Surge-Protective Devices (SPDs)
SPDs clamp voltage spikes before they reach your equipment, and the Code now requires them on dwelling services to protect all the sensitive electronics we live with.
Why this code exists
Modern homes are full of electronics with delicate power supplies; a single surge from the grid or a nearby strike can wipe out thousands of dollars of gear.
An SPD is a fast valve that shunts a surge to ground before it fries your electronics. Type 1 goes ahead of the main, Type 2 at the panel, and they are cheap insurance for everything downstream.
I love that this finally became mandatory. We wire whole houses full of computers, heat pumps, and chargers, and a ten-dollar surge event can take out all of it at once.
When it applies
Surge protection now belongs on dwelling services because modern homes are full of electronics, appliance control boards, and EV gear that a single transient can cook. The SPD clamps the spike before it reaches the branch circuits, and it installs at or near the service equipment with short, straight leads.
Common mistakes
- Leaving long, looped conductors on the SPD leads, which kills its clamping response.
- Treating the SPD as a substitute for grounding and bonding instead of a complement to it.
- Skipping the end-of-life indicator check after a nearby strike.
In the field
I mount a Type 2 SPD right at the panel with the leads as short as the enclosure allows. On solar retrofits I add protection on the PV side too, since that gear sees plenty of transients.
The first surge device I installed at a panel paid for itself a month later when lightning hit a transformer down the street and the whole block lost TVs — except my customer's.
See full history
- 2002 — SPD requirements consolidated into Article 285.
- 2020 — SPDs mandated on dwelling-unit services.
Related codes
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