Why is 120/208 volt 3 phase used mostly in commercial buildings, and not 120/240v like residential single phase homes? (My question is not about the single or 3 phase, but about 208 vs 240v). What is the 3 phase 208v wild leg usually used for in commercial panels?
Also since romex (NM) is not allowed in commercial buildings, has anyone ever seen an application where 277/480v was used, using NM? NM insulation is good for 600v.

3 phase is used because it pushes a large motor so very well. Also, for heavy loads, 3 phase gets you 1.73 times more power for 1 additional wire, thereby saving dollars on the cost of the conductors. Residential loads are not heavy and they have few large motors, so it is not economical to run 3 phase to homes.

Since when is NM not allowed in commercial buildings?

53........
120/208 is used mostly because of not only having the luxury of 3 phase for motor applications, but it also avails you to 3 seperate 120 volt circuits from the transformer secondary. If you are using 120/240, you would only have two, because it is a center-tapped single phase. Thanks to the connection of the "grounded" conductor, which would be your common on your three phase "wye" secondary, you have the opportunity for more circuits on a much more balanced load. The more balanced the load, the less you are asking of your neutral. Hope this helps.

I know why they use 3 phase instead of single phase. I was wondering why they use 208v instead of 240v?

walrus,
From as long as I can remember about 30 years doing elec work, NM is not used in commercial buildings. (except some rare "against the code" handy man work).

Not to be a smart aleck, but if its against code please cite the code section(s)prohibiting the practice of using NM cable in commercial buildings.

"What is the 3 phase 208v wild leg usually used for in commercial panels?"

You are more confused than you realize. Let's assume you do understand the 240/120v 1ph system: a single transformer secondary winding with a grounded center tap. The two hots are condicered 180 deg. opposed to each other.

You could do the same thing with two separate 120v-secondary transformers: join one end of each (in phase!) and the junction becomes the center tap, which we ground. That gives us two points that each provide 120v to ground, and 240v between them.

A Y-system is basically the same thing, except we have three 120v-to-ground secondaries, all joined at one end, the grounded neutral. The hots are only 208 between one another, because of the 120-deg. phase angle.

A center-tap-grounded Delta system is actually one transformer connected exactly as a single-phase 240/120v transformer, plus two more, also 240v hot-to-hot, connected as an equilateral triangle; the Delta.

The phases are normally identified as A, B, and C, with A and C as the two ends of the center-tapped secondary, and the third hot as B, aka the "wild leg". A and C are each 120v to ground, but B is 208 to ground, so not useable for line-to-neutral loads.

An "open Delta" system omits one of the two extra transformers. It still works, but there is a bit less "stability" in keeping the voltage steady with varying loads; in other words, a greater source impedance.

The Y-system is mostly used where there is mostly line-to-neutral loads. The Delta is best for high-current 3-phase loads, and the open Delta (generally on the way out of use) is where there is a combination of large 240/120v 1-ph loads, with a bit of 3-phase loads.

The corner-grounded Delta is just what it says; a triangular connection with one phase grounded. It's used where only line-to-line loads are used, and no secondary has the center tap connected to anything.

On wye systems:
208V line-line gives (3) 120V line-neutral circuits
240V line-line gives (3) 136V line-neutral circuits.

On delta systems:
240V line-line gives (2) 120V line-neutral and (1) 208V line-neutral circuits.

So if your intent is to have as many 120V cicuits as possible then you want 208Y/120.

Great thread.

Can never know enough about transformers, how they work, and what applications they are used in. However, I'm still a bit confused about phase angles. I just finished a course on AC principles, bought a Mike Holt book about the subject, and I'm still confused. I know more about it now than I did 3 months ago for sure. It's difficult for me to understand the relationship between inductance and capicitance, and to apply them.

Thanks for all the insight.

[This message has been edited by ShockMe77 (edited 04-23-2006).]

Larry, Nice job explaining those differences
may be using some of that, if I can with some of my guys. Thanks!


Phil

"anyone ever seen an application where 277/480v was used, using NM? "

well i did see few place did use the NM on 277/480 circuit but tell ya the truth it was kinda spooky on me but if my memory serve me right in history way back in WW2 era they did use the NM alot on 480 volt circuit to save steel that time but currently it about nonexitsing now .

ps if this person ask about 277/480 volt system this is very common on large commercal/industrail area and quite few store did ran on 277/480 due large number of lighting system

Merci, Marc

[This message has been edited by frenchelectrican (edited 04-23-2006).]

norcal, Look in the NEC 2005, Art 334.10, 334.12.
Have you ever seen NM in a commercial building? I haven't,except for someones handymans work, rarely though.
--------------------------------------------Larry Fine, In many commercial/industrial panels there will be 3 phase, 2-120v to ground legs, and 1-208v to ground leg, but they will measure 208 volts between each hot leg. I know the transformers are set up that way, but when would you use the 208v to ground, leg. Most commercial motors are 3 phase 120/208/v or 277/480v, or sometimes 120/208v single phase.
--------------------------------------------
frenchelectrician, maybe in WW2, but I've never seen 277/480v using NM today.

If you have 2 legs that measure 120 volts to ground and one that measure 208 volts to ground the voltage between the legs will be 240 volts not 208 volts. This is a delta system.

If there is 208 volts between each of the legs then all 3 will measure 120 volts to ground. This is a wye system.

The 208 volt leg on the delta system is only used for 3 phase loads.

Curt

anyone ever seen an application where 277/480v was used, using NM? "

I have seen it used ..and we ripped every bit of it out. It was in a HUGE warehouse rite outside of Philly. The place had a drop ceiling with 2 x 4 layins. well over 300 of them. Somebody wired them all with romex, 12-4 romex to j-boxes then three 12-2s to a group of 8 or 10 fixtures. All the J-boxes and wire was just laying on the ceiling tiles and grid. J-boxs were nasty .. lose wire nuts, bare wires... What a mess ... spent weeks in the place re-wiring. In Lots of Places in PA ..ANYONE with insurance can be an electrical contractor, and it shows with some of the work I have seen there in the past 20 years.

caselec, forgot, your right about the 240v between phases on a wild leg system.
But still not sure what that 208v wild leg is doing on the 3 phase motors?

Motors don’t know that the wild leg is 208 volts to ground. Three phase motors don’t require a neutral. All they see is 240 volts between each of the legs.

Curt

caselec, I realize all that, but what is the 208v to ground (wild leg) used for usually?

Can't happen. Not in one system.

The first scenario (2 x 120v and 1 x 208) is a center-tap-grounded Delta or open-Delta, period. 240 line-to-line, and don't use the high leg (208v) for line-to-neutral loads.

The second (3 x 120v) will be a 208v hot-to-hot Y system, period. All three hots will measure the same to neutral (and thus, to ground).

The third hot wire for 3-phase loads, and never for line-to-neutral loads, period.

Larry Fine,
Not sure I'm getting it... so the wild (208v) leg never gets used to neutral, what is it there for then? Doesn't seem like it would do anything on a 3 phase motor if your measuring 240v between each phase.

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.

Here:


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

Scott, I just did this exact setup in a major restaurant remodel, where the service is open Delta. We have a 125a 3ph panel for the two RTU's (HVAC) and a 200a 1ph panel for everything else.

The original plans called for a new 208/120Y system with two 3ph panels, but I bumped heads with the engineer and we redesigned the service. Saved my customer $5-6K in new transformer costs.

That's a bad idea. It's a relatively high-impedance source, not real stable, and I'm quite sure it's not an approved method of obtaining 208v.

It is always fun to read and listen to guys discuss topic that I can easily learn from - very good!!!

On the topic of why NM cable is so inferior in commercial occupancies, I would like to hear from the guys who think it is so inferior as to why it is so. The NEC permits it whether you like it or not.
I am interested as to why you think it is so worthless. No personal opinions, give me some facts.

I thought two transformers indicated an ungrounded system? It actually means that the system is an "open delta"? It can still be a grounded system?

I agree with Pierre, good thread. I'm taking notes.

Larry, you've done an EXCELLENT job explaining 3-phase and transformers. I've never heard a more concise and easy to understand explanation anywhere.

PCBelarge, the reason I believe Romex is inferior can be summed up in two words:

FIRE & ELECTROCUTION

Here's my list of best to worst wiring methods:

1. RMC
2. IMC
3. EMT
4. MC
5. AC/BX
6. PVC Conduit
7. Romex
8. K&T

The problem with Romex is it offers NO protection. You can easily drive a nail or screw through even CORRECTLY (according to code) installed Romex. Even staples can be a problem.

Metal ducting can pinch Romex and said ducting can become energized. Didn't we recently read about such an accident on this forum about a guy in FL who was electrocuted installing a dryer when the ducting became energized because of nicked Romex?

The flat, parallel nature conductor placement with most Romex cable is also an issue. In the event of overload, the parallel cooper in Romex will just keep getting hot until something ignites -- similar to heating elements. Not so with the twisted nature of MC and AC and to some degree, cable installed in conduit -- chances are when the wiring heats up, it will melt the insulation, immediately tripping the OCPD. And if fire should occur, there is the additional safety factor of it happening inside the flex or conduit. By the way, with MC and AC, the brown kraft paper will stop the fire from spreading up through the cable.

A residential rewire (where the walls are not ripped down) with Romex presents all sorts of problems. First is removal of old wiring. With conduit, obviously it's a breeze. With MC or AC, you can tug on cable and pop out the staples inside the wall. It's VERY difficult doing any of that with Romex.

Fishing unprotected Romex inside walls is even more problem prone. Code says fished Romex is considered "protected" but who knows where it will end up? Will it be snaked around a sharp nail or screw? MC or AC offers "instant" protection against this. What if snaked Romex ends up against a hot water pipe or heating duct? What may happen over time? Again, not as much of a problem with MC or AC.

Romex is also too easy to kink.

There IS a reason why Romex isn't used in commercial construction, and most local codes prohibit it. Heck, Chicago sparkys have to do residential in pipe. NYC recently allowed Romex, but almost no new construction I've seen takes advantage of it.

The only good thing about Romex is it can be made into a beautiful "looking" install, like we've seen on this forum, where it looks like it was done with an iron. But conduit looks even better. MC and AC is hard to get looking nice.

Romex is nothing more than glorified zip cord.


Joe

quote:
--------------------------------------------------------------------------------
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.
--------------------------------------------------------------------------------

That's a bad idea. It's a relatively high-impedance source, not real stable, and I'm quite sure it's not an approved method of obtaining 208v.

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

It's a relatively high-impedance source, not real stable ...

Can you expand on those two points? Are you concerned about a large single phase load shifting the neutral?

Thanks, LarryC

Yeah, Larry, why not? Not trying to be a joker, but I have done it on rare occassion in the past. (when pinched) Not that it is my favorite way to go or anything. But don't know of any prohibition against it? Code or otherwise. The way I see it, it is simular to obtaining different voltages from the same autotransformer.

I do know that not all high-legs are 208 either, many, depending on where that neutral tap ends up are often off ~10 volts or more. If thats the case, those loads end up being single phase from high-leg to A or C at 240.....

As for 277/480 on romex. I too think that is just a bad idea. (Unless you provide anyone working in the building hot gloves and a course in CPR.) To me the difference there is a "little shock" or full arithmia, and a burn to go with the bite. Although not against the NEC, most of the places that I work in wont allow it in commercial, and I personally would be frightened by 277 in Romex. It is too fragile in terms of insulation integrety IMO for that hazardous a voltage. Little shock, and spark -vs.- death, and fire.

Regarding using the 208 high-leg for 208 volt loads...
I think it is generally accepted that single phase 208 volt equipment is designed for 2 hot legs of a wye system. Although "generally accepted" dosen't constitute a code rule, using a single pole "slash-rated" circuit breaker would be a violation of 240.85. Although I have used "straight rated" 2-pole CBs on delta systems, I'm not sure they are available as a single pole device.

Just George;


Two Transformers NORMALLY indicates an Open Delta "Vee" configuration.
It can be either a 3 Wire or 4 wire type, and for 3 Wire ones, they can be either Grounded or Ungrounded.

Same goes for "Full / Closed" Deltas, which use Three individual Transformers. All "Flavors" are available too!

Typically if the setup is a 4 Wire Delta, the "Center Transformer" on Closed Deltas is larger than the outer ones, and for Open Deltas, one Transformer is larger.

The "Larger" Transformer is the one used for the 1 phase 3 wire portion of the system - and utilizes the center tap on its secondary winding for the Grounded Neutral.

With that being said, I do need to include a DISCLAIMER regarding Two Transformers on a single pole, vs. the type of system created.

TWO TRANSFORMER DISCLAIMER

The usage of Two individual Single Phase Transformers on a Single Pole, may also be one of the following connections:

<OL TYPE=A>


[*] Single Phase 3 Wire - Parallel Systems: whereas each Transformer's Secondary feeds a separate service,


[*] Single Phase 3 Wire - Parallel Transformers: to increase the ampacity available from only a single Transformer. I have only seen it done 3 times. Each Transformer's Primary and Secondary are connected together in Parallel,


[*] 3 Phase Open Delta "Tee" - Typically used on dry type Transformers of 15 KVA and less, these may be 3 or 4 wire secondaries and typically are configured to "Represent" a 208Y/120V 3 Phase 4 Wire system - although they may be setup in many ways,


[*] 2 Phase Open Delta "Tee" - similar to that described above in "C", these include the "Scott 'T' Connection", but typically when used for 2 Phase systems, they are connected as 3 and 4 wire (no "Neutral") systems,


[*] 3 Phase 3 Wire "Open Wye" - typically has a conductor which is "Common" to both sides (Primary and Secondary), which is connected to the common star point of the ends of each windings' pair.
</OL>

Scott35

edited for spelling blunders

[This message has been edited by Scott35 (edited 05-08-2006).]

Thanks for that explanation, Scott.

Ok, on the Delta 3 phase transformer with a ground center tap. I see how you get the 208v going to a roughly phase and a half. But then it seems if you measured from phase to ground on the other 2 phases it would be something less than 120v.
Also, in this type of 3 phase panel wouldn't you have the 208v every 3rd breaker down?

Yes, you do. (Have 208 on every 3rd breaker.) That's why it was suggested that you have two panels--one for the three-phase loads, and a second, single-phase three-wire panel connected A-neutral-C for the single-phase, 120/240 loads.


[This message has been edited by SolarPowered (edited 05-09-2006).]