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Joined: Dec 2004
Posts: 1,064
D
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Why can't the IG be terminated on the Nuetral bar of the service panel again?

250???


Dnk....

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Joined: Jan 2004
Posts: 1,507
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Okay- What are doing for transformer overcurrent?? I say we protect the primary at 125% of it's capacity and forgo protection of the secondary. If that's the case we can protect the primary at 125% x 90 = 112a. so we use a 125a overcurrent protection. gives me a #1/0 wire. Now we don't have to protect the secondary. This overcurrent protects the transformer. If we go over 125a. on the primary we would have to add secondary protection.

Added edit:
I'm going on the basis that we have a 480v. 3Ø primary and it is a 75kva xformer. Sorry bout that guys.

[This message has been edited by George Little (edited 09-09-2005).]


George Little
Joined: Aug 2003
Posts: 1,374
R
Moderator
George, I prefer the same method you describe.


Ryan Jackson,
Salt Lake City
Joined: Jan 2003
Posts: 4,391
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iwire Offline OP
Moderator
Well that would depend on what the transformer is feeding.

If ut feeds a panel it is likely the panel will need protection and that might as well be incorporated into the transformer protection as well.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Joined: Mar 2002
Posts: 582
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Ron Offline
Member
Due to panelboard OCPD requirements, or just that there is a lot of 120V loads that due to load transfer/movement may result in a lot of imbalance for a short period of time, detection of a 120V overcurrent is made more accurate by secondary protection. Remember the primary to secondary is not bolted, so a significant single phase load change does not directly effect the primary current per phase in exactly the same ratios (ie primary phase A to secondary phase A).


Ron
Joined: Mar 2002
Posts: 582
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Ron Offline
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One other benefit of having the primary and secondary protection is the increased flexibility in the primary OCPD to grow larger (250% max) to overcome high inrush, possibly due to k-ratings or other design consideration.
Transformer inrush per ANSI standards is approximately 12xFLA or 1080A, so the primary breaker may trip, even if the actual load is low. Inrush is unrelated to actual connected load.

Edited for calculation error.

[This message has been edited by Ron (edited 09-09-2005).]


Ron
Joined: Oct 2000
Posts: 2,723
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I would say the most "NEC-ish" problem in the Transformer Image would be the Grounding Electrode Conductor does not terminate directly to the "X-0" terminal.

Next would be the "what appear to be" too small EGCs - at least for the Secondary side's feeders the EGC looks a wee bit small.

Odd to me (at least) is the Phase arrangement here - more specific, the color code arrangement, vs. terminals.

Starting from the left, we have:
<OL TYPE=1>

[*] An "X" terminal tagged Blue (would be typically X-3),


[*] An "H" terminal tagged Orange (would be typically H-2),


[*] The common point of the Secondary coils - "X-0",


[*] An "X" terminal tagged Red (would be typically X-2),


[*] An "H" terminal tagged Brown (would be typically H-1),


[*] An "XH" terminal tagged Yellow (would be typically H-3),


[*] An "X" terminal tagged Black (would be typically X-1).
</OL>

Not that this is any violation, just kind of an odd arrangement to me.
Anyone find it strange?

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Jul 2002
Posts: 717
M
Member
Scott, here is a self quote from my post on this."Question #2 is I cannot make out what is in the conduit on the bottom left side. It appears to be the gec. If so where is it terminated and would the bonding jumper sent to the bushing on that conduit entry need to be the same size as the gec? " I am still wondering if I am looking at this picture wrong, or am I missing something?

Joined: Jan 2003
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iwire Offline OP
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Thanks everyone for the responses so far.

Scott noticed what I noticed, the GEC lands at the bonding point not on XO directly which IMO is a violation of the current wording in 250.30(A)(2)(a)

Just part of 250.30(A)(2)(a)
Quote
shall be used to connect the grounded conductor of the derived system to the grounding electrode

IMO as it is in the picture the GEC is being used to connect the bonding jumper to the electrode.

My HI buddy, I believe what we have is this.

1.25" primary feeder with EGC.

2" Secondary tap with EGC

The GEC conductor disappears into the back somewhere. It probably runs without conduit out one of the holes in the grill.

I had the advantage of seeing this much larger, I reduced the picture size before posting.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Joined: Dec 2000
Posts: 4,294
Member
The way I do these is to install a triple lug on X0.
One conductor is the secondary grounded conductor (neutral).
The next conductor is the GEC.
The third is the bonding jumper.

I have "made do" with double lugs by passing the GEC through the terminal and bonding the tag end to the xfrmr case, but this comes very close to encroaching into the "No conductors above this point" area.

It looks to me like the GEC, which is landed in the group at the bottom left (There's only one conductor that I can't account for and it appears to be going through one of the slots in the transformer bottom) is undersize for GEC, possibly a #6.

Thanks for the thread, Bob. Some of my coworkers are still mystified by transformer grounding, and maybe this will help.

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