Sizing transformer grounded service conductor

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EGENG

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Shaking off the holiday rust and looking for a back check on a matter.

I have a delta-grounded wye 34.5kV/600V 5.5MVA transformer where the low voltage side is connected to four separate inverter based generators. By their nature, the generators do not have neutral loads and the neutral is not brought to any of the units. The transformer has the grounded service conductor (neutral) from the X0 bushing to a ground bar in the transformer. Supply side bonding jumpers are brought from the ground bar to each generators switchgear along with the ungrounded phase conductors. The SSBJs are sized using Table 250.102(c)(1) and the 12.5% rule for the ungrounded phase conductors to each generator and run from the ground bar at the generator to the ground bar in the transformer. The connection from the ground bar in the transformer to the ground ring around the transformer was sized by table 250.66.

My inclination for sizing the grounded service conductor from the X0 bushing in the transformer to the grounding bar in the transformer, despite carrying no neutral current, would be to follow NEC250.24(C). This section points to sizing the connection by table 250.102(c)(1). In this situation, I am inclined to add up all the ungrounded phase conductors between the four separate generator units, in this case, 5 600kcmil per phase for a total of 5 * 4 generators = 20 * 600kcmil = 12000kcmil. And since the cross sectional area is therefore larger than the table, using 12.5% rule for a equivalent grounded service conductor size of 1500kcmil (or 3 500kcmil) between the transformer X0 and transformer ground bar.

I would appreciate everyone's feedback if I made a slip somewhere.
 
Table 250.102(C)(1). note
"1. If the ungrounded supply conductors are larger than 1100 kcmil copper or 1750 kcmil aluminum, the grounded conductor or bonding jumper shall have an area not less than12 1∕2 percent of the area of the largest ungrounded supply conductor or equivalent area for parallel supply conductors. The grounded conductor or bonding jumper shall not be required to be larger than the largest ungrounded conductor or set of ungrounded conductors."
 
I've always taken "set of ungrounded conductors" to represent the parallel conductors of a phase. In this case, 5*600=3000kcmil. Is that incorrect?
 
For bonding at Xformer, 20*600*12.5% kcmil, since it is greater 600Kcmil, you can use 1-600Kcmil for bonding/SSBJ.
From Xformer to gen (ATS). (you should have disconnect/ECB in between) 1/0 SSBJ with each set.
 
Engser18 said:
Table 250.102(C)(1). note
"1. If the ungrounded supply conductors are larger than 1100 kcmil copper or 1750 kcmil aluminum, the grounded conductor or bonding jumper shall have an area not less than12 1∕2 percent of the area of the largest ungrounded supply conductor or equivalent area for parallel supply conductors. The grounded conductor or bonding jumper shall not be required to be larger than the largest ungrounded conductor or set of ungrounded conductors."
Click to expand...
I'm interested in if it's clearly defined somewhere but in note 1 of the table which you posted above, it highlights the 12.5 percent rule for the largest ungrounded supply conductor or equivalent area of parallel conductors, whichever is larger. I've always taken the second sentence to mean you need to do something similar, for the grounded conductor. Admittedly, it doesn't make much sense when 12.5% of any single conductor would be smaller than the one conductor. Curious how else the set of ungrounded conductors would apply.
 
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