Surge Protection (SPDs)
Surge Protective Devices clamp transient overvoltages from lightning, switching events, and motor stops. NEC 285 governs application. NEC 230.67 (2020+) now requires Type 1 or 2 at every service.
What SPDs Do
Surge Protective Devices clamp transient overvoltages from lightning, switching events, and motor stops. Without SPDs, transients reach equipment as 2-10× nominal voltage spikes for microseconds — destructive to electronics.
| Source | Magnitude | Duration |
|---|---|---|
| Direct lightning strike (rare on building) | 30,000 - 200,000 A | Microseconds, single shot |
| Indirect lightning (induced) | 500 - 10,000 V transient | Microseconds |
| Utility switching | 2-3× nominal V | Cycles to seconds |
| Capacitor switching | 1.5-2× nominal V | ~ 1 cycle |
| Inductive load switching (motor stop) | 10s of kV depending on size | Microseconds |
| Welding equipment | kV transients | Repetitive |
SPD Types — NEC 285
| Type | Location | UL standard | Where used |
|---|---|---|---|
| Type 1 | Line side of service disconnect (between utility transformer and service main) | UL 1449 Type 1 | Hardwired to service entrance — handles direct lightning. Required by NEC 230.67 for some services. |
| Type 2 | Load side of service disconnect — at service equipment or panelboard main | UL 1449 Type 2 | Most common. Whole-building protection. Often combined with main breaker. |
| Type 3 | Point-of-use — > 30 ft from service | UL 1449 Type 3 | Plug strips, UPS input, sensitive equipment |
| Type 4 | Component (no enclosure) | UL 1449 Type 4 | OEM use inside equipment — not field-installed |
NEC 230.67 — SPD Required at Every Service (2020+)
The 2020 NEC introduced 230.67, which requires Surge Protective Devices on most new services. The 2023 NEC expanded the requirement.
| Service | NEC 230.67 (2020) | NEC 230.67 (2023) |
|---|---|---|
| Dwelling unit ≤ 1000V | Type 1 or Type 2 SPD required at service equipment | Same — required |
| Other occupancies | Not addressed | SPD required if essential systems present (e.g., fire pumps, life safety) |
| Industrial / commercial > 1000V | Not addressed by 230.67 (good practice still applies) | Same |
SPD Ratings to Look For
| Rating | Meaning | Typical values |
|---|---|---|
| kA per phase | Maximum surge current the SPD can handle (8/20 µs waveform) | 40, 80, 120, 160, 200, 300, 400 kA per phase |
| VPR (Voltage Protection Rating) | Voltage that passes through SPD during a surge — what the equipment sees | Lower is better. 600V VPR for 480V service is excellent. |
| Nominal Voltage (Vn) | System voltage SPD is designed for | 120, 208Y/120, 240, 480Y/277, 600Y/347, 12.47kV |
| MCOV (Maximum Continuous Operating Voltage) | Sustained voltage SPD can withstand without operation | ~ 115% of nominal |
| SCCR (Short Circuit Current Rating) | Available fault current SPD can be installed in | 10, 22, 65, 100, 200 kA |
| Nominal Discharge Current (In) | Current the SPD can repeatedly discharge without damage | 5, 10, 20 kA per phase |
Cascading SPDs
Best practice: layered protection. Type 1 at service handles the biggest surges; Type 2 at distribution panels reduces remaining; Type 3 at sensitive equipment provides final filtering.
| Layer | Location | Typical kA | VPR |
|---|---|---|---|
| 1 — Service entrance | Type 1 at MV switchgear or service equipment | 120-300 kA | 1500-2000V (480Y/277V) |
| 2 — Distribution | Type 2 at major distribution panels | 40-120 kA | 1000-1500V |
| 3 — Branch / point-of-use | Type 3 at sensitive equipment | 10-40 kA | 600-900V |
SPD Technology — MOV vs SAD
| Technology | Mechanism | Pros | Cons |
|---|---|---|---|
| MOV (Metal Oxide Varistor) | Zinc oxide ceramic — non-linear V/I | Cheap. Handles high energy. Self-resetting. | Wears out with surges. Eventual end-of-life. Visual indicator required (NEC). |
| SAD (Silicon Avalanche Diode) | Solid-state semiconductor | Faster clamping. Tighter VPR. | Lower energy capability. More expensive. |
| GDT (Gas Discharge Tube) | Spark gap in gas-filled tube | Very high current handling. | Slow response. High let-through during firing. |
| Hybrid | Combines MOV + SAD + filter | Best of all worlds. | Most expensive. |
Worked Example 1 — Atlas DC1 SPD Cascade
Example 01 · Atlas DC1 spineLayered SPD protection from MV through to rack PDU
| Location | SPD type | Spec | Why |
|---|---|---|---|
| 12.47 kV MV switchgear | MV surge arrester (not "SPD" per UL — different standard) | 15 kV class, 12 kV MCOV, 10 kA discharge | Handles direct lightning entry from utility primary |
| 480V SWGR-A main | Type 1 SPD | 200 kA per phase, VPR 1500V, hardwired ahead of main breaker | Whole-building protection. NEC 230.67 even though commercial. |
| 480V distribution panels | Type 2 SPD | 120 kA per phase, VPR 1200V | Reduces surges reaching downstream equipment |
| UPS-A1 input | Type 2 SPD | 80 kA, VPR 1000V | Protects UPS rectifier (most sensitive component) |
| UPS-A1 output (415V) | Type 2 SPD | 40 kA, VPR 800V | Protects critical IT downstream |
| PDU-A1 distribution panel | Type 2 SPD | 40 kA, VPR 800V | Last stage before IT racks |
| Rack PDU strips | Type 3 SPD (integral) | 10 kA, VPR 600V | Final point-of-use filtering |
| Generator paralleling cabinet | Type 2 SPD on each gen output | 40 kA, VPR 1500V | Protects gen alternator from switching transients |
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SPD diagnostics matter
MOVs degrade with each surge. NEC 285.4 requires SPDs to have a status indicator (red/green LED). Atlas DC1 SPDs are monitored via Modbus/SNMP — when status changes, the BMS alerts maintenance for replacement before the next surge can reach equipment.
Worked Example 2 — Residential Service SPD (NEC 230.67 Compliance)
Example 02 · Alternate scaleSingle-family home · 200 A 1φ service · 2020+ NEC compliance
- NEC 230.67 (2020): Type 1 or Type 2 SPD required at the service equipment of every dwelling unit. Mandatory.
- Type 2 selection: 200 A panel, 120/240V 1φ. SPD ratings: 40-80 kA per phase, VPR ≤ 1500V on 240V (or ≤ 600V on 120V circuits).
- Mounting: Hardwired to a 2-pole 30A breaker in the panel. Some panels have factory-installed integral SPD.
- Status indicator: Green LED when active, red LED when end-of-life. Per NEC 285.4.
- Cost: Whole-house SPD: $50-200 retail, $300-500 installed. Saves $$$ in claims after the first nearby strike.
Drill — Quick Self-Check
Work each problem mentally; reveal to check. Goal: reflex, not deliberation.
Drill 1 · SPD types
Type 1, 2, 3 difference?
Type 1 = line side; Type 2 = load side; Type 3 = point-of-use
Cascading layers reduce VPR each step.
Drill 2 · NEC 230.67
Required at every dwelling service since which NEC?
2020 NEC
Type 1 or Type 2 SPD.
Drill 3 · MOV vs SAD
Which has tighter VPR?
SAD (silicon avalanche diode)
MOV = cheaper, higher energy capability.
Drill 4 · VPR — meaning
Voltage Protection Rating: what does it mean?
Voltage that gets through SPD during a surge
Lower is better.
Drill 5 · Cascade design
Layered SPDs — service kA vs point-of-use kA?
Service: 100-300 kA; Point-of-use: 10-40 kA
Each layer absorbs some, reduces let-through.
If You See THIS, Think THAT
| If you see… | Think / use… |
|---|---|
| "SPD" or "Surge Protective Device" | NEC 285. Clamps transient overvoltages. |
| "NEC 230.67" | Mandatory SPD at every dwelling service since 2020 NEC. Some commercial in 2023. |
| "NEC 285" | Surge protective device application rules. |
| "Type 1 SPD" | Hardwired ahead of service disconnect. Handles biggest surges. |
| "Type 2 SPD" | Load side of service. Most common. Whole-building protection. |
| "Type 3 SPD" | Point-of-use. Plug strips, UPS input. |
| "VPR" | Voltage Protection Rating. Lower is better. What gets through SPD. |
| "MOV" (Metal Oxide Varistor) | Most common SPD technology. Wears out with each surge. |
| "SAD" (Silicon Avalanche Diode) | Faster, tighter clamping. Lower energy. |
| "Hybrid SPD" | MOV + SAD + filter. Premium. |
| "MCOV" | Max Continuous Operating Voltage. ~ 115% of nominal. |
| "SPD status indicator" | Red/green LED. Required by NEC 285.4. Visual end-of-life signal. |
| UL 1449 | SPD product standard. Verify Type listing matches application. |