12KV Loop

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JdoubleU

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We have what some call a campus Distribution loop. Our campus has a main with 6 breakers. Breaker 1 and breaker 6 are looped. There is a open in the middle of the loop. So essentially part of the campus is fed from breaker 1 and part from breaker 6. This allows flexibility when it come to shutting down buildings and refeeding other so as to have minimal amount of buildings down while one is being worked on. My question is if the loop is closed how is the current split? Say breaker 1 had 20 amps and breaker 6 had 30 amps, when the loop is closed would breaker 1 and 6 both have 50 amps on it.
 
We have what some call a campus Distribution loop. Our campus has a main with 6 breakers. Breaker 1 and breaker 6 are looped. There is a open in the middle of the loop. So essentially part of the campus is fed from breaker 1 and part from breaker 6. This allows flexibility when it come to shutting down buildings and refeeding other so as to have minimal amount of buildings down while one is being worked on. My question is if the loop is closed how is the current split? Say breaker 1 had 20 amps and breaker 6 had 30 amps, when the loop is closed would breaker 1 and 6 both have 50 amps on it.

No, both breakers would have something close to 25 amps on them.
 
If 1 & 6 are on the same bus, the current will split about 50% (25A) on each because the total load hasn't changed.

But the system may not be designed to run that way. With the loop closed, a 12 kV cable fault will trip both breakers and all locations will loose power. With an open loop, only a portion will be down for a while. Closing the loop also places voltage on the load side of an open breaker. That could be a safety issue.

If the main bus has a tie breaker between 1 & 6 or if 1 & 6 can be connected to different sources, closing the loop could be a problem due to high short circuit currents, backfeed, or paralleling two out of phase sources.
 
If the loads are evenly distributed around the loop and the open circuit voltages of each source are equal, the the current will divide based on the source impedance of the two sources. If they are the same source, the currents will divide based on the resistance of the loop wires.
Either way, a 50-50 split is a good starting point.

Tapatalk!
 
The current would divide about evenly assuming the cables have similar lengths and impedance.


The loop can be closed as long as both originate from the same buss and are of the same size. A fault will trip devices outside of what normally would and the fault current may be larger than normal if the calculations were based on the impedance of a radial normally open network.
 
6a: current flow is very low.

6a: current flow is very low.

50A?
We have the same 12KV loop. We also have some big new switchgear and some big old fused switchgear. We have a dedicated feed from utility substation. I had a tour of the substation one time and found out we were only drawing 6-8 amps on the #4's

Little wires in / big wires out.
Basically the current flow is very low.
Co-ordination studies :) should be in place for one scenario or another and you should conform to the studies.
Best practice IMO is to leave the loop open at the most convenient switch. For example we have some four position sectionalizing switches that need a shotgun stick and are more difficult to throw than a handle. I leave one of the handles open better. But the 4pos $ is a better place to park the boots if you want to really dead end, cap test or lock out the power for vault work.

As an aside, I had an Irish guy working for me years ago and he told me in Ireland that all residential cts were wired up in a loop.
That is, the power went out and around and the ct came back and landed under the same screw. You can wire up alot more load like that as the current flow is generally lower. In fact, in Ireland, he said, you can wire unlimited res outlets under a 20a whatever their voltage is.

It's kind of like wiring up some garden lights and putting the xfmr in the middle and then looping back and around.
That will basically give you half the flow each way as the power searches out the easiest way home.
 
As an aside, I had an Irish guy working for me years ago and he told me in Ireland that all residential cts were wired up in a loop.
That is, the power went out and around and the ct came back and landed under the same screw.
Besoeker can tell you (and has in other threads) that this is common practice throughout the UK (and the former UK, of course.) :)

There have been several lengthy discussions about the pros and cons, mostly coming down to the fact that it is used as part of a wiring system that does other things differently in ways that work together with loop wiring. (Fuses at receptacles or plugs, for example.)
There has also been some discussion about whether that kind of installation (at under 600V) is actually allowed or prohibited by the NEC. The discussion of just what constitutes parallel conductors can go on for months. :)
 
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