Whole-House Surge Protection
Why this matters
The morning after a thunderstorm the call list fills with dead boards: a furnace that will not start, a garage door opener that ignores the remote, a pool controller with a blank screen. Some of those were surges. Some were a wiring short or a board that was going to die that week anyway, and a tech who writes "lightning damage" on every one of them hands the customer's insurer a claim the evidence does not support. Since the 2020 National Electrical Code, there is also a code question attached: in a jurisdiction that adopted that edition or later, replacing a dwelling's service equipment brings a surge-protective device requirement with it.
This card owns the code requirement and what surge damage actually looks like. Device categories and the install steps are in the existing Surge Protection Reference; this card does not repeat them.
What the code requires, and where
NEC 230.67, in the 2020 edition and later, as adopted by your jurisdiction. The 2020 edition requires that all services supplying dwelling units be provided with a surge-protective device (SPD), a device that limits transient overvoltage by diverting or limiting surge current. Its parts, as written in the 2020 edition:
- The requirement: a surge-protective device on every service supplying dwelling units.
- Location: an integral part of the service equipment or immediately adjacent to it, with an exception allowing it at the next level of distribution equipment downstream toward the load.
- Type: a Type 1 or Type 2 device.
- Replacement: where service equipment is replaced, the requirement applies. This is the clause service shops meet most. A panel change on a 1970s house in a 2020-NEC jurisdiction now includes an SPD, whether or not the customer asked.
The 2023 edition extended the rule to more occupancies and changed some details, so read the edition your authority having jurisdiction (the AHJ, the inspector or office that enforces the code) actually adopted. Adoption lags: some states and towns still enforce the 2017 edition or earlier, where 230.67 does not exist and an SPD is optional. The installation rules for SPDs live in Article 242 in the 2020 edition (Article 285 in earlier ones).
Who it binds. The NEC is a model code published by NFPA. It becomes law only when a state or local government adopts it, and it reaches the electrician through the permit. An HVAC or appliance tech replacing a board is not doing service equipment work and 230.67 does not attach to that job. Service equipment replacement is licensed electrical work in most jurisdictions and needs a permit; if your shop is not licensed for it, the recommendation goes to an electrician.
Type 1, Type 2, point-of-use, in one paragraph each
The categories come from UL 1449, the safety standard SPDs are listed to. A listed device carries a voltage protection rating (VPR), the let-through voltage measured in the standard's test, and a nominal discharge current (In), the surge current it survives repeatedly. Lower VPR passes less voltage downstream.
- Type 1 devices are permanently connected and listed for installation on either side of the service disconnect, including the line side, without their own overcurrent device. They suit a meter-main or a service where the panel has no spare breaker space.
- Type 2 devices are permanently connected on the load side of the service overcurrent device, typically landing on a two-pole breaker in the panel. This is the common residential installation.
- Type 3 is point-of-use: plug-in strips and receptacle devices at the equipment, listed for installation at least 10 meters of conductor length from the service panel. They catch what the service device lets through and what arrives on paths it does not see.
A service SPD does not protect against surges that enter on another conductor (cable TV coax, telephone, a long outdoor control wire) or that are generated inside the house by motors and switching. That is why the layered approach exists, and the existing reference covers it.
Lead length and bonding: the two install details that decide whether it works
Two details decide whether a whole-house device does its job, and both get skipped.
Lead length. Surge current is fast-rising, and every inch of conductor between the SPD and the bus adds inductance, which adds voltage on top of the device's VPR. Manufacturers' instructions call for the shortest, straightest leads the enclosure allows, often with the device mounted on the nearest knockout and its breaker placed at the top of the bus. A device mounted a few feet away on long looped leads can let through far more than its label says.
Intersystem bonding. If the cable company's coax ground and the power system's grounding electrode are not bonded together, a surge raises one ground relative to the other and the current crosses through whatever equipment connects both, usually a TV, a modem or a smart thermostat's network gateway. NEC 250.94, as adopted, requires an intersystem bonding termination at the service for communication systems to bond to. A whole-house SPD on a house whose coax ground runs to a separate rod with no bond has left the most common damage path open.
What surge damage looks like on boards and controls
The evidence that separates a surge from an ordinary failure is physical and it is on the board. Photograph it before anything is removed.
| What you find | What it suggests | How much weight it carries |
|---|---|---|
| A metal oxide varistor (MOV, the disc-shaped component across the line input) scorched, cracked or split | The board's own surge clamp absorbed more energy than it could | Strong. MOVs fail this way from overvoltage |
| Blown input fuse, everything else intact | A transient, or a short somewhere downstream | Weak by itself. Prove the low-voltage circuit is not shorted before calling it a surge |
| Burned trace or pitting at the terminal where a long outdoor wire lands | Energy arrived on that wire, induced by a nearby strike | Moderate to strong, especially on the outdoor-unit terminals of an air handler board |
| Several unrelated devices failed the same night | A common event on the supply or ground | Strong as a pattern, even when each board alone is ambiguous |
| Corrosion, swelling, or one failed component with no event | Age, moisture or a manufacturing defect | Points away from surge |
| Repeated fuse failure after replacement | A wiring fault | Points away from surge; find the short |
Long low-voltage runs are surge antennas. A thermostat cable running 40 feet along an exterior wall to a condenser collects induced voltage from a strike nearby, and the damage shows at the indoor board's outdoor terminals. A whole-house SPD at the panel does not clamp that path; an SPD rated for the outdoor unit, or low-voltage protection at the board, does.
Worked example: the morning after
The night before, a strong thunderstorm passed over a 1978 house. In the morning the furnace does not start, the garage door opener is dead, and the cable modem was replaced by the internet provider at 8 am. The house has a 100-amp panel with no SPD, and an electrician has already quoted a 200-amp service upgrade for next month. The jurisdiction enforces the 2020 NEC.
The HVAC tech's sequence:
- Safety before the board. Furnace service switch off, and the circuit's absence of voltage proven at the board terminals with a tester that is checked on a known live source before and after. The 120 V side of a furnace is the hazard here, not the 24 V side.
- Look and photograph. The board's 3-amp fuse is open. The MOV on the line input is intact, unmarked. The R and C terminals are clean. At the terminals where the condenser's control wire lands, one trace is darkened over about a quarter inch, visible in the photo.
- Rule out a short. With the condenser control wires lifted, the tech reads resistance across the outdoor leg of the low-voltage circuit: the contactor coil reads in the normal range for a coil of that type (manufacturer's data), no short to ground. With the wires still lifted, a new 3-amp fuse goes in, the blower door is back on so the door switch and the cover keep hands off the 120 V terminals, power is restored, and the board starts the inducer and lights the burner normally. With power off again and verified dead, the wires are relanded; power back on, cooling call from the thermostat, fuse holds, contactor pulls in.
- Read the evidence honestly. One blown fuse alone would not justify "surge." But the darkened trace at the outdoor terminals, the opener and modem failing the same night, and a clean low-voltage circuit together support a transient arriving on the outdoor control wire.
What goes on the ticket. "Board fuse open; darkened trace at outdoor control terminals (photo 3); low-voltage circuit tested, no short; fuse replaced and held through heating and cooling calls; garage opener and modem reported failed same night by customer. Findings consistent with a transient on the outdoor control wiring. Board operating; trace damage may shorten board life." That sentence describes what was found. It does not tell the insurer what caused the storm damage, which is the adjuster's call.
The code thread. The tech tells the customer that the 200-amp upgrade replaces the service equipment, so under 230.67 in the 2020 edition the electrician's permit will include a Type 1 or Type 2 SPD at or beside the new panel. Two things the code does not cover that the tech names: the coax ground bond (the tech can see the coax grounded to its own rod at the side of the house, with no visible bond back to the power ground), and the outdoor control wire, where a device protecting the condenser circuit is a separate recommendation.
What would change the answer. If the fuse had blown again with the condenser wires lifted, the short is in the indoor wiring or the board, and "surge" is off the ticket. If the jurisdiction still enforced the 2017 NEC, the SPD would be a recommendation, not a requirement on the upgrade. If the MOV had been split, the board would be replaced rather than returned to service, because the clamp that protects it is gone.
The failure mode. Writing "lightning" on every post-storm board makes your tickets worthless as evidence; an adjuster who sees it once on a board with a shorted thermostat wire discounts every ticket you write after.
How to verify you got this right
- Every post-storm board ticket carries photos taken before removal, the fuse and MOV condition, and a low-voltage short check.
- Service replacements in adopting jurisdictions show the SPD on the permit and the inspection card.
- Installed devices are checked on each maintenance visit: most carry a status indicator, and a device whose indicator shows it has failed has stopped protecting even though power stays on.
- The intersystem bond is checked whenever the panel is open.
References
- NFPA 70, National Electrical Code, 230.67 (2020 edition) and Article 242, as adopted by your jurisdiction; 250.94 intersystem bonding
- UL 1449, Standard for Surge Protective Devices (Type 1, 2 and 3; VPR; nominal discharge current)
- Device manufacturer installation instructions for lead length and breaker sizing
- See related: Surge Protection Reference; Lightning Protection Systems and the Trades That Touch Them; Hail and Outdoor Equipment