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Monday, August 21, 2017
Question:
Good Morning, We have an Assisted Living Facility that is fed from a Utility transformer and a Generator back-up. We have the service Equipment fed through a 4 pole ATS from both sources of power. The MDP (Service Equip.) has a Main Bonding jumper and the Generator has a System Bonding Jumper because the neutrals are being switched at the 4 pole ATS. Question is at the second service for the Fire Pump is fed from the Utility transformer and Generator via the Fire Pump’s 3 Pole ATS where the neutrals are Landed on a common bus. It seems to me, like it really should be a 4 pole ATS in the F.P. Controller otherwise how do I determine whether the Generator is a separately Derived system or not?
Thanks, Kelly Wofford
A
Answer:
Hey Kelly thanks for your question. If I understand your example correctly the 4 pole Automatic Transfer Switch (ATS) on the supply side of the Main Distribution Panel (MDP) is the service equipment and it must be identified as service equipment as stated in 230.66. Notice that transfer switches are not allowed on the supply side of the service disconnecting means as stated in 230.82.The main bonding jumper needs to be installed in the service equipment as stated in 250.24(B). Connecting the grounded conductor to the non-current carrying metal parts of equipment or to equipment grounding conductors downstream of the service equipment such as in your (MDP) is not allowed as stated in 250.24(A)((5).
Without seeing the actual installation or a drawing it is difficult to visualize the generator location and its connections to the fire pump controller. For separately derived systems in general a system bonding jumper must be located at a single point as stated in 250.30(A)(1). If there is a system bonding jumper at the generator then another one is not allowed at the transfer switch.
Please send in a follow up question with more details about this installation and we will try to address them.