a wye-wye transformer

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maskling

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yes it is a wye-wye dry type S0 from square d. labels: high side, 220/380, low side, 120/208, 40KVA. low side powers a panel with electronic non-continuous single phase loads and UPS's in a gov't facility. the hook up diagram inside is missing. it's taps are labeled H0-H1-H2-H3 and X0-X1-X2-X3.

whomever hooked it up brought 3 phases and a ground from the panel feeding the high side. they hooked the ground to H0 and H0 is jumpered to X0. XO is tied to the enclosure as per NEC, bonding of separately derived systems.

PROBLEM(?)-there are 9 amps on the green wire feeding H0. the other end of this green is on the ground bar of the distribution panel.

extensive research on square d's tech library fails to find any info on this particular model. i think it is custom made. but square d references a different wye-wye tx they make and call for H0 and X0 to be tied together with a removable strap.

even if i were to bring a neutral to this tx, assuming i have to tie H0 to X0, and i bond X0 to the EGC per code, i will still have amperage on the ground, correct? the neutral i run and the ground will simply share the imbalance from the secondary, no?

there is no way to balance the secondary given the single phase loads it carries.

it would seem to me that there is no way to get the ground unloaded. anyone know anything good? surely the code prohibits this nonsense? anyone? bueller?
 
It seems you would bring a neutral from the supply panel to HO, remove all other connections to HO. Ground the enclosure with the EGC from the supply panel.

The secondary side you would treat like any other Wye secondary.


I have never wired this type of transformer but I see no reason for any bonding connections to HO and it would be violation to do so.
 
that was

that was

my first thought, too. but the ground would still carry the secondary side imbalance back to the panel. my idea is to get amperage off of the ground.

what nec violation is there in tying h0 and x0? chapter, verse?
 
maskling said:
my first thought, too. but the ground would still carry the secondary side imbalance back to the panel. my idea is to get amperage off of the ground.
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No, the secondary side current does not want to go back toward the supply panel. It wants to go to back to the transformer.

Any grounded SDS has the secondary side neutral connection connected to the EGC from the supply panel.

That secondary side neutral connection should also have a grounding electrode, usually building steel if available.

what nec violation is there in tying h0 and x0? chapter, verse?
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250.24(A)(5).

You would be bonding the supply neutral to ground at the transformer if you bonded HO to XO as XO must be bonded to ground in most circumstances.
 
In other words, supply the primary's H0 with the supply's neutral conductor, but do not tie anything else to that connection. Keep it isolated as you would with a sub-panels non-bonded neutral bus.

Now, the secondary's X0, the derived neutral, should be run to the load panel's neutral bus as usual, plus be bonded to the transformer cabinet, the panel enclosure, and a grounding electrode, like a service.

Added: Note: only bond the neutral in one place, either in the transformer or in the breaker panel enclosure, but not both (unless you're using PVC between them.)
 
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Type SO simply means you have a dry type transformer. You need to find the serial number or model number.

Very rarely do we ever connect to the H0 terminal with any conductor; the normal practice is to leave it floating, unless you are a utility.
 
jim dungar said:
Very rarely do we ever connect to the H0 terminal with any conductor; the normal practice is to leave it floating, unless you are a utility.
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But, unlike with a Y-D unit (I've learned ;)), don't you need the H0 connected to maintain the three secondaries at 120v during unbalanced operation?
 
LarryFine said:
But, unlike with a Y-D unit (I've learned ;)), don't you need the H0 connected to maintain the three secondaries at 120v during unbalanced operation?
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Yes, that would be one reason, that there are L-N secondary loads.
 
A load unbalance on the secondary of the wye-wye transformer will cause a current in the neutral of the primary. If the only connection from the supply panel to H0 is the EGC, then this unbalance current will flow in the EGC. This is undesirable. If the secondary system serves 4-wire loads, then I think you need a grounded circuit conductor from the supply panel to H0 and a separate EGC not connected to H0.
 
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