Type 1 devices install on the line side of the service disconnect — utility side, before the main breaker. Because nothing upstream protects them, they're tested without external overcurrent protection and built to take direct-strike-class energy. In a solar context, a Type 1 at the service entrance is the first wall of defense against grid-side surges, and it's the only type permitted ahead of the main disconnect.

Type 2 devices install on the load side of the service disconnect — at panels, combiner boxes, and inverter terminals — and require external overcurrent protection. This is the workhorse type in PV systems: a Type 2 SPD at the inverter's AC output and another at the DC combiner covers the two doors surges walk through. Most DIN-rail solar SPDs, including the MidNite units we stock, are Type 2 (or Type 1CA/2CA component assemblies).

Type 3 devices are point-of-use protectors — receptacle strips and device-level suppressors — installed at least 30 feet of conductor downstream from the service so the upstream impedance has already knocked the surge down. They supplement Type 1/2 devices; they never replace them. A plug strip is not a surge protection strategy.

705.11 and related sections govern where PV interconnects to the service, and the surge protection conversation follows the interconnection point: a supply-side (line-side) tap puts your inverter's AC terminals on the unprotected side of the main breaker unless a Type 1 SPD covers the service. A load-side breaker connection inherits whatever service-level SPD exists — but that device protects the house from the grid, not the inverter from the array.

690.35 and the Article 690 general requirements don't mandate SPDs in every PV system outright, but the code increasingly assumes them, and many AHJs require DC-side SPDs on ground mounts and long home runs as a condition of approval. More importantly, lightning protection for PV arrays (where a lightning protection system is installed per NFPA 780) requires SPDs at both ends of the DC run — array and inverter — to bond the LP system and the PV system against each other's transients.

The 2020 NEC added 230.67: all services supplying dwelling units must have a Type 1 or Type 2 SPD at the service. The 2023 code extended the requirement to feeder replacements and panel changes — swap a panel on a house and you trigger the SPD requirement. For solar installers doing panel upgrades as part of a PV project, this is now a line item on every quote, not an upsell.

MCOV must exceed the array's maximum open-circuit voltage at the coldest design temperature — not the nameplate Voc at STC. Module Voc rises roughly 0.3%/°C below 25°C for typical crystalline silicon (check the datasheet's temperature coefficient; NEC 690.7 Table 690.7(A) gives the official correction factors, up to 1.25× at −40°C). A string with a 480 V STC Voc in a climate that sees −10°C corrects to roughly 480 × 1.14 ≈ 547 V — so the 600 V SPD is the correct device, and the 500 V unit is a smoke machine waiting for January.

The AC side is simpler: match the SPD's MCOV to the service voltage class with headroom for sustained overvoltage. For a 120/240 V split-phase residential service, the standard MCOV is 150 V line-to-neutral / 300 V line-to-line. For 120/208 V three-phase, 150/300 V class devices apply; for 277/480 V commercial services, spec 320 V L-N / 640 V L-L MCOV. The service-entrance SPD (Type 1 or 2) should carry Imax of at least 40 kA per phase for a dwelling and 65–100 kA for commercial services with rooftop arrays.

Residential job: two strings of 14 modules, STC Voc 49.5 V each → 693 V string Voc... no — this is why we do it on paper. Real case: strings of 10 × 48.2 V = 482 V STC, coldest recorded −15°C, module coefficient −0.29%/°C. Temperature delta: 40°C below STC → Voc multiplier 1 + (0.0029 × 40) ≈ 1.116. Corrected Voc ≈ 482 × 1.116 ≈ 538 V. Spec a 600 V MCOV DC SPD at the inverter (never a 500 V unit), plus a 150/300 V Type 2 SPD at the main panel covering the AC side, plus — because the house got a panel upgrade — the 230.67 service SPD requirement is satisfied by that same Type 2 device. Three line items, under $400 of material, protecting a $9,000 inverter and a $30,000 array.

Here is where most SPD installs fail, and it has nothing to do with the device. Every inch of lead between the SPD and the bus adds inductance, and a fast-rising surge sees that inductance as voltage: roughly 1,000 volts of added let-through per meter of lead at surge risetimes — call it ~25 V per inch as a planning number. A 1,500 V VPR device on 30 inches of spaghetti passes 2,200+ V to your inverter. Keep total lead length — both the line tap and the ground return — under 10 inches, twist the leads together, and land the ground directly on the panel's ground bar, not daisy-chained through an enclosure stud. I've scoped inverters that died with a perfectly good SPD mounted 3 feet away; the device did its job and the leads undid it.

The SPD's ground is the surge's exit path, and it has to be short, fat, and direct. Use the SPD's specified conductor size (commonly 10–6 AWG), bond to the same ground bar the inverter EGC uses, and never coil excess lead — a coil is an inductor by definition. The SPD ground is not a current-carrying conductor in normal operation, so it doesn't follow 250.122 sizing; it follows the manufacturer's instructions, which are conditions of the listing.

Serious installations layer SPDs so each stage absorbs what it can and passes a smaller remnant downstream:

• Stage 1 — Service entrance: Type 1 or heavy Type 2, 65–100 kA, at the main panel. Eats grid-side surges and lightning-induced service transients.
• Stage 2 — Inverter AC and DC terminals: Type 2 devices at the inverter's AC disconnect and DC combiner. This is the layer that saves the inverter specifically. Pair it with a proper safety disconnect and the XW power distribution panel or equivalent gutter space so lead lengths stay short.
• Stage 3 — Sensitive electronics: Type 3 point-of-use for monitoring gateways like the Conext Insight Home, network gear, and control boards.

MidNite's MNSPD line is the default in the North American off-grid and residential PV world for a reason: the 300 V and 600 V DC versions match common battery and string voltages, they're compact enough to mount inside combiner and e-panel enclosures, and the price is right for putting one on every box. We stock MidNite alongside the rest of our surge protection lineup.

Schneider's surge line covers the AC side comprehensively, and it integrates naturally with Schneider-based systems — if the job is built around an XW platform with the XW conduit box and distribution panel, a Schneider AC SPD keeps the ecosystem and the warranty conversation simple. For whole-home service protection on generator-backed properties, the Champion PSP whole-house unit (120 kA) is the value pick we sell alongside standby generator packages.

Phoenix Contact and ABB dominate utility-scale DC protection up to 1,500 V. Eaton's residential and light-commercial SPDs are solid, widely stocked at supply houses, and their CHSP series is a common 230.67 compliance answer. Whatever the brand, the checklist doesn't change: UL 1449 listing, MCOV matched to corrected system voltage, leads under 10 inches.