Sizing secondary conductors.

[david luchini]

They may be feeders, they may be secondary conductors but they are still taps.

The reason this becomes important is when someone wants to tap these taps.

I think there is a common misconception that the code says you "cannot tap a tap." It doesn't say that anywhere.

What it does say is that conductors supplied under the provisions of 240.21(A) through (H) shall not supply any other conductor except through an OCPD. This includes not only taps and transformer secondary conductors, but also conductors from generator terminals and battery conductors.

 

[Smart $]

Consider a 175A continuous load supplied by a 75kVA transformer (208V). Applying 215.2 to the OCPD and the downstream feeder, you would need an OCPD that is not less than 219A, so you use a 225A c/b, and you would need a feeder conductor that has a minimum SIZE that has an ampacity not less than 219A before the application of any adjustment/correction factors, so you use 4/0.

Now look at the transformer secondary conductors applying only 240.21(C). You will need the secondary conductors to have an ampacity that is not less than 225A. What is the ampacity of 3/0 THWN at say 55degF (maybe a refrigerated warehouse)...It would be 230A, and meet the requirements of 240.21(C).

So you could have 3/0 from the transformer to breaker, and require 4/0 downstream from the breaker.

I see that... but as I said, 110.14(C) still applies. So you can't use a conductor that has a pre-adjusted, -corrected ampacity less than 225A... and that would still be 4/0@75?C. Adjustment and correction factors only apply to the field wiring method, not in the enclosure where terminations are made.

 

[david luchini]

I see that... but as I said, 110.14(C) still applies.

I don't see where 110.14(C) changes anything. The conductors are selected so as not to exceed the lowest temperature rating of any termination, conductor or device.

 

[Smart $]

I don't see where 110.14(C) changes anything. The conductors are selected so as not to exceed the lowest temperature rating of any termination, conductor or device.

When you determine minimum size under 215.2(A)(1), you use NC+1.25C... before the application of adjustment and correction... and that's because you're sizing for terminations. What happens after the conductors leave the enclosure is where you apply adjustment and correction... but you do no factor continuous by 125% for this determination.

If 215.2(A)(1) doesn't apply to secondary conductors, you still have to determine minimum size for terminations, and 240.21(C) says the ampacity cannot be less than the OCPD rating. That's where 110.14(C) comes into play. As I said before, conditions of use beyond the enclosure are where adjustment and correction are applicable, not within.

The terminations must be able to handle the OCPD rating's worth of current without exceeding the maximum rated termination temperature. In essence, it's minimum sizing, but there is no factoring for continuous loads because the minimum size determination is based on the OCPD rating, not the load.

 

[david luchini]

When you determine minimum size under 215.2(A)(1), you use NC+1.25C... before the application of adjustment and correction... and that's because you're sizing for terminations. What happens after the conductors leave the enclosure is where you apply adjustment and correction... but you do no factor continuous by 125% for this determination.

If 215.2(A)(1) doesn't apply to secondary conductors, you still have to determine minimum size for terminations, and 240.21(C) says the ampacity cannot be less than the OCPD rating. That's where 110.14(C) comes into play. As I said before, conditions of use beyond the enclosure are where adjustment and correction are applicable, not within.

The terminations must be able to handle the OCPD rating's worth of current without exceeding the maximum rated termination temperature. In essence, it's minimum sizing, but there is no factoring for continuous loads because the minimum size determination is based on the OCPD rating, not the load.

I don't follow your logic. The ambient conditions would apply to the terminations as well as the conductors, assuming the enclosure is located in the same ambient conditions. Otherwise you would have to determine the conductor and the terminations separately (ie, you'd have to use a 4/0 termination with the #3/0 with an ampacity of 230,) and I don't see where the Code directs you to do that.

I don't see that the 3/0 (and the 3/0 terminations) with an ampacity of 230 would exceed the 75deg temperature rating with a load of 175A.

But of course, it's all moot, as 215.2 applies to the transformer secondary conductors as well as the feeder on the load side of the OCPD.

 

[Smart $]

I don't follow your logic. The ambient conditions would apply to the terminations as well as the conductors, assuming the enclosure is located in the same ambient conditions.

Well, for the sake of discussion we'll say it does... but you are correct in may not be in the same ambient temperature environment. But I'm going to counter your limitation on the criteria and further limit the conditions to say the secondary conductors never leave the enclosure. Secondary terminals, 310.104 single conductors, and OCPD are in the same enclosure. Just exactly how do you apply 'derating' to these conductors?

 

[iwire]

I think there is a common misconception that the code says you "cannot tap a tap." It doesn't say that anywhere.

It does not say 'you can't tap a tap' but that concept is reality not a misconception.

What it does say is that conductors supplied under the provisions of 240.21(A) through (H) shall not supply any other conductor except through an OCPD. This includes not only taps and transformer secondary conductors, but also conductors from generator terminals and battery conductors.

Let's stick with transformer secondaries for now.


Is it your suggestion that I could run say one set of 500 kcmil from a transformer and tap that 500 with 3 AWG a number of times to supply a number of disconnects?

That would violate this.

2011 NEC

240.21(C) Transformer Secondary Conductors. A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.

There is no permission to tap the taps.

 

[david luchini]

Is it your suggestion that I could run say one set of 500 kcmil from a transformer and tap that 500 with 3 AWG a number of times to supply a number of disconnects?

I'm not saying that at all. That would violate 240.21.

Are you saying that it would be permissible to tap from a 200A feeder (3/0) with #3 conductors to a 100A OCPD, and then on the line side of the OCPD run another sets of 3/0 to another load? You haven't "tapped the tap", as the new 3/0 would be protected by the 200A feeder OCPD. But that would still violate 240.21

There is no permission to tap the taps.

There is no permission supply any other conductor except through on OCPD, from a conductor supplied under the provision of 240.21(A) through (H), whether those conductors are taps, transformer secondaries, conductors from generator terminals, battery conductors, etc.