Transformer secondary

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hhsting

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Glen bunie, md, us
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I have the attached installation in which transformer secondary conductors 2 sets of 4#41/0+1#1/0AWG Gnd are in wire trough. The trough is tapped*with: 4#1/0AWG+1#1/0AWG Gnd to P1, 4#1/0AWG+1#1/0AWG Gnd to P2, 4#1/0AWG+1#1/0AWG Gnd to P3.*

The furthest distance from transformer to panel P3 is 15 feet and Panels P1, P2,P3 are grouped each with 150A main circuit breaker.

I am trying to make inspector understand how I got 2 sets of 4/0AWG which is if I take each secondary conductor back to transformer that's each 1/0AWG to transformer and 1/0AWG ampacity is 150A and three of them would be 3x150 = 450A. 2 sets of 4/0AWG is equivalent*to 450A and same as taking conductors back individually.

However the inspector is telling me that it does not comply per NEC 2014 Article 240.21 cannot tap a tap and also that secondary tap conductors are not suppose to extend beyond panel or overcurrent protection served. Inspector is telling me either add secondary*450A breaker between trough and transformer to make them in feeders load side of breaker.

Am I correct in my explanation and don't need to provide anything or inspector is correct and need to add secondary 450A breaker between trough and transformer?

*

12de40220d320c0c4d9ac8f92aff01a5.jpg


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augie47

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Tennessee
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State Electrical Inspector (Retired)
I'd agree with the inspector:
240.21 Conductors supplied under the provisions of 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4.

also known as "you can't tap a tap"
 

charlie b

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Lockport, IL
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My reading of the second sentence in 240.21 tells me the inspector is correct. You used 240.21(C) to create a transformer secondary tap. You can't tap that tap. However, a possible alternative is to put overcurrent devices immediately adjacent to the trough, instead of internal to the panel downstream. I don't know if this would be physically possible for your installation, but at least I think it will comply with that "second sentence" I mention above.
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
or run a set of conductors from the transformer for each panel.
 

victor.cherkashi

Senior Member
Location
NYC, NY
probably daisy chain all panels with 2 sets of 4/0AWG could be solution. in this case it is not tap of tap because you don't reduce conductor size. am I right? can you splice tap conductor with same size?

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david luchini

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Connecticut
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probably daisy chain all panels with 2 sets of 4/0AWG could be solution. in this case it is not tap of tap because you don't reduce conductor size. am I right? can you splice tap conductor with same size?

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That would not comply with 240.21 or 240.21(C)
 

hhsting

Senior Member
Location
Glen bunie, md, us
Occupation
Junior plan reviewer
Let say I go for set of 1/0AWG going back to the transformer which would equal to having 3 sets of 1/0AWG going from transformer into trough and then feeding each panel.

So the question now I have more than 3 current carrying conductors (12 conductors) in trough but I am confused since one set would feed first panel leaving 8 conductors to feed in trough until next panel etc. What article to use in this case and do I need to do amapcity correction?
 

david luchini

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Location
Connecticut
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Let say I go for set of 1/0AWG going back to the transformer which would equal to having 3 sets of 1/0AWG going from transformer into trough and then feeding each panel.

So the question now I have more than 3 current carrying conductors (12 conductors) in trough but I am confused since one set would feed first panel leaving 8 conductors to feed in trough until next panel etc. What article to use in this case and do I need to do amapcity correction?

See 376.22(B). It does not appear that you would need ampacity adjustment in the wireway.
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
See 376.22(B). It does not appear that you would need ampacity adjustment in the wireway.

but you might if there is a conduit between the transformer and panel. If there is one and its over 24" long, you would use 320.15(b)(3).
There are numerous possibilities... as david pointed out, if you ran a wireway all the way to the transformer (or the interconnecting nipple was less than 24" you would not need to worry about derating, or you could run a seperate raceway for each panel to the transformer.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Let say I go for set of 1/0AWG going back to the transformer which would equal to having 3 sets of 1/0AWG going from transformer into trough and then feeding each panel.

So the question now I have more than 3 current carrying conductors (12 conductors) in trough but I am confused since one set would feed first panel leaving 8 conductors to feed in trough until next panel etc. What article to use in this case and do I need to do amapcity correction?

As mentioned, the wireway will likely not need derating. However, if the raceway from the wireway to the xformer is more than 24" long you will have to derate. See 310.15(B)(3)

Edit to add...Sorry, I see augie just posted the same info.
 

hhsting

Senior Member
Location
Glen bunie, md, us
Occupation
Junior plan reviewer
but you might if there is a conduit between the transformer and panel. If there is one and its over 24" long, you would use 320.15(b)(3).
There are numerous possibilities... as david pointed out, if you ran a wireway all the way to the transformer (or the interconnecting nipple was less than 24" you would not need to worry about derating, or you could run a seperate raceway for each panel to the transformer.
I am assuming you mean NEC 310.15(b)(3) because I dont see 320.15(b)(3).

310.15(b)(3) says for length longer than 24 inch and are not installed in raceways. So they can be shorter than 24 inch and not install in raceway to not to use 310.25(b)(3). No?

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hhsting

Senior Member
Location
Glen bunie, md, us
Occupation
Junior plan reviewer
I am assuming you mean NEC 310.15(b)(3) because I dont see 320.15(b)(3).

310.15(b)(3) says for length longer than 24 inch and are not installed in raceways. So they can be shorter than 24 inch and not install in raceway to not to use 310.25(b)(3). No?

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Its confusing first paragraph of 310.15(b)(3)(a) says length longer than 24 inch and not installed in raceway but then you read on 310.15(b)(3)(a)(2) adjustment factors shall not apply to conductors in raceway not exceeding 24 in. Not sure so doesnt matter install in raceway or not?

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david luchini

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Connecticut
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Its confusing first paragraph of 310.15(b)(3)(a) says length longer than 24 inch and not installed in raceway but then you read on 310.15(B)(3)(a)(2) adjustment factors shall not apply to conductors in raceway not exceeding 24 in. Not sure so doesnt matter install in raceway or not?

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310.15(B)(3)(a) says where the number of current carrying conductors in a raceway exceeds three, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a)...Adjustment Factors.


310.15(B)(3)(a)(2) says that adjustment factors shall not apply to conductors in raceway having a length not exceeding 24in.
 
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