transformer tap

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What do you all think of .......
75 kva trans 480-120/208 pri-100amp
secondary tap inside transformer to two seperate 100 amp panelboards both w/main disconects at 100 amp

tap length is approx 10ft and 15ft to the other

reasoning --- existing electric room, currently there are two seperate trans .
and hardly any more room w/ respect to clearances

is this a code violation(I cant seem to find one

thanks
 

bob

Senior Member
Location
Alabama
Look at 240.92(B)(2)(1...4). According to your post, the 75 kva has a FLA of 208 amps. You may have up to 6 OC devices if the sum of the rating does not exceed the transformer rating. You need to also look at 240.21(C)(2) .. (3) for 10 ft and 15 ft taps..
 

micromind

Senior Member
Considering how many of these I've done form engineered drawings, I can't see any sort of problem with it. Just make sure the two panels are grouped together, you should be fine.

I may get hammered on this one, but I believe that the wire from the xfmr to the panel mains need only be sized to the rating of said breakers. (the tap rule).
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
We have to look at two things here. First we have to look to 450.3 for the protection of the transformer itself and then at 240.21(C) for the protection of the secondary conductors. The full load primary current is ~90 amps. Table 450.3(B) tells us that if the primary OCPD is rated at 125% of less of the rated primary current that no seconday protection is required for the transformer. The use of a 100 amp OCPD on the prirmary meets this rule.
Assuming that this is not an industrial installation the protection of the secondary conductors is covered by 240.21(C)(6). As long as the secondary conductors have an ampacity of at least 100 amps you are in compliance with the code rules. There is no requirement that the panels be located next to or even within sight of each other or the transformer.
Don
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
don_resqcapt19 said:
Assuming that this is not an industrial installation the protection of the secondary conductors is covered by 240.21(C)(6). As long as the secondary conductors have an ampacity of at least 100 amps you are in compliance with the code rules. There is no requirement that the panels be located next to or even within sight of each other or the transformer.
Don
Does that also mean the panels need not have individual 100a mains?
 

ron

Senior Member
Don,
What are your thoughts about the second part of:

240.21(C)(1) .... Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

Since this is a multiphase transformer, I think it needs secondary OCPD's.
 

Mike01

Senior Member
Location
MidWest
Mcb

Mcb

I believe the panels would be required to have mains based on the code section (drawing a blank) that regards lighting and branch circuit appliance panel boards, also rembering that this is a separately derived system the ground conductors are sized based on the are of the conductors table 250.66 not 250.122.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The original post specified 100 amp mains in the panels. My post is based on that installation. Sorry for any confusion.
Don
 
ron said:
Don,
What are your thoughts about the second part of:



Since this is a multiphase transformer, I think it needs secondary OCPD's.


Art 450 deals with the transformer winding. There are times when the protection scheme of transformers may well work for the protection scheme of the conductors related to the transformer.

With that said, I agree with what Ron has posted.
 

bob

Senior Member
Location
Alabama
ron said:
Since this is a multiphase transformer, I think it needs secondary OCPD's.
241.21(C) says that secondary conductors may be installed without OC protection if installed per 241.21(C()(1) thru C(6). Under C(2), 1b does not require OC protection if the conductor rating is not less than the rating of the device supplied by the conductors, in this case a 100 amp panel. This is
contingent that the rest of C(2) is met. Is it the opinion of the group that a main OC is still required?
 
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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
bob said:
Under C(2), 1b does not require OC protection if the conductor rating is not less than the rating of the device supplied by the conductors, in this case a 100 amp panel. This is contingent that the rest of C(2) is met. Is it the opinion of the group that a main OC is still required?
Well, I believe the "device supplied by the conductors" refers to an OCPD, and not a main-lug panel, so I'd say "yes."



Yes.
 
Thanks for all the thoughts. I prefer to have main disc in each panel from design/safety stand point. I belive it was coming down to a personal choice from plan check.... My reasoning was based on space available and existing conditions to tap the secondary in the transformer. He preferred to see 1-225 amp panel board fed w/ 4/0's and then feed the 2nd panel from this one w/ a 100 amp....

He did mention of a past bad experience and may be what led him in this direction

Worst part is it cost about 1 week and another trip to the engineer.

If anyone is still listening where is your preferred location to bond in the transformer or in the panel.... I have always been partial to the panel due to accesibility ....

thanks
 

dahualin

Senior Member
electricjireh said:
Thanks for all the thoughts. I prefer to have main disc in each panel from design/safety stand point. I belive it was coming down to a personal choice from plan check.... My reasoning was based on space available and existing conditions to tap the secondary in the transformer. He preferred to see 1-225 amp panel board fed w/ 4/0's and then feed the 2nd panel from this one w/ a 100 amp....

He did mention of a past bad experience and may be what led him in this direction

Worst part is it cost about 1 week and another trip to the engineer.

If anyone is still listening where is your preferred location to bond in the transformer or in the panel.... I have always been partial to the panel due to accesibility ....

thanks

If I am going to design the system, I will design two feeders from the 75KVA transformer to two panels with 125amp main circuit breaker. The feeder will be 4#1, #6G, 1-1/4"C. I think my design is less expensive than what you mentioned.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Larry,
Well, I believe the "device supplied by the conductors" refers to an OCPD, and not a main-lug panel, so I'd say "yes."
If that is the case why does 240.21(C)(2)(1)(b) say:
b. Not less than the rating of the device supplied by the secondary conductors or not less than the rating of the overcurrent-protective device at the termination of the secondary conductors, and ...
That being said (C)(2) does not apply to this installation as the secondary conductors are over 10' long. Also even after you have met the requirements of 450.3 for the transformer secondary protection and 240.21(C) for the secondary conductor protection, you still have to meet the rules in 408 for the panel protection.
Don
 

danickstr

Senior Member
judging by the variety of interpretations and your time being a week out, I would put a call in to the AHJ and get an OK on what you want.
 

Smart $

Esteemed Member
Location
Ohio
ron said:
Don,
What are your thoughts about the second part of:
240.21(C)(1) .... Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.
Since this is a multiphase transformer, I think it needs secondary OCPD's.
Well it does need secondary OCPD, but not because of 240.21(C)(1)... at least not directly.

240.21(C)(1) is only pertinent if we try to qualify the secondary conductors as protected by the primary OCP. It does not qualify as this is a multiphase xfmr. Hence, the secondary conductors must be protected under one of the other subparagraphs. They may be protected under (2) through (6). However, the requirements for protecting the panelboard must also be met. That takes us to 408.36(B) [2008] or, I believe it's 408.36(D) [2005], which states the protection for a panelboard must be on the secondary side of a xfmr-supplied panelboard. The only exception here is compliance with 240.21(C)(1), where being this is a 3? system, the panelboard cannot be protected by the xfmr's primary OCPD.
 
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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
don_resqcapt19 said:
If that is the case why does 240.21(C)(2)(1)(b) say:
b. Not less than the rating of the device supplied by the secondary conductors or not less than the rating of the overcurrent-protective device at the termination of the secondary conductors, and ...
In that case, I want to change my answer to "no".


No.




I just never thought of a panel, or its lugs, as being a 'device'. I think of a breaker, a switch, a contactor, etc.
 
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