What is the difference between Type 1, Type 2, and Type 3 SPDs?

Type 1 connects on the line side of the service disconnect and handles the highest surge energy. Type 2 installs on the load side at the main panel or sub-panel. Type 3 is a point-of-use device near the equipment and must sit at least 10 m of conductor downstream of a Type 1 or 2.

What is MCOV and why does it matter?

MCOV is the maximum continuous operating voltage an SPD can ride out without conducting. It must exceed the highest expected steady-state line voltage — roughly 1.25 times nominal — so the device does not leak current and overheat during normal voltage swells.

What is nominal discharge current (In)?

In is the surge current an SPD can divert repeatedly without degrading, tested per UL 1449. Service-entrance devices need a high In (often 20 kA) because they face direct lightning energy; point-of-use devices can use a lower In.

Does the NEC require surge protection now?

Yes, in many cases. NEC 230.67 requires SPD protection for most dwelling-unit services, and 700-series articles require it for some emergency and critical systems. This tool helps select a compliant device, but always check the edition adopted locally.

Can a Type 3 device alone protect my equipment?

No. A Type 3 plug-in or point-of-use SPD is a complement, not a substitute. It must coordinate with a Type 1 or Type 2 device upstream so the higher-energy surge is clamped before it reaches the sensitive load.

Surge Protective Device (SPD) Sizing Calculator

A surge protective device diverts transient overvoltages — from lightning or switching — away from your equipment. Choosing the wrong type or a too-low MCOV leaves gear exposed or causes nuisance failures. This tool recommends the SPD type, MCOV, and discharge rating from where the device sits and the system voltage.

How it works

The recommendation follows the IEEE C62.41 exposure categories and NEC Article 285 installation rules.

Category C — service entrance. Highest surge energy. Use a Type 1 (line side of the service disconnect) or a Type 2 on the load side, with a high nominal discharge current (20 kA or more).
Category B — feeders and sub-panels. Moderate energy. Use a Type 2 on the load side of the main overcurrent device.
Category A — branch circuits and receptacles. Lowest energy, far from the service. Use a Type 3 point-of-use device, installed at least 10 m of conductor downstream of a Type 1 or 2.

The minimum MCOV is set so the device rides out normal voltage swells:

MCOV >= 1.25 x nominal line-to-neutral voltage

For a 120 V circuit that is about 150 V; for 277 V it is about 346 V.

Worked example

A 277/480 V wye sub-panel feeds office lighting and you want surge protection at that panel. The line-to-neutral voltage is 277 V, the location is a feeder (Category B), so the tool recommends a Type 2 SPD with a minimum MCOV of about 346 V and a nominal discharge current of 10–20 kA.

Notes and tips

Lead length matters more than the SPD’s headline rating. Keep the connecting conductors short and straight — every extra inch adds let-through voltage.
Coordinate stages: a Type 2 at the panel plus a Type 3 at sensitive equipment gives a lower clamping voltage than either alone.
Check the Voltage Protection Rating (VPR) on the label; a lower VPR means the device lets less surge voltage through to the load.

Every recommendation is a planning aid; confirm the final device against the manufacturer’s data and the adopted NEC edition.