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Posted By: iwire Transformer Installations - 09/08/05 10:59 AM
I figured we could talk about Separately Derived Systems, (SDS) specifically SDS derived from transformers.

Here is a great picture of an transformer installation.

[Linked Image]

I am not interested in 'slamming' the installer I would like to discuss the NEC requirements regarding this installation.

Personally I think this is a nice job and only have one minor code issue with it.

What do the rest of you think of this?

For those of you who have not installed transformers join in, ask questions. No one is going to get flamed for trying to learn new things. [Linked Image]

Bob
Posted By: Ryan_J Re: Transformer Installations - 09/08/05 12:57 PM
Bob, I can't tell for sure, but is the bonding jumper undersized on the secondary side?
Posted By: iwire Re: Transformer Installations - 09/08/05 01:52 PM
Ryan just call me Homer. [Linked Image]

I don't know what I was thinking. I used a picture that I have no info on. [Linked Image]

DOH!

For the sake of things lets say we have a 75 KVA transformer 480-208Y/120.

Where would we get started figuring out conductor sizes assuming we want the maximum capacity from this 75 KVA unit?

Bob
Posted By: SolarPowered Re: Transformer Installations - 09/08/05 03:46 PM
OK, I'll take a stab at this--

Input: 75,000VA / 1.73 / 480V = 90A. Call it #3 or #4, depending on whether you play by the "next larger standard breaker size" rule.

Output: 75,000VA / 3 / 120V = 208A. If you call this 200A, it will take 3/0 cable. I don't know if the rules would allow fusing this at 225A; if so, you'd want to use 4/0.

The above assuming 75 degree wiring.

I would imagine that either the input or the output fusing is supposed to limit the TX to 75 KVA; I don't know what the rule is for this. In a vacuum, I'd say a 90A breaker on primary feeder would do the job.
Posted By: DSpanoudakis Re: Transformer Installations - 09/08/05 03:47 PM
Well, on a SQ D slide-rule my teacher gave us last semester, it states that a 75 KVA can hold a maximum of 208A at 208V, 180A at 240V, 90A at 480V, and 72A at 600V.

Then, going from 310.15, 4/0 for 208V, 3/0 for 240V, #3 for 480V, and #4 for 600V.

I barely finished my 1-year program, I'm barely going to start my Apprenticeship. Someone correct me if I'm wrong.
Posted By: DSpanoudakis Re: Transformer Installations - 09/08/05 03:49 PM
SolarPowered, which formula are you using to calculate that?
Posted By: SolarPowered Re: Transformer Installations - 09/08/05 03:55 PM
VoltAmps = Volts * Amps
Posted By: SolarPowered Re: Transformer Installations - 09/08/05 04:11 PM
I suppose that answer was rather sparse. I also used the relationship that the VA of a three-phase circuit is sqrt(3) * the VA of a single phase circuit, at the same number of amperes per conductor.
Posted By: iwire Re: Transformer Installations - 09/08/05 04:28 PM
Thanks guys.

At this point I think we have ampacity of the transformer worked out.

On the primary side we have a full load current of 90 amps

On the secondary side we have a full load current of 208 amps.

Now we need to figure conductor and breaker sizes keeping in mind that we want the maximum capacity from this transformer.

The connected load to this transformer is 75 KVA continuous.

Bob
Posted By: SolarPowered Re: Transformer Installations - 09/08/05 05:23 PM
My problem here is protection of the transformer. Using normal breakers, with a continuous, 100% load, you need to oversize by 25% because of the 80% rule. That would suggest 90*125% = 112.5A on the primary, and 208*125% = 260A on the secondary.

Also, there is some loss in the TX. Supposing 5% loss, you need 94.5A in to get 75kVA out.

So, my thought would be to use 100% rated breakers, with #3 fused at 100A on the input, and with 4/0 fused at 225A on the secondary.

This doesn't really limit the TX to 75 kVA. A 90A, 100% breaker would limit the input to 75kVA, but you won't get 75kVA out of it. Assuming this isn't a trick question where the premise is false, you said that the load is 75kVA; therefore, I have to assume that the TX is designed and listed to have an input large enough to supply that load. Given that, I'd feel comfortable with an adjustable, 100% breaker set to 95A supplying the primary. I have no idea if that would meet Code. And I'll stay with 4/0 at 225A, 100%, on the secondary.
Posted By: Ryan_J Re: Transformer Installations - 09/08/05 06:24 PM
Perhaps I'll post an article I wrote on transformers here after this dies down a bit [Linked Image]
Posted By: iwire Re: Transformer Installations - 09/08/05 06:27 PM
Ryan that would be great, I started this thread for education purposes (my own education included) and your input is more than welcome.

I do like your idea of letting this run as is for a bit.

Bob



[This message has been edited by iwire (edited 09-08-2005).]
Posted By: Ryan_J Re: Transformer Installations - 09/08/05 09:17 PM
In my opinion, creating a true Isolated Grounding system is best done by running an IG to the transformer itself.

How do I size it, since it is not an equipment bonding conductor (according to a freind of mine on panel 5), and therefore 250.30(A)(2) doesn't apply?
Posted By: iwire Re: Transformer Installations - 09/08/05 10:30 PM
Ryan you lost me on that one, I do not see how an IG and 250.30(A)(2) would come up in the same conversation.

I have always used Table 250.122 to size IG conductors.

Typically that would mean a 12 AWG all the way back to the transformer XO terminal would be adequate as the OCP that IG will need to open will be a 20 amp. Very few IG circuits are larger than 20 amps.

Generally I have used a 6 AWG back to XO from a IG bar in the panel just because a 12 landing on XO of a 75 KVA transformer looks bizarre.

Thinking on this some more and it seems a strict reading of 250.122(B) would require the IG for a 20 amp circuit run back with the feeder to be the same size as the feeders ungrounded conductors. [Linked Image]

Bob
Posted By: Ryan_J Re: Transformer Installations - 09/08/05 10:35 PM
What happens if you have a phase to IG fault in the secondary raceway? Granted, that is a remote possiblity, but you could have an issue trying to open the primary protection device with a 12 AWG conductor.

I don't think the code addresses this. I am trying to come up with a proposal to address it, but I am at a loss as to how to do it.
Posted By: iwire Re: Transformer Installations - 09/08/05 10:52 PM
What happens?

Hmmm, if I run a 12 AWG all the way back I would say the 12 AWG IG opens at the time of fault and the enclosures of all the IG equipment become 'hot' for just a moment or possibly permanently if the small IG opens on the line side of the fault and welds to an ungrounded conductor on the load side of the fault.

Not real good. [Linked Image]

Perhaps a simple proposal that says the IG that is allowed to run back to XO must be sized per 250.122 for whatever OCP protects the the conductors in the same raceway.
Posted By: jw electric Re: Transformer Installations - 09/09/05 12:18 AM
No person shall operate a motor vehicle on a road or highway at a speed that is greater than is reasonable and prudent to the existing conditions.

I just for the life of me can't figure why in this world that who ever did this job would want them black wires flopping around between them windings like they are. Is this a problem? Could this cause some kind of induction in them wares? Would this cause them wares to get hot?

Now Bob you promised not to whop me ifen my questions hant no good.

smiley face goes here

edited to explain: My Granddaughter is taking drivers ed and dared me to post a lesson in here

[This message has been edited by jw electric (edited 09-08-2005).]
Posted By: iwire Re: Transformer Installations - 09/09/05 12:25 AM
Welcome new member [Linked Image] once your in the 'club' we will let you know the secret smile codes. [Linked Image] [Linked Image] [Linked Image]

You will have to ask the factory about the black wires up around the coils.

Usually a transformer like this will not allow field wiring above the H and X terminals.

Go here https://www.electrical-contractor.net/ubb/faq.html#smilies to learn the smile rules.

Bob
Posted By: jw electric Re: Transformer Installations - 09/09/05 12:32 AM
Thank you Bob [Linked Image]

You said ask questions so i tried to ask a stupid one for my first, did I do good?
[Linked Image]
Posted By: iwire Re: Transformer Installations - 09/09/05 12:36 AM
When I see you ask a question like that I usually wait for the other shoe to drop. [Linked Image]

A guy as knowledgeable as yourself will many times have something up his sleeve. [Linked Image]

Bob
Posted By: Roger Re: Transformer Installations - 09/09/05 12:40 AM
Welcome to the forum Mike. [Linked Image]

Roger
Posted By: jw electric Re: Transformer Installations - 09/09/05 12:44 AM
Thank you Roger [Linked Image]
Posted By: Ryan_J Re: Transformer Installations - 09/09/05 12:57 AM
My welcomes as well, Mike.

Quote
Perhaps a simple proposal that says the IG that is allowed to run back to XO must be sized per 250.122 for whatever OCP protects the the conductors in the same raceway.

Thats the problem, there is no OCPD protecting the IG and phase conductors on the secondary side, other than the primary OCPD. I wonder if the proposal should use 250.122 based on the primary feeder, or 250.66 based on the derived conductors. I would hate to have to go to 250.122 for the IG and 250.66 for the EGC on the secondary side. Just one more rule to try to remember.
Posted By: macmikeman Re: Transformer Installations - 09/09/05 02:45 AM
I have two questions on this installation. Question#1 is the transformers usually have a line above which no field wiring is allowed, but are not those primary conductors coming in from above?. 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?
Posted By: sierra electrician Re: Transformer Installations - 09/09/05 03:05 AM
I have installed many transformers just like the one shown. I used TA lugs just like that. My question is, when you perform PM and you can not shut down power, your going over for tight and the lug rotates......does it scare the heck out of you? It does me!
To answer myself kinda, I wish the Mfg's of Xfmr's supplied TA lugs with shoulders and Dividers between termination ponits.

Rob
Posted By: Ron Re: Transformer Installations - 09/09/05 03:58 AM
So back to the original discussion.
You've decided that it is a 75 KVA transformer 480D-208Y/120
On the primary side we have a full load current of 90 amps
On the secondary side we have a full load current of 208 amps.

If we are to provide primary and secondary protection, the primary protection can't
exceed 250% [450.3(B)]. I usually choose about 125%. So I pick a 125A trip breaker for primary protection.

Primary conductors are chosen 125% of full load, so if terminations are 75 degree,
3-#2awg copper + 1-#6awg copper Gnd. I like a separate EGC even though you would be installing it in EMT or RGS,

Secondary protection (within 10' of secondary windings) is (calculated at 260A) I choose 300A trip.

I start by choosing secondary conductors as 4-350kCMIL copper + 1-4awg copper Gnd.
Since there is 4 current carrying conductors in the secondary conduit, I check that 80% of 350A (ampacity of 350kCMIL @ 90 deg) is 280A, so since it is not 300A to match the secondary protection, I will change the secondary wire size to 4-400kCMIL copper + 1-4awg copper Gnd. Again 80% of 380A is ok with the OCPD, so it's ok.

From 250.66, I choose a 1/0 awg copper for the n-g bonding jumper and grounding electrode.

Hopefully this gets the conversation going again. [Linked Image]
Posted By: georgestolz Re: Transformer Installations - 09/09/05 04:52 AM
Now, in my toils as a knuckledragger, I believe reading that the tranformer that comprises an SDS must be grounded to building steel and the water pipe.

If I wasn't headed to bed, I'd look it up. Off the top of my head, is that right?

I believe it's found in 250.30(Z)(zz)...
Posted By: SolarPowered Re: Transformer Installations - 09/09/05 05:55 AM
Now that Ron has pointed out the section that covers overload protection of transformers, I see that I was far too conservative in my breaker ratings. [Linked Image] Good job, Ron! (Although, I don't know it those are the right answers, either. But at least they're reasoned from applicable provisions in the code.)

I note that, unless there are high harmonic currents, the neutral on the secondary doesn't count as a "current-carrying conductor", so there are only three CCCs in that conduit. [Linked Image]
Posted By: Ron Re: Transformer Installations - 09/09/05 11:48 AM
I'm used to designing feeders for systems that contain lots of non-linear loads, so I just did it from habit. I couldn't help myself [Linked Image]
Posted By: Ron Re: Transformer Installations - 09/09/05 12:08 PM
Ryan,
The IG conductor is in addition to the required EGC. So I'm not counting on the IG conductor to clear a fault, just its mythical job of reducing noise.
When I'm asked to provide an IG circuit, I provide an IG panel with an IG sized to match the EGC.
Since an IG is a design issue/choice, I don't think the NEC should get involved in the size.
Posted By: Dnkldorf Re: Transformer Installations - 09/09/05 12:30 PM
Why can't the IG be terminated on the Nuetral bar of the service panel again?

250???


Dnk....
Posted By: George Little Re: Transformer Installations - 09/09/05 12:46 PM
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).]
Posted By: Ryan_J Re: Transformer Installations - 09/09/05 01:13 PM
George, I prefer the same method you describe.
Posted By: iwire Re: Transformer Installations - 09/09/05 01:32 PM
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.
Posted By: Ron Re: Transformer Installations - 09/09/05 04:31 PM
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).
Posted By: Ron Re: Transformer Installations - 09/09/05 04:54 PM
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).]
Posted By: Scott35 Re: Transformer Installations - 09/10/05 01:54 AM
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
Posted By: macmikeman Re: Transformer Installations - 09/10/05 02:42 AM
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?
Posted By: iwire Re: Transformer Installations - 09/10/05 11:31 AM
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.
Posted By: electure Re: Transformer Installations - 09/10/05 12:44 PM
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.
Posted By: Scott35 Re: Transformer Installations - 09/10/05 06:51 PM
Quote


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 do the same as Electure, for the X-0 terminal.
When Parallel Feeders are used, then it's time for a 4 position Lug.

What really sucks for terminating to those larger Lugs, is when the terminal studs are sooooooo low and close to the bottom mesh of the Transformer - such as the Transformer we are using for this thread's example Guienna Pig.

Federal Pacific's Transformers have the terminations nice and high, which makes it soooooo much nicer to land large Feeders; whereas the typical GE, MGM and ACMEs are about 4" above the bottom mesh!

Quote


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.


Not sure if we can use the Primary OCPD to protect the Secondary Feeders on this particular Transformer - as it is a 208/120 VAC Secondary - not a "Single Voltage Secondary".
I think this would be in Article 240, maybe something like section 240-3, and about ¾ of the way down through the list of items (Transformer Secondary Conductors' Overcurrent Protection).

Shooting from the hip here, as my code books are "Not Readilly Accessible At This Time"
(read: NEC and CEC books are in the Van; I'm too lazy to go get one at this time...)

Article 450 would only mention Primary or Secondary OCPD to be no more than 125% FLA - with certain exceptions, so there's no need to venture into the "400's" to reference this Secondary Voltage thingee.

Feel free to debunk this and reply with many, many swear words if I am wrong about the OCPD stuff.

Quote


. 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?


Here's something overlooked so far - and a good question too!

Would the Bonding Bushing for the GEC's Conduit need to be bonded with a Conductor equally sized to the GEC, or will a Conductor sized similar to the EGC used with the Secondary Feeders be Okee-Dokee?
Would a "Wedge Screw Locknut" be acceptable, in lieu of a Bonding Bushing?
("Wedge Screw Locknut" = Locknut with a set screw cast into it at an angle, which is "Wedged" against the enclosure when the Locknut is completely set).

Bonding the Conduits for the Feeders is a No-Brainer (as far as sizes go that is!).

Blast away [Linked Image]

Scott35
Posted By: sandsnow Re: Transformer Installations - 09/17/05 06:57 AM
Remember when all xfmrs had the "no conductor zone" down low inside. I'm seeing more and more with the zone extending way high. Here is one:

edit: Posting the pics myself did not work. What it showed was the lugs at the top of the coils and the cable height sticker even with the lugs. A lot more room to work the cables.

[This message has been edited by sandsnow (edited 09-17-2005).]
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