high-leg (or wild-leg

[Hameedulla-Ekhlas]

Greeting all,
Reading Jan.Top-30 posted answer and question I found one thing about High leg and it is NEC artical [408.3(F)].

If a 4-wire, delta-connected, three-phase (high-leg) system supplies a panelboard, the high-leg (or wild-leg) conductor which operates at 208V to ground must be terminated to the ?B? phase of the panelboard.

It is a new thing I have heard but I am really interested to be clear more for me.

My question:
1- Is this only in delta connection or in wye connection too?
2- What is the doing advantage of this?
3- In delta 3ph 208/120 how one phase should be 208 volt and why it is not in wye connection.

Is there any explaination reference except NEC regarding this.

Hope to advise me soon.

 

[ohmhead]

Greeting all,
Reading Jan.Top-30 posted answer and question I found one thing about High leg and it is NEC artical [408.3(F)].

If a 4-wire, delta-connected, three-phase (high-leg) system supplies a panelboard, the high-leg (or wild-leg) conductor which operates at 208V to ground must be terminated to the “B” phase of the panelboard.

It is a new thing I have heard but I am really interested to be clear more for me.

My question:
1- Is this only in delta connection or in wye connection too?
2- What is the doing advantage of this?
3- In delta 3ph 208/120 how one phase should be 208 volt and why it is not in wye connection.

Is there any explaination reference except NEC regarding this.

Hope to advise me soon.

Well its not new in a delta .

Only in a delta not in a wye transformer.

Its not a advantage to me its a waste but thats my thoughts.

A delta in the USA has 240 volts leg to leg and 120 volts to grounded phase center .

Its not in a wye transformer because of the way it is designed the windings are not connected to give you that extra voltage from grounded center tap like on a delta C winding if you look at a delta it has the extra voltage on B by part of C winding connected to and its out of phase with the others by 90 degs in addition to its phase location in transformer i see no advantage to this its just the way we made transformers work in the past by using three seperate transformers on a pole to give us power and by connection of these.

The only good part i guess is one could replace a bad transformer meaning one phase easy on a pole or many years ago they made more single phase transformers than three phase much easyer to work with .

 

[iwire]

1- Is this only in delta connection or in wye connection too?

Just delta.

2- What is the doing advantage of this?

The only advantage is to the utility, it can save them the cost of one single phase transformer in open delta or can reduce the transformer sizes in closed delta.

 

[iwire]

Schematic drawing of the 3 Phase 4 Wire Open Delta.

Layout drawing of the 3 Phase 4 Wire Open Delta.



The above from Scott " 35 " Thompson

http://www.electrical-contractor.ne.../ubb/showflat/Number/177458/page/1#Post177458

 

[Hameedulla-Ekhlas]

thanks all and very nice for replying and it is really usefull.

 

[TOOL_5150]

And the 208v is pretty much unusable in both a delta and an open delta, right?

~Matt

 

[kwired]

advantages are mostly in favor of power company. The ability to supply several single phase customers and the occasional three phase customer all from the same transformer bank, the ability to bring only two primary phases to a remote area and use an open delta transformer bank. I have run into many small (older) retail stores that have all single phase except for air conditioning is three phase.

It can be a little advantage for user if majority of load is motors or other three phase power loads and not line to neutral loads, 240 volts = a little more wattage per amp than 208. Most heating type equipment is designed for 240 volts especially if it is single phase equipment. If connected to a 208 volt system you will get less heat from this equipment.

Where I live there is not just 240 volt delta systems there is also 480 volt delta systems. (not corner grounded systems, there is a high leg) These are usually supplying limited loads like irrigation equipment.

 

[George Stolz]

And the 208v is pretty much unusable in both a delta and an open delta, right?

~Matt

To my knowledge, yes. You'd have to install a breaker rated for 277v into a panel rated for 120/240v. Physically.

Apparently, by Bob's diagram, a closed delta would deliver a stable high leg voltage, but that's the first I've heard that (not that that means anything).

 

[mivey]

And the 208v is pretty much unusable in both a delta and an open delta, right?

~Matt

Only because of finding the proper equipment, not because there is something "wrong" with the voltage.

 

[mivey]

Apparently, by Bob's diagram, a closed delta would deliver a stable high leg voltage, but that's the first I've heard that (not that that means anything).

Proper transformer sizing would deliver stable voltages in open or closed delta.

 

[hurk27]

Here our POCO will not install 4-wire deltas, Y's are the norm, or 3-wire delta.
but if you are installing one, and your POCO is ok with supplying it, make sure you land the high leg as phase "C" in the meter can/CT and "B" in the panel marked orange. as per 110.15 (2002 don't have my 2005 at home:roll

this is because of the way the meter reads the high leg

 

[hillbilly1]

Our poco loves them, because they can use smaller transformers for the same kw three phase load. One of the reasons behind having it on "B" phase is so it is always the middle buss bar whether looking at it from the front or rear of a switch gear.

 

[Minuteman]

It's also easy for a poco to just add transformer, if there suddenly be comes a need for 3 phase. I did a old 2 story building that we remodeled into a law office. The new elevator and a new A/C was the only 3 phase load. So poco just added a second transformer.

 

[shamsdebout]

I thought it was interesting in the book: Electrical Power Systems Technology, it says the voltage from between the "wild-phase and the neutral will be in excess of 300 volts!
- Has anyone ever heard this before?

It says this on page 261,262 on this book
http://books.google.com/books?id=bo...&resnum=1&ved=0CEAQ6AEwAA#v=onepage&q&f=false

 

[hurk27]

I thought it was interesting in the book: Electrical Power Systems Technology, it says the voltage from between the "wild-phase and the neutral will be in excess of 300 volts!
- Has anyone ever heard this before?

I don't know where that book got that the voltage will exceed 300 volts, but it will be 208 volts from "B" phase to the grounded conductor

 

[frenchelectrican]

I thought it was interesting in the book: Electrical Power Systems Technology, it says the voltage from between the "wild-phase and the neutral will be in excess of 300 volts!
- Has anyone ever heard this before?

I don't know where that book got that the voltage will exceed 300 volts, but it will be 208 volts from "B" phase to the grounded conductor

The 208 volts on wild leg connection is from 240D120 system however if you have 480 volts verison then the wild leg voltage will be right about 415 { give or take couple volts } so that why on 480 volts it not too common to find a wild leg there unless you got a centre tapped 480/240 volt transfomer.

Merci,Marc

 

[hurk27]

The 208 volts on wild leg connection is from 240D120 system however if you have 480 volts verison then the wild leg voltage will be right about 415 { give or take couple volts } so that why on 480 volts it not too common to find a wild leg there unless you got a centre tapped 480/240 volt transfomer.

Merci,Marc

While true on a 480 4 wire delta (very rare here in the states), the book link in shamsdebout post clearly is talking about a 240/120 4-wire delta, and even has a diagram to the effect, the way the author describes the circuit he is adding the 120 volts from the center tap to either "A" or "C" phase with the 240 volts between "A or C" and "B" phase, which we know is wrong which should be 120*1.732=208

 

[lakee911]

the ability to bring only two primary phases to a remote area and use an open delta transformer bank.

Can you elaborate on this? Still has X1, X2, and X3 (A, B, and C), so how is it only 2 phases?

 

[e57]

I find this diagram to be misinforming.... Neither provide stable voltage if we are talking about the B phase. The closed delta Neutral center tap to B is not usable as 208, and due to increased impedance reduces reliability of OCP. (As it was never intended to be used B-N) The voltage that can be measured is an mathematical anomaly.

 

[kwired]

Can you elaborate on this? Still has X1, X2, and X3 (A, B, and C), so how is it only 2 phases?

The secondary has X1, X2, X3, XO as usual.

The primary only uses H1, H2, HO on an open delta primary transformer bank even if all three phases are present.

If it were a closed delta (with three transformers) then all three primary phases are used plus the primary neutral is also bonded to the secondary XO.