit breaker that's primarily there to protect wiring from overcurrent, a disconnect's main job is providing a definite, visible break in the circuit that anyone can confirm is open, which is why it's treated as a distinct code requirement rather than just another overcurrent device, in the same fire-and-life-safety spirit as the setback rules covered in the fire code setback loss calculator.
Why AC and DC disconnects are sized from different starting points
The AC disconnect sits on the inverter's output side and is sized from the inverter's AC current, calculated from its wattage rating and voltage. The DC disconnect sits on the array's input side and is sized from the combined DC current of all strings feeding into it, following the same current logic used in the combiner box sizing calculator and parallel strings current calculator. These are genuinely separate calculations even though both apply the same 125% continuous factor, drawn from the same conductor ampacity limits in NEC Table 310.16, before rounding to a standard switch size.
How disconnects differ from a transfer switch
It's easy to conflate disconnects with transfer switches since both involve switching equipment in a solar installation, but they serve different purposes. A disconnect fully isolates a circuit, cutting power entirely. A transfer switch instead routes power between two active sources, like utility grid power and battery backup power, without necessarily de-energizing anything, the distinction covered in the transfer switch sizing calculator. Many systems need both, and confusing one for the other during planning can lead to missing a required piece of equipment.
Where disconnects typically get installed
A DC disconnect commonly sits at or near the inverter's DC input, sometimes integrated directly into the inverter itself on newer models. An AC disconnect typically sits near the utility meter or main service panel, in a location accessible to utility personnel without needing to enter the building. Exact placement requirements vary by local code and utility, so it's worth confirming before finalizing an installation plan, and the connector and wiring inspection cost estimator covers what it costs to keep the wiring at both locations in good shape over time.
What this tool doesn't cover
This calculator handles amperage sizing only. Disconnects also need a voltage rating that matches or exceeds the system's operating voltage, and some jurisdictions specify additional requirements like a lockable, visible-break design, similar to the compliance details in the rapid shutdown compliance cost estimator. Always cross-check the final product selection against your local electrical code and installer's recommendation, or start with the Solar Energy Industries Association's plain-language solar overview if you're new to how this equipment fits into the bigger picture.