Transformer reverse connected...

[iwire]

So then I theoretically I would need a single phase disconnect because only 2 of the phases would have fused protection and the 3rd phase (grounded phase) with white tape would pass through my disconnect? Correct???

Yes that is one way but I do agree with Don that you better be sure of the listing.

Another way to do this is a 3 phase fused disconnect but on the grounded phase you install a 'dummy fuse'. Bussman sells listed 'non-fuse gizmos' to install in the grounded phase position.

 

[chevyx92]

Yes that is one way but I do agree with Don that you better be sure of the listing.

Another way to do this is a 3 phase fused disconnect but on the grounded phase you install a 'dummy fuse'. Bussman sells listed 'non-fuse gizmos' to install in the grounded phase position.

Ok thats good to know. So on a corner grounded delta 480v system, you will have Brown for phase A, White or gray for phase B(since thats the grounded phase) and Yellow for phase C. There will be no Orange correct? I'm learning a lot here as I've never seen or done this before.

 

[iwire]

Chris as I have pointed out I have not done this before either. What I know I have learned from the forums.

Anyways the only required color is the white on the grounded conductor. The other colors are your choice / local preference etc.

If I recall member 'sandsnow' posted some pictures of a corner grounded delta service.

In that case they used breakers so it was acceptable to have a 3 pole breaker with say Brown, White, Orange....it looked very strange. (You can use an over current device on the grounded conductor if all other poles will open at the same time...in other words fuses no, breakers yes.)

I will see if I can find that thread.

 

[Ragin Cajun]

SquareD sells a 208V Delta primary, 480/277V wye secondary transformer line in sizes from 15 to 500 KVA.

That's the simple way to go. Slight cost difference over the normal, but it's worth it since you don't have a wierd set-up. I may have a new car lot lighting project that I may need to use one.


RC

 

[ceb]

I just did what you are wanting to do for a overhead crane. It sounds like it would blow up the transformer but it works.here is my install I needed 480v 3ph for crane only. service to building was 208v 3 ph I bought a 480v prim. 208v sec. SUITABLE for reverse connection. I set my 3ph disco. I ran my grounding cond. from the ground lug in disco. to ground rod to lug on steel beam to ground lug on transformer shell to a double lug on h2 all one continuous run of wire. My primary 208v on x1 x2 x3 my secondary out on h1 h2 h3 NOTHING on xo (per acme transformer tech. support) voltage between h1,h2 480v h1,h3 480v h2,h3 480v h2 to ground 0.

 

[dnem]

Chevy,

I wanted to voice agreement with everything that Bob said. . Reread all of his posts. . He covers alot of the issues with OCPD and OLs and XO and marking. . This stuff doesn?t come up commonly when you?re manning a limited number of jobsites as a contractor. . As an inspector, I see about a dozen different jobsites each day on average. . I see things attempted that I would have never attempted as a contractor. . And I sometimes scratch my head and call other inspectors. . We have a couple guys around that are very experienced.

I?ve inspected ungrounded delta, corner tapped delta, and center tapped delta [with the hi leg] systems. . There?s unique things about each of them. . I?ve even seen a corner tapped delta installation from the 40s that used the building steel as the grounded phase. . Basically, the corner tapped delta is treated like a single phase system. . The corner tap conductor is treated as you would treat a neutral. . 240.22 would apply.

David

 

[dnem]

A couple years ago I was doing an inspection at a Laundromat that was adding a couple sun coffins in a few back rooms. . They were 3 phase cord and plug units with big monster cords on them. . I stuck my wiggy in one of the plugs and got what appeared to be 480 phase to phase. . 480 A to B. . 480 A to C. . 480 B to C. .Then I tried phase to ground and nothing. . I immediately thought they had forgot to connect the equipment ground. . As I continued the inspection, it turned out the installation was not according to prints. . They had 380 volt European coffins that they were supplied from a separate small transformer that had an ungrounded delta secondary. . There wasn?t any problem with the grounding path. . Ungrounded delta has no voltage to ground.

I called a very experienced inspector from a nearby city and he came out to take a look. . In the end they redrew the prints and corner grounded the transformer. . The grounded phase was marked white at every termination and the overcurrent stayed as-is because it was multipole 3 phase breakers [240.22(1)].

Strange animal, never saw it again, but it worked fine and passed code. . In that case it was a special delta to delta transformer that is used for European voltage equipment. . No XO to think about.

David

 

[chevyx92]

I just did what you are wanting to do for a overhead crane. It sounds like it would blow up the transformer but it works.here is my install I needed 480v 3ph for crane only. service to building was 208v 3 ph I bought a 480v prim. 208v sec. SUITABLE for reverse connection. I set my 3ph disco. I ran my grounding cond. from the ground lug in disco. to ground rod to lug on steel beam to ground lug on transformer shell to a double lug on h2 all one continuous run of wire. My primary 208v on x1 x2 x3 my secondary out on h1 h2 h3 NOTHING on xo (per acme transformer tech. support) voltage between h1,h2 480v h1,h3 480v h2,h3 480v h2 to ground 0.

My boss disagrees with me on the fact that I have to corner ground the delta and leave the XO floating. He's telling me to run delta in and delta out, and to bond the XO to frame and building steel. I have no need for neutral loads so there won't be a neutral in my supply conductors. He's telling me not to corner ground, that it will be fine. He's saying that I have to bond XO to frame and building steel to stablize the voltages. I am getting confused here. What's the proper way to do this????? I agree with you guys and he doesn't agree with me.

 

[LarryFine]

He's saying that I have to bond XO to frame and building steel to stablize the voltages.

Even I, who still puzzles over leaving a primary X-0 floating, knows using building steel instead of an insulated grounded circuit conductor is a big no-no.

 

[Jljohnson]

My boss disagrees with me on the fact that I have to corner ground the delta and leave the XO floating. He's telling me to run delta in and delta out, and to bond the XO to frame and building steel. I have no need for neutral loads so there won't be a neutral in my supply conductors. He's telling me not to corner ground, that it will be fine. He's saying that I have to bond XO to frame and building steel to stablize the voltages. I am getting confused here. What's the proper way to do this????? I agree with you guys and he doesn't agree with me.

I have seen this done the way your boss wants you to do this. It "functions" OK until the utilty loses a phase for whatever reason. When this occurs, XO tries to become X-whatever phase is missing. This creates alot of heat on the GEC conductor to the building steel. I have been called out to troubleshoot issues like this twice in past few years. Once the GEC conductor actually melted the PVC conduit that was protecting it from physical damage. Best thing to do.... order the Square D reverse fed listed transformer. 2nd best....follow the advice given by Bob, he is right on with how this should be done.

 

[infinity]

My boss disagrees with me on the fact that I have to corner ground the delta and leave the XO floating. He's telling me to run delta in and delta out, and to bond the XO to frame and building steel. I have no need for neutral loads so there won't be a neutral in my supply conductors. He's telling me not to corner ground, that it will be fine. He's saying that I have to bond XO to frame and building steel to stablize the voltages. I am getting confused here. What's the proper way to do this????? I agree with you guys and he doesn't agree with me.

How will bonding the primary XO make the secondary grounded also? The secondary would still operate as an ungrounded system. That would trigger, IMO more costly other code requirements for using an ungrounded system.

 

[jim dungar]

My boss disagrees with me on the fact that I have to corner ground the delta and leave the XO floating. He's telling me to run delta in and delta out, and to bond the XO to frame and building steel. I have no need for neutral loads so there won't be a neutral in my supply conductors. He's telling me not to corner ground, that it will be fine. He's saying that I have to bond XO to frame and building steel to stablize the voltages. I am getting confused here. What's the proper way to do this????? I agree with you guys and he doesn't agree with me.

Do not bond X0 to the building steel. The transformer primary will be connected the same as a motor that has wye connected windings. The center point of motor windings are always left "floating" with no concern about stabilizing the voltages.

Also, and more importantly, I know of no transformer manufacturer that does not warn/advise against connections to the X0 when "reverse" connected.

 

[chevyx92]

Even I, who still puzzles over leaving a primary X-0 floating, knows using building steel instead of an insulated grounded circuit conductor is a big no-no.

It's not "INSTEAD". He's saying to bond the XO, only difference is there would be no actual grounded conductor being used. If it were hooked up normally you would have the building steel ground bonded with frame and XO. So it's not instead.

 

[ceb]

Do not bond X0 to the building steel. The transformer primary will be connected the same as a motor that has wye connected windings. The center point of motor windings are always left "floating" with no concern about stabilizing the voltages.

Also, and more importantly, I know of no transformer manufacturer that does not warn/advise against connections to the X0 when "reverse" connected.

If your boss dosent agree with this tell him to call ACME Elect. at 1-800-334-5214 ask for tech. support. They will tell him as they told me when I questioned grounding one phase. YOU CONNECT NOTHING TO XO.For you type in corner grounded transformer in search you will find more than enough info. good luck

 

[benaround]

How much more money does the proper transf. cost ?? I have never seen

a manafacturer recommend reverse connecting, but many advise against it.

When I look at a transf. name plate it will say;

PRI. 480vac
SEC 120/208vac

If the primary voltage in the building is 120/208vac, how is this a good NEC

install.

Why would the manf. even make a 208pri, 480sec, if they didn't have to??

I think this is a " we always do it this way " thing that's still being done, I'm

sure it works just fine, just like electric heat on a high leg does. Just talking

code here, that's what the Forum is all about.

 

[iwire]

I have never seen

a manafacturer recommend reverse connecting, but many advise against it.

From Square D.

Can Transformers be backfed (used in reverse)?

Answer

1. Backfeeding is not recommended for general purpose transformers smaller than 3kVA, and is not allowed for any Industrial Control Transformers of all sizes, because windings are compensated. Backfeeding will result in lower than expected output voltage.

2. Backfeeding causes very high excitation inrush, making coordination to breakers or fuses difficult without exceeding the limits set by the National Electrical Code. Avoid backfeeding wherever possible for this reason.

3. If a Delta-Wye transformer is to be backfed so that the Wye side is the input, do not connect the neutral terminal to the primary system neutral, nor should the neutral terminal be connected to ground.

4. If the transformer has taps, when backfed the taps do not help compensate for poor (other than nominal) source voltage to provide the appropriate magnetic flux levels in the core as they are designed to do.

 

[chevyx92]

If your boss dosent agree with this tell him to call ACME Elect. at 1-800-334-5214 ask for tech. support. They will tell him as they told me when I questioned grounding one phase. YOU CONNECT NOTHING TO XO.For you type in corner grounded transformer in search you will find more than enough info. good luck

Let me ask this. By grounding the "B" phase that phase will have zero volts correct? If thats correct how would a 3 phase motor work on this corner grounded system? You would have phases A,C and the B is the grounded one with zero volts. Wouldn't the 3 phase motor be single phasing on this setup???

 

[ceb]

No, I cannot tell you the high octane explanation behind all of this( there are those here that are far more learnered than I) but after reading what I could find and talking with the transformer people it works. What readings I have are a to b 480v a to c 480v b to c 480v. now from b to ground 0 a to ground 480 c to ground 480. All of my phases have voltage but (what I can understand) I now have ground fault protection if something were to short. If I were to not have grounded one leg and there were a short I would have energised the crane frame and the steel beams the tracks are mounted to. The bottom line is when I powered up the transformer it did not go "boom" when I closed the disconnect to the crane it did not start by its self and fly through the end of the building. The only thing that happened is when I mashed the down button for the hoist it went up, I switched leg a and b and it now doses what it supposed to. Also,as posted which ever leg you ground you must put in a "dummy" fuse on the leg that you ground as to not lose the ground.

 

[LarryFine]

Let me ask this. By grounding the "B" phase that phase will have zero volts correct? If thats correct how would a 3 phase motor work on this corner grounded system? You would have phases A,C and the B is the grounded one with zero volts. Wouldn't the 3 phase motor be single phasing on this setup???

Yes, B phase would have zero volts relative to earth, but the voltages among the three conductors remains the same.

This is no different than feeding a motor with 120v 1ph. Whether one of the circuit conductors is grounded or not will make no difference to the motor.

 

[chevyx92]

Ok I think I have a real good picture of how you're suppose to corner ground a delta. 3 phase plus EGC in with no neutral, 3 phase plus EGC out no neutral. XO will not have any connections to it. Take and ground "B" phase of delta side to frame of Xfmr and building steel and EGC. "B" phase will be taped white and have no fused protection per code. And the disconnect has to be rated for a corner grounded system. Have I got this all correct???? Also are there any links of diagrams or pics or official documentation as to the way we are hooking this up?? Thanks.