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Tuesday, August 29, 2017
Question:
Re: CQD answer published Tuesday, August 22, 2017 -Services and Generators
Good Morning, in regards to Monday’s 8-21-17 Code Question, I see now I did not word the question correctly.
The Utility Transformer feeds the Service Main Distribution Board (MDP). MDP then feeds normal power to a 4P ATS and the Generator feeds emergency power to the 4P ATS. The MDP gets a Main bonding Jumper and since the ATS is a 4 Pole, then the Generator (Separately Derived System) requires a System Bonding Jumper. Done.
But then It get confusing. The Utility Xfmr. also feeds the Fire Pump Controller (3 Pole ATS) for normal power. The generator supplies an emergency feed to the F.P Controller (3P-ATS), apparently they do not make 4 Pole Fire Pump Controller ATS’s since the F.P. motors are 3 Phase 480 volt and a neutral is not needed.
Question 1. Since 250.24(C) requires us to bring the grounded conductor (neutral) to all Service Disconnecting means. I now have a generator that feeds a 4 pole ATS via MDP and a 3 pole F.P/ Controller ATS. Since 1 ATS is 4 pole with switched neutrals and one is 3 pole with Norm. & Emergency neutrals bonded together. Wouldn’t that be a violation of 250.30(A)
Question 2. If I did not have to bring a neutral from the Utility Xfmr. & from the generator to the 3P Fire Pump Controller ATS that would solve my dilemma. Is there something in the code that would allow me to do that and not violate 250.24(C) ?
Thanks in advance for your help,
Kelly Wofford
A
Answer:
Hey Kelly thanks for the additional information.
As you indicate a grounded conductor is required to be connected to each service disconnecting means grounded conductor terminal or bus as stated in 250.24(C). A main bonding jumper is installed in your MDP and also in the Fire Pump Controller because they are (and need to be) both service equipment. See 695.4(B)(3) for more about the fire pump disconnecting means.
Q1. The generator grounded conductor connection to the MDP is switched by the 4 pole ATS so the generator is a separately derive system as defined in Article 100. The 3 pole Fire Pump ATS is OK because there is no grounded conductor (neutral) from the generator to that ATS installed and connected to the service grounded conductor .If you did it would violate 250.24(A)(5).
Q2. You do need a grounded conductor from the utility transformer to the 3 pole Fire Pump ATS as mentioned in the answer to Q1 and as stated in 250.24(C). Notice that for separately derived systems 250.30(A)(3) uses the word "if" and does not require a grounded conductor to be installed. An affective fault current path must be provided either by an equipment grounding conductor or more likely a supply side bonding jumper depending on the overcurrent device location for the generator conductors.
If this doesn't help enough please send in an additional question.