1 Scope
NOTE This standard covers the materials, ratings, configuration, installation, and testing of permanently connected surge protective devices applied to low-voltage AC power systems rated 1000V and below. (1.1)
NOTE The work includes service-entrance SPDs, distribution and branch-panel SPDs, and point-of-utilization SPDs, together with their mounting, conductor connections, status indication and monitoring, and field verification. (1.2)
NOTE An SPD limits transient overvoltages by diverting surge current and clamping the voltage that reaches connected equipment; it does not protect against sustained overvoltage, undervoltage, or interruption. (1.3)
1.4SPDs shall be selected, applied, and installed in accordance with NFPA 70 Article 242, Part II (Surge-Protective Devices, 1000V or Less).
1.5SPDs shall be listed and labeled to UL 1449 by a Nationally Recognized Testing Laboratory.
NOTE An SPD is one element of a coordinated overvoltage-protection strategy, not a standalone product; its value depends on correct type selection, correct voltage and mode configuration, coordinated cascading between stages, and short connecting conductors. (1.8)
NOTE A surge protective device is the modern term for what was historically called a transient voltage surge suppressor (TVSS); the two terms describe the same device, and UL 1449 governs both. (1.9)
NOTE Where a lightning protection system is installed, the SPD work shall be coordinated with it, but the air terminals, down conductors, and bonding of the lightning protection system are a separate scope covered by
Lightning Protection.
(1.10) 2 Referenced Standards
2.1Equipment, materials, and installation shall comply with the latest adopted edition of the following standards and codes.
| Standard |
Title |
| NFPA 70 |
National Electrical Code (Article 242 — Overvoltage Protection; Part II — Surge-Protective Devices 1000V or Less) |
| NFPA 70E |
Standard for Electrical Safety in the Workplace |
| UL 1449 |
Surge Protective Devices |
| NEMA LS 1 |
Low Voltage Surge Protective Devices |
| IEEE C62.41.1 |
Guide on the Surge Environment in Low-Voltage (1000V and Less) AC Power Circuits |
| IEEE C62.41.2 |
Recommended Practice on Characterization of Surges in Low-Voltage (1000V and Less) AC Power Circuits |
| IEEE C62.45 |
Recommended Practice on Surge Testing for Equipment Connected to Low-Voltage (1000V and Less) AC Power Circuits |
| IEEE C62.72 |
Guide for the Application of Surge-Protective Devices for Low-Voltage (1000V or Less) AC Power Circuits |
| IEEE 1100 |
Recommended Practice for Powering and Grounding Electronic Equipment (Emerald Book) |
| ANSI/NETA ATS |
Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems |
2.2Where the contract documents, the adopted building code, or a referenced standard conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
NOTE UL 1449 is the listing standard that establishes the SPD Type classification and the measured ratings (VPR, In, and SCCR) used throughout this standard, while NFPA 70 Article 242 governs where each Type may be installed. (2.3)
NOTE NEMA LS 1 is a legacy specification reference for low-voltage SPDs that predates the current UL 1449 and IEEE C62 framework; it is cited for continuity of terminology and is not relied upon where it conflicts with UL 1449 or NFPA 70. (2.4)
3 Submittals
3.1 Action Submittals
3.1.1The Contractor shall submit the following for the Engineer's review prior to procurement and installation:
- Product data for each SPD, identifying the Type (1, 2, or 3), the system voltage and configuration it is configured for, the protection modes provided, and the mounting arrangement (integral or external)
- The UL 1449 listing, showing the listed VPR per mode, the nominal discharge current (In) rating, and the short-circuit current rating (SCCR)
- The per-phase or per-mode surge current capacity (kA, 8/20 µs) for each SPD
- Manufacturer's wiring diagram showing the connection to the phase conductors, neutral, and ground, and the recommended and maximum connecting conductor lead lengths
- A schedule identifying every SPD on the project, its location, its Type, and the equipment it serves, demonstrating the coordinated multi-stage arrangement
- Manufacturer's installation instructions, including conductor sizing and any required or permitted overcurrent device
☐ Product data with Type, system voltage, modes, and mounting
☑ UL 1449 listing (VPR per mode, In rating, SCCR)
☐ Per-phase / per-mode surge current capacity
☐ Wiring diagram with recommended and maximum lead lengths
☐ Project SPD schedule showing coordinated stages
☐ Manufacturer installation instructions
3.1.2Installation of any SPD shall not proceed until the corresponding submittals are reviewed and returned.
NOTE The submitted VPR, In, and SCCR shall be the values appearing on the UL 1449 listing, not marketing or catalog summary values, because only the listed values are verified by the testing laboratory. (3.1.3)
3.2 Closeout Submittals
3.2.1The Contractor shall provide the following at substantial completion before the SPD work is accepted:
- Field test and verification reports confirming each SPD indicates a healthy (protected) status on every phase after energization
- As-built SPD schedule reflecting the installed locations, Types, and the equipment served
- Operation and maintenance data describing the status indicators, the module replacement procedure, and the meaning of the monitoring contacts and alarm
- Warranty documentation for each SPD
☑ Field verification reports (per-phase healthy status confirmed)
☑ As-built SPD schedule
☑ Operation and maintenance data
☑ Warranty documentation
4 Quality Assurance
4.1 Listing and Labeling
4.1.1Every SPD shall be listed and labeled to UL 1449 by a Nationally Recognized Testing Laboratory.
4.1.2The SPD label shall display the SPD Type, the VPR per protected mode, the nominal discharge current (In) rating, and the short-circuit current rating.
4.1.3An SPD that is not listed to UL 1449, or whose label does not display the ratings required by this standard, shall not be installed.
NOTE UL 1449 listing is mandatory under NFPA 70 242.6; an unlisted suppressor has no verified VPR and no verified end-of-life safety behavior and shall be rejected. (4.1.4)
4.2 Manufacturer Qualifications
4.2.1SPDs shall be the product of a manufacturer regularly engaged in producing UL 1449 listed surge protective devices.
4.2.2All SPDs of a given Type and application on the project should be furnished by a single manufacturer so that ratings, status indication, and monitoring contacts are consistent across the facility.
4.3 Installer Qualifications
4.3.1SPD installation shall be performed by electricians experienced in connecting surge protective devices and supervised by a licensed electrical contractor.
NOTE The installer shall be familiar with the lead-length and routing requirements of this standard, because correct installation is decisive for SPD performance. (4.3.2)
5 Environmental and Service Conditions
NOTE An SPD shall be suitable for the electrical system, ambient conditions, and enclosure environment at its point of installation. (5.1)
5.2 System Voltage and Configuration
5.2.1The SPD shall be configured for the nominal system voltage and grounding configuration of the circuit it protects.
5.2.2An SPD configured for one system voltage or grounding arrangement shall not be applied to a different one.
208Y/120V, 3Φ 4-wire (solidly grounded wye)
240/120V, 1Φ 3-wire (split-phase)
480Y/277V, 3Φ 4-wire (solidly grounded wye)
240V delta, 3Φ 3-wire
600Y/347V, 3Φ 4-wire
NOTE Applying a wye-configured SPD to a delta or otherwise ungrounded system places full line-to-line voltage across the line-to-ground modes, which the SPD is not rated to sustain and which causes premature failure; the maximum continuous operating voltage must be matched to the actual grounding arrangement. (5.2.3)
5.3 Enclosure and Ambient
Integral to host equipment (no separate enclosure)
NEMA 1 (indoor general purpose)
NEMA 4X (washdown / corrosive / wet)
NEMA 12 (industrial, dust-tight)
NEMA 3R (outdoor)
40°C (standard rating)
50°C
55°C
5.3.1An externally mounted SPD enclosure shall match or exceed the environmental rating of the equipment it serves.
NOTE Where an SPD is integral to the host equipment, it shares that equipment's enclosure and a separate enclosure rating does not apply. (5.3.2)
6 SPD Type and Application
NOTE UL 1449 classifies permanently connected and point-of-use SPDs into Types that define where the device may be installed relative to the service disconnect. (6.1)
NOTE Type 1 SPDs are permitted on the line (supply) side of the service-equipment overcurrent device, and also on the load side; Type 2 SPDs are permitted only on the load side of the first overcurrent device; Type 3 SPDs are point-of-utilization devices installed at least 10 m (30 ft) of conductor length from the service. (6.2)
NOTE Type 1 is the only Type permitted to connect on the supply side of the service disconnecting means. (6.3)
6.4 Service-Entrance SPD
○ Type 1 (line side or load side of service disconnect)
● Type 2 (load side of service disconnect only)
6.4.1An SPD shall be installed at the service entrance to protect the facility from externally originating surges.
6.4.2The service-entrance SPD shall be a Type 1 or a Type 2 device.
6.4.3Where the SPD connects on the supply side of the service disconnecting means, it shall be a Type 1 device.
6.4.4For services supplying dwelling units, and for the additional sleeping-occupancy categories added by the 2023 NEC including dormitory units, hotel and motel guest rooms and suites, and patient sleeping rooms of nursing homes and limited-care facilities, a Type 1 or Type 2 SPD shall be provided in accordance with NFPA 70 230.67.
NOTE The 230.67 service SPD shall be located within or immediately adjacent to the service equipment, and where it is replaced the service shall not be re-energized until the replacement SPD is installed. (6.4.5)
NOTE A Type 2 SPD installed on the load side of the main is the most common service-entrance choice for commercial work because it is housed in or adjacent to the panel and requires no separate overcurrent device on the line side; a Type 1 is selected where supply-side connection or the highest surge-current capacity is required. (6.4.6)
6.5 Distribution and Branch-Panel SPD
● Provided at designated distribution and sub-panels
○ Not provided (service-entrance SPD only)
6.5.1A Type 2 SPD shall be installed at distribution and branch panelboards that serve sensitive, electronic, or critical loads.
6.5.2The distribution-panel SPD shall be located on the load side of the panel's main or feeder overcurrent device.
NOTE A second stage of protection at the distribution panel reduces the residual surge voltage that the service-entrance device lets through and clamps surges that originate downstream within the building. (6.5.3)
6.6 Point-of-Use SPD
○ Provided at designated sensitive equipment
● Not provided
6.6.1Where required to protect specific sensitive equipment, a Type 3 SPD shall be installed at or near the point of use.
6.6.2A Type 3 SPD shall be installed at least 10 m (30 ft) of conductor distance from the service or separately derived system disconnect, in accordance with its listing and NFPA 70 242.16.
NOTE The conductor distance required for a Type 3 device provides the series impedance that decouples it from the upstream SPD and allows the two to coordinate rather than fight over the same surge. (6.6.3)
6.7 Coordinated Multi-Stage Protection
6.7.1Where more than one stage of SPD is provided, the stages shall be coordinated so that the higher-energy upstream device diverts the bulk of the surge and the downstream device clamps the residual let-through voltage.
6.7.2The service-entrance SPD shall have the highest surge current capacity and nominal discharge current, with progressively lower capacities permitted at distribution and point-of-use stages.
Single stage — service entrance only
Two stage — service entrance plus distribution panels
Three stage — service entrance, distribution, and point-of-use
NOTE A layered, coordinated installation mirrors the IEEE C62.41.1 location categories, with the highest-energy device at the service (Category C), an intermediate device at distribution (Category B), and a low-energy device at the point of use (Category A); a point-of-use device installed without an upstream service device receives the full surge energy and fails. (6.7.3)
7 Electrical Ratings
NOTE The electrical ratings determine whether an SPD actually protects the connected load; they shall be selected together and coordinated with the system and the host equipment. (7.1)
7.2 Voltage Protection Rating
NOTE The voltage protection rating (VPR) is the rounded-up measured let-through voltage of the SPD when subjected to the UL 1449 6 kV, 3 kA, 8/20 µs combination wave, reported per protection mode. (7.2.1)
NOTE The VPR is the single most important protection metric because it is the voltage that actually reaches the connected equipment; a lower VPR provides better protection. (7.2.2)
330 V
400 V
500 V
600 V
700 V
800 V
400 V
500 V
600 V
700 V
800 V
900 V
7.2.3The SPD VPR for each protected mode shall not exceed the values selected for the project.
NOTE The Contractor shall not substitute a high surge-current device with a poor VPR for a device with the specified VPR; surge current capacity governs durability, while VPR governs protection. (7.2.4)
NOTE Specifying a high kilo-amp surge rating while ignoring VPR is the most common SPD specification error and yields a durable device that passes through damaging voltage to the load. (7.2.5)
7.3 Maximum Continuous Operating Voltage
7.3.1The maximum continuous operating voltage (MCOV) is the maximum steady-state RMS voltage the SPD can be continuously energized at without conducting, and it shall comfortably exceed the nominal voltage of the protected mode.
● Manufacturer standard MCOV for the selected system voltage
○ Elevated MCOV (for systems with chronic overvoltage or voltage swell)
7.3.2The MCOV shall be matched to the system voltage and grounding configuration so that the line-to-ground modes are not overstressed on ungrounded or impedance-grounded systems.
NOTE An MCOV set too close to nominal causes nuisance conduction and premature aging from normal voltage variation, while an MCOV matched to the wrong grounding configuration fails outright; the MCOV is typically 15 to 25 percent above the nominal mode voltage. (7.3.3)
7.4 Nominal Discharge Current
NOTE The nominal discharge current (In) is the peak 8/20 µs current the SPD can survive for 15 surges and remain functional, and it is a measure of the device's durability over repeated events. (7.4.1)
7.4.2The service-entrance SPD shall have a nominal discharge current of not less than 20 kA, the distribution SPD not less than 10 kA, and the point-of-use SPD not less than 3 kA, consistent with IEEE C62.72 application guidance for the respective location categories.
NOTE The In rating is distinct from the one-time surge current capacity; In describes how many repeated surges the device tolerates while continuing to protect, which is what determines service life in a real surge environment. (7.4.3)
7.5 Surge Current Capacity
7.5.1The surge current capacity is the maximum one-time peak 8/20 µs current the SPD can withstand, expressed per phase or per mode, and it shall be selected for the surge exposure of the installation.
50 kA
80 kA
100 kA
120 kA
160 kA
200 kA
240 kA
300 kA
400 kA
50 kA
80 kA
100 kA
120 kA
160 kA
7.5.2The surge current capacity shall be stated on a consistent basis, either per phase or per mode, and the Contractor shall verify which basis a submitted value uses before comparing devices.
NOTE Per-phase and per-mode surge current values are not interchangeable, and a per-phase value is typically about twice the per-mode value; comparing a per-phase rating from one product against a per-mode rating from another is a frequent procurement error. (7.5.3)
NOTE A surge current capacity in the range of 100 to 200 kA per phase is a defensible service-entrance target for most facilities, with higher values selected for sites in lightning-prone regions or with high surge exposure. (7.5.4)
7.6 Short-Circuit Current Rating
7.6.1The short-circuit current rating (SCCR) of the SPD shall equal or exceed the available fault current at its point of connection, in accordance with NFPA 70 242.8.
10 kAIC
22 kAIC
25 kAIC
42 kAIC
65 kAIC
100 kAIC
200 kAIC
Per drawings — one-line diagram (deferred by default)
7.6.2The SPD SCCR shall be coordinated with the available fault current determined by the short-circuit study per Short Circuit Study. NOTE An SPD with an SCCR below the available fault current can fail destructively when an internal component reaches end of life and faults, rather than disconnecting safely; matching the SCCR to the panel rating is a code requirement and a safety requirement. (7.6.3)
8 Protection Modes
NOTE The protection modes are the pairs of conductors between which the SPD provides suppression: line-to-neutral (L-N), line-to-ground (L-G), neutral-to-ground (N-G), and line-to-line (L-L). (8.1)
NOTE An all-mode SPD provides suppression in every available mode, while a partial-mode SPD provides only some modes, typically omitting L-G or N-G. (8.2)
● All-mode (L-N, L-G, N-G, and L-L)
○ L-N, L-G, and N-G (no L-L)
○ L-N and L-G only (partial mode)
8.3SPDs shall provide all-mode protection unless the contract documents specifically permit a reduced mode set.
NOTE An all-mode device suppresses both common-mode (to ground) and normal-mode (between conductors) surges, whereas a partial-mode device leaves the unprotected modes to pass surge energy to the load. (8.4)
8.5 Neutral-to-Ground Mode on Separately Derived Systems
8.5.1SPDs applied to separately derived systems, including transformer secondaries and on-site generator outputs, shall include the neutral-to-ground (N-G) protection mode.
NOTE Omission of the N-G mode on a separately derived system is a frequent and consequential error, because the local neutral-to-ground bond at the derived source means common-mode surges are not otherwise suppressed and pass directly to downstream equipment. (8.5.2)
NOTE The Contractor shall coordinate the bonding of the separately derived system, including the system bonding jumper and grounding electrode connection, with
Grounding And Bonding.
(8.5.3) 9 Construction and Components
9.1 Suppression Components and Replaceable Modules
● Field-replaceable modules (no rewiring required)
○ Non-replaceable (whole unit replaced at end of life)
9.1.1SPDs intended for distribution equipment should provide field-replaceable surge modules so that an end-of-life module can be replaced without rewiring the device.
NOTE Field-replaceable modules reduce downtime and labor at end of life, because the depleted suppression element is swapped rather than the entire device disconnected and rewired. (9.1.2)
9.2 Thermal Disconnect and Overcurrent Protection
9.2.1Each SPD shall include an internal thermal disconnect that isolates a failing or overheating suppression component before it can overheat or ignite.
9.2.2The SPD shall include integral overcurrent protection or disconnection consistent with its UL 1449 listing.
NOTE The thermal disconnect is the core end-of-life safety mechanism of a modern SPD; a metal-oxide varistor that has degraded from repeated surges draws increasing current and must be disconnected before it fails thermally. (9.2.3)
9.3 Status Indication
● Per-phase status indicators (protected / failed)
○ Single overall status indicator
9.3.1Each SPD shall provide visible status indication showing whether protection is healthy or has been lost.
9.3.2SPDs in three-phase equipment shall provide per-phase status indication so that the loss of protection on a single phase is visible.
NOTE Without per-phase indication, the failure of one phase's suppression can go unnoticed while the device still appears generally healthy, leaving that phase unprotected indefinitely. (9.3.3)
9.4 Monitoring and Alarms
● Form C dry contacts for remote / BMS monitoring
○ No remote contacts
○ Audible alarm with mute on protection loss
● No audible alarm
○ Surge event counter provided
● Not provided
9.4.1SPDs at monitored facilities shall provide Form C dry contacts to signal protection status to a remote panel or building management system.
NOTE A failed SPD that is not monitored or visibly inspected leaves equipment unprotected until the next manual check; remote contacts and alarms surface the failure so the device is replaced promptly. (9.4.3)
NOTE A surge event counter records cumulative surge activity, which supports maintenance planning and post-event forensics where surge exposure is a concern. (9.4.4)
10 Installation
10.1 Mounting
● Integral / factory-installed in the host equipment
○ External, mounted directly on the host equipment
○ External, adjacent to the host equipment
10.1.1SPDs shall be mounted integral to, on, or immediately adjacent to the equipment they protect.
NOTE Integral, factory-installed SPDs are preferred over external devices because the factory bus connection minimizes the connecting conductor length that governs let-through voltage. (10.1.2)
10.1.3An externally mounted SPD shall be located so that its connecting conductors are as short and straight as possible.
10.2 Conductor Lead Length
10.2.1The connecting conductors between the SPD and the protected conductors shall not be any longer than necessary and shall avoid unnecessary bends, in accordance with NFPA 70 242.24.
10.2.2Total connecting conductor lead length should be kept under 24 in., and as short as practical.
10.2.3Connecting conductors shall be routed straight, without coils, loops, or sharp bends, and the line, neutral, and ground leads should be kept close together.
NOTE Conductor lead length is the dominant installation factor in SPD performance, because the inductance of the connecting conductors develops a voltage during the fast-rising surge that adds directly to the SPD's VPR; an SPD on long leads can let through nearly twice the voltage of the same SPD on short leads, defeating a low-VPR device. (10.2.4)
NOTE Coiling excess conductor or routing the leads through sharp bends increases inductance and raises let-through voltage, and shall be avoided. (10.2.5)
10.3 Conductor Sizing
14 AWG (NEC minimum)
12 AWG
10 AWG
10.3.1Connecting conductors shall be not smaller than 14 AWG copper or 12 AWG aluminum, in accordance with NFPA 70 242.28.
10.3.2Connecting conductors should be 10 AWG or 12 AWG for mechanical robustness and lower impedance where the lead length and terminations permit.
10.4 Overcurrent Coordination
10.4.1The Contractor shall provide any overcurrent device required or permitted for the SPD in accordance with the manufacturer's listing and instructions.
NOTE A Type 1 SPD does not require an external overcurrent device, because its disconnection means is internal to the listed device. (10.4.2)
NOTE Where the manufacturer's instructions specify or permit an external overcurrent device for a Type 2 SPD, it shall be sized per those instructions; an oversized device may fail to clear a degraded SPD. (10.4.3)
11 Testing
11.1 Factory Verification
11.1.1Each SPD shall be production-tested by the manufacturer in accordance with UL 1449 prior to shipment.
11.1.2The Contractor shall furnish the manufacturer's certification that each SPD passed its production tests.
11.2 Field Verification
11.2.1After energization, the Contractor shall verify that each SPD indicates a healthy, protected status on every phase.
11.2.2Where an SPD provides monitoring contacts, the Contractor shall verify that the contacts correctly report protection status to the remote panel or building management system.
☑ Per-phase status indication confirmed healthy
☐ Monitoring contacts confirmed reporting at remote panel
☐ Audible alarm function confirmed (where provided)
☐ Connecting conductor lead length verified within limit
11.2.3The Contractor shall verify by inspection that the installed connecting conductor lead length is within the specified limit and that the leads are routed without coils or sharp bends.
NOTE Field verification of an SPD is largely a verification of correct installation and healthy status, because the suppression performance itself is established by the UL 1449 listing and cannot be re-tested in the field without specialized surge generation. (11.2.4)
11.2.5The Contractor shall correct any SPD that does not indicate healthy status or whose installation does not meet the lead-length and routing requirements, and shall re-verify.
12 Identification and Labeling
12.1Each SPD shall be labeled to identify it as a surge protective device, its Type, and the equipment or circuit it protects.
12.2Where an SPD is required for compliance with NFPA 70 230.67, the service equipment shall be marked to indicate that the SPD is part of the service and shall not be re-energized without it.
NOTE The meaning of the status indicators and the module replacement procedure shall be available at the device, either on a label or in the operation and maintenance data. (12.3)
13 Delivery, Storage, and Handling
13.1SPDs shall be delivered in the manufacturer's original packaging with listing marks intact.
13.2SPDs shall be stored indoors in a clean, dry location and protected from moisture, corrosive atmospheres, and physical damage until installed.
NOTE SPDs shall not be installed if their enclosures or modules show evidence of impact, moisture intrusion, or other damage. (13.3)
14 Warranty
5 years from substantial completion
10 years from substantial completion
Manufacturer's standard limited warranty
● Not required
○ Manufacturer connected-equipment warranty where offered
14.1SPDs shall be warranted by the manufacturer against defects in materials and workmanship for the specified period.
14.2The Contractor shall warrant the installation, including the connecting conductor routing and the correct status indication of each device, for the project warranty period.
NOTE Where a manufacturer offers a connected-equipment protection guarantee, it may be required, with the understanding that such guarantees carry conditions on installation and coordination that the Contractor shall satisfy. (14.3)
15 Spare Parts
○ None
● One spare module set for each distribution SPD type installed
○ 10% of installed modules (minimum one set)
15.1Where SPDs use field-replaceable modules, the Contractor shall provide spare modules of the same type and rating as those installed.
15.2Spare modules shall be stored in the electrical room in their original packaging and identified with the SPD type and rating they serve.
NOTE Stocking spare modules allows a depleted SPD to be restored to service quickly after a major surge event, rather than waiting on procurement while equipment is unprotected. (15.3)