Mis-matched transformer/panel -or not?

[bbouchard79]

An electrical contractor on a job I'm working on was asked to provide a 480 delta 120/208 Wye 150 KVA dry type transformer & a 400Amp 42 circuit distribution panel. He is planning on using 500kcmil's on the secondary side of the transformer to feed the panel which is MLO. The panel is located less than 10ft from the transformer. After searching on-line I found a Mike Holt article Titled: Transformer Installation made easy -sort of. In the example given, one of the transformers was a 480 delta 120/208 Wye 112.5 KVA. After performing the various sizing & rating steps a conductor size of 700kcmil?s was determined. This alarmed me because we have a 150KVA transformer which I think would have larger conductors if the transformer & load were matched correctly.If the electrical contractor raised the size of the conductors to 700kcmil?s and provided a 400 amp OCPD on the secondary side of the transformer would this be a acceptable installation? The example in the article is almost an exact match except the transformer we have on site is larger.

Thank you in advance.

 

[Ragin Cajun]

A 150 KVA 120/208V transformer requires a minimum ampacity of 521A on the secondary, likely 2-400 MCM per phase.

RC

 

[Smart $]

... He is planning on using 500kcmil's on the secondary side of the transformer to feed the panel which is MLO. ...

408.36 violation. The panelboard is required to have supply-side overcurrent protection, with three exceptions. Two of the exceptions require the panelboard be used as service equipment, and the other one requires two protective devices on the supply.

 

[ron]

Guessing that you will have adequate secondary overcurrent protection (or overcurrent protection for the MLO panelboard) at some point, you can surely use less capacity from the XFMR than 150kVA, but you need to have the adequate overcurrent protection.
A D-Y XFMR cannot be adequately protected by primary protection only.

 

[e57]

An electrical contractor on a job I'm working on was asked to provide ~~~~

After searching on-line I found a Mike Holt article Titled: Transformer Installation made easy -sort of. In the example given, ~~~~

This alarmed me because ~~~~

If the electrical contractor raised the size of the conductors ~~ would this be a acceptable installation? ~~~

Thank you in advance.

Why does this sound we take questions from persons outside the trade +/or second guessing someones elses work? One would assume the work is being inspected? Specified? Things we can't see from here????

Anyway we don't know the load, and the panel is 400a but don't know if there is protection prior to it and what size it is, or might be? Last time I checked 42 was more than 6...