Guides SPD (surge protective device) selection: recommends the appropriate SPD Type for the installation location, computes a minimum recommended MCOV (Maximum Continuous Operating Voltage) rating from the system voltage and grounding configuration (using the worst-case line-to-ground voltage the system can present), and checks whether a candidate SPD's SCCR rating meets the available fault current at the installation point.
Standards followed
NEC 230.67 — surge protection at service equipmentNEC Art. 242 — overvoltage protection (SPD requirements)NEC 285.6 / 285.7 — SPD ratings and use with other equipment
What it can do
- SPD Type recommendation by installation location (service entrance, downstream panelboard, or point of use).
- Minimum recommended MCOV from system voltage and grounding type, using a standard safety margin over the worst-case line-to-ground voltage.
- SCCR adequacy check against the available fault current at the point of installation.
Example problem
480Y/277V solidly grounded wye service, candidate SPD rated 320V MCOV / 65kA SCCR, 25kA available fault current.
- Reference (line-to-neutral) voltage = 480/√3 ≈ 277.1V
- Minimum recommended MCOV = 277.1 × 1.15 ≈ 318.7V — the 320V candidate SPD clears this.
- 65kA SCCR ≥ 25kA available fault current — adequate.
ADEQUATE — 320V MCOV and 65kA SCCR both clear the requirement.
The 1.15× MCOV margin is a commonly used manufacturer/IEEE-guidance safety margin over the worst-case line-to-ground voltage, not a single literal NEC-mandated multiplier — always cross-check against the specific SPD's UL 1449 listed voltage options, since MCOV ratings are only available in the manufacturer's standard steps. For ungrounded or high-resistance grounded systems, the full line-to-line voltage is used as the reference since a ground fault can drive the line-to-ground voltage up toward that value.