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How do you properly size the transient voltage surge suppression TVSS for lightning strike potential?

Steve’s question is regarding lightning protection for a small commercial 240 1 phase, 100A panel. He asks; How do you properly size the Transient Voltage Surge Suppression (TVSS) for lightning strike potential?

Hi Steve, thank you for your excellent question regarding proper sizing of Transient Voltage Surge Suppression (TVSS) surge protection systems in electrical panels.

Proper Transient Voltage Surge Suppression (TVSS) sizing as stated in UL1449, 2nd Edition, February, 2007 is based on the Short Circuit Current Rating (SCCR) of the electrical panel to be protected.  All you need to do is find the Short Circuit Current Rating (SCCR) rating of the panel, and install a Transient Voltage Surge Suppression (TVSS) system equal or greater in rating.  The Over Current Protection Device (OCPD) or circuit breaker must be installed in accordance with manufacturer’s requirements.  If the Over Current Protection Device (OCPD) rating is not stated, it is to be sized based on the units supply conductors.  Transient Voltage Surge Suppression (TVSS) systems must also be installed on the load side of the panel, which generally means to install the Over Current Protection Device (OCPD) at the top of the panel closest to the primary feed.It is of course important to remember that Transient Voltage Surge Suppression (TVSS) systems in particular require good grounding, as the surge will be dumped directly to ground (which is unlike a regular surge protectors that use the neutral as well as the ground).  In fact, there are some serious concerns about installing Transient Voltage Surge Suppression (TVSS)  systems without proper grounding, as it may be more dangerous to install them on ungrounded systems than if they had not been installed at all.

Adherence to NEC Article 250 is very important.  We recommend that you have at least two (2) dedicated 10-ft ground rods installed at the panel and in compliance with NEC 250 for your TVSS system.

Here are a few of the codes that relate to your task:

  • National Electrical Code NEC Article 240.4 requires that Transient Voltage Surge Suppression (TVSS) be installed with an Over Current Protection Device (OCPD) such as a circuit breaker.
  • NEC Article 310.15 requires that the Over Current Protection Device (OCPD)  be properly rated to the size of the TVSS feed conductors, even if it is not listed on the device.
  • NEC Article 285.6 requires that the Short Circuit Current Rating (SCCR) of the Transient Voltage Surge Suppression (TVSS) be greater than the Short Circuit Current Rating (SCCR) of the panel to be protected.
  • NEC Article 250 requires proper grounding and is especially important for Transient Voltage Surge Suppression (TVSS) installations.

Best regards,
The E&S Grounding Solutions Engineering Team

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