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#195074 - 07/09/10 09:32 AM 3 Phase Delta - Neutral required?  
Grover  Offline
Joined: Jan 2005
Posts: 109
Sebago, ME, USA
Customer wants to be able to cross connect between 2 1000 A 3 phase, 480 V generators to be able to operate in case of catastrophic generator failure. All load are 3 phase delta connected. 200HP load is softstart.

Is there any requirement to carry a neutral conductor(s) (NEC or MSHA), or can I just carry 3 conductors and EGC?

Thanks for help!


Work Gear for Electricians and the Trades

#195076 - 07/09/10 01:14 PM Re: 3 Phase Delta - Neutral required? [Re: Grover]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,142
I assume you mean they have 3 way 3 phase transfer equipment.
If this is corner grounded I don't see a reason to even have a "neutral" from either generator it is one of the phases. The service grounding point (MBJ) and the corner ground in each generator would suffice to ground the service.
If you center tap ground this, then you do need to deal with the neutral somehow. (SDS/non-SDS rules)

In fact if this is corner ground and you "float" the generator bond, you could bolt the grounded phase (white wire) and use 2 pole transfer equipment.

Greg Fretwell

#195077 - 07/09/10 01:49 PM Re: 3 Phase Delta - Neutral required? [Re: gfretwell]  
mikesh  Offline
Joined: Jun 2006
Posts: 613
Victoria, BC, Canada
You say there is no need for the neutral but how are the generators wired? Y or Delta? IF they are connected internally Y then you must take the neutral at least to the main disconnecting means and ground it or you will have issues in a fault. After the main switch the bonding system will carry fault back to the ground point to operate the protection.
Without a star point then the ground is more like bonding and a ground fault detector must be installed somewhere appropriate.

I have not covered a lot of the details regarding the transfer switch and whether a 4 or 3 pole transfer switch 1 or 2 electrode systems, open or closed transition transfer switches etc.

#195080 - 07/09/10 06:29 PM Re: 3 Phase Delta - Neutral required? [Re: mikesh]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,142
You only need GF protection if this is ungrounded. I was assuming corner or center tap ground since he said delta.
Corner ground scares people but in a lot of cases it is the simplest way to go if you are not serving L/N loads.
(only 2 ungrounded conductors and a 3 wire feeder)
Wye is just handy because of the resulting 277.

Greg Fretwell

#195081 - 07/09/10 06:57 PM Re: 3 Phase Delta - Neutral required? [Re: gfretwell]  
renosteinke  Offline
Cat Servant
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
We're well beyond the scope of the NEC here; perhaps the answer is best found in an NESC forum.

After all, with two generators, how can you monitor for phase loss?

I suspect the customer lacks faith in nhis PoCo because of power quality issues. In this day of varieble frequency drives, induction heating, UPS systems, fluorescent lighting, etc ... I am personally of the opinion that delta systems in general, and ungrounded deltas in particular, is long gone. The guy is experiencing voltage swings because .. he is creating them!

I'm not clear on the theory, but it appears that the neutral of a wye system does far more than just give you 277v lighting.

Full disclosure is in order here. My employer. based in part on my opinion, has committed millions to convert part of out facility from ungrounded delta to grounded wye. Talk to me in two years - I'll either be the smartes kid in town, or unemployed!

#195082 - 07/09/10 07:15 PM Re: 3 Phase Delta - Neutral required? [Re: renosteinke]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
EUSERC Hates ungrounded systems...

Because of all of the issues triggered by sub-system faults.

Faults like bad motor capacitors, bad windings in either transformers or motors can produce astonishing voltage peaks in ungrounded systems.

Now that Congress has legislated high efficiency solid-state switching devices ( electronic ballasts ) and VFD's are going everywhere grounded systems are the only way to go.

The only ungrounded systems that make any sense are those industrial plants that just can't afford an uncoordinated shut-down. (A paper mill being a prime example.)

I hate ungrounded control circuits. The back-EMF swings of full voltage contactor logic all too often trigger chatter.


If you need a reference neutral for an ungrounded delta think in terms of a zig-zag transformer with controlled impedance. In which case, I'd have it engineered.


#195084 - 07/09/10 07:48 PM Re: 3 Phase Delta - Neutral required? [Re: Tesla]  
mikesh  Offline
Joined: Jun 2006
Posts: 613
Victoria, BC, Canada
As my colleagues have stated. Ungrounded systems are fraught with hazards. It is far more common to find that industrial sites are using resistive or impedance grounded systems. The have the advantage of tolerating ground faults long enough to isolate them and fix them without all the voltage and insulation issues associated with a ground fault on an ungrounded system.
In Canada our industrial voltage is 600 instead of 480 which puts even more stress on an ungrounded system. in normal operation a balanced 600 volt delta service floats around 347 volts to ground but if any phase grounds the voltage is now 600 in the other 2 ungrounded conductors which puts a bigger stress on all the components and insulation.

#195086 - 07/09/10 08:54 PM Re: 3 Phase Delta - Neutral required? [Re: mikesh]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,142
I don't think we have been told this was an ungrounded system, only that they did not extend a neutral conductor.
Back in the glass house computer room days they never extended the neutral beyond the service disconnect or secondary of the transformer and these were usually wye systems. The only time you really need to extend the grounded conductor is in a corner delta and that is because it is one of the phases, albeit a "white" one. The others can stop at the main bonding jumper.

Greg Fretwell

#195087 - 07/10/10 12:06 AM Re: 3 Phase Delta - Neutral required? [Re: gfretwell]  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA

In a nutshell, if the connected loads do not require a Circuit which includes the System Grounded Conductor (there are no L-N Loads), the Grounded Conductor is not required to be brought past the point of initial Bonding to the GES (Grounding Electrode System).

From what I am interpreting this topic, it appears your Client has (2) 480Y/277V, 750KW Generators.

Is the plan to have both Gennys sync'ed, running in Parallel - then isolate One Genny if mechanical failure occurs;
or will One of the Two Genny be kept stand-by + isolated until needed?
(just curious)

I think I have an idea of the System Transfer action planned.
The Switchgear has (or will have) a section with a Manual Transfer Switch, and the Genny(s) connect to the gear via cords?

Anyhow, If there will be any L-N (277V) loads to be driven when the System is powered by the Gennys - such as during utility outages, then bring the Star-Point Common Grounded Conductor to the Gear, along with the (3) Ungrounded Conductors + an EGC.

If the Gennys ("Generators") are Delta Connected, there will not be a "Neutral"; only a Grounded Conductor (if Corner Grounded), or a Center Tap.
In the case of the Corner Grounded Delta, definitely bring the Grounded Conductor - as it will be Phase "B".
Also bring an EGC.
If Center Tapped, bring all (3) Ungrounded Conductors, along with an EGC bonded to the Center Tap + GES.

I attached Two Images of Switchgear containing a MTS (Manual Transfer Switch) + Genny Cord Whips, for reference.

Let us know the details and such.


Attached Files
USFS_MTS01.jpg (64 downloads)
Image #1: Switchgear with MTS (Manual Transfer Switch). Handle is in closest section
USFS_MTS02.jpg (64 downloads)
Image #2: Overview of MSB. MTS is at the end

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#195144 - 07/13/10 08:43 AM Re: 3 Phase Delta - Neutral required? [Re: Scott35]  
Grover  Offline
Joined: Jan 2005
Posts: 109
Sebago, ME, USA
Thanks for all the input guys! My customer is playing a "progressive disclosure" game - I'll be back with more details....

All my loads are Delta - no L-N loads. No utility power - I have a couple of dry transformers for power when the gensets are off at night - manual transfer switches to connect those loads to PoCo. No corner grounds either.

One genset feeds a SquareD panelboard; the other feeds an A-B MCC.


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