busway application issue

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texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Here is my dilemma.
Existing service rated busway presently fed by a delta/wye bank located directly under the busway. For those that are familiar, the busway termination is what most call a transformer tap section where the A,B,C busway connections are spread out about 15 feet to allow the open transformer conductors to go straight up to the bus and there is a N connection at the bus next to each phase so that the busway neutral becomes the tie point for the transformer bank. Seems all well and good and this was common in the days of open single pot Y transformer banks to feed a busway.
Now the plan is to remove the wye bank and re-feed the busway from a feeder VIA a cable tray. The neutral bond will be removed at the load end service equipment.
My question is that since now the (4 set parallel) feeder will come VIA cable tray to be terminated to the old transformer tap section, the proposal is to terminate all neutrals at the end and all the phases to each phase connection on the busway. This results in the phases and neutral connections to the busway spread out along about a 15' distance. Will this connection arrangement dramatically increase the short circuit impedance?
My opinion is that a busway terminal box needs to be used (as opposed to a a transformer tap section) so that all phases and neutral terminate in the same location to avoid impedance issues.
 
Here is my dilemma.
Existing service rated busway presently fed by a delta/wye bank located directly under the busway. For those that are familiar, the busway termination is what most call a transformer tap section where the A,B,C busway connections are spread out about 15 feet to allow the open transformer conductors to go straight up to the bus and there is a N connection at the bus next to each phase so that the busway neutral becomes the tie point for the transformer bank. Seems all well and good and this was common in the days of open single pot Y transformer banks to feed a busway.
Now the plan is to remove the wye bank and re-feed the busway from a feeder VIA a cable tray. The neutral bond will be removed at the load end service equipment.
My question is that since now the (4 set parallel) feeder will come VIA cable tray to be terminated to the old transformer tap section, the proposal is to terminate all neutrals at the end and all the phases to each phase connection on the busway. This results in the phases and neutral connections to the busway spread out along about a 15' distance. Will this connection arrangement dramatically increase the short circuit impedance?
My opinion is that a busway terminal box needs to be used (as opposed to a a transformer tap section) so that all phases and neutral terminate in the same location to avoid impedance issues.

I think what you are worried about is that there is an potential current loop of non-zero area between any fault current flowing in the non-grounded conductors that are in the buss-way and the fault current in the grounded grounded conductors laying in the cable tray? Yes, there is an impedance associated with that open loop area. The number would be pretty small assuming there is not a log/lot of metal inside the loop ( well, a log of metal would also be bad. ) I can't say as to whether the increase in impedance would be 'dramatic'.

Any metal inside the loop is an issue also as it will be subjected to inductive heating should there be fault currents or unbalanced currents on the neutral.
 
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