Most facilities today have surge arrestors on the building supply (service). These are typically bought on a budget mentality that results in a waste of the entire investment. It doesn't have to be that way. In fact, correct specification and installation will provide a very high return on investment (ROI).

The first thing to consider is that these systems need to be part of a larger surge protection strategy. They will protect only against the larger spikes (reducing them to a lower voltage) from outside your facility.

With this in mind, we turn to 280.4, which provides the minimum requirements for surge arrestor selection. Essentially, you have to select a surge arrestor that is listed for this use and that is sized to at least the maximum continuous operating voltage at the point of application:

  • For solidly-grounded systems, it's the phase-to-ground voltage.
  • For impedance or ungrounded systems, it's the phase-to-phase voltage.

This is the minimum safety requirement. The circuit protection requirements aren't covered by the NEC, as their determination requires an engineering analysis for the specific system and location. Manufacturers and their authorized resellers can help you with this analysis.