2000KVA Transformer secondary taps

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rjc3po

Member
Hello,
I am confused about the rules for secondary feeder taps on a transformer.
Basically, I have a 4160/480, Delta/Wye, 2000KVA transformer. I would like to feed 2x480VAC MCC line ups (1x1600amp bus and 1x1200 amp bus).
I was planning on placing the transformer directly outside an existing building adjacent to the MCC line ups (the MCCs are within the building). This however may be changing for other reasons.

My question is:
If I plan to connect the feeds for both MCCs "inside" the transformer secondary enclosure itself and run the two separate feeds directly to the MCCs in there own conduits, are these still considered "taps". If not are there length limits or cable size issues that I need to consider?

I suspect that the new transformer location may be outside of the 25ft length limit specified in 240.21. Other than putting a low voltage switchgear line up in between the transformer and the MCCs (1 feed from the transformer to the LV switchgear with 2 sub feeds to the LV MCCs) what other options do I have?

Thanks
Robert
 

zog

Senior Member
Location
Charlotte, NC
Hello,
I am confused about the rules for secondary feeder taps on a transformer.
Basically, I have a 4160/480, Delta/Wye, 2000KVA transformer. I would like to feed 2x480VAC MCC line ups (1x1600amp bus and 1x1200 amp bus).
I was planning on placing the transformer directly outside an existing building adjacent to the MCC line ups (the MCCs are within the building). This however may be changing for other reasons.

My question is:
If I plan to connect the feeds for both MCCs "inside" the transformer secondary enclosure itself and run the two separate feeds directly to the MCCs in there own conduits, are these still considered "taps". If not are there length limits or cable size issues that I need to consider?

I suspect that the new transformer location may be outside of the 25ft length limit specified in 240.21. Other than putting a low voltage switchgear line up in between the transformer and the MCCs (1 feed from the transformer to the LV switchgear with 2 sub feeds to the LV MCCs) what other options do I have?

Thanks
Robert
Keep in mind you will have some serious arc flash issues on your MCC's without a main breaker.
 

rjc3po

Member
The MCCs do have main breakers housed within that MCC itself.
But I do not have a breaker at the transformer itself.
Would you not always have that same type of arc flash issue with any outside transformer installation? I mean you will always have you primary and secondary protection at some other location...
 
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iMuse97

Senior Member
Location
Chicagoland
c3po: I think ZOG was just pointing out that you probably need to design for a main breaker (or two) before the MCCs, since the arc flash issues will be a big limiting factor on who is even allowed into the MCC room, much less the massive amounts of PPE they would have to put on. It's just good design for many reasons to have that main, and allows you to have MCC breakers with a more reasonable interrupting rating, and less chance for catastrophic failure that destroys the whole MCC.
 

zog

Senior Member
Location
Charlotte, NC
c3po: I think ZOG was just pointing out that you probably need to design for a main breaker (or two) before the MCCs, since the arc flash issues will be a big limiting factor on who is even allowed into the MCC room, much less the massive amounts of PPE they would have to put on. It's just good design for many reasons to have that main, and allows you to have MCC breakers with a more reasonable interrupting rating, and less chance for catastrophic failure that destroys the whole MCC.
Thats exactly was I was saying. There is an IEEE white paper on the issue, to big to post but I will email it to you if you want.
 

iMuse97

Senior Member
Location
Chicagoland
As to your main question: I think you have already found the option--installing a main breaker. I've looked without success for another.
 

augie47

Moderator
Staff member
Location
Tennessee
If the MCC has a withstand rating equal or greater than the transformer AIC, and if his secondaries meet the requirements of 240.21 (C)(4), then I see no violation. I would not argue the fact that an upstream overcurrent device, especially one that is current limiting would not be a good idea, but I can't see the Code requirement as long as the installation meets tap rules and the MCC is adequately rated.
 

iMuse97

Senior Member
Location
Chicagoland
Agreed, Gus. The OP mentioned he may have to exceed this 25' limit, if b/c of other considerations; in that case it seems the main OCPD would be required, and would assist other design issues related to Arc Flash also. Again I see your point, and reviewed 240.21 to re-learn myself on the '08 Code b/c the customer may be hesitant to pay for something that they already paid for in the design of the MCC to have an appropriate withstand rating. And to C3PO: Welcome to the Forum! :)
 
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rjc3po

Member
I think that it all comes down to what Augie47 mentioned 240.21(c)(4)
Outside secondary conductors.

First we must see if this applies and 240(C) tells me that multiple sets of secondary’s are ok when it says “or each set of conductors feeding separate loads”

240.21(C) Transformer Secondary Conductors. A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without over-current protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.
Now on to 240.21(C)(4)

240.21(C)(4) Outside Secondary Conductors.
Where the conductors are located outdoors of a building or structure, except at the point of load termination, and comply with all of the following conditions:
(1) The conductors are protected from physical damage in an approved manner.
(2) The conductors terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors. This single over-current device shall be permitted to supply any number of additional over-current devices on its load side.
(3) The over-current device for the conductors is an integral part of a disconnecting means or shall be located immediately adjacent thereto.
(4) The disconnecting means for the conductors is installed at a readily accessible location complying with one of the following:
a. Outside of a building or structure
b. Inside, nearest the point of entrance of the conductors
c. Where installed in accordance with 230.6, nearest the point of entrance of the conductors



1. I am running both sets of secondary conductors in conduit so... that's ok

2. My MCC main breaker (adjustable trip) is rated for the bus rating and my secondary conductors are of slightly higher ampacity than the main breaker in the MCC...so... that too would be ok.
3. I have circuit breakers in my MCC that count as a disconnecting means... so that's ok too
4. That too should be ok in my application as both MCCs are in the same room and adjacent to each other on the side of the building where the transformer is closest to..


It seems like a home run… And if this is the case.. When does the 25ft rule come in to play for outside transformers? It would seem that there is not a rule concerning the secondary lengths for outside transformers..

Does anyone out there see it differently? Please comment..
 
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rjc3po

Member
Also would it not be possible to specify current limiting fuses ahead of my MCC Circuit breaker (but within the MCC) to deal with the arc flash and AIC issues? This may be a silly question but I just thought I would ask.
 

augie47

Moderator
Staff member
Location
Tennessee
The outside conductor and 25 ft tap rule are two different animals, each with their own set of requirements as with all the tap rules.

You need to assure your equipment has a withstand rating equal to or greater than the available fault current delivered by the transformer.
Since you are supplying motors there would be a "motor contribution" so this determination might involve an enginering study

If the available AIC is greater than your MCC rating then some current limiting device such as C.L. fuses might be necessary. Zog's input in post #3 can not be emphasized enough.

My first step would be to obtain the withstand rating of the MCC and the AIC of the transformer to give you a rough idea if there is a potential problem.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Engineer
The transformer does not have an AIC rating. It has available Short Circuit Amps (SCA).

The withstand rating of the MCC is only one component to be considered. The Amps Interrupting Capacity (AIC) of the existing MCC main breaker must also be investigated.

Current limiting fuses can not be used to reduce the amount of SCA at the MCC unless they have a 'series combination' rating with the MCC main breaker and the protective devices in the starter buckets.

Current limiting fuses do not always reduce arc flash incident energy.

A high level of incident energy at a location does not mean PPE must be worn 'just to be in the room'. PPE is only required when the equipment is being interacted with.

I would recommend separately mounted main devices.
 

iMuse97

Senior Member
Location
Chicagoland
I also thank you all for your comments.

I've entered this discussion to keep the thread going and learn something, and keep the OP going.

Having worked on a few of these things (installation, PM and such), I've kept my eyes open as to design,
and have yet to see one without the separately mounted main, as you mentioned, Jim.

Again, thanks to the OP (rjc3po) and to all have and/or will comment.
 

rjc3po

Member
When you guys refer to a seperatly mounted main.. You are talking about a stand alone line up of low voltage switchgear and not just the incomming vertical section containg the MCC disconnect... Is that correct?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Engineer
When you guys refer to a seperatly mounted main.. You are talking about a stand alone line up of low voltage switchgear and not just the incomming vertical section containg the MCC disconnect... Is that correct?
I mean a main device that is barriered enough to prevent an arc on the cross bus, in a vertical section, from propagating into the main device and flashing across its line and load connections. Some manufacturers are starting to build equipment this way, but in general it requires a stand alone structure.
 

augie47

Moderator
Staff member
Location
Tennessee
and Jim, thanks for your comments over mine. You have tried to teach me the correct terminology and I appreciate it. I don't ignore you, it's just hard to teach an old dog new tricks :grin:, and lacking your learned input, I did want to make sure the OP addressed the AIC issue.

I have one question. You mentioned AIC of the MCC main. If the MCC has a withstand rating, would the main not automatically be within that parameter ?
 
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