Double Ended Substation Kirk Key Issue
Double Ended Substation Kirk Key Issue
Many good responses here, but no one mentions that on a 4000 amp main totaling up 4000 amp worth of breaker ratings (not load, but ratings) doesn't mean the 4000 amp bus is loaded to 4000 amps. As a matter of fact, it better not be loaded to 100% of the all breaker ratings. Just because it's a double ended substation, doesn't mean each breaker should be or needs to be rated to carry ALL of the code calculated loads of both sides when you close the tie breaker, but that is frequently done if the criticality of the systems warrants it. The actual running load on 4000 amps worth of breaker ratings is probably somewhat less than 2000 amps, but you can check this with the panel meter and see what you've got. The Code Calculated Load for the 4000 amps worth of breakers might be as high as 2800 - 3000 amps (75% of the breaker rating is pretty high), but I'd be surprised if it were much more than that. Closing the tie on a total load of 3500 to 3800 amps would not present any problem for the main breaker carrying that total load, but if it were much more than that, you'd want to consider load shedding prior to closing the tie breaker. Also, the main question, if both of your mains are fed from the same transformer there is no problem closing the tie before opening a main as long as you're sure they're phased properly, which is sounds like you've properly checked. The available fault current shouldn't increase, because you're still dealing with the same transformer let-through current, and the feeders are still sharing the loads pretty much the same as they were before. Just make sure there isn't a generator associated with feeding either or both sides of the substation, because that would definitely be out of phase with the utility source, and would be disasterous if you closed a tie on that source.