480 volt corner ground delta questions

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sdbob

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I'm installing electric for a well in a very remote rural area. This service (480 volt 3-phase) will also feed a transformer giving the customer 120/208 Y for some receptacles and lighting. One day into this project I look up on the utility pole next to our service (service drop not yet connected) and notice the utility has one phase tied to the steel messenger cable of the 4-wire drop. Red flags flew, as I have read about but never worked with a corner grounded system before. My boss has never heard of it and has no idea what I'm talking about.

My questions are:

Are straight rated 480 volt breakers (as opposed to slash rated) sufficient?

Will a standard delta - wye transformer function normally with a grounded primary phase?

Should 2-pole or 3-pole mains be used?

I hate to sound ignorant but apparently this system is so rare in this area nobody I've talked to has worked with it before. In fact, I called Hammond Transformer tech support and the first guy had never heard of a corner grounded delta and the head engineer he referred me to gave me an under enthusiastic "It should work" in a heavy foriegn accent. With 480 volts to ground on 2 phases I want to be as up to speed as possible before I hook any of this equipment up.

Any help is appreciated.
 
By the way the utility wants the B phase marked RED. This is a grounded conductor, shouldn't it be white or gray?

Given the right circumstances I could see someone very easily mistaking this for a single phase service. Wierd.
 
There is no problem with a corner grounded system feeding a delta-wye transformer.

The utility gets to have the B phase colored red on their side of the service entrance equipment. You need to follow the NEC on your side and color the B phase conductor white or grey.

This is another good example of why coloring coding is an aide only. Voltage testing must always be performed and labeling is highly recommended.
 
One day into this project I look up on the utility pole next to our service (service drop not yet connected) and notice the utility has one phase tied to the steel messenger cable of the 4-wire drop.
Click to expand...


Why would a corner grounded Delta system have a 4 wire drop?
 
Tyoically this is grounded B-phase system because that's the way most people have done it. It doesn't matter per the NEC, it's just the way most manufacturers build it and how it is listed in catalogs.

Make sure you have a grounded B-phase system. See Infinity's question. It should have a 3 wire drop if it is grounded B-phase

You have to use single voltage rated c/b's

Make sure the equipment is specificaly ordered as 480V 3 phase 3 wire grounded B-phase. If you leave out the grounded B-phase you could get the wrong equipment. 2 or 3 pole mains are ok. Double check the ratings on equipment before punching holes etc.

Fuses are not permitted except where used as overload protection for motors.

Customer wiring must be white for the grounded phase
 
Are straight rated 480 volt breakers (as opposed to slash rated) sufficient?
Straight rated breakers are required.
Will a standard delta - wye transformer function normally with a grounded primary phase?
Yes.
Should 2-pole or 3-pole mains be used?
You can use either. Note that some single phase equipment is suitable for use on corner grounded systems. Check the equipment listing.

Don
 
I, too, have some question as to whether this is a 3-wire corner-grounded Delta or a 4-wire high-leg Delta system. There needs to be a closer look at the transformer secondary connections, or have voltage readings taken.

Bob, take another look at which transformer conductor is grounded. If it's the center of three on one transformer, it's the neutral of a high-leg Delta. If it's one of the jumpers connecting two secondaries, it's corner-grounded.
 
If it is indeed a 4 wire system why would we assume that it would a 4 wire Delta? Around here the utility would only supply a 277/480 volt 4 wire Wye system. Where would you even use a 4 wire Delta system?
 
Never heard of a Hi leg delta w/ 480 between phases.

Been thinking more on this. To determine system:

If there are three pots on the pole and there is a wire from each pot connected to a common point, then you have a Wye system.

If each pot has a wire connecting to the next one then it's a delta. There's probably a better way to describe that, I'm just blanking out.

If there are two pots up there, it's an open delta. Never heard of an open delta w/ a grounded corner.

More info on grounded Bphase equip.: (I don't know how to make pretty hyperlinks yet)
http://ecatalog.squared.com/techlib/docdetail.cfm?oid=09008926800b38e9
 
240/480V Delta... I have seen such Animal!

240/480V Delta... I have seen such Animal!

SDG&E has - in the past - used a rather odd Delta setup for 480 VAC 3? systems.

The configuration consists of a typical Closed Delta network, however, the system is Grounded via a center tap on one Pot - typical to the 240/120V 3? 4 Wire Delta - only the Center Tapped Conductor is not used for any Active Circuit, and is only bonded to the GEC at the Service and at the Transformer.

First time I dealt with such system, it threw me big time!
Using my Wiggy, the L-L Volatge was "480-ish".
L-G Voltage was "lower than what is normally seen on a 480Y/277V 3? 4 Wire System - this was on the "First" and "Second" L-G tests.
Third L-G test brough the solenoid up almost like it was measuring 480 VAC!

Using a DVM with the Wiggy connected in Parallel, readings for L-G tests were "235-245V" on 2 of the 3 Lines, and "410-415V" on the 3rd line.

Definitely one for the "Whaddaheck!?!?!?!?" books! :eek:

Scott35
 
It's definetly a corner grounded delta. This feeds a well pump in the middle of nowhere, and apparently is not uncommon in this area in this application. In fact, when I finally found the correct meter base from the only guy in town who knew what I was asking for, he asked, "What are you wiring, a pump?" I'll try to post a few pics in the next few days.


Infinity
Why would a corner grounded Delta system have a 4 wire drop?

SDG&E?s Service Standards book states: ? On overhead service installations the service ground conductor, provided as the redundant 4th wire corner ground conductor of a 480 volt delta service, will be connected to the customer?s identified equipment grounding conductor at the point of service connection (drip loop). Customer shall identify B-phase red in color.

Where would you even use a 4 wire Delta system?

The fourth wire appears to be a redundant B-phase.


sandsnow
Never heard of a Hi leg delta w/ 480 between phases.

There isn?t a ?high? leg. A and C phase have 480 volts to ground, B phase has 0 volts to ground, at least at the main service where it?s bonded.


It looks like my last question is this:

Inside the meter can, there are three test blocks just like on a standard three phase commercial meter base. B-phase, however, has the line and load shorted together at the top (B-phase does not buss through the meter, it just has a small jumper to a 5th jaw. There are two full size lugs on an insulated mount with a bonding jumper to the cabinet.

I would think b-phase must bond in the meter enclosure, but am unclear whether it should go through the shorted test block first and then bond or bond before it lands on the test block. Anyone?

By the way this thread has been extremely helpful. I appreciate you guys helping me out here.

Bob
 
sdbob said:
In fact, when I finally found the correct meter base from the only guy in town who knew what I was asking for, he asked, "What are you wiring, a pump?"
Click to expand...

I love it when that happens!

SDG&E?s Service Standards book states: ? On overhead service installations the service ground conductor, provided as the redundant 4th wire corner ground conductor of a 480 volt delta service, will be connected to the customer?s identified equipment grounding conductor at the point of service connection (drip loop). Customer shall identify B-phase red in color.

Where would you even use a 4 wire Delta system?

The fourth wire appears to be a redundant B-phase.
Click to expand...

It also sounds like the "neutral" shouldn't be bonded in this service.


sandsnow
Never heard of a Hi leg delta w/ 480 between phases.

There isn?t a ?high? leg. A and C phase have 480 volts to ground, B phase has 0 volts to ground, at least at the main service where it?s bonded.
Click to expand...

That was in response to a mention by Scott of it being done.


It looks like my last question is this:

Inside the meter can, there are three test blocks just like on a standard three phase commercial meter base. B-phase, however, has the line and load shorted together at the top (B-phase does not buss through the meter, it just has a small jumper to a 5th jaw. There are two full size lugs on an insulated mount with a bonding jumper to the cabinet.

I would think b-phase must bond in the meter enclosure, but am unclear whether it should go through the shorted test block first and then bond or bond before it lands on the test block. Anyone?
Click to expand...

It sounds to me like the meter is referencing the B phase just for voltage, as if it were two legs and a neutral. If you think about it from the grounded conductor's point of view, it sorta makes sense.
 
sdbob said:
SDG&E?s Service Standards book states: ? On overhead service installations the service ground conductor, provided as the redundant 4th wire corner ground conductor of a 480 volt delta service, will be connected to the customer?s identified equipment grounding conductor at the point of service connection (drip loop). Customer shall identify B-phase red in color.
Click to expand...

Something about their requirement doesn't make sense. I don't remember ever seeing a service drip loop with an equipment grounding conductor. There shouldn't be one there to bond to. There might be a neutral, but I have never used one in a corner grounded service before.

Jim T
 
Yea, I know it doesn't make sense. Just like changing the phase color from red to white doesn't make sense.

I'm gonna take the (red) B-phase through the meter test blocks to an insulated lug bonded to the enclosure with a jumper. I'll land the "redundant" grounding conductor from the riser on the meter enclosure. I'll then take the B-phase (now white) to the 3-phase service disconnect along with A and C from the test blocks. Inside the disconnect I'll again land B-phase on an insulated buss bonded to the XO enclosure with a screw (along with my GEC), than through the disconnect using a slug instead of a fuse. Out of the load side of the XO I'll bring ABC and a grounding conductor from the XO enclosure.

From there I keep the B-phase isolated from ground, running underground to a backboard to a 1-phase, 60 amp, 600 volt disconnect, tapping two 20 amp branch circuits from the load side of that to their respective 1-phase XO's. One feeds the well pump, the other my transformer. The 60 amp XO also gets a grounding electrode to its enclosure.

Anybody see a problem with this?

22 years in the trade and I feel as ignorant as I did when my 8th grade winter dance date kissed me, lol.
 
I was back out there today. Here are some pics; it's kinda hard to see detail given the file size restrictions.

Clearly there are four wires in the POCO drop, and two of them are tied together, the messenger and the phase tagged red. Hopefully the utility transformer connections can be seen by someone who understands them. I do not.

I ended up using 3-phase disconnects and instead of using a slug in the b-phase I moved the lug from the load side of the fuse to where the top of the fuse would be. To me that seemed like a better solution, removing the possibility of someone downstream replacing the slug with a fuse. We'll see what the inspector says.

I bonded B-phase at the main service (pole) and fed the main disconnect on the backboard with the 3 phases and a grounding conductor, keeping b-phase ungrounded at the backboard which is a separate structure. A ground rod at the backboard serves as the grounding electrode for both the 208 volt transformer and the 480 volt main disconnect.
 

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Pic 1: That's definitely Delta, but the primary looks to be Y! (Doesn't matter with the fuses open anyway.)

Pic 2: Yep, red and bare from the same wire, all right.

Pic 3: No wonder you have no power! Got any Tomics?

Pic 4: I like the shovel-shaped grounding electrode.
 
Good job in the service. Your inspection should go smooth.

The B-phase red when we require it white is akin to the Hi-Leg on C-phase (req'd by POCO) and NEC req's it on B-phase
 
Scott,

The voltages you measured are exactly right for the system you describe. Draw a 480V delta phasor diagram to scale for visual proof.

Rattus
 
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