Tap Rule Question

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rayinva

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Hello guys first time poster here thank you in advance for any information.

I have a commercial set up in a shopping center and they are trying to add a 400 amp 480 volt service onto an existing 3000 amp service. The architect is drawing the plans calling for me to set the CT cabinet and feed off the busway with 600 copper to the CTs. All good. He is then making me set two 200 amp fused disconnects to run the secondaries into the space. My issue is he is saying I will have to run 600s to each disconnect. I was wondering why I could not run paralell 3/0 from the CTs to each disco seeing as they are under 10 feet away from the CT cab and will be fused as such at that point.
 
I would contact the engineer but you could certainly parallel 3/0 copper but if the plans call for 600 kcm then I would get an okay from the engineer. There shouldn't be any reason not to parallel- it is compliant

Why would you need parallel 3/0 if you have 2- 200 amp discos. That is not considered a parallel run unless the start and end at the same points
 
rayinva said:
... He is then making me set two 200 amp fused disconnects to run the secondaries into the space. My issue is he is saying I will have to run 600s to each disconnect. ...
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rayinva said:
Poor choice of wording on my part. - mean to say run. A set of 3/0 to each disco off the ct.
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That would be compliant, provided these are truly service conductors.

The architect (???) maybe thinking like David... but tap rules of 240.21 do not apply to service conductors.
 
word games :D
Where is the service overcurrent protection ?
It would appear your 3000 amp switch is your service disconnect making the load side buss/conductors taps.
If so, 240.21(B)(1) would require your tap conductor ampacity to be 1/10 the ampacity of the OCP ahead of the tap or 300 amps. Thus your 3/0s would not be adequate.
 
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You do have an interesting dilemma...
assuming we are addressing a tap, to obey the 10% rule your tap conductor would need to be a minimum of a 250 kcmil which is larger than most 200 amp switches will accept.
If you tap with a 600 as requested, you can't "tap a tap" so tapping off the 600 is prohibited.\
My first thought would be to look at the availability of pin connectors and see if you could install a pin connector on a 250 and terminate in your 200 amp disconnect with that. (Look at Burndy YE29P46X92)

Incidentally.. welcome to the Forum
 
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rayinva said:
The 3000 amp switch feeds a bus system that the other services are fed from.
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david luchini said:
What you want to do would violate 240.21. The architect has it correct.
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augie47 said:
word games :D
Where is the service overcurrent protection ?
It would appear your 3000 amp switch is your service disconnect making the load side buss/conductors taps.
...
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Depends on whether it is just a 3,000A-rated switch or 3,000A fused [service] disconnect...???

Being CT's are on the load side, I'm leaning toward just a switch, but needs to be confirmed for definitive resolution...
 
Smart $ said:

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Depends on whether it is just a 3,000A-rated switch or 3,000A fused [service] disconnect...???

Being CT's are on the load side, I'm leaning toward just a switch, but needs to be confirmed for definitive resolution...
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I don't see how just a switch being located on the supply side of the service disconnect would comply with 230.82. I often see shopping centers where the meters/cts are on the load side of the service disconnecting means.
 
david luchini said:
I don't see how just a switch being located on the supply side of the service disconnect would comply with 230.82. I often see shopping centers where the meters/cts are on the load side of the service disconnecting means.
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See 230.82(3).

Not saying otherwise to your second sentence. Just asking for confirmation one way or the other.
 
david luchini said:
I don't see how 230.82(3) would apply. The 3000A switch would disconnecting all conductors in the building from the service entrance conductors, not disconnecting individual meters.
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It seems to me that, as related to the OP, if the 3000 amp switch was decided to be a "meter disconnect" then by virtue of 230.82(3) it could be deemed to be "ahead of the service disconnect(s)" and his connection to the buss would not be taps but service conductors. Confusing to me as it could, in fact, disconnect the service, but not be a "service disconnect"
Locally, the decision is often made by who required it, who controls it and often who if anyone locks it.
I asked about OCP earlier as in my experience these meter disconnects often have no OCP although that normally introduces a fault current problem.
 
I am assuming that you are making an unprotected cable connection from the busway to the CT, in which case the engineer is technically correct. However, for large mall type feeders in our jurisdiction we have allowed these taps to be considered service taps even though technically they are feeder taps. We allow this because in some cases, especially where the building is served at 480/277 volts, it is impracticable, for example, to require a minimum 400 amp tap conductor (where derived from a 4,000 amp feeder) to terminate into a 100 ampere disconnecting means for a small tenant space. In your case the 3,000 amp switch is probably necessary to comply with the maximum number of disconnects permitted for a service. To even consider that this switch would not have overcurrent protection so that it could technically qualify as a meter disconnect switch is the best argument for this policy that I can think of.
 
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