TTP (Theory To Practice)

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scott

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Situation -

Two phase 277/480volt 3 wire power is fed to meter base from utility. Service disconnect installed with ground rod and GEC at site made pedestal/stand. Load side of service disconnect runs to hung 25kva single phase transformer with 480 volt primary and 3 wire center tapped 120/240volt secondary. GRC is used between the disonnect and transformer, and only the phase(ungrounded) conductors are run to the XFMR. The installation is outdoors and used to feed a distribution center wich subsequently feeds a DSL/phone cabinet on a concrete pad (about 4 feet away).

The distribution center is designed for 120/240 volt single phase power, with a main and space for 20 1 pole circuits, self contained surge protection, and power indicators. The pad has a #2 ground ring built beneath it, and is properly bonded to the distribution center and the DSL equipment.

A 3 wire feeder is run between the distribution center and the transformer (SDS). In the distribution center, a N-G bond is installed via manufactured screw.

The other end of the feeder is connected as follows...

L1 - X1 L2 - X4 X2 - X3
N - case ground (at XFMR)

The primary is connected as follows...

phase A - H1 phase B - H2

Questions
1) What would the voltage to ground be from line 1 in the distribution center?

2) What would the voltage be from line 1 to line 2 in the distribution center?

3) What would happen if a line to nuetral load were to be utilized out of the distribution center?

ps - this is just for fun - no wrong answers, I want to share a recent experience of mine with others.

Enjoy your day!
 

hurk27

Senior Member
Re: TTP (Theory To Practice)

Ed I think what Scott was trying to say is the poco fed the meter with a single phase off a 480/277 Y 3 phase transformer 3 wire two hots and the neutral.
 

roger

Moderator
Staff member
Re: TTP (Theory To Practice)

Scott, are you saying the secondary winding(s) are not grounded? (just the case) Did you mean L2 is connected to x2 and x4 which are bugged together and x3 is floating?

Sorry, but I'm a little confused. Don't say it, I know, I 'm allways confused :D

Roger
 

scott

Member
Re: TTP (Theory To Practice)

Sorry gentlemen, I tried to be clear in my post, but I can't post drawings like Ed can. I also appologize for reposting so late, but my internet server has been giving me fits lately.

Two ungrounded conductors and one grounded conductor are line side from the utility at 277/480 volts (two of three phases + neutral). The neutral is dropped off at the service disconnect which has the GEC and BJ in it.

GRC conduit is run to single phase 480 volt primary with only the two ungrounded conductors connected to the primary.

-line 1 (phase A) connects to H1
-line 2 (phase B) connects to H2
-equipment ground provided from disconnect to XFMR by GRC nipple

Transformer is intended to be an SDS, with 120/240 volt single phase power derived (3 wire with center tapped neutral). The transformer, meter, and disconnect are all mounted on a strut rack located outdoors.

Two hots and a neutral run underground about 5 feet to a pad mounted pedestal. The concrete pad holds the power pedestal, DSL equipment, and a ground ring. N-G bond takes place at disconnecting means in pedestal, as well as GEC from ground ring for the SDS.

The transformer secondary is connected as follows...

-line 1 (pole A) connects to X1
-line 2 (pole B) connects to X4
-X2 and X3 are connected together
-neutral from/to pedestal is connected to transformer case

Same questions apply if I am clearer than mud?!?... ;)

Enjoy your day!
 

gwz2

Senior Member
Re: TTP (Theory To Practice)

Question 1) 120 volts.

Question 2) 240 volts.

Question 3) You said a three wire feeder between the tranformer and the Distribution Panelboard (DP) installed in RMC (344.).

The Grounded Conductor ( Neutral ) is bonded to the transformer enclosure.

The Grounded Conductor is bonded to the DP with the SUSE bonding screw.

The Grounded Conductor ( Neutral ) and the RMC are in parallel. This violates 250.30(A)(1) last sentence of text paragraph.

I would consider this a violation, even with-out a Grounded Conductor Load.

It is also my position that the RMC between the transformer and the DP should have bonding bushings ( or equal ) although the 2002 NEC does not mention this. The 1999 NEC 250-30(a)(4) could be interpreted to require the bonding bushings.
 

scott

Member
Re: TTP (Theory To Practice)

Some clarification...

The raceway form the XFMR to the distribution center/pedestal is PVC, with a short liquid-tite transition on the XFMR side.

The feeder from the XFMR to the pedestal is 3 wire. Two hots and one neutral for the intended 120/240volt single phase supply to the pedestal.

The GRC raceway is the equipment ground on the primary side of the transformer.
 

roger

Moderator
Staff member
Re: TTP (Theory To Practice)

Scott, it sounds as though the winding is not grounded at all.

If I'm interpretting correctly this would mean,

#1 Line 1 would have a low reading maybe in the nieghborhood of 60v give or take with no load present.

(If you applied a load, this would read more in balance)

#2 Line 1 to line 2 would read 240.

#3 maybe nothing but a dangerous situation, current would not be going back to source. (circuits would probably work)


X2 X3 must be bonded. I may still be missunderstanding, Sorry

Roger

[ March 01, 2003, 07:48 PM: Message edited by: roger ]
 

don_resqcapt19

Moderator
Staff member
Re: TTP (Theory To Practice)

Scott,
You can't connect the grounded conductor from the transformer to ground at both the transformer and the distribution center. It can only be connected to ground at one locattion and an EGC will be required between the two. If the raceway between the transformer and the distribution center was nonmetallic, then Exception #1 to 250.30(A)(1) would permit the bonding in both locations.
As far as the voltages they should be 120 L1 to ground and 240 line to line. There would be no problem with line to neutral loads, except that the rigid conduit is in parallel with the neutral, but all of the voltages should be normal.
Don
 

gwz2

Senior Member
Re: TTP (Theory To Practice)

Since the conduit between the transformer enclosure and the DP is non-metallic and the bonding required by 250.30(A)(1) and the grounding required by 250.30(A)(2) is done at the at the DP, ( the green Manufacturer's screw ) then an EGC must be installed from the DP to the transformer enclosure and the Grounded Conductor Bonding Jumper (BJ) that is installed at the transformer to the transformer enclosure must be removed to prevent a parallel path of the Grounded Conductor and the EGC between the transformer enclosure and the DP enclosure.
[ Actually, the paralleling of the Grounded Conductor, not the paralleling of the EGC ].

Problem is, the NEC is not clear on the sizing of this particular EGC between the Transformer enclosure and the DP enclosure. Is this EGC sized per 250.28(D) or per 250.122.
This particular EGC is not on the 'load side' of the OCPD.

Again, the 1999 NEC 250-30(a)(4) would permit 250-28(d).

[ March 01, 2003, 09:08 PM: Message edited by: gwz2 ]
 

gwz2

Senior Member
Re: TTP (Theory To Practice)

Rather than another edit of my last post, am making a new post.

The EGC of my post for a 25KVA 1? 3W 120/240V system would not be much extra cost in comparison for the same EGC's for a 3000A SDS. The length of the EGC's and the size required in each of the multiple raceways could drastically increase the cost.

Do it right the first time.
 

roger

Moderator
Staff member
Re: TTP (Theory To Practice)

Don, If the winding has no reference to groung the voltage in an idle state would not read 120v to L1. Read voltage on L1 and L2 of an isolated control transformer to ground for an example.

Roger
 

gwz2

Senior Member
Re: TTP (Theory To Practice)

Didn't the original post metnion " A N-G bond is via installed manufactured screw"?

That would be the Ground reference point.
 

roger

Moderator
Staff member
Re: TTP (Theory To Practice)

GWZ, yes, but that is at the DP, and the way I'm reading Scotts discription the secondary winding has no physical connection to any grounds or service neutral.

Roger
 

gwz2

Senior Member
Re: TTP (Theory To Practice)

I read the original post, after "Situation":

Paragraph 3;

1) 3W feeder between BP and the transformer.

2) A N-G bond via manufactured screw.( at the DP ).

Paragraph 5;

1) L1 is tied to X1.
2) L2 is tied to X4.
3) N is tied to X2 - X3.
4) N is bonded to the transformer case.

******

With the N bonded to case at the transformer and to the DP with the Manufactured screw and the 2AWG grounding at the DP, there voltage between the ungrounded conductor(s) and the grounded conductor.

Problem is 250.30(A)(1) and 250.30(A)(2) are violated by being grounded at two points.

I certainly would expect that the 25 KVA transformer case is grounded to Ground Rods ( etc ) by the Service Equipment disconnect and with the 480V 2W feeder being in RMC (344), 250.118(2), 250.134(B).

Since non-metallic conduit is used between the transformer and the DP, two thihgs should be done.

1) Install and EGC between the DP and to the transformer case.

2) Remove the N to Case bond at the transformer.

Problem with 1) is 'what size' should that EGC be?
Of course the proper term for that conductor would be " Equipment Bonding Jumper" (EBJ) because it is bonding the transformer case to the source of grounding the secondary side of the SDS at the DP.

[ March 02, 2003, 07:14 AM: Message edited by: gwz2 ]
 

scott

Member
Re: TTP (Theory To Practice)

The installation in question is not my work, I found it while performing grounding checks on phone sites for the phone company. I wanted to share my unfortunate experience with others so that they might see the problems of not following the code, and see that a piece of equipment that has been working for years is not necessarily safe (poor assumtion on my part).

Anyway-

The meter/service disconnect/transformer rack is 5 feet away from the poured concrete pad, and is located outdoors.

Would not the rack and the pad be considered two different structures?

A 3 wire feeder to the pedestal/distribution equipment would be allowed by 250.32. Each structure has a disconnecting means rated for use as service equipment, a ground electrode, and a GEC that is bonded in the main disconnect enclosure.

The SDS has secondary protection within 10 feet according to 240.21.

The SDS bond is made at the first disconnecting means as allowed by 250.30.
 

gwz2

Senior Member
Re: TTP (Theory To Practice)

Getting stickier.

Seems the grounding electrode(s)(250.50) of the Service Equipment (250.92) 250.97), the SDS Primary EGC (250.134(B), the SDS enclosure (250.96), the SDS Secondary 250.134(B), the Distribution Panelboard, and the requirements 800.40 bonded together as per 250.58, beit 250.32(A), and (B)(1), and (B)(2), and (D) and 250.32(E).

Others' comments ?
 

don_resqcapt19

Moderator
Staff member
Re: TTP (Theory To Practice)

Scott,
So you are saying that the rule in 250.30(B)(2) over rides the rule in 250.30(A)(1). Not sure what one really applies here, I guess I'd accept either one. There should be no problems with the operation of this installation, but you posts seem to imply that there was an opperation issue. What happened?

Roger,
Even if there is not a grounded to grounding bond at the transformer, there is one at the distribution panel. The system is bonded to the grounding electrode system.

Don
 

roger

Moderator
Staff member
Re: TTP (Theory To Practice)

Don, I see that now. What I kept missing was where the winding made any connection to the neutral. The way I was reading it there was only a connection to the XFMR case, and then to the distribution point.

Roger
 

Ed MacLaren

Senior Member
Re: TTP (Theory To Practice)

The sketch below is my interpretation of the existing installation, as Scott described it.



Ed

[ March 02, 2003, 10:30 PM: Message edited by: Ed MacLaren ]
 
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