Surge protection (SPD) for solar

A surge protective device (SPD) is inexpensive insurance for the most expensive parts of a solar system. It clamps down voltage spikes from lightning or utility transients before they fry an inverter or battery. Knowing the SPD types and where each goes keeps the whole array protected. Here is the practical picture.

What an SPD does

Surges are brief, high-voltage events — a nearby lightning strike, a utility switching transient, or a fault elsewhere on the grid. An SPD provides a low-impedance path to ground only during these events:

• At normal voltage it does nothing and passes no current.
• During a surge it conducts, diverting the excess energy to ground.
• After the surge it returns to its non-conducting state.

This protects inverters, batteries, monitoring electronics, and connected appliances from damage.

SPD types

The NEC and UL 1449 classify SPDs by where they can be installed relative to the service overcurrent device:

• Type 1 — rated for the line side of the main service disconnect (before the main breaker). Built to survive larger, direct surges, including lightning-related events entering from the utility.
• Type 2 — installed on the load side of the main breaker, protecting downstream equipment such as inverters and subpanels.
• Type 3 — point-of-use devices near individual equipment, used as supplemental protection.

Type 1 and Type 2 are the ones that matter most in solar. A Type 1 device can go at the service; Type 2 devices commonly protect the inverter and combiner.

AC side and DC side

Solar systems have both AC and DC circuits, and both can carry surges:

• AC side — an SPD at the service or inverter AC output protects against grid-borne transients and helps shield the whole home.
• DC side — long PV conductor runs on the roof act like an antenna for induced surges. A DC-rated SPD near the inverter or in the combiner protects the inverter's DC input. DC SPDs must be rated for the system's DC voltage — never use an AC-only device on a DC circuit.

Many string inverters include integrated Type 2 SPDs; check the datasheet before adding external ones.

Where it belongs in the design

Placement follows the principle of protecting equipment as close to the threat as practical:

• Type 1 or 2 at the service to guard the whole installation.
• Type 2 (AC and DC) at the inverter, the equipment most worth protecting.

The SPD, its type, and its location appear on the one-line diagram. Grounding matters here too — an SPD is only as good as its connection to the grounding system, covered in NEC 250.4 grounding and bonding basics.

Match the SPD type to its position — Type 1 at the service, Type 2 protecting the inverter on both AC and DC — and confirm DC voltage ratings against the datasheet. A licensed electrician or PE should review the final design. See the full drawing list in what is in a solar permit set.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.