Mike Dolt Enterprises
Mike Dolt Enterprises
User: utilguy, Location: southeastern georgia
Asked on: Monday, May 2, 2011 at 2:16 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:52 am EDT
Question: We're installing an ATS between two 120/208 transformer services to an IT network bldg - one from utility, other from a university dist system (which is essentially another feeder from the utility). Typically the services from the transformers would have the neutral grounded at the transformer. There will be a service disconnect between each transformer and the ATS. Do I need to treat the univsersity one as a separately derived system and use a 4 pole switch, or can I treat it like two utility feeds, use a 3 pole and do the bonding in the service disconnects?thanks
You must treat the university feed as a separately derived system and use a 4-pole switch.The NEC defines a separately derived system as a system that is not connected to a grounded neutral system. In this case, the university feed is not connected to the utility's grounded neutral system. Therefore, it must be treated as a separately derived system.Using a 4-pole switch will ensure that all four conductors (two hots, neutral, and ground) are disconnected when the ATS is in the off position. This will prevent any current from flowing between the two systems.If you were to use a 3-pole switch, the neutral conductor would not be disconnected when the ATS is in the off position. This could allow current to flow between the two systems, which could create a safety hazard.Bonding the neutral conductor in the service disconnects is not sufficient to prevent current from flowing between the two systems. The bonding connection must be made at the ATS.Here is a diagram of how the ATS should be connected:[Image of an ATS connected to two separately derived systems]The hot conductors from each system are connected to the ATS input terminals. The neutral conductors from each system are connected to the ATS neutral terminal. The ground conductors from each system are connected to the ATS ground terminal.The ATS output terminals are connected to the load. Flag for review