tapping a tap ?

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timz

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We have a 480 volt single phase primary to 240/120 volt single phase secondary,100kva transformer. 417 secondary amps. Off the secondary we are bringing 500 kcmil into a 10x10x6 ft wireway. In the wireway the 500 kcmil is terminating into a power block. From the power block 4/0 will be brought down into 3 individual 200 amp single phase 240 volt disconnects. My question is, are we tapping a tap by coming off the power block in this way. All lenghts of the tap conductors are within 10 ft.
 
what is the total lenght of the 500 + 4/0's??.


WHat is the lenght of the Trought??

Charlie
 
Well, your first violation is that your secondary conductors are too long.

Aside from that, I see no problems. You can do what you are doing as long as you meet 240.21(C)(6)(1), which I am assuming you will. What is the primary overcurrent device rating?
 
I don't think that is allowed. See the first paragraph of 240.21, and for example, the #5 of 240.31(B) (3) (5)
 
The 4/0 conductors have to run direct from XFMR to line side of fusible switch. Then you can comply with either 240.21(C) 2 or 6

The problem comes in having adequate space in the XFMR to do that. If you don't then the 500kcmil would have to terminate in a single overcurrent device or a MLO panelboard. In the MLO panelboard install two 200amp C/B's.

The 500kcmil conductors in your example do not meet the conditions of any of the rules in 240.21(C). The closest one is 240.21(C)(2). That requires the conductors to be in a raceway which shall extend to the enclosure of a switchboard, panelboard, control device or open switchboard. Your 500kcmil do not do that. they end in a wireway.

Your 4/0 conductors in your example do not comply with any of the feeder taps in 240.21(B) since there is no OCPD ahead of them.
 
I have to ask

if the total conductor lenght , from trans sec to the OCP in panels is 10 feet or less why not change the sec lugs and run all 4/0 's????


Charlie
 
ryan_618 said:
What section of 240.21 is being violated? If the 4/0 conductors are protected, the 500's certainly are!
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ryan_618 said:
None of what is described is a tap.
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I disagree.
240.2 Defintions

Tap Conductors. As used in this article, a tap conductor is defined as a conductor, other than a service conductor, that has overcurrent ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4​

But we'll put this on the back burner for now as there is a bigger problem with the installation as described...

First there is no protection for the secondary conductors as far as the NEC is concerned...

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

The secondary described hereinabove is not a single-phase 2-wire secondary—"240/120 volt single phase secondary". That is, unless the OP'er clarifies the installation as not using the center tap on the secondary winding.

The combined effectiveness of the first encountered devices do not qualify as protection for the secondary 500kcmil conductors either, as 3 x 200A = 600A. Therefore, the 500kcmil secondary conductors are not protected to NEC requirements.
 
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ryan_618 said:
None of what is described is a tap. They are transformer secondary conductors. If you can keep them under 25' you will be legal.
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That's interesting. I've always called "secondary conductors" taps since they don't have overcurrent protection at the supply. I am assuming you are calling conductors installed per 240.20(B) 1 through 5 "tap conductors". Then anything installed per 240.20(C) 1 through 6 you are refering to as "transformer secondary conductors".

I'll buy that. Looking at the items in (C), none actually say "tap".

But I still don't think what is being described is allowed because of the first paragraph of 240.21:

No conductor supplied under the provisions of 240.21(A) through (G) shall supply another conductor under those provisions, except through an overcurrent protective device meeting the requirements of 240.4

The 500's supply the 4/0 through a terminal block, not a OCP device. And surely the 500's and 4/0 aren't considered the same conductor.

Steve
 
timz said:
I always thought that any conductors off the secondary of a xfmr is considered a tap?
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Yes they are taps... but they are transformer taps, and distinguished as such from feeder taps, busway taps, motor circuit taps, service-entrance conductor taps, ...did I leave any out?
 
I would have to say that the title of this thread has the right idea they just used the wrong words.
I do believe that the secondary conductors have been tapped.

As outlined in the original poster these conductors are;
timz said:
The trough is 10x10x 6 ft and the total lenght of the 500's and the 4/0' are 20 ft
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Therefore section 240.21(C)(6) would apply
(6) Secondary Conductors Not Over 7.5 m (25 ft) Long. Where the length of secondary conductor does not exceed 7.5 m (25 ft) and complies with all of the following:
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(1) The secondary conductors shall have an ampacity that is not less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent device protecting the primary of the transformer.
(2) The secondary conductors terminate in a single circuit breaker or set of fuses that limit the load current to not more than the conductor ampacity that is permitted by 310.15.
(3) The secondary conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.
Click to expand...

The original poster is using a 200 amp on the primary but just for the fun I will comply with 450.3(B) and install a 300 amp fuse on the primary. This will make the secondary conductor bigger.
(1) 300 divided by 3 is 100, the ratio is a 2 to 1 so 100 times 2 is 200 or the smallest secondary conductor would have to be sized at no less than 200 amps.
(2) These conductors are required to land on single disconnect
(3) These conductors must be protected.

The original poster has a ?Hot Trough? type of set up like is used on a service and I don?t see this as being allowed on the secondary side of a transformer.
 
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