counted as current carrying

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nakulak

Senior Member
I can't find one that permits it, but also can't find one that prohibits this either. Am suggesting that it is a similar situation to the OP. IMO if a conductor is not going to carry current at the same time as another conductor then the heat produced within a raceway would be the same as if the non carrying conductor were not there which is the intent of the deration to begin with. I can see some arguement to this but if there would be some type of interlock to keep both loads from operating at the same time then why not? A similar situation exists for feeder and service calculations as well as neutrals carrying unbalanced current. The real question is what are current carrying conductors for this particular situation?

actually, a similar does not exist for feeders and service conductors, because they are allowed to be smaller for non coincident loads - see 220.51 and 220.60. IMSHIO, derating for noncoincident conductors is ridiculous.
 

kwired

Electron manager
Location
NE Nebraska
derating for noncoincident conductors is ridiculous.
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Why? if you have 10 neutrals in a raceway all carrying unbalanced load I think most of us agree that it is permitted to count these as non current carrying. It is because the heat produced within the raceway will never be more than what is among the current carrying conductors being balanced.

How is this different from three way circuits or non coincident loads - especially if interlocked or of such a different characteristic that they will not operate at the same time - only a fixed number of conductors will create heat in the racewayat any particular time.

The heat produced is the reason for derating the conductors not how tight they are packed in the raceway. Same rules apply to 10 #12's in 3/4 pipe as does the same 10 #12's in a 4 inch pipe, may not ever see it but same rules apply.
 

kwired

Electron manager
Location
NE Nebraska
the similarity with feeder and services is that only the larger of the non coincidental load need to be accounted for in the calculations.
 

nakulak

Senior Member
I must have misunderstood your previous post, we are in agreement that derating noncoincident loads is crazy.
 

ivsenroute

Senior Member
Location
Florida
IMO they are assuming a common sense application of the wording .... only count the conductors that simultaneously carry current.

Yes, but I still believe it needs clarification. Common sense goes a long way but we may not be familiar with the engineering involved in figuring derating and conduit fill.

Remember, it is still space taken inside the conduit and less air space for air to convect heat.

I appreciate the common sense approach/assumption but there may be more to the story than we know.
 

iwire

Moderator
Staff member
Location
Massachusetts
Remember, it is still space taken inside the conduit and less air space for air to convect heat.

In my opinion space has nothing at all to do with it, this is evidenced by the fact four 14 AWGs would be subject to derating in a 4" EMT.

But I do agree, it could be worded better.
 

Jim W in Tampa

Senior Member
Location
Tampa Florida
IMO they are assuming a common sense application of the wording .... only count the conductors that simultaneously carry current.

Since when has common sence allowed us to alter nec ???????
I do not read anywhere that we can base current carrying conductors on them being used. If they are there and can carry current they are current carrying conductors.
I do agree they should not be but simply does not say that.
 

iwire

Moderator
Staff member
Location
Massachusetts
Since when has common sense allowed us to alter nec ???????

It is not being altered they just read it differently then you do.

I can see your reading of it but I can also see their reading of it.


I do not read anywhere that we can base current carrying conductors on them being used.

And I do not see anything that says we cannot.


If they are there and can carry current they are current carrying conductors.

So an EGC would also have to be counted as it to may carry current at times.
 

nakulak

Senior Member
how can you count a conductor as a "current carrying conductor" if it is not carrying current ?

For example, in a 3 wire 4way leg, you have a neutral and 2 travelers. At any point in time you have 2 current carrying conductors, there are NEVER 3 current carrying conductors in that scenario, because only 2 are carrying current at any point in time. I cannot see how that can be disputed ?
 

Umlaut

Member
To me, "current carrying" means a conductor which is intended to, sometimes or always, carry current. I think a "current carrying conductor" is the opposite of an equipment ground conductor. And that's how it can be disputed--there's a sensible interpretation of the phrase which meets the apparent wording and fits reasonably with other definitions.

I found an article ECM magazine which speaks to this issue. It says that, in two-wire circuits, "the neutral and ungrounded conductors of a 2-wire circuit are considered current carrying."

I think that in the three-way switched circuits we've been discussing, you have a switched ungrounded conductor, an opposite switched ungrounded conductor, and a neutral. That is, there are three current-carrying conductors.

This is a really interesting discussion; I've always thought this way about the issue, and I'm a little surprised that people would consider that exclusive switching would exempt conductors from the counts. I suppose I've always, then, over-sized boxes and conduit as a result, so I'd be interested to learn that I could definitively use smaller counts and smaller hardware as a result.
 
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roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
I suppose I've always, then, over-sized boxes and conduit as a result, so I'd be interested to learn that I could definitively use smaller counts and smaller hardware as a result.

Conduit and box sizing is based on fill not CCC's.

For example, the conductor adjustment for a set number of CCC's will be the same in a 4" conduit as it would in a 1/2" conduit.

Roger
 

iwire

Moderator
Staff member
Location
Massachusetts
I think that in the three-way switched circuits we've been discussing, you have a switched ungrounded conductor, an opposite switched ungrounded conductor, and a neutral. That is, there are three current-carrying conductors.

There are three conductors that may carry current but only two at any given moment. As far as conductor derating it only counts as two current carrying conductors.

I'm a little surprised that people would consider that exclusive switching would exempt conductors from the counts.

As far as the count for derating the group that writes the code feels exclusive switching makes a difference. See the ROP I posted previously in this thread.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The previous discussions on this topic that I can recall were all about a slightly different issue, specifically: How do you count multiple switch legs feeding _different_ loads but supplied from the same circuit.

In the previously discussed case, _all_ the conductors in the conduit carry some amount of current at the same time, but you can _never_ have a case where an excessive number of conductors would be fully loaded. Consider a bank of 5 switches, wired as switch loops using conduit. You have a single unswitched conductor coming to the bank in the conduit, and 5 switched conductors returning in the same conduit. From a physics point of view, it is totally clear to me that this arrangement produces less heat than 2 fully loaded conductors, and my 'common sense' tells me that for purposes of derating this arrangement should count as 2 CCCs. But I don't think that this interpretation would be accepted by most.

In the current discussion, we have a situation where we at least have an absolute on or off condition for the conductions in question.

Back to hard examples:
A) conduit with 3 way:
3 conductors, 1 unswitched carrying 100% of load, 1 switched carrying 100% of load, 1 switched carrying 0% of load
B) conduit with bank of 5 switched conductors from single circuit
6 conductors, 1 unswitched carrying 100% of load, 5 switched carrying 20% of load each.

Per this discussion, most would count situation A) as 2 CCCs and B) as 6 CCCs.

Per the physics of the situation, assuming the same total load and conductor size, situation B produces less heat.

Per my gut feeling for 'what is right' they should both be counted as 2 CCCs.

Personally, I think that Umlaut is reading the code as written, but since the code making panel is giving an interpretation that make better sense, I'm happy to go along :)

-Jon
 

Umlaut

Member
As far as the count for derating the group that writes the code feels exclusive switching makes a difference. See the ROP I posted previously in this thread.

Thanks, I found it.

Does the ROP mean that the committee that writes the Code knows, then, that the wording is confusing and ambiguous to its audience, and hasn't moved to clarify it? Only people who know of and understand the ROP, at this point, know the true meaning of the section.

It seems like the interpretation hinges on the definition of "current carrying", which doesn't seem to appear in any of the definition articles. Wouldn't simply adding "concurrently current-carrying", "unswitched current carrying", "non-exclusive current carrying" conductors help?

Why did the committee unanimously vote to fail to improve their product?
 
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