Secondary Side of Transformer

[Designer371141]

My current employer has been connecting multiple motor control centers to a single transformer by connecting all feeders to the common taps of the transformer. I cannot find any code areas where this is legal to do. Does anyone know and can you provide me a code reference which designates how may feeders can be taken off the secondary side of the transformer, our current project is calling for four separate sets of feeders to four separate motor control centers with common transformer, each feeder is rated for 1600 amps. He is insisting on seeing a code reference which does not allow this practice.

 

[augie47]

If it's a customer transformer (as opposed to a utility service transformer), 240.21(C)
allows multiple secondary connections to a transformer provided the provisions of 240.21(C)(1) thru (6) are met as well at 450.3 for proper transformer protection.

The same holds true for a service transformer only there are a lot more variables to address.

 

[charlie b]

Take a look at table 450.3(B), and in particular at its Note 2. The practice is legal. But there are limits. One limit is the number of connections. The other limit is the total of the ratings of all the overcurrent devices connected to the one transformer. My inclination is to wonder about connecting four 1600 amp MCCs to a single transformer. You would have to be looking at a 4MVA transformer, if the voltage is 480.

Welcome to the forum.

 

[Designer371141]

Additional Information

Additional Information

My current employer has been connecting multiple motor control centers to a single transformer by connecting all feeders to the common taps of the transformer. I cannot find any code areas where this is legal to do. Does anyone know and can you provide me a code reference which designates how may feeders can be taken off the secondary side of the transformer, our current project is calling for four separate sets of feeders to four separate motor control centers with common transformer, each feeder is rated for 1600 amps. He is insisting on seeing a code reference which does not allow this practice.

This is a utility transformer, so far they are planning on using a 1500Kva, and utilize the maximum allowable under NESC for primary protection.

 

[augie47]

This is a utility transformer, so far they are planning on using a 1500Kva, and utilize the maximum allowable under NESC for primary protection.

Then all the corresponding rules of Art 230 apply. The restriction on the number of sets of service conductors in 230.71 with 230.2(C) and 230.40 taken into account.
It is certainly possible, but a close look at Art 230 would be in order. (such as 230.2, 230.71, 230.70, etc.)

The devil will be in the details.

 

[RB1]

If the MCC's are grouped at one location there is no problem as far as code is concerned. You may have a problem terminating that many conductors on the transformer bushings. In our area you would be limited to 12 conductors per phase max using NEMA stacking spacers. If the POCO uses a secondary connection pedestal it shouldn't be a problem. Better confirm with the utility.