I am hoping I can get some feedback from you guys concerning the correct wire sizing for a 112.5 KVA transformer. We are planning on feeding it from a 480/277 panel with a 150amp breaker. we are going to install a 200amp disconnect before the transformer and reduce the fuse size to 150 also. The length is 300' from the 480/277 panel to the disconnect. I am saying that we can use #1 THHN and a #2 GND, but my boss is saying we need to run #1/0 THHN. What about bonding bushings? At the panel, or when we enter the transformer from the disconnect? We are planing on installing a 300 amp 120/208 panel off the secondary side with # 350MCM. My boss doesn't do alot of commerical work, and on most jobs I worked on in the past that information is already laid out for you from the shop. Is there anything else that we are missing? I do believe I saw something about grounding the transformer to the building steel also. Just trying to help the guy out. Any thoughts???
The capacity of #1 copper is 130 Amps. Your transformer operating at 100% load will draw 135 Amps.
Depending on what you have connected to it will determine whether you will ever approach this value.
Since the next standard size of breaker is 150 Amps, you are permitted to use that size breaker on the #1 conductor provided your total connected load does not exceed 130 Amps. The ECG only needs to be a #6 on the primary side of this transformer.
Bonding bushings are required where concentric or eccentric knockouts are encountered (over 250 Volts), or where flexible conduit is used. However, you may not need those if a separate EGC is installed with your circuit conductors in the flex.
Correct Wire Sizing for Transformer
Posted - 12/15/2009 : 00:18:47
