As you may have read in another thread the guy I am working for doesn't do much commerical work, and I am trying to help him out. He hired me to work on one of his commerical jobs and has been asking me some questions about other jobs he's quoting. I am trying to help so he gets the jobs and keeps me working, so bear with me please. One of his customers has just purchased 4 machines that require 480volt 3 phase. The building is only equipped with 120/208 3 phase. We contacted the utility and there is no 480 volt 3 phase readily available without spending somewhere between $45,000 and $ 60,000. The customer has a 1200amp service. I am going out to the facility in the morning to take some amp readings on the current service. My first thought is to get a transformer and go from 3 phase 120/208 to 3 pahse 480/277. How do I know if the existing service will be able to handle the load that the new equipment will draw? Is there a limit on how big a transformer we can use to accomplish this task? My thought is to get on big enough to set a 480/277 panel and then feed the equipment from it, and he is thinking on installing two breakers in the existing distribution panel and feeding multiple transformers? Any ideas on this topic?
You can either use stand-alone transformers or one larger unit that feeds a distribution panel.
Transformers can be wired "in reverse" but if you do this, you might end up with a "delta-480" service with no neutral conductor. I have heard of instances where one phase is intentionally grounded, creating a corner-grounded delta.
Also, be aware that you do not need or use a neutral conductor on the feeds to a transformer so connected.
Take the largest at 125% and add the other 2 motors at FLA or around 195 amps X 480 X Root 3 or 165 KVA. The 165 kva is the load for the panel and transformer and the secondary conductors. 148 KVA is the load on the sevice for that calculation. This is rough and based on CEC but watts are watts even in the US. Check with an NEC expert but I think these numbers should get you on the right track.
120/208 service feeding 480 machine
Posted - 12/15/2009 : 00:33:41
