Transformer replacement questions

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charlie b

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Lockport, IL
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Retired Electrical Engineer
Two somewhat odd questions. I will have two transformers operating in parallel. Each secondary will be protected by a 2000 amp fuse and a network protector. The calculated neutral current is well below the ungrounded conductor?s current, so I won?t need the neutral to have the same ampacity as the ungrounded. The engineer designing this system wants to use the following:
? 5 conductors for phase A, each 750 MCM copper.
? 5 conductors for phase B, each 750 MCM copper.
? 5 conductors for phase C, each 750 MCM copper.
? 3 conductors for the neutral, each 750 MCM copper.

All of these will be single-conductors, and all will be routed within the same enclosure that connects the network protectors to the downstream busway. The distance within the enclosure is under 2 feet, so I don?t expect derating issues. The conductors are not part of a cable system, and I would not call the connecting enclosure a ?raceway.? So I don?t think 310.4(C) comes into play.

The two transformers are not identical, in that one is 500 KVA and the other is 1000 KVA. This is an existing situation; we are not creating or changing that disparity. We are replacing a PCB-liquid filled transformer, and the specifications for the new transformer will require matching impedance with its future roommate.

Question 1: Is it code compliant to have a different number of conductors for the phase conductors and the neutral conductors.

Question 2: Is there a code requirement that would be violated by having differently rated transformers?
 

hurk27

Senior Member
If this was installed in multiple raceways, I would say 300.3(B)(1) Would require equivalent sized to the 3 750kmil conductors. but since technically it is with a single enclosure, I don't see a problem as long they are protected for the available fault current.

And 310.4 only requires that each "made" parallel neutral to be made up with the same size conductors, and it doesn't have to be the same amount as the phase conductors:

310.4 or grounded circuit conductor shall not be required to have the same physical characteristics as those of another phase
 
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hurk27

Senior Member
I would say operating transformers in parallel will each require over current protection for the rating of each transformer, if you loose one transformer then the other would be left holding the whole load and would be a hazard.
See the definition in 450.2 then read 450.3

other then that I don't see a problem.
 
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RUWired

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Location
Pa.
Question 1: Is it code compliant to have a different number of conductors for the phase conductors and the neutral conductors.

Question 2: Is there a code requirement that would be violated by having differently rated transformers?

Charlie, if the conductors are service conductors, article 250.24(C)1 requires the minimum size grounded conductor to be 12-1/2% of the (5) 750 kcmil conductors.

If the conductors are feeder conductors, article 215.2(A)1 requires the minimum grounded conductor to be sized per table 250.122 based on the OCPD.

Article 450.7 allows transformer to be in parallel providing.....
Transformers shall be permitted to be operated in parallel and switched as a unit, provided the overcurrent protection for each transformer meets the requirements of 450.3(A) for primary and secondary protective devices over 600 volts, or 450.3(B) for primary and secondary protective devices 600 volts or less.

I would think that impedance, phase angle and polarity would be more important than size.

Rick
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
To RUWired: We are dealing with the "grounded," as in "neutral," not the "grounding electrode conductor."

To Mike: This is on the owner's side of the utility connection point.

To whom it may concern: The overcurrent protection for both transformers is the same: 2000 amps. That is the highest allowable value for a 500 KVA (at 208/120 volts), and it is less than the rated current of a 1000 KVA.
 

RUWired

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Location
Pa.
To RUWired: We are dealing with the "grounded," as in "neutral," not the "grounding electrode conductor."
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Charlie the articles i quoted are the minimum grounded neutral conductor sizes for service and feeder conductors.
250.24(C)
(1) Routing and Sizing. This conductor shall be routed with the phase conductors and shall not be smaller than the required grounding electrode conductor specified in Table 250.66 but shall not be required to be larger than the largest ungrounded service-entrance phase conductor. In addition, for service-entrance phase conductors larger than 1100 kcmil copper or 1750 kcmil aluminum, the grounded conductor shall not be smaller than 12-1/2 percent of the area of the largest service-entrance phase conductor.

215.2(A)1
The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel.

Rick
 

Hameedulla-Ekhlas

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Location
AFG
Question 2: Is there a code requirement that would be violated by having differently rated transformers?

According to some engineering books.

The necessary for parallel operation

Transformers of unequal kVA ratings will share a load practically ( but not exactly) in proportion to their ratings, providing that the voltage ratios are identical and the percentage impedances (at their own kVA rating) are identical, or very nearly so in these cases a total of than 90% of the sum of the two ratings is normally available.

It is recommended that transformers, the kVA ratings of which differ by more than 2:1, should not be operated permanently in parallel.


Other condition necessary for parallel operation

1- All paralled units must be supplied from the same network.
2- Secondary cabling from the transformers to the point of paralling have approximately equal length and characteristics.
3- Voltage difference between corresponding phase must not exceed 0.4%
 
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