I wired my first dry type transformer. My superior came to me afterwards and expressed that I needed to run a bonding jumper from the case of the transformer to Xo. The question I have is why is this important? 480v/208v-120v

Ground reference. Secondary of the transformer needs a bonding jumper to give some reference to ground otherwise the voltage between ground and any other terminal could be anything from 0 to 208v

Because your transformer is an SDS it requires that the neutral be bonded to a GEC on the secondary side. This can be done in the transformer or in the enclosure containing the first disconnect.

Angel_Electric:

The Transformer's case needs to be bonded to "Both Sides", in case of a Ground Fault.
What I mean here is the case is bonded with the following:

1. An Equipment Grounding Conductor brought in with the "Primary Feeders" (whatever Branch Circuit is feeding the Transformer),
2. A System Bonding Jumper from the Grounded Conductor on the "Secondary Side" (whatever is leaving the Transformer - AKA: the Separately Derived System, or "SDS")

With the Bond described in "A" above, a Ground Fault from a "Primary Feeder" to the Transformer's case will have a low Impedance path, and (hopefully) trip the Over Current Protection Device (OCPD) on the Feeders for the Transformer.

With a solid Bond, such as described in "B" above, a Ground Fault from a "Secondary Feeder" to the Transformer's case will be able to (hopefully) load up the Primary Feeder's OCPD and eventually cause it to trip.

WITHOUT the Bonding described in "B" above, the Transformer's case may remain energised if a Ground Fault occured internally on a Secondary Feeder.

FYI: Even an Ungrounded System with Ungrounded SDS's will have Transformer cases bonded to a given Grounding Electrode System, plus connected to Equipment Grounding Conductors - only difference is there will be no Bonding Jumper between the SDS and the Grounded Equipment or Grounding Electrode System.

Here are a few Schematics for references:

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Fig. 1: Single Phase Transformer - with 3 Wire Center Tapped Secondary.

Center Tapped Conductor is the Separately Derived System's Grounded Conductor.

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Fig. 2: 3 Phase 4 Wire Wye Secondary - "X0" originates at the "Star Point" connection for all three Secondary Coils (Windings).

This "Star Point" is tapped, and the Conductor which is derived from the star point is the Common Neutral Conductor.
Grounding that tapped Conductor makes it the Separately Derived System's Grounded Conductor.

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Fig. 3: 3 Phase 4 Wire Delta Secondary.

The Grounded Conductor is also a Center Tap, as described in "Fig.1" above.

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Hope this is helpful.

Scott

You demystified the confusion. I Appreciate the pics. Also having seen yet another acronym spelled out has kept any further confusion from occurring. Thanx

Without the bonding jumper, the neutral is free to float, and though you may still see 120V phase-to-neutral on all phases, you might see one phase at 200V and another at 50V when you're expecting all to still be roughly 120V from ground. This tends to really mess with some equipment, especially surge supressors which are looking at voltage-to-ground and will discharge on the high leg.

The same is true of oil-filled transformers, as well; whenever there's a Y-secondary, you need to ground X0*.

* caveot for some generator/ATS setups, but that's getting a bit off track.