Future PI 110.15

[tortuga]

Based on our discussions on loading 3-phase open delta systems with single phase loads it seems there is a need to identify which set of wires is the open phase.
It is import for the electrician to know which set of phase wires / busbars are the open phase so they can make an accurate determination of the load as adding single phase loads to the open phase has more affect on transformer loading than the other two sets.

110.15 Identification of Delta-connected Conductors. High-Leg Marking.

(A) On a 4-wire, delta-connected system where the midpoint of one phase winding is grounded,
only the conductor or busbar having the higher phase voltage to ground shall be durably and permanently marked by an outer finish that is orange in color or by other effective means.
Such identification shall be placed at each point on the system where a connection is made if the grounded conductor is also present.

(B) On a open delta-connected system where one primary phase winding is not present, the set of conductors or busbars having the open phase shall be permanently marked by an effective means.
The means of identification shall be documented in a manner that is readily available and shall be permanently posted at each point on the system where a service, feeder or branch circuit originate.

 

[Hv&Lv]

Based on our discussions on loading 3-phase open delta systems with single phase loads it seems there is a need to identify which set of wires is the open phase.
It is import for the electrician to know which set of phase wires / busbars are the open phase so they can make an accurate determination of the load as adding single phase loads to the open phase has more affect on transformer loading than the other two sets.

110.15 Identification of Delta-connected Conductors. High-Leg Marking.

(A) On a 4-wire, delta-connected system where the midpoint of one phase winding is grounded,
only the conductor or busbar having the higher phase voltage to ground shall be durably and permanently marked by an outer finish that is orange in color or by other effective means.
Such identification shall be placed at each point on the system where a connection is made if the grounded conductor is also present.

(B) On a open delta-connected system where one primary phase winding is not present, the set of conductors or busbars having the open phase shall be permanently marked by an effective means.
The means of identification shall be documented in a manner that is readily available and shall be permanently posted at each point on the system where a service, feeder or branch circuit originate.

Looks to me like this will be difficult at best.

All the different utility’s out there you would have them to mark the series connection to the MB/Cabinet.
We don’t do that. We just connect the high to the right side, the other two wires land where they land..

But worth a try.

 

[electrofelon]

I was also thinking this would be difficult to make happen for utility transformers as it involves coordination and cooperation of the utility, but then again we have to know things like available fault current which requires input from the POCO.

 

[jim dungar]

Looks to me like this will be difficult at best.

All the different utility’s out there you would have them to mark the series connection to the MB/Cabinet.
We don’t do that. We just connect the high to the right side, the other two wires land where they land..

But worth a try.

Is this supposed unbalanced loading really a problem for utilities?
We have lots of these open delta systems in this state. I don't remember ever hearing of more problems caused by customers adding loads than with any other utility system. Do we have a solution in search of a problem?

 

[electrofelon]

Is supposed unbalanced loading really a problem for utilities?
We have lots of these open delta systems in this state. I don't remember ever hearing of more problems caused by customers adding loads than with any other system.

I guess we have to define what we are trying to accomplish. Presumably the POCO's already throw in a "derating" (theoretically 86.6%) for the transformers when they do their sizing? There doesnt seem to be a consensus on avoiding the open phases for single phase loads. Some people say it is important, some say its not, some say no one does it, some say people do indeed do it where they're at.....

 

[Hv&Lv]

Is this supposed unbalanced loading really a problem for utilities?
We have lots of these open delta systems in this state. I don't remember ever hearing of more problems caused by customers adding loads than with any other utility system. Do we have a solution in search of a problem?

It’s not an issue with us at all.
Earlier I was saying single phase loading doesn’t need to be done to the open phases because of changing loads causing voltage regulation issues on the secondary side.

 

[electrofelon]

It’s not an issue with us at all.
Earlier I was saying single phase loading doesn’t need to be done to the open phases because of changing loads causing voltage regulation issues on the secondary side.

so if voltage regulation is the issue, then should the NEC go as far to say no single phase loading on the open phases, or just require the identification and let the electrician decide whether to utilize the opens for single phase loads?

 

[Hv&Lv]

so if voltage regulation is the issue, then should the NEC go as far to say no single phase loading on the open phases, or just require the identification and let the electrician decide whether to utilize the opens for single phase loads?

I would say no. The regulation depends on the loading. You have small single phase loads that aren’t significant in the big picture I don’t think it would be a problem.

We were discussing loading the transformers to their full kVA.

 

[tortuga]

Syncro laid out the math in this post:

Trouble finding straight 240v breaker vs 120/240 breaker

There is no real derating necessary for using the 'open' phase. Typical formula for sizing the unequal transformers are: Power pot = .58T Lighting pot = .58T + S Where T is the total three phase load and S is the total single phase load. Note how the transformer sizing takes into account the...

forums.mikeholt.com

So seems like there is good cause for the electrician to know what set of phases is the open phases and its probably pretty easy to implement, just say orange - blue is the open set, or L2-L3 etc.

 

[kwired]

I was also thinking this would be difficult to make happen for utility transformers as it involves coordination and cooperation of the utility, but then again we have to know things like available fault current which requires input from the POCO.

And POCO doesn't always give you the actual figure but rather some figure they give to everyone on same size transformer even though supply side conditions are going to be different as well as transformer impedance could vary even on same voltage and kVA ratings.

 

[kwired]

Syncro laid out the math in this post:

Trouble finding straight 240v breaker vs 120/240 breaker

There is no real derating necessary for using the 'open' phase. Typical formula for sizing the unequal transformers are: Power pot = .58T Lighting pot = .58T + S Where T is the total three phase load and S is the total single phase load. Note how the transformer sizing takes into account the...

forums.mikeholt.com

So seems like there is good cause for the electrician to know what set of phases is the open phases and its probably pretty easy to implement, just say orange - blue is the open set, or L2-L3 etc.

Kind of a design issue IMO and not what NEC is supposed to be about.

Utility supplied source - is sort of their problem.

Separately derived source - probably a wise decision to look more carefully at loading. At same time open delta is not very common for separately derived sources, but if you were to use it you likely are supplying a rather specific load anyway.

 

[don_resqcapt19]

Syncro laid out the math in this post:

Trouble finding straight 240v breaker vs 120/240 breaker

There is no real derating necessary for using the 'open' phase. Typical formula for sizing the unequal transformers are: Power pot = .58T Lighting pot = .58T + S Where T is the total three phase load and S is the total single phase load. Note how the transformer sizing takes into account the...

forums.mikeholt.com

So seems like there is good cause for the electrician to know what set of phases is the open phases and its probably pretty easy to implement, just say orange - blue is the open set, or L2-L3 etc.

For the slash rated breaker, the only issue is the high leg. That is already required to be identified. In an open delta, the high leg would always be on the open phase, as the transformer with the center tap supplies all of the line to neutral loads.

 

[electrofelon]

For the slash rated breaker, the only issue is the high leg. That is already required to be identified. In an open delta, the high leg would always be on the open phase, as the transformer with the center tap supplies all of the line to neutral loads.

Right but we don't (usually ) know which non high leg phase is the other one.

 

[jim dungar]

Right but we don't (usually ) know which non high leg phase is the other one.

And, other than theoretically, this doesn't seem to be a real world problem.
Significant load additions should already be coordinated with the utility for services and when doing load calculations for existing feeders.

Have you ever heard of any utility asking for per phase loading when they supply open-delta services?

 

[don_resqcapt19]

Right but we don't (usually ) know which non high leg phase is the other one.

My comment only addressed the slash rated breaker issue, and was just pointing out that we already know where we can install slash rated breakers.

 

[Hv&Lv]

And, other than theoretically, this doesn't seem to be a real world problem.
Significant load additions should already be coordinated with the utility for services and when doing load calculations for existing feeders.

Have you ever heard of any utility asking for per phase loading when they supply open-delta services?

Correct. We don’t care about that.
Give us the single phase loads and the three phase loads only, not phase loading

 

[electrofelon]

And, other than theoretically, this doesn't seem to be a real world problem.
Significant load additions should already be coordinated with the utility for services and when doing load calculations for existing feeders.

Have you ever heard of any utility asking for per phase loading when they supply open-delta services?

Correct. We don’t care about that.
Give us the single phase loads and the three phase loads only, not phase loading

Right I am not too concerned about the utility equipment. Pretty much every utility has a clause requiring them to be notified of added loads. What I cant seem to get a straight answer on is whether the open phases should be avoided for customer equipment concerns.

 

[Hv&Lv]

Right I am not too concerned about the utility equipment. Pretty much every utility has a clause requiring them to be notified of added loads. What I cant seem to get a straight answer on is whether the open phases should be avoided for customer equipment concerns.

I thought I answered that in post 63 of your thread on open delta.

In real world situations, it’s not a problem that I have seen
In you hypothetical question of using the full kVA of the two pots, it could be an issue

 

[jim dungar]

Right I am not too concerned about the utility equipment. Pretty much every utility has a clause requiring them to be notified of added loads. What I cant seem to get a straight answer on is whether the open phases should be avoided for customer equipment concerns.

The customer loading would only be a problem if it caused voltage regulation issues. If this were to occur, the process would be to contact the utility, just like is done with any other service configuration.

Open-delta configurations are not more likely to have voltage issues than any other undersized transformer. Electricians seem to worry about the 'missing phase' more than power engineers and utilities do, this may be due to old wives tales and anecdotal comments in their basic electricity courses.

 

[electrofelon]

Thank you guys for the comments.