SURGE PROTECTORS (SPDs) – What They Are, How They Work, What You Need To Know


Surge Protection. Why, where, and wtf


NEC 2020 code requirement changes 

The National Electric Code (NEC) sets the standard for electricians and electrical work across the US, with the arrival of it’s 2020 edition adding some substantial changes to the codes on surge protection. The most notable of these changes can be found in “Services” article 230.67, which states that any new or replaced service supplying a dwelling unit must be provided with a Type 1 or Type 2 Surge Protection Device. Residential Services will no longer pass inspection in most jurisdictions without a surge protection device either inside the service panel or immediately adjacent to… Which is kind of a big deal. In addition, NEC has moved the entire article of Surge Protection Devices from 285 in the 2017 edition over to 242, renaming the article Overvoltage Protection in order to incorporate surge arrestors over 1000 volts. Lastly adding a helpful table (Table 242.3) listing other sections of the NEC that mention surge protections for specific uses, such as fire pumps and data centers. 

What is a surge?

A power surge or “transient over-voltage” is a spike in voltage to an electrical system that can reach thousands of volts so instantaneously that it has to be measured in fractions of a second. There are multiple types of surges, the most common is caused by large loads with-in the electrical system (e.g., Air Conditioning units, elevators and motors). These surges created by appliances and motor loads are typically oscillatory, or one event that remains just long enough to cause a “rippling effect” in the system and will shorten the life of sensitive electronics. The larger less frequent surges are caused by changes to the grid by the power company and lightning strikes to the grid itself. These surges are typically much larger impulse transients, meaning one  spike that dissipates immediately, these large impulse surges can be detrimental to a building’s electrical system.

Surge Protection Devices

To help decrease the damage caused by surges, Surge Protection Devices (SPDs) are installed “upstream” at the beginning of the electrical system/circuit they are intended to protect.  SPDs use voltage-dependent resistors (VDRs or varistors) designed to allow standard voltage to pass through normally until the maximum continuous operating voltage (or MCOV) is surpassed. The spike causes the VDR to “shunt” or to create a new path of least resistance and redirect voltage back to ground. No SPD can ever fully protect against 100% of the surge, some voltage/current will always continue through to the load, but if the SPDs are installed correctly the residual surge will not be large enough to cause damage. For the highest level of protection, it is recommended to use the  “cascading principle”, which means to layer the surge protection devices starting at the service entrance with whole-home protection, then individually “down stream” onto major appliance loads, then to the more location-specific point-of-use styles 

The 4 Types of SPDs  

Type 1– Permanently connected device to be installed either on the supply (line) side or load side of a service overcurrent protection device, this is considered “whole home” protection an is most effective in surges directly from the grid 

 Type 2 -Permanently connected device to be installed on the load side of an overcurrent protection device, either at the service entrance, feeder supplied structure (separate garages, guest house etc.) or a separately derived system (emergency generators, transformers, solar service etc.). If installed at the very top of a service entrance panel, this type can be considered whole home protection or further down as branch circuit protection. When installed at major appliances, this type is most effective in decreasing surges from with-in the electrical system

Type 3– Point of use, these are to be installed on the load side of a branch circuit overcurrent device. Usually seen in the form of power strips or receptacles, these devices are installed directly at specific  locations to protect sensitive electronics like computers or AV equipment.

Type 4– Component type SPDs, installed into equipment by the manufacturer.

Spikes not Strikes

After all of this big brain info on transient over-voltage, it’s extremely important to note that Surge protection will most likely do nothing to help protect against lightning strikes to the actual home or building itself. To protect against direct lightning strikes you need a Lightning Protection System which is a whole other ball game that involves… some stuff. Perhaps Dustin will make a video on it one day!


Stay safe out there, Love from the ATX