A recent training program discussed the requirements for connecting surge protection at service equipment. With the variety of opinions on this issue, it seemed appropriate to provide some information about connecting surge protective devices (SPDs) in a manner compliant with the National Electrical Code (NEC). Prior to the 2011 NEC, SPDs were known as transient voltage surge suppressors (TVSSs). Recent changes in the product standard UL 1449, Standard for Surge Protective Devices, resulted in a clearer delineation between surge arresters and surge protective devices. It also resulted in a change in nomenclature. NEC Article 285 covers surge arresters and surge protective devices rated for systems of 1 kilovolt (kV) or less, while Article 280 covers surge arresters for systems above 1 kV. Read below for the essential installation surge protection requirements for electrical systems operating at less than 1 kV.

Product certification

To determine the applicable NEC requirements for installing a surge protection device, criteria must be established. Determine whether the SPD will be installed on the supply side of the service or on the service overcurrent protection device’s load side. In accordance with 285.5, SPDs must be listed by a nationally recognized testing laboratory, which means it will include specific installation instructions that must be followed. Listed SPD provide extensive important information in the instructions and labeling of the device. One requires a marked short-circuit current rating that must not be exceeded.

Types of surge protective devices

There are four types of SPDs available. NEC Article 100 defines them as follows—type 1: permanently connected SPDs intended for installation between the secondary of the service transformer and the line side of the service disconnect overcurrent device; type 2: permanently connected SPDs intended for installation on the load side of the service disconnect overcurrent device, including SPDs located at the branch panel; type 3: point of utilization SPDs; and type 4: component SPDs, including discrete components and assemblies.

Line-side connections

Based on the above definitions, type 2 and 3 SPDs are typically installed on the load side of the service equipment. Type 4 SPDs are generally recognized components of an overall assembly and are not suitable for field installation. The only SPDs permitted for installation on the service’s supply side are type 1; they are also permitted for use on the load side of the service equipment. Section 285.23 allows for installation on the service’s supply side. This section also references 230.82(4), which indicates that SPDs are one of various types of equipment that can be connected to the service’s line side.


The conductors installed for connection to an SPD must not be any longer than necessary and have to be installed in a manner that avoids bends as much as possible. The installation instructions often specify the minimum size for conductors, length of conductors and maximum rating of overcurrent device. Note that the Code indicates that the smallest size conductor for line or grounding connections is 14 AWG copper or 12 AWG aluminum or copper-clad aluminum.


SPDs are typically connected between any two conductors of a system or service such as between the ungrounded conductor and the grounded conductor, the ungrounded conductor and the equipment grounding conductor, or the ungrounded conductor and the grounding electrode conductor. They are required to be connected to ground (the earth) by attachment to the service grounded conductor (often a neutral), to the grounding electrode conductor for the service, directly to the grounding electrode, or to an equipment grounding terminal within the service equipment enclosure. The grounding connections must be made in a manner as specified in 250.8 and must meet the installation instructions provided with the device. If the conductors connecting to the grounding electrode are installed in a ferrous metal raceway or enclosure, additional bonding, in accordance with 250.64(E), must be provided.

New requirements forthcoming

Currently, SPDs are generally not a mandatory requirement in the NEC, but if installed, the Code rules apply. New requirements for surge protection installation have been incorporated into the 2014 NEC. When it is adopted, surge protection must be installed on panelboards and switchboards that are part of an emergency system, legally required standby system or critical operations power facility. This new rule will help ensure that emergency electrical distribution systems continue to deliver reliable power to vital life safety loads during voltage-surge events. Voltage surges resulting from lightning strikes or due to various sources within an electrical system, such as switching of power electronic devices, can damage or destroy the electrical distribution equipment and the loads supplied. Electrical equipment damage can be from a single lightning strike nearby or the cumulative effects of multiple voltage surges from sources within an electrical system.


For more detailed information on type 1, type 2, type 3 and type 4 SPDs, see UL 1449, Standard for Surge Protective Devices.

About the Author

Michael Johnston

Executive Director of Standards, NECA
Michael Johnston is NECA’s executive director of standards and safety. He is chair of the NEC Technical Correlating Committee. He served as a principal representative on NEC CMP-5 representing IAEI for the 2002, 2005, and 2008 cycles and is currently...

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