Single phase new service disconnect three phase service

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dema

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Indiana
We are adding a new small building to a campus. An electrical contractor has requested that we do something odd. I am the PE, design engineer on the job, and I can't think of how this would be illegal. But it is rather strange.

I had specified a new single phase 100A service on the request of the owner.

The existing campus has two 120/208V three phase 400A service disconnects. The EC is requesting that he tap off the metered feed of one of these disconnects and add a third service entrance disconnect placed on the same wall. Then he would run three phase out to the construction site, use three phase for his construction equipment, and then leave the third phase feeder in a junction box (that can't be left live I'm sure - but having trouble with code references on all this.) and run single phase (120/208) to the panelboard.



That is the whole question. My befuddlement is - this isn't tap rule because it is upstream of the service disconnect. Right?

We aren't allowed to leave abandoned cable. Is this cable abandoned? Or a "spare"? How does the code address spares that are intentionally pulled for future use (or occasional use) vs being abandoned cable?

Is there a problem with running a single phase service disconnect off of a three phase service?

Thank you for any help on this.
 
It would be a service entrance conductor tap. The tap rules of Article 240 do not apply to service conductors. It's typically not a problem because service disconnects for a single building are required to be grouped, so the taps are generally in the service equipment or in adjacent enclosures or raceways.

There is no requirement to pull the abandoned conductor, or even that it be deenergized for that matter.

The problem that may surface is any associated with 120/208 vs 120/240, i.e. the line-to-line equipment compatibility and no reduced neutral. On the latter, the contractor would have to install a neutral conductor the same size as the ungrounded conductors.
 
it is a little odd but I do not see that any of it violates code.

there is no requirement I know of to remove unused conductors of this type.

personally, i would be inclined to just install a 3 phase PB in the building. the cost difference is not enough to worry about. you might even save money by being able to run MWBC from it.
 
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