Does my ADU need a whole-home surge protector?

Usually, yes. The 2023 NEC adopted in California requires a Type 1 or Type 2 surge-protective device (SPD) for services that supply dwelling units (Section 230.67), and the 2023 edition added a matching rule for feeders that supply dwelling units (Section 215.18). Whether your ADU has its own meter or runs off a subpanel, your plans should show where the SPD goes.

If the ADU has its own service

• The SPD must be built into the service equipment or installed immediately next to it.
• It must be a Type 1 or Type 2 device.
• If the service equipment is replaced later, the surge requirement applies to the replacement.

If the ADU is fed by a subpanel

Section 215.18 requires an SPD where a feeder supplies a dwelling unit. The device goes in or next to the distribution equipment on the load side of that feeder that contains the branch-circuit breakers serving the dwelling, which for most ADUs is the ADU subpanel. The requirements include:

• A Type 1 or Type 2 SPD.
• A nominal discharge current rating of at least 10kA.
• The rule applies again whenever that feeder-supplied equipment is replaced.

Article 225, which covers outdoor feeders running to separate buildings, also contains surge protection requirements, so a detached ADU is addressed in both places.

Where interpretations differ

Electricians and plan reviewers sometimes debate whether one SPD at the main house service can cover a nearby ADU subpanel, or whether each feeder-supplied panel needs its own. The text of Section 215.18 points to the panel served by the feeder, so the cautious approach is to show an SPD at the ADU panel and let plan check confirm. Your local building department's interpretation controls.

What an SPD does, and what it doesn't

A panel-mounted SPD limits voltage spikes from utility switching, nearby lightning and large motors cycling on and off. It helps protect the electronics built into heat pumps, induction cooktops, EV chargers and solar inverters, which are standard equipment in many all-electric ADUs. It does not make the unit lightning-proof, and plug-in protection for sensitive electronics is still worth having.

What this means for your electrical plan

The single-line diagram and panel schedule should identify the SPD type, its rating and its location, plus the breaker that feeds it if it is breaker-connected. It is a small item, but a missing SPD is an easy correction for a plan reviewer to flag under the current code. See how an ADU subpanel works, the most common electrical plan check corrections and our ADU electrical guide.

Sources: larimer.gov, ecmweb.com, mikeholt.com, dgs.ca.gov.