Open Delta 120/240 Volt Service

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jkim780

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On an open delta 120/240 volt service, is it permissible by NEC to downsize the high leg service conductors and install a fused switch with reduced fuse in the b phase? Please provide NEC provisions.

Thanks for your answer in advance.

[ October 14, 2003, 02:58 PM: Message edited by: jkim780 ]
 
Re: Open Delta 120/240 Volt Service

The NEC is a permissive document. In other words if it doesn't say you can't, then you can. I don't know where you would be prohibited from doing what you propose but I don't understand why you would want to downsize one phase. :confused:
 
Re: Open Delta 120/240 Volt Service

Charlie:

On a high-leg delta service, all the single phase 120V stuff has to go on two phases, right? Then assuming all the 3 phase equipment are evenly divided, isn't one phase going to be loaded lighter than the other two?

I thought the power companies often used 3 pole-mounted xformers for this type of service. I even thought they usually used 2 larger transformers, and a smaller one for the high leg.

Steve
 
Re: Open Delta 120/240 Volt Service

If you want to talk about the way an electric utility does it, they use a smaller and a larger transformer in an open delta configuration. The larger one is for the lights and receptacles and the local slang is the "lighter" because it feeds the lights. The smaller on is for the higher phase to ground and picks up the three phase loads (mostly motors). The lighter picks up all the single phase and the three phase loads while the smaller one just picks up the three phase loads, that is why it is smaller. Also since it "kicks up" the high phase, the local slang name for the smaller transformer is the "kicker".

If the amount of three phase loads are great enough, the delta will tend to collapse and it needs to be propped up with another "kicker". In a closed delta configuration, most of the single phase goes on the "lighter" and the three phase load is split evenly among the three transformers.

As examples, it is common for a 50 kVA and a 25 kVA to be used in an open delta bank. It is also common for 2 - 75 kVAs and a 100 kVA to be in a closed delta bank.

I have never seen but one bank where there were two larger transformers and one smaller transformer. This happened because one of the transformers failed and we did not have another one of the correct size. In order to get the customer back into service, we used the next larger size which was the same size as the "lighter".
 
Re: Open Delta 120/240 Volt Service

I can't believe as verbose as I got, I still didn't say as much as I needed to say.

When splitting loads for an open delta configuration, 100% of the single phase load goes on the "lighter", 57.735% of the three phase loads go on the "lighter" and the "kicker".

When splitting loads for a closed delta configuration, 66.667% of the single phase load goes on the "lighter" and 33.333% of the single phase load goes on each of the "kickers". 33.333% of the three phase loads go on the "lighter" and each of the "kickers". :D
 
Re: Open Delta 120/240 Volt Service

I have my Delta's confused. I must have been thinking about a closed Delta. I now realize an open delta only has two transformers (although I can't find anything about them in the handbook).

After looking at a diagram for a high-leg delta, I realize I was backwards on that too - two kickers and one lighter, not vice versa.

I'm learning :D

Charlie, if you don't mind answering one more question: where is an open delta grounded? In the center tap of the lighter?

STeve
 
Re: Open Delta 120/240 Volt Service

Yes, that is why the configuration works the way it does. You will read 240 volts from phase to phase to phase, 120 volts to the grounded conductor from A and C phase, and 208 volts from the grounded conductor to B (high phase or wild leg) phase. :D
 
Re: Open Delta 120/240 Volt Service

Cash register, kicker, lighter, and wild leg.....good stuff they don't teach you in school :D

Steve
 
Re: Open Delta 120/240 Volt Service

Thank you charle, and steve. But to be honest with you, I have no idea what you guys are talking about. What do you mean by 2 trnasformers in open delta sytem? Did you mean 2 different windings or did it actually mean that open delta sytem utilize 2 different transformer?
 
Re: Open Delta 120/240 Volt Service

A good way to visualize the configuration is to imagine a wye primary and delta secondary configuration with three separate transformers. This is normally done with pole mounted transformers by the electric utility.

Center tap one of the transformers on the secondary side and ground it. Bringing out that conductor is normally called a neutral but it is not, it is the grounded conductor.

Now, bring out the other three points of the delta. These will be the phase wires and the one opposite the grounded center tap will be the "high" phase or B phase.

On the primary side, connect the center point of the wye to the same ground as the grounded secondary conductor. This is also connected to the neutral that feeds the bank.

Now, remove one of the transformers without disturbing the connection of the other two. Do not remove the transformer with the grounded center tap.

What remains is an open delta bank as described previously. :D
 
Re: Open Delta 120/240 Volt Service

Charlie, who decided that the grounded conductor of a center grounded Delta is not a neutral?

I know that the NEC doesn't think it is, but if the two legs it is referenced to were evenly loaded, what would the reading be on this conductor?

BTW I'm not trying to start anything, I just can't figure why this is not considered a neutral as far as theory is concerned.

Roger
 
Re: Open Delta 120/240 Volt Service

If you take the A & C phases plus the grounded conductor and feed a bunch of dwelling units, the grounded conductor is the neutral for those services. It is not the neutral when you introduce the B phase into the mix. In a wye configuration, you get the same voltage from any phase to the neutral and the neutral carries the maximum unbalanced current. In a delta configuration, the same statement doesn't apply. BTW, I am not involving harmonics in the above discussion. :D
 
Re: Open Delta 120/240 Volt Service

Charlie, I won't keep this going after this post but, When would the B phase ever be used with the grounded conductor? (you guys don't allow it :) )

When we are talking about the winding this neutral (yes I'm going to still call it a neutral :D

Roger
 
Re: Open Delta 120/240 Volt Service

Of course you are going to call it a neutral, so does almost everyone else. It is important for everyone to understand what a neutral is and why. Once we get past that point, lets all call it a neutral. :D

When would the B phase ever be used with the grounded conductor? (you guys don't allow it)
Click to expand...
Yep, you are right. Your circuit is either a single phase 120 or 240 volt or a 240 volt three phase circuit without the "neutral". :p
 
Re: Open Delta 120/240 Volt Service

OK Charlie, I lied, 1 more. :D

I call it a neutral because it is a neutral. If both sides of the the winding are loaded the same this can be removed and there is no effect to the loads.

A two wire circuit doesn't have a neutral but this does.


Now, we can discuss removing one winding of a "closed delta" and we have an "open delta" this is still a "system"

Now let's remove two windings of a closed delta and leave the kicker and we still have a "system"

How does this "system" difer from any residential "single phase service"?

My question is still, who decided and when was it decided this conductor used in this way (center grounding one winding of a delta) is not a neutral.

I promise this is my last post in this thread with an answer or not. :D

Roger

[ October 15, 2003, 09:04 PM: Message edited by: roger ]
 
Re: Open Delta 120/240 Volt Service

The way I look at the delta, it is a 3 phase system, Just because you use only the two conductors and the center tap of one transformer does not change the make-up to the 3 phase system and the Grounded conductor does not have the same voltage potential to each fo the undgrounded conductors of the derived system.
It is not a neutral.
 
Re: Open Delta 120/240 Volt Service

Roger, don't stop on my account. :D

Roger and Glenn, IMO, drop the idea of a system unless you build it as a SDS. Start at the service, if it is a 120/240 volt, three phase, four wire, you do not technically have a neutral for that service. If you have a 120/240 volt, single phase, three wire, you do technically have a neutral for that service.

Both services may be fed from the same transformer bank.
 
A Neutral Conductor Is A Conductor That Has An Equal Potential Difference Between It And The Other Output Conductors Of A 3 Wire Or 4 Wire System. So A Delta 4 Wire 120/240v Would Not Have A Neutral Conductor.
 
NICK D said:
A Neutral Conductor Is A Conductor That Has An Equal Potential Difference Between It And The Other Output Conductors Of A 3 Wire Or 4 Wire System. So A Delta 4 Wire 120/240v Would Not Have A Neutral Conductor.
Click to expand...

The code panel apparently doesn't agree with your conclusion.

NEC Committee Report on Proposals 2007 page 26
5-36 Log #1554 NEC-P05
Panel Meeting Action: Accept in Principle
Add the following (two) definitions to Article 100 as follows:
Neutral Conductor. The conductor connected to the neutral point of a system that is intended to carry current under normal conditions.
Neutral point. The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system, or midpoint of a single-phase portion of a 3-phase delta system, or a midpoint of a 3-wire, direct current system.
FPN: At the neutral point of the system, the vectorial sum of the nominal voltages from all other phases within the system that utilize the neutral, with respect to the neutral point, is zero potential.

So in the 2008 code there will be wording that says:
Neutral Conductor. The conductor connected to the neutral point of a system that is intended to carry current under normal conditions.
Neutral point. The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system, or midpoint of a single-phase portion of a 3-phase delta system, or a midpoint of a 3-wire, direct current system.
FPN: At the neutral point of the system, the vectorial sum of the nominal voltages from all other phases within the system that utilize the neutral, with respect to the neutral point, is zero potential.

The high leg doesn't utilize the neutral so you don't take into consideration the fact that the voltage from highleg to neutral is different than the other legs.

Also, altho the neutral point definition doesn't mention a corner grounded delta, the fact that the FPN uses the word "other" makes me wonder about how they're looking at a corner grounded delta.
FPN: At the neutral point of the system, the vectorial sum of the nominal voltages from all other phases within the system that utilize the neutral, with respect to the neutral point, is zero potential.

The conductor attached to the corner grounded termination of the delta is a grounded conductor. It also is a phase conductor. But are they implying that it is also a neutral conductor ? What other reason would they have added the word "other" ?

David
 
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