Type 3 Surge Protective Device - 30' from service requirement

[gregmillsmec]

Why does the NEC require a minimum of 30' conductor between a Type 3 (branch circuit) surge protective device and service disconnect (see article 285.25)?

Theoretically, I don't understand why the surge device would need to be separated from the service.

Any insight would be appreciated.

Thanks,
-greg

 

[augie47]

Welcome to the Forum:

The link below will guide you to ROP 5-349 which is the reference given for the Code change. I read it til my eyes crossed and did not locate the reason but it is lengthy and may give you some insight.


http://www.nfpa.org/assets/files/PDF/ROP/NEC2008Article285-338.pdf

 

[tom baker]

A short answer is we are using the impedance of the building wiring to reduce the magnitude of the surge. And I am guessing it has a relationship with how the SPD is tested at UL. If the surge is less, then the device does not have to be as robust.

 

[gregmillsmec]

Thanks!

-greg