Parallel Service Conduits

[charlie b]

A utility provides 12.5kV to a large site. The owner brings the 12.5kV to various transformers throughout the site. Each subsequent transformer is the beginning of a new SDS. The one in which I am interested is a 12.5kV to 120/208, 1500 KVA, with a 4000 amp secondary fuse. There are 11 parallel conduits, each with 350 MCM phase and grounded conductors, and a #4 ?ground.? I see at least one code violation ? you need a 500 to serve a 4000 panel with 11 parallel conduits.

I do not yet know if the Main Bonding Jumper lands on the neutral bus at the switchgear or on the neutral point of the transformer. Three specific questions.
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• <font size="2" face="Verdana, Helvetica, sans-serif">I believe that if it lands at the switchgear, then the five wires from the transformer to the main panel are three phases, one neutral, and one EGC (i.e., to bond the case of the transformer to the case of the mail panel). That tells me that each conduit must have a full-sized EGC, and that the existing installation, with a #4 in each conduit, would be another code violation. My first question: Is this right?</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">On the other hand, if the ?grounded-to-neutral connection? happens at the transformer, then the five wires from the transformer to the main panel are three phases, one neutral, and one MBJ. That tells me the MBJ must be sized at 12.5% of the total phase conductor area. 11 times 350 times 12.5% is around 480 MCM. My second question: Do I need to run a 500 in each of the 11 conduits, or can I run it in only one, or can I divide the 480 by 11 (result 43,800 CM), and use a #3? Regardless of the answer, I still have a code violation. My third question: Is this right?</font>

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[ June 25, 2004, 02:18 PM: Message edited by: charlie b ]

 

[don_resqcapt19]

Re: Parallel Service Conduits

Charlie,
What is being used as the raceway?
If the neutral to ground bond is in the panel, I agree that the conductors are EGCs, but how do we size them? Table 250.122 is based on the size of the OCPD that protects the associated circuit conductors, but we are on the line side of that OCPD. Do we use the primary OCPD times the turns ratio to size this conductor?
If the bond is at the transformer, then I agree that the conductors are bonding jumpers (note: the term main bonding jumper is only used for services), and I would size it per 250.102(C). I know that this is not exactly the case in question, but that is how I would do it.
This is an area of the code that needs work. The system installation changes based on who owns the transformer. The code should permit this installation to be made in the same manner as when the transformer is a service. The ownership of the equipment does not change the laws of physics.
Don

 

[charlie b]

Re: Parallel Service Conduits

Thanks, Don. Now I don?t feel as bad about having been confused.

Since I posted the question, I have learned that the information shown on the drawing as ?existing? was not determined on the basis of an as-built drawing or a personal inspection. I must consider the information to be unreliable, and will take steps to have the configuration verified.

 

[bob]

Re: Parallel Service Conduits

Charlie
I think Don is correct in that this type of installation is not handled well by the NEC. In reality there is a 15kv switch some where and it is the main disconnect for this system. Beyond that the 12 kv conductors are feeders and the circuit to the transformer is the same as if the primary was 480 volts. WE would have 3 phase conductors and a EGC.
If this transformer was owned by the utility there would be a ground at the transformer connected to the Xo terminal and bonded to the frame. We would install the phase conductors and neutral connected to the Xo terminal. The neutral would be bonded to the main disconnect and it would act as the EGC. This is covered in 250.24B. 250.24B2 states that the neutral is based on the size of the phase conductors in the conduit rather that the total CM or all conductors.
If this is correct you would not need a EGC.
250.30.A5 says that 250.66 is used to size the EGC. It is not clear but 250.30.A6 allows the EGC to be sized by the conductors in each conduit.

[ June 27, 2004, 04:18 PM: Message edited by: bob ]