240.C6, NEC 2011, TX low side unprotected conductors

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rayp1

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Location
Phoenix, Az, USA
Situation: Electrical room has switchboard with breakers feeding 25 dry type transformers. TX Sizes are 225kVA and smaller. The TX secondary feeds vary in length from 200 to 700 feet feeding electrical panels. Conductors will be installed in certified equipment grounding conductor cable tray. The electrical room is space limited. The option of placing a breaker at each transformer is now deferred to a switch board containing all 25 low side transformer breakers. This extends the unprotected secondary conductors beyond the 25 feet up to max of 50 feet for some of the secondary conductors to the proposed switchboard containing the breakers for the secondary runs to remote panels. The project is non-public access with 24/7 maintenance staff.

My question is: What if we don't install this switchboard containing low side transformer breakers. What are the risk?
 

augie47

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State Electrical Inspector (Retired)
Welcome to the Forum.

Don't exactly understand how you plan to terminate all secondaries in one switchboard, but more directly addressing your question, take a look at 240.92 for supervised industrial applications to see if that helps.
 

david

Senior Member
Location
Pennsylvania
The TX secondary feeds vary in length from 200 to 700 feet feeding electrical panels. The option of placing a breaker at each transformer is now deferred to a switch board containing all 25 low side transformer breakers. This extends the unprotected secondary conductors beyond the 25 feet up to max of 50 feet for some of the secondary conductors

My question is: What if we don't install this switchboard containing low side transformer breakers. What are the risk?

You state you have extended the tap rule from a max of 25 ft. to 50 ft. already, and you are asking what could happen if you eliminate the secondary over current protection at 50 ft. and extend that to 200 ft and some at 700 feet before secondary over current protection.

Am I clear that is what you are asking?
 

rayp1

Member
Location
Phoenix, Az, USA
Situation: Electrical room has switchboard with breakers feeding 25 dry type transformers. TX Sizes are 225kVA and smaller. The TX secondary feeds vary in length from 200 to 700 feet feeding electrical panels. Conductors will be installed in certified equipment grounding conductor cable tray. The electrical room is space limited. The option of placing a breaker at each transformer is now deferred to a switch board containing all 25 low side transformer breakers. This extends the unprotected secondary conductors beyond the 25 feet up to max of 50 feet for some of the secondary conductors to the proposed switchboard containing the breakers for the secondary runs to remote panels. The project is non-public access with 24/7 maintenance staff.

My question is: What if we don't install this switchboard containing low side transformer breakers. What are the risk?

Thank you.
 

rayp1

Member
Location
Phoenix, Az, USA
The dry type transformers are installed in basically two areas. One area is tight with 300kVA size and other less size tx. The 300kVA area needs to provide more accessible work area so some kind of properly supported stacking maybe necessary, but the NEC limits this to 50kVA size tx.

Meeting the criteria of NEC 240.92 as recommended by others on this Forum will take care of the secondary runs up to 100 feet.
 

don_resqcapt19

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Illinois
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retired electrician
If the transformers are not stacked, then there should be room directly above the transformer to install the required secondary conductor OCPD.

As far as the NEC limiting the stacking of transformers to those rated 50 kVA or less, I am not aware of that rule. Can you cite the code section.
 

rayp1

Member
Location
Phoenix, Az, USA
If the transformers are not stacked, then there should be room directly above the transformer to install the required secondary conductor OCPD.

As far as the NEC limiting the stacking of transformers to those rated 50 kVA or less, I am not aware of that rule. Can you cite the code section.


Remember handbook comment under 450.13(B)
 

don_resqcapt19

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retired electrician
Remember handbook comment under 450.13(B)
I thought you were in a room, not a hollow space. The rule in 450.13(B) applies to locations like above a drop ceiling.
Also, handbook comments are not code. They are the opinions of the paid staff of the NFPA. They are not written or reviewed by the members of the Code Making Panels.
 

charlie b

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Lockport, IL
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I do not understand the question. Here is what I think I am reading:

  • There is a switchboard, call it ?SWBD A,? and it has 25 feeder breakers.
  • Each feeder breaker provides power to a dry-type transformer. Thus, you have 25 transformers.
  • You plan to run the secondary conductors from each of the 25 transformers to a second switchboard. Call it ?SWBD B.?
  • You will have 25 feeder breakers on SWBD B, each of which serves a branch circuit panel located 200 to 700 feet away.

Is that much right? If so, it does not make sense. SWBD B cannot have 25 separate incoming feeds. Perhaps you mean that the second ?switchboard? is really just an enclosure that has the ability to mount 25 individual breakers. Each of these breakers gets a feed from one of the transformers on its line side, and provides the feed to a single panel on its load side. Is that what you are trying to describe? I have never heard of a manufacturer creating such an enclosure.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Situation: Electrical room has switchboard with breakers feeding 25 dry type transformers. TX Sizes are 225kVA and smaller. The TX secondary feeds vary in length from 200 to 700 feet feeding electrical panels. Conductors will be installed in certified equipment grounding conductor cable tray. The electrical room is space limited. The option of placing a breaker at each transformer is now deferred to a switch board containing all 25 low side transformer breakers. This extends the unprotected secondary conductors beyond the 25 feet up to max of 50 feet for some of the secondary conductors to the proposed switchboard containing the breakers for the secondary runs to remote panels. The project is non-public access with 24/7 maintenance staff.

My question is: What if we don't install this switchboard containing low side transformer breakers. What are the risk?

you do not get to just extend the taps beyond the allowed distances. it is just not permitted.

a 225KVA 3 phase transformer will be 270 FLA if 480V and 625FLA if 208V. That is not small conductors or small OCPD. They will take some room.

I cringe when I hear terms like certified equipment grounding conductor cable tray. I don't know what it means or how it would be relevant to the questions being asked.

Why not just move some of the transformers downstream somewhere to get some more room?
 
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Smart $

Esteemed Member
Location
Ohio
...

I cringe when I hear terms like certified equipment grounding conductor cable tray. I don't know what it means.

...
Cable tray identified as a sufficient EGC for the conductors it contains... with limitations per Article 392. As far as the thread topic is concerned, it doesn't mean much other than the secondary conductors are not considered outside the building (i.e. outside secondary taps).
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Cable tray identified as a sufficient EGC for the conductors it contains... with limitations per Article 392. As far as the thread topic is concerned, it doesn't mean much other than the secondary conductors are not considered outside the building (i.e. outside secondary taps).

is this term found in the NEC? I do not recall the word certified is used in this way in the code. a quick word search in the 2005 edition did not show the word certified used in other than the name of a company and in describing inspector training.
 
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Smart $

Esteemed Member
Location
Ohio
is this term found in the NEC? I do not recall the word certified is used in this way in the code. a quick word search in the 2005 edition did not show the word certified used in other than the name of a company and in describing inspector training.
Term is not used in the NEC. If certified, probably under another standard, perhaps ANSI, NECA, NEMA. I'm not aware of any UL standard and haven't looked to discern. AFAIK, cable tray is not required to be listed or labeled under the NEC. As I recall reading somewhere, I'm thinking a NEMA publication, where tray is suitable for use as an equipment grounding conductor (without the addition of bonding jumpers), it has to be labeled as part of the NEMA standard's criteria... but don't quote me on that :happyno:
 

GoldDigger

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Placerville, CA, USA
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Retired PV System Designer
Term is not used in the NEC. If certified, probably under another standard, perhaps ANSI, NECA, NEMA. I'm not aware of any UL standard and haven't looked to discern. AFAIK, cable tray is not required to be listed or labeled under the NEC. As I recall reading somewhere, I'm thinking a NEMA publication, where tray is suitable for use as an equipment grounding conductor (without the addition of bonding jumpers), it has to be labeled as part of the NEMA standard's criteria... but don't quote me on that :happyno:
Maybe another case of the infamous "identified as suitable for" with no guidance on who gets to make that identification?
 

Smart $

Esteemed Member
Location
Ohio
Maybe another case of the infamous "identified as suitable for" with no guidance on who gets to make that identification?
TTBOMK, identified for any particular use is typically made by the manufacturer... ultimate judgement of suitability falls on the AHJ, perhaps a PE as an intermediary.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I was in a new plant being built in Iowa Tuesday. For some reason they had installed bonding jumpers around all the joints on the steel cable tray. Given the joints were all bolted together, it seemed excessive and pointless to me. I have never seen cable tray with so many bonding jumpers before. Usually I see them on both ends or where two cable trays cross, but never at every joint.

They also ran a ground wire inside the cable tray that was bonded to the cable tray. Someone who was really worried about grounding I guess.

For the most part the largest load is being fed by a 400A CB so I don't think they ran into any limit to the maximum EGC size the cable tray is rated for.
 
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Smart $

Esteemed Member
Location
Ohio
I was in a new plant being built in Iowa Tuesday. For some reason they had installed bonding jumpers around all the joints on the steel cable tray. Given the joints were all bolted together, it seemed excessive and pointless to me. I have never seen cable tray with so many bonding jumpers before. Usually I see them on both ends or where two cable trays cross, but never at every joint.

They also ran a ground wire inside the cable tray that was bonded to the cable tray. Someone who was really worried about grounding I guess.

For the most part the largest load is being fed by a 400A CB so I don't think they ran into any limit to the maximum EGC size the cable tray is rated for.
Yeah, I've seen that too. I just think what a waste of money... and perhaps brain matter :happyyes:
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
Meeting the criteria of NEC 240.92 as recommended by others on this Forum will take care of the secondary runs up to 100 feet.
:thumbsup:Increasing the size of the secondary conductors to qualify under .92 as protected by the primary side OCPD may well be the most economical way to proceed. The wire cost for that many runs will be high, but if there is space for the larger wiring it will solve a lot of other space problems.
 

iwire

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Staff member
Location
Massachusetts
Maybe another case of the infamous "identified as suitable for" with no guidance on who gets to make that identification?

For what it is worth


Identified (as applied to equipment). Recognizable as suitable for the specific purpose, function, use, environment, application, and so forth, where described in a particular Code requirement.

FPN: Some examples of ways to determine suitability of equipment for a specific purpose, environment, or application include investigations by a qualified testing laboratory (listing and labeling), an inspection agency, or other organizations concerned with product evaluation.
 
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