Feeder calc. w/cont. & derating

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Charlie, thank you so much for your input. I can see the light :) This all makes sense to me now that you have explained it in a methodical manner. At no point can the conductor's ampacity at it's "operating" temperature ever exceed 125% of the continuous load plus the noncontinuous. But when we have correction factors and adjustments, we start with the conductor's temperature rating and we never need to apply the 125% factor to the load, continuous or not. The conductor ampacity must be able to satisfy both conditions. And yes, Charlie, I believe that was the thought process of the instructor I had. It just went a wee bit over my head :p
 
You might be seeing the light, but you are not correctly expressing what you see.
greeny said:
At no point can the conductor's ampacity at it's "operating" temperature ever exceed 125% of the continuous load plus the non-continuous.
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Yes it can. That is like saying you are not allowed to have more money in your wallet than you need to pay for dinner. You can use a conductor with a 1000 amp ampacity in a circuit with a load of 1 amp continuous and 1 amp non-continuous. Perhaps not a good economic decision, but it?s allowed.

greeny said:
But when we have correction factors and adjustments, we start with the conductor's temperature rating and we never need to apply the 125% factor to the load, continuous or not.
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I don?t think you are seeing the right light. What I have said, and by the way I am still waiting for the experts to weigh in on this concept, is,

  • You DO have to perform the calculation of ?125% of continuous plus 100% of non-continuous.?
  • What I am beginning to change my viewpoint about is what to do with that number.
  • At the very least, you must look at Table 310.16 (in the 60C or 75C column), and you must select a conductor that has at least that value of ampacity.
  • Next, you take the tabulated ampacity of your selected conductor (and here you can use the 90C column), and apply the applicable corrections and adjustments.
  • That calculated ampacity number must be equal to, or higher than, the calculated load.
  • It now appears to me that the calculated ampacity of the selected conductor need not higher than the results of calculating ?125% of continuous plus 100% of non-continuous.?

greeny said:
The conductor ampacity must be able to satisfy both conditions.
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No. Essentially, you are doing THREE separate calculations, and performing ONE separate table look-up:

  • Calculate the load, per Article 220. Call this, ?Result #1.?
  • Calculate ?125% of continuous plus 100% of non-continuous.? Call this, ?Result #2.?
  • Calculate the ampacity of the selected conductor, by applying an ?adjustment? for 4 current-carrying conductors, and by applying a ?correction? for ambient temperature. Call this, ?Result #3.?
  • Look up the tabulated value of ampacity for the selected conductor, keeping in mind that you can?t use the 90C column. Call this, ?Result #4.?
What I am suggesting, for the consideration of the membership, is that,

  • Result #4 must be equal to or higher than Result #2.
  • Result #3 must be equal to or higher than Result #1.
  • Result #3 need not be higher than Result #2. That is the new concept, or at least it is new to me.
 
Before derating (using the 75 degree column) the conductor must be >= 100% of non continuous and 125% of continuous.

After derating (using the 90 degree column) the conductor must be >= 100% of non continuous and 100% of continuous.

This is what I understand is being suggested.

I've heard this in the past, but I've never really had to apply it.
 
hardworkingstiff said:
Before derating (using the 75 degree column) the conductor must be >= 100% of non continuous and 125% of continuous.

After derating (using the 90 degree column) the conductor must be >= 100% of non continuous and 100% of continuous.

This is what I understand is being suggested.

I've heard this in the past, but I've never really had to apply it.
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I agree.

I'm still trying to figure out exactly why there are 4 current carrying conductors. A three phase, 4 wire FEEDER to a sub-panel, which serves single phase, non-harmonic loads would only have 3 current carrying conductors in the conduit. Can anyone provide some examples as to how this situation is going to apply in the real world. My designs would never allow a single phase branch circuit to be run in the same conduit as a feeder circuit.
 
hardworkingstiff said:
Before derating (using the 75 degree column) the conductor must be >= 100% of non continuous and 125% of continuous.
Click to expand...
hardworkingstiff said:

After derating (using the 90 degree column) the conductor must be >= 100% of non continuous and 100% of continuous.

This is what I understand is being suggested.
Click to expand...

An elegant summary. Once again, I am not asserting that this is the correct interpretation. But it is an interpretation that I had not previously heard of, nor thought of on my own.
 
Charlie,

I believe that you've reasonably expressed the concept of the instructors that the OP was reporting.

IMHO their line of reasoning is only partially correct.
For branch circuits, consider 210.19(A)(1) and 210.20(A).
For feeders, consider 215.2(A)(1) and 215.3

It is pretty clear that _both_ the conductors and the OCPD must have ratings of 125% continuous load + 100% non-continuous load. It is also pretty clear that this requirement is for the benefit of the OCPD devices, not the conductors. The Exceptions to the above rules involving the use of 100% rated OCPD are the basis of my reasoning. Conductors may be used at 100% continuous loading, as long as properly rated OCPD is used.

The line of reasoning being discussed here suggests that conductors must have ratings of 125% continuous + 100% non-continuous _prior_ to derating and adjusting for things like temperature and conduit fill, however after such derating and adjustment they need only have 100% rating for both. I agree with this interpretation of 210.19(A)(1) and 215.2(A)(1).

However 210.20(A) and 215.3 still apply, which means that the OCPD must have a rating of 125% continuous plus 100% non-continuous. This means that all of the requirements of article 240 are in play, and the conductors with their adjusted and derated capacity must still be properly protected by the OCPD.

In the particular example given, you end up with conductors of 180A ampacity feeding a 180A load, on a 200A breaker. This is entirely kosher because of 240.4(B) and 240.6(A).

But consider instead this same panelboard, but rather than 100A non-continuous plus 80A continuous loading, give it 160A of continuous loading, again high harmonic single phase loads where the neutral must be counted as a current carrying conductor. And lets tweak things a bit by putting the feeder into a 90F environment.

215.2(A)(1) that the conductors have an ampacity not less than that required to serve the load, and in addition requires the use of 200A conductors prior to derating and adjustment, that being 125% * 160A. For example, 3/0 THHN. The derated and adjusted ampacity of the 3/0 THHN conductors is 225 (300.16) * 0.94 (310.16) * 0.8 (310.15(B)(2)(a) ) = 169A, which is sufficient for the load. So (by the current reasoning) we are good with both parts of 215.2(A)(1).

However 215.3 still requires the use of 200A OCPD.

But 169A conductors are _not_ properly protected by a 200A OCPD, even though they are sufficient for the load served, and meet the requirements of 215.2(A)(1).

-Jon
 
Good point, Jon, and well reasoned. The rating of the OCPD does seem to muddy the waters a bit.

Anyone else want to offer a viewpoint?
 
If you have the NEC handbook, there is an example following 210.20.

It seems to indicate you need to do both: multiply the continuous load by 125%, then diveid by the adjustment factor of 80%. So a 25A continuous load would require a condutor with a 40A ampacity.

Is it possible the handbook authors have also misinterperted the phrase "before the application of any adjustment or correction factors".

Steve
 
kingpb said:
Steve,

The only problem I see with your reference is that 210 is for Branch Circuits, and this is a Feeder.
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Well, the original post didn't say if they were branch circuits or feeders. I think they could be either. Anyhow, the code language is exactly the same word for word (except for feeder vs. branch circuit). Compare 210.19 vs. 215.2.

So I think the calculations were intended to be done the same for branch circuits or feeders.
 
Winnie, excellent example. It shows the correlation between 215.2a1 and 215.3 in a way that I've never given much thought.
Also, Charlie, I'm completely on-board with these interpretations as I've stated in my previous post. I did misword a sentence. When I said "exceed", I should have said "be less than". wording = wrong; thought process = correct. I stand by the rest of my statements. And I did have a similar example on my master's test, which I know I got correct.
 
Steve66, that is a good find. That example absolutely implies that we must apply 125% to the continuous loads in conjunction with any adjustment and correction factors. And I agree that it doesn't make a difference whether it's a feeder or branch circuit. The NEC wording is the same. And that wording is very ambiguous.
I personally don't agree with that example.
125% of 25 amps is 31.25(ampacity needed at 60` for terminations).
25/(.8*.96)= 32.55(ampacity needed at 75`for THWN).
So the conductor must be able to carry both 31.25@60` and 32.55@75`. #8 works. AND I feel it satifies 210.19A1, because the ampacity of the conductor is more than 125% of continuous load BEFORE correction factors.
 
greeny said:
Steve66, that is a good find. That example absolutely implies that we must apply 125% to the continuous loads in conjunction with any adjustment and correction factors. And I agree that it doesn't make a difference whether it's a feeder or branch circuit. The NEC wording is the same. And that wording is very ambiguous.
I personally don't agree with that example.
125% of 25 amps is 31.25(ampacity needed at 60` for terminations).
25/(.8*.96)= 32.55(ampacity needed at 75`for THWN).
So the conductor must be able to carry both 31.25@60` and 32.55@75`. #8 works. AND I feel it satifies 210.19A1, because the ampacity of the conductor is more than 125% of continuous load BEFORE correction factors.
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In the handbook example, thye took 25*1.25 to get 31.25 amps. Then, they divided the 31.25 by 0.8 to get a required conductor ampacity of about 39 amps. But the handbook has been known to be wrong on other occasions.

I agree that the wording is very confusing. Like Charlie metioned, it says "before", but doesn't tell you what to do after.

I don't think we can decide which way the NEC meant to do this calculation without an opinion from someone at NFPA.

Steve
 
In sizing the cable, it needs to be able to carry the current that is required by the load after all adjustments. So it is useless to really size the cable until you have properly selected the OCPD. Then the cable is whatever it needs to be.

The NEC is "minimum required", not absolute.
 
Charlie,

Sorry, I had to edit (re-word) my original question.

If we have a calculated branch circuit load of 200A, why is there a fuss over the feeder and demand factors?
Plus I don't understand why the 4 wire feeder?
 
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Dave, if we have a "calculated load" of 200 amps, we will have already taken any applicable demand factors into account, before starting to select the feeder size. I do not understand what you are asking.

Also, the issue with a 4-wire feeder, in this thread, is that the neutral of a 3-phase system is being counted as a current-carrying conductor, due to the large percentage of harmonic loads.
 
greeny said:
This all makes sense to me now that you have explained it in a methodical manner. At no point can the conductor's ampacity at it's "operating" temperature ever exceed 125% of the continuous load plus the noncontinuous. But when we have correction factors and adjustments, we start with the conductor's temperature rating and we never need to apply the 125% factor to the load, continuous or not. The conductor ampacity must be able to satisfy both conditions. And yes, Charlie, I believe that was the thought process of the instructor I had. It just went a wee bit over my head :p
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I disagree with that 100%. The addition of the extra 25% has nothing to do with the conductor ampacity. It is added so there is more copper to act
as a heat sink for the breaker + the breaker can not be operated at more that 80% of its rating for cont loads. Look at exception 1 is both 210.19A and 215.2A. When a breaker rated for 100% load the additional 25% is not required.
The title of 210.19 and 215.2 "Minimum Capacity and Size". The required Minimum Size is 125% cont. + 100% non-cont. That is the minimum. If there are adjustments to be made, its because the ampacity of the conductor is reduced in these environments. The minimum conductor capacity calculated above is still valid but the conductor rating is not adequate. A larger conductor must be selected using the Temperature adjustments and table 310.15(B).2a when required.
 
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