3 phase panel on an open delta service?

[alabaza]

I am designing a project at a building that has 120/240V, 3 PHASE, 400A electric service. I called the electric utility company and confirmed that there is (2) pole mounted transformers on the outside pole to provide open delta 3 phase service to the building(it means the neutral is center tapped between two phases). My question is that i am providing a new 3 phase, 120/240V branch circuit panelboard inside the buidling to feed some loads, but i will have 120v receptacles and light connected to this panel, i know that for ex. phase A or B with the neutral can be used to feed the 120v circuits, what about phase C with neutral? how am i supposed to coordinate the panel schedule in this situation? what if the contractor connected phase C with neutral instead? how is he supposed to know which ones have the center tap?

thank you for the input.

 

[itsmoked]

Measuring between the various phases will present 240V
However measuring between the each of the phases and the neutral or ground will present 120, 120, and something MUCH higher like 190V. That is typically called the "wild leg" or the "stinger" or the "high leg".

You just schedule the wiring to allow ONLY three phase or 240V loads to be connected to the stinger. ALL single phase loads must avoid connection to the stinger.

It's very common and not a problem.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

This is addressed in the code (110.15, 230.56 and 408.3E and F), as well as the design of the panel (or should be). On a full Delta system, the "high" leg is supposed to be B phase and marked with orange tape. One phase bus in the panel will be arranged such that you cannot attach single pole breakers to it, therefore there is no valid way to use it with the neutral.

"Will work for (the memory of) salami"

 

[waross]

It is common for the transformers to be disimilar. One transformer may be larger than the other transformer to handle the single phase loads.
The second transformer may not be as large but be only large enough to service the three phase loads.
It is possible that you have a 100 KVA transformer to supply 400 Amps single phase and perhaps a 5 KVA, 10 KVA or 15 KVA transformer to service a small amount of three phase loading.
If this is the case you must be careful to connect your panel so that the single phase loads are fed from the larger transformer.
You may be fortunate and have two equally sized transformers.
Best to check with the utility.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Compositepro]

Another option is to place a single phase panel next to the three phase panel and connect all your single phase branch circuits there. It frees up space in the three phase panel and will be less confusing for future "electricians".

This link would be of interest to you

http://ecmweb.com/qampa/stumped-cod...ying-high-leg-3-phase-4-wire-connected-system

 

[alabaza]

I appreciate the responses , it all helped , i just don't understand the logic behind having such system, it just makes thing much more confusing for future upgrades and creates an unbalanced system!

 

[itsmoked]

The reason is very clear. It costs the power company considerably LESS money to implement, while actually providing everything the customer needs. The customer's electrical staff just needs to understand this very common scheme.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The wyepen delta is a dependable system.
The four wire wye:closed delta is nasty. Circulating currents, back feeds, blown primary fuses. I have seen many four wire wye:closed delta banks with one primary fuse blown and the bank working happily as an open delta.
A four wire wye:full delta bank will backfeed into a line to ground fault anywhere on the primary distribution circuit. This often takes out one of the primary fuses.
If you need 240 Volts three phase power, then open delta is the way to go.


Bill
--------------------
"Why not the best?"
Jimmy Carter