• What is the key point to make the decision of paralleling?
  ** Reliability and redundancy.

• Do you use 4 pole or 3 pole ATSs on 120/208V Y systems

** To determine which configuration (3-pole or 4-pole) to select, attention must be paid to the system grounding scheme and ground fault protection requirements. If ground fault detection is required on either source, a 4-pole transfer switch is necessary in most cases. Per NFPA 70: National Electrical Code 230.95 Ground Fault Protection (GFP) of equipment is required at the service disconnect (utility breaker) for systems with all of the following:

Solidly grounded wye electrical service.
More than 150 volts to ground (277/480 or 347/600 VAC).
Over current device rating of 1000 A or more.
NEC 700.6 (D) and 701.6 (D) requires Ground Fault Indication (GFI) at the emergency source.

Ground fault sensing depends on being able to sense ground fault current. To accurately sense ground fault current, it must return to its source on a known path relative to ground fault current transformers (CTs). In basic emergency standby systems there are two rules to follow to meet these requirements:

There can only be one neutral/ground connection on any neutral bus at one time.
Ground fault sensors (the CTs) must be downstream (or on the load side) of the bonding connection.

To meet both rules when connected to either the normal or the emergency source, the neutral must be switched using a 4-pole transfer switch