ECN Forum Forums General Topics of the Trade General Discussion Area 208, 240, & 277 volt questions

As mentioned, it is one leg of the 3 phase. It is not used to neutral. The fact that it has a reference to neutral is besides the point.

For example, say you hooked up three single phase pots in a 4 wire delta configuration. Each of the pots forming the "high" leg are conventional single phase pots, so they each have an X2 bushing that is simply not used. But if you took a voltmeter and read from them to neutral (or any other phase) you would still get a reading. It's just not a usable voltage.

Off topic a little. Another reason the delta's are being phased (no pun intended ) out is that they are more susceptible to ferro-resonance problems. As distribution voltages go higher and higher, this has become more of a problem.

Al, ignore the voltage to ground for a moment.

In a Delta system, there are three transformer secondaries that each have 240v between the end trerminals. Connect them in a triangle, and you have three phase wires.

Until you ground a wire somewhere, the voltages are "floating" relative to ground, but there is always 240v between any two phases.

If you ground one of these hot wires, you have a "corner-grounded" Delta. There is still 240v between any two phases, but now there is also 240v between either of the two non-grounded phases and ground.

Now, let's say that one of these three 240v secondaries has a center tap that is grounded (instead of one phase), exactly like the 1-phase transformer feeding your house.

Now, you will have 120v between these two phases and ground, but the third phase will have 208v to ground. This is due to the 1-1/2 secondaries (240 and 120), "adjusted" by the 120 deg. between phases.

This "adjustment" accounts for why there is 208v between phases on a Y system, instead of 240v. Because the phase angle is greater than 90 deg., the net result of adding the 240v and 120v is a subtraction.

The point is that, to a 3-phase load, it doesn't matter whether, or where, any wire of the system is grounded.

Larry, so depending on where the center tap is built on the transformer (changing the phase angle), the 208 voltage would change. That makes sense. So there is no real purpose for the wild leg, since it does not affect a 3 phase motor, or is there another application it is used, right?
Thanks,
Al

Al, here is a graphic to accompany the others explanations.

A high leg is an unavoidable and unused voltage point to the grounded conductor on a Delta configuration of transformer windings.



Concerning NM in Commercial buildings


So yes, NM can be used in commercial buildings if the prohibitions covered in 334.12 are taken into consideration.

Roger



[This message has been edited by Roger (edited 04-24-2006).]

Roger,
That is a great explanation of the delta transformer.

On the NM issue in 334.12, not clear as to what is considered a commercial building that NM can be used.
Thanks,

The main place you will see 3p "wild leg" delta is where you have a a mix of light industrial bays that generally get along fine on regular single phase. When you have a customer who needs a little bit of 3 phase for motors you can give it to them for the price of one more transformer. You have the same basic single phase service you have in a home with one extra transformer and that may be quite a bit smaller than the single phase transformer for the 3d phase.

Just to add to the previously made posts, here is my 2¢:


Depends on the Customer + the time period when the Building was developed.

If the "area" where the commercial client leases the Building happens to have customers on the distribution system who use more large Motor loads than "Lighting" loads, these Buildings most likely would have been fed from a 4 Wire Delta.

Same client's Building - if larger than 20,000 square feet for example, may have been fed from either a 480Y/277 Volt 4 Wire Wye, or a 480 Volt 3 Wire Ungrounded delta - or even a 240 Volt Corner Grounded 3 Wire Delta.

Other areas and design bases of Buildings - large and small, may be fed from 208Y/120 Volt 4 Wire Wye setups.

All depends on the customer types, the buildings' designs, the existing / available PoCo systems, and most importantly, the time period of development + area.

Currently in my area (Southern California) for a "No Large Motor Use Commercial Project", the only way to get a completely new 240 Volt Delta System - either 3 or 4 wire, open or closed, is to have a very special reason for it + pay $$$ for it,
or maybe threaten the Design Engineers with Guns or such (just joking!)

Even when the connected equipment becomes majority 3 phase Induction Motors, it's under special permission.

Same goes for anything of the 480 and 600 Volt 3 Phase Deltas - Grounded and Ungrounded.

The usage of 4 wire Wye services for Commercial / light Industrial customers is more useful than Deltas.
These systems are well suited for the "common" commercial enviroment where a majority of equipment will be Line to Neutral load connections, and less than 40% of connected loads will be heavy L-L-L or L-L loads.
If there is a need for a full 240 Volt rated system, then it's the client's obligation to install an SDS.

Along with this, there is an issue of people getting screwed by the "Stinger Leg" of a 4 wire delta, along with misapplications of connected equipment to 4 Wire Open Deltas - causing the PoCo's Transformers to release smoke.

I personnaly do not mis PoCo supplied Delta Systems - had dealt with many in the past, but mostly because of the crazy things that occur when people that have little or no knowledge of them, start messing with them; or connect equipment to them.

The disadvantage of 4 wire Wye connected secondaries is Zero-Sequenced Harmonics reflected into these windings are not canceled out - as they would be in a Delta connected Secondary - but this "problem" can be overcome with proper design and selection of connected equipment, and increased Apparent Power sizing for Transformers (or using "K Rated" Transformers).

The problems of applying "too low of voltage" to equipment is quickly going away.
Back in the 1970's, I remember all too well of the "it needs to be 240 volts, not 208 volts" conflicts.
This was applied to Heating type equipment, as well as Motor Loads, Welding Loads and Battery Charging Loads - all of which had no previsions to set jumpers on a "208 Volt" tap, or even a "200 Volt" tap - only had "220 Volts", "240 Volts", "460 Volts", "480 Volts", "575 Volts" and "600 Volts".

Eventually, Manufacturers started to give "a 200/208 Volt Option" for equipment, and doing so made them more appealing to customers, as they did not have to install any "special SDS Transformer" to run that equipment, and they could run it directly from their existing electrical system.


This is simply a "result" of having a 4 Wire Delta system, using Transformers with 240 Volt Secondary Windings connected in a "Parallel Delta" configuration, and having one Transformer's Secondary Winding "Center Tapped" to obtain the Neutral Conductor for a 120/240 System.
The "Stinger Leg" or "High Leg/Phase" is not an item of design - and is not intended to be used for Line to Neutral load connections!

It becomes present in Panelboards (as the Conductor color coded "Orange") when the Panelboard is a 3 Phase type, where there will be a combination of 3 Phase 3 Wire Branch Circuits and 1 Phase 3 Wire Multiwire Branch Circuits used.

A very common design technique for Buildings with 4 Wire Deltas, was to use 1 Phase 3 Wire Panelboards - fed with Lines "A", "C" and the Center Tapped Neutral, where only 1 Phase loads were used (like in Office spaces and such), and using 3 Phase Panelboards - fed with Lines "A, B & C" only - no Grounded Conductor (Neutral), where only L-L and 3 Phase loads were used (like in the warehouse shop area or for HVAC equipment).


I have only seen a few small commercial locations, where a previous EC had used NM Cable.
It looks very odd, but it is not illegal - per the NEC.

If the Design Specifications for a certain project does not allow NM Cable, but the NEC (or ammended version of model code) will allow NM Cable for that given Commercial Project, the Design Specifications of the Contract Documents wins!

Scott35

Al, it would be wrong to say that the high leg serves no purpose. It's useless for line-to-neutral loads.

The high leg is one third of the 3-phase syetem. There are three equal hot lines; the high leg is one of them. It's only "high" because of where, in the triangularly-connected three windings, the grounding wire (a.k.a. the main bonding jumper) is connected.

If it were connecte directly to one of the hot wires instead, then we'd have a corner-grounded Delta. Absolutely nothing is different among the different Delta types (Delta, Corner-grounded Delta, high-leg Delta, etc.) as far as the 3-phase system is concerned.

There is 240v. bewteen any two of the phase wires; 3-phase loads use all three, whether one of them is "high", grounded, etc. Only the voltage between each and earth changes with each method.

Larry, where did anyone say the (A for sake of conversation) high leg point did not have a purpose or was not a point of a system connection.

It was pretty clear to me that it was (for this thread) only prohibited in ungrounded to grounded conductor situations.


When it is used for "phase to phase" circuits it is just another equal winding as far as voltage is concerned.

Roger



[This message has been edited by Roger (edited 05-07-2006).]

No use for high leg to neutral???? Some high-bay lighting and other HID ballasts can make good use of it.... There are also some single phase, dual rated equipment that can use it too. For instance, commercial kitchen equipment. Not off the shelf stuff often, but it is out there.