Application Paper AP158001EN Effective August 2015 Determining the overcurrent protection device size when installing an SPD Introduction One of the most common questions heard by technical support and product representatives is “What size of breaker do I need to use when installing an SPD?”. This technical applications note addresses the installation requirements for the need of an overcurrent protection device (OCPD) with a surge protective device (SPD). Applicable NFPAT 70 standards, National Electrical CodeT (NECT) rules, Authority Having Jurisdiction (AHJ), ANSI/ULT 1449 ratings, and installation practices are discussed. SPD design capabilities Many SPDs are built with thermally protected metal oxide varistors (MOV). These MOVs are the heart of the SPD itself. The MOVs detect a transient high voltage surge and quickly shunt the surge to ground. Because the MOV cannot distinguish between transient events and sustained overvoltage events, it must have a way of removing itself from the system during the overvoltage event. This keeps the SPD from entering into a thermal runaway situation that could cause internal and external damage. The thermally protected MOVs have internal components that disconnect the MOV from the system during sustained overvoltage and overcurrent occurrences. The high temperature associated with these events causes the MOV temperature rise to a critical value that initiates a mechanical separation of the conducting components. Because the SPD itself is internally protected, it does not require external overcurrent protection in the form of a circuit breaker or fuse. Other types of SPDs exist that do not have component level protection on the MOVs or are built with a different type of component such as silicon avalanche diodes (SAD). SPDs that are built with these types of components typically require external overcurrent protection because they are incapable of taking themselves offline during an overcurrent situation. If an OCPD is required for the operation of the SPD itself, this will be indicated in the device’s UL listing 1 and the manufacturer will clearly state this requirement in the installation instructions. Always consult the installation instructions before installing an SPD. 1 To verify SPD UL Listing requirement, go to UL Online Certification Directory. NEC Article 240.4 (overcurrent protection) protection of conductors Regardless whether the SPD requires an OCPD, the installation methods must meet all applicable electrical codes. Article 240.4 of the NEC states all conductors, other than flexible cords, flexible cables, and fixture wires shall be protected with an OCPD in accordance with their ampacity rating specified in Article 301.15, unless otherwise permitted or required in Article 240.4(A) through (G). This article applies to conductors extending beyond the switchboard, switchgear, or panelboard that are used to connect an SPD to an electrical system. Connecting an externally mounted or side mounted SPD directly to a system bus without an OCPD is a direct violation of the NEC and can lead to safety and liability concerns. Acceptable OCPDs are listed circuit breakers and fuses. NEC Article 310.15 (conductors for general wiring) ampacities for conductors rated 0–2000 V Article 310.15(B)(16) defines the rating of the conductor ampacity that drives the maximum rating of the OCPD to be used to protect the electrical conductors. The rating is based on the ampacity of the conductors. For example, #10 AWG conductors at 60 °C will require an OCPD of 30 A. NEC Article 240.21 SPD installation using Feeder Tap Rule Article 240.21(B)(1), which is known as the 10 ft Feeder Tap Rule, allows an SPD to be installed without overcurrent protection at the point of connection. Strict rules as defined in the above referenced paragraph must be adhered to in order to install an SPD utilizing the Feeder Tap Rule. Summary When installing SPDs, NEC, local electrical codes, and UL requirements must be followed. The proper OCPD must be used upon installation in order to meet the NEC requirements and maintain the ANSI/UL 1449 Listing of the SPD. In many instances the reason for external overcurrent protection is dictated by NEC to protect the conductors, not the SPD itself. SPDs containing an internal OCPD can be connected directly to the electrical bus only if the installation meets all of the requirements defined in the NEC Article 240.12(B)(1), the Feeder Tap Rule, the Authority Having Jurisdiction (AHJ), or by strictly following the manufacturer’s installations instructions on OCPD recommendations. Eaton 1000 Eaton Boulevard Cleveland, OH 44122 United States Eaton.com © 2015 Eaton All Rights Reserved Printed in USA Publication No. AP158001EN / Z16959 August 2015 Eaton is a registered trademark. All other trademarks are property of their respective owners.
