How does a whole-house surge protector work?

A whole-house surge protector, which electricians call a surge-protective device or SPD, is wired to the main breaker panel and gives a sudden voltage spike on the incoming power line a short path to ground. The spike is shunted away at the edge of the house instead of travelling through every circuit to your appliances. It is one layer of protection, not the whole answer: it does nothing about surges that come in on the cable, phone or antenna line, so plug-in protectors at your most sensitive electronics still matter.

What a surge is

Household power is meant to be a steady 120 volts. NIST's homeowner guide explains that lightning, short circuits, a pole knocked down by a car or some other accident can make the voltage jump to hundreds, even thousands of volts.^[1] A surge lasts only a few millionths of a second, but that is enough to destroy or upset an appliance.^[1]

Lightning is the famous cause, not the only one. NIST notes that switching surges happen when electrical loads are turned on or off inside the home, and during the power company's normal operations.^[1] Leviton, a maker of these devices, says utility work and air conditioners or washers and dryers cycling on and off cause small surges inside a home every day, and that these may not cause obvious damage but can shorten the life of equipment over time.^[3]

What the device does with it

The name "surge protector" oversells it slightly. NIST points out that they neither suppress nor arrest a surge; they divert it to ground, where it can do no harm.^[1] In simple terms, the unit stays out of the way at normal voltage and conducts only during the spike, so the excess energy goes to ground instead of into your appliances.

Two things follow from that:

• Grounding is everything. NIST warns that the best surge protector can be useless if grounding is not done properly.^[1] If the home's grounding and bonding are poor, the surge has nowhere good to go.
• Short wires work better. NIST advises that the protector should sit close to the service panel, within about one foot, or its voltage-limiting effect is degraded.^[1] One manufacturer's installation notes put it simply: long leads hurt performance.^[4] That is why the unit is usually mounted right beside or inside the panel, as in the photo above.

Type 1, Type 2 and Type 3

Surge devices are sorted by where they connect. Per Leviton's code summary, Type 1 devices are intended for installation between the secondary side of the service transformer and the line side of the service equipment, and protect against external surges caused by lightning or utility capacitor bank switching.^[2] Type 2 devices go on the load side of the service equipment and protect against residual lightning energy, motor-driven surges and other surges generated inside the house.^[2] Type 3 devices are point-of-use surge receptacles for computers, televisions and other expensive electronics.^[3]

For a home's service, Leviton summarizes that Article 230.67 of the 2020 National Electrical Code requires a Type 1 or Type 2 device for all services supplying dwelling units, installed integral to or immediately adjacent to the service equipment, including where service equipment is replaced.^[2] NIST describes the common in-panel arrangement, with the protector connected on the load side of a pair of dedicated spare breakers in the panel.^[1]

Reading the label

NIST explains that joules are a simplified measure of the surge energy a protector can absorb without damage to itself, and maximum surge current may give better information than the joule figure.^[1] It also advises to make sure the product has been tested by a product safety testing organization such as UL, ETL or CSA.^[1] As an example of what you will see, one manufacturer's whole-house units are listed to UL 1449 as Type 2 devices and come in surge capacities of 60,000, 100,000 and 140,000 amperes.^[4]

The device wears with use, so it has indicator lights. On that same product line, their green lights go out, a red service light flashes and an alarm beeps when the unit needs replacing.^[4] Other brands use different signals; the instruction card tells you which. Our answer on how long a surge protector lasts covers the wear in more detail.

What it does not do

NIST's guide is frank about the limits. It says a protector at the service entrance is enough for appliances connected only to power, but not for appliances also connected to a telephone, cable or satellite line.^[1] With those, a surge on one of those systems can damage the appliance through a difference in voltage between the two systems, even when each system has its own protector.^[1] The fix is a plug-in protector that routes both the power and the coax or phone line through the same box.

That does not make the panel device pointless. An important function of the service-entrance protector is to divert large surges on the power line before they enter the house,^[1] which makes the plug-in protectors' job easier.

Nor is it lightning protection for the building. Lightning rods protect the structure of the house and help avoid fires, but they do not prevent surges in the wiring.^[1] And the National Weather Service is blunt: Typical surge protectors will not protect equipment from a lightning strike.^[5] If something was damaged despite a protector, see why electronics get damaged in a storm even with a surge protector.

Where Virginia's code stands

Fairfax County's technical bulletin on the 2021 Virginia Uniform Statewide Building Code says a surge-protective device is required at the service panel under Section E3606.5 of the 2021 code, and that the requirement also applies to replacement service equipment.^[6] In practice that means a new home, or a panel or service replacement, includes one. For a panel that is staying put, Virginia's maintenance code says no provision of it requires alterations to existing equipment unless conditions meet its definition of an unsafe structure,^[7] so adding a unit is your choice. Our answers on whether you need one and what it costs in Virginia help with that decision.

When to call a licensed pro

A whole-house unit connects inside or right beside the panel, on live service equipment. Virginia defines an electrician as a tradesman who does the construction, repair, maintenance, alteration or removal of electrical systems regulated under the building code,^[9] and no individual may work as a tradesman unless licensed.^[10] CPSC tells homeowners not to remove the service-panel cover, because that is a job for a qualified electrician.^[8] Ask the electrician which type and rating suits your panel, where it will mount, whether your grounding and bonding need attention first, and whether your locality wants a permit.

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