Transformer secondary conductors

[ggunn]

Because each set of secondary conductors would then individually comply with 240.21(C). [Note: I've done zero installations with transformers. My comments are based on carefully integrating 15+ years of what I've read here with what I read in the NEC directly.]

So, in your opinion, could we size these secondary conductors for the 200A OCP devices? If so, I don't see what the difference would be, electrically, from the way I have it drawn, if I just removed the 600A conductors between the transformer and a tap feeding the three 200A discos.

 

[wwhitney]

So, in your opinion, could we size these secondary conductors for the 200A OCP devices? If so, I don't see what the difference would be, electrically, from the way I have it drawn, if I just removed the 600A conductors between the transformer and a tap feeding the three 200A discos.

Yes, if you run 3 sets of 200A secondary conductors from the transformer secondary terminals, each of which individually complies with 240.21(C), then that's copacetic.

BTW, this install has gotten involved enough, between the two threads, that I suggest posting a complete single line diagram for more eyes on the design.

Cheers, Wayne

 

[jim dungar]

So, in your opinion, could we size these secondary conductors for the 200A OCP devices? If so, I don't see what the difference would be, electrically, from the way I have it drawn, if I just removed the 600A conductors between the transformer and a tap feeding the three 200A discos.

Use the 240.2 definition of a tap.

Your transformer secondary single set of conductors needs to be 600A rated and run to the line side of each 200A switch.
Or
Your three sets of secondary conductors must be rated at 200A and run directly from the transformer terminals to the line side of each switch.

In either case you need to follow the rules of 240.21(C).

 

[ggunn]

Yes, if you run 3 sets of 200A secondary conductors from the transformer secondary terminals, each of which individually complies with 240.21(C), then that's copacetic.

BTW, this install has gotten involved enough, between the two threads, that I suggest posting a complete single line diagram for more eyes on the design.

Cheers, Wayne

Here it is.

 

[wwhitney]

Let's see if this inline image is sufficiently legible, as I expect it will get more view. Cheers, Wayne

 

[jim dungar]

That new sketch will not comply with 240.21(C). You cannot tap a tap.
The 250kCMIL must be brought to each switch, or the #3/0 from each switch must be brought back to the transformer terminals.

In note 1, 240.21(B)(1) is not applicable as these are considered transformer secondary conductors not feeders.
The transformer has a 4 wire output so no part of 240.21(B) would apply to its secondary side conductors.

 

[ggunn]

In note 1, 240.21(B)(1) is not applicable as these are considered transformer secondary conductors not feeders.

Looking at the definition of a feeder in article 100 I do not see where a transformer (the source of a separately derived system) secondary conductors are not feeders.

 

[ggunn]

...or the #3/0 from each switch must be brought back to the transformer terminals.

If I do that would the tap rules in 240.21(B) apply to the #3/0 conductors (ampacity, conductor length, etc.)? I still do not see the electrical difference between what you are suggesting and connecting them to the other end of two sets of 250kcmil.

Also, I didn't see that attaching three sets of conductors to the end of two sets of 250kcmil secondary conductors would be tapping a tap. I see this kind of thing done with service conductors all the time, although I do realize that the tap rules don't apply to service conductors. Be that as it may, I have often heard that comment about not tapping a tap but I have never seen that in the NEC; can you point me to it?

 

[don_resqcapt19]

If I do that would the tap rules in 240.21(B) apply to the #3/0 conductors (ampacity, conductor length, etc.)? I still do not see the electrical difference between what you are suggesting and connecting them to the other end of two sets of 250kcmil.

Also, I didn't see that attaching three sets of conductors to the end of two sets of 250kcmil secondary conductors would be tapping a tap. I see this kind of thing done with service conductors all the time, although I do realize that the tap rules don't apply to service conductors. Be that as it may, I have often heard that comment about not tapping a tap but I have never seen that in the NEC; can you point me to it?

240.21 Location in Circuit.
Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 240.21(A) through (H). Conductors supplied under 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4.

 

[don_resqcapt19]

Looking at the definition of a feeder in article 100 I do not see where a transformer (the source of a separately derived system) secondary conductors are not feeders.

They are a special case of feeder conductors and as such have their own set of rules in 240.21(C) Transformer Secondary Conductors.

 

[jim dungar]

If I do that would the tap rules in 240.21(B) apply to the #3/0 conductors (ampacity, conductor length, etc.)? I still do not see the electrical difference between what you are suggesting and connecting them to the other end of two sets of 250kcmil.

Also, I didn't see that attaching three sets of conductors to the end of two sets of 250kcmil secondary conductors would be tapping a tap. I see this kind of thing done with service conductors all the time, although I do realize that the tap rules don't apply to service conductors. Be that as it may, I have often heard that comment about not tapping a tap but I have never seen that in the NEC; can you point me to it?

Your parallel 250kCMIL are rated for 600A. Your 3/0 are rated for 200A. Connecting them together is a tap per 240.2.

 

[wwhitney]

If I do that would the tap rules in 240.21(B)

It would be 240.21(C).

Have you read the first paragraph of 240.21(C)? It lays out that 240.21(B) is not allowed for transformer secondary conductors, and that you are allowed multiple sets of secondary conductors, each of which much comply with 240.21(C).

One thing I do find lacking in 240.21 is an explicit statement to the effect that the transformer secondary counts as a (new) source of supply.

Cheers, Wayne

 

[ggunn]

Up front: Thank you guys for helping me out with this and thanks for your patience. I have been designing PV systems for years but this is a bit of an adventure outside my comfort zone.

Your parallel 250kCMIL are rated for 600A. Your 3/0 are rated for 200A. Connecting them together is a tap per 240.2.

I asked in the other thread about the 480/277V tap regarding the smaller conductor sets going to the disconnect and the panel board - which is the feeder and which is the tap - and I got the answer that they were both taps. Fair enough, but if I can attach two sets of conductors to a tap, then why not three? Why is two sets of smaller conductors connected to the end of a feeder not tapping a tap but three sets is?

 

[wwhitney]

I asked in the other thread about the 480/277V tap regarding the smaller conductor sets going to the disconnect and the panel board - which is the feeder and which is the tap - and I got the answer that they were both taps. Fair enough, but if I can attach two sets of conductors to a tap, then why not three? Why is two sets of smaller conductors connected to the end of a feeder not tapping a tap but three sets is?

Here's a textual version of your one line, with labels on each line of the various terms in use. Each line of text is a cross section with one item or several similar parallel items, and there is connectivity from each line of text to the next one.

Utility
400A conductors (Service Conductors)
400A OCPD
400A conductors (Feeder, not a tap)
(100A, 300A) conductors (Feeder taps)
(100A, 300A) disconnects
300A conductors (Feeder, not a tap, omitting the 100A branch henceforth)
Transformer primary
Transformer secondary
600A conductors (Feeder, secondary conductors, no OCPD at source of supply)
600A OCPD (not shown, but required per 240.21(C))
(200A, 200A, 200A) conductors (Feeder taps)
(200A, 200A, 200A) disconnects

The point is that the 600A conductors connected to the transformer secondary do not have OCPD at their point of supply (which is the transformer secondary), and thus do not comply with the first sentence of 240.21. Therefore they may not supply another conductor except through OCPD, and must meet one of the subsections of 240.21, in this case 240.21(C). Thus you need the 600A OCPD I listed.

If you change it to:

. . .
Transformer secondary
(200A, 200A, 200A) conductors (Feeders, secondary conductors, no OCPD at source of supply)
(200A, 200A, 200A) disconnects

Then that complies with 240.21(C), and you don't need the 600A OCPD.

Cheers, Wayne

 

[jim dungar]

I asked in the other thread about the 480/277V tap regarding the smaller conductor sets going to the disconnect and the panel board - which is the feeder and which is the tap - and I got the answer that they were both taps. Fair enough, but if I can attach two sets of conductors to a tap, then why not three? Why is two sets of smaller conductors connected to the end of a feeder not tapping a tap but three sets is?

Follow the definition of a tap in 240.2.

You can connect as many conductors as you want in any combination of series or parallel and you will not have a tap as long as they are all protected by an appropriatelyy sized OCPD at the beginning of the run.

In this case you have (2) 250kCMIL connected in parallel, for all intents this becomes a single conductor. You also have (3) 3/0 which are not connected in parallel.
Connecting 200A conductors to 600A conductors is a tap per 240.2.

The 600A conductor originates at a transformer secondary so it is subject to 240.21(C) regardless if you call it a feeder or not.

 

[don_resqcapt19]

Up front: Thank you guys for helping me out with this and thanks for your patience. I have been designing PV systems for years but this is a bit of an adventure outside my comfort zone.



I asked in the other thread about the 480/277V tap regarding the smaller conductor sets going to the disconnect and the panel board - which is the feeder and which is the tap - and I got the answer that they were both taps. Fair enough, but if I can attach two sets of conductors to a tap, then why not three? Why is two sets of smaller conductors connected to the end of a feeder not tapping a tap but three sets is?

Even one set is a tap if the connected conductors do not have an ampacity at least equal to the upstream OCPD.

 

[ggunn]

So, this works?

Even one set is a tap if the connected conductors do not have an ampacity at least equal to the upstream OCPD.

Hence my question; why is the 480V tap OK? 400A conductors come in to the gutter, 300A and 100A conductors go out. If what you are saying is accurate, it seems to me that I would have to run the 400A conductors all the way to the disco as a continuation of the feeder.

 

[ggunn]

My question about the 480V tap notwithstanding, would this be compliant?

 

[don_resqcapt19]

So, this works?

Hence my question; why is the 480V tap OK? 400A conductors come in to the gutter, 300A and 100A conductors go out. If what you are saying is accurate, it seems to me that I would have to run the 400A conductors all the way to the disco as a continuation of the feeder.

Because the 400 amp conductors are protected at their ampacity. Those are feeder conductors and the taps start where you connect the 100A and 300A circuits. The conductors on the secondary of the transformer are not protected at their ampacity as there is no OCPD. They must terminate at an OCPD. So for your project, the easiest code compliant method is to run three sets of transformer secondary conductors.

 

[ggunn]

Because the 400 amp conductors are protected at their ampacity. Those are feeder conductors and the taps start where you connect the 100A and 300A circuits. The conductors on the secondary of the transformer are not protected at their ampacity as there is no OCPD. They must terminate at an OCPD. So for your project, the easiest code compliant method is to run three sets of transformer secondary conductors.

Got it. Thanks for your patience; I'm asking these questions because the what is easier to remember if I understand the why.

Could I have connected three sets of lower ampacity conductors instead of two to the end of the feeder in the 480V gutter without "tapping a tap"?