The choice is mostly dependent on the neutral grounding method.
A four pole ATS is used when the neutral must be switched to accommodate grounding requirements.
Many methods of measuring ground fault currents require the use of a four pole ATS.
The code requirement is indirect in that if the ground fault protection requires a switched neutral, then you must use an four pole ATS.
Bill
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"Why not the best?"
Jimmy Carter
In contract with a solid neutral system where a 3-pole ATS is normally used, a 4 poles ATS for is used on a separately derived power system that ultimate is associated with the earthing (grounding) for the emergency system and the ground-fault protection scheme.
Below is a schematic & a switch rendering indicating the neutral pole switched by the ATS.
[sup]>>>>>> [sub]REFERENCES: NEC Sections 230.95,250.24(A)(5), 700.6 (D), K MacKenzie & J. Stark[/sub]<<<<<<[/sup]
