SkillsUSA preliminary contest Industral Motors Controls Written test Q7

[fmtjfw]

What is the number of CCC in a conduit containing a grounding wire, two 3-phase feeders and a neutral with linear loads? [Hint: linear is not nonlinear] The hint was to help them find it in the index of NEC

Also Code References?

 

[don_resqcapt19]

I expect that my answer was not really the one that you were looking for.

 

[fmtjfw]

I expect that my answer was not really the one that you were looking for.

Nope, are you counting the neutral because it "might" be shared?

 

[Smart $]

310.15(B)(5)(a)

The neutral is for two [feeder] circuits. Not counting the neutral only applies when it carries unbalance current of the same circuit.

Even where the neutral carries unbalance of same [phase-balanced] circuit, code does not prohibit it from being counted.

 

[GoldDigger]

310.15(B)(5)(a)

The neutral is for two [feeder] circuits. Not counting the neutral only applies when it carries unbalance current of the same circuit.

Even where the neutral carries unbalance of same [phase-balanced] circuit, code does not prohibit it from being counted.

If you run two neutrals, you do not have to count either of them, but if you only run one you must count it. :angel:
In which case 6 would be the correct answer, but the neutral would have to be double sized and that would impact the fill. (Which was never asked.)

 

[don_resqcapt19]

Nope, are you counting the neutral because it "might" be shared?

Your post in the thread about the total number of conductors in the raceway said it was shared.
That being said, the question does not give enough information to be sure of selecting the correct answer. My assumption that the grounded conductor is shared between the two feeder circuits, is just as valid as the assumption that the grounded conductor is only associated with one of the feeder circuits.
These types of questions led to many discussions with my instructor in apprentice class...I am sure they were all happy when I moved on to the next instructor.

 

[iceworm]

I'm not interested in the poll or the question. But I am curious:

Under what conditions or design criteria would one ever use one neutral for two 3ph feeders?

How would one size the neutral?

 

[Smart $]

If you run two neutrals, you do not have to count either of them, but if you only run one you must count it. :angel:
In which case 6 would be the correct answer, but the neutral would have to be double sized and that would impact the fill. (Which was never asked.)

The neutral could be associated with either or both feeder circuits. The neutral does not necessarily have to be doubled in sized... depends on the amount of calculated unbalanced current.

Technically, the only absolutely correct answer without more information is 7 CCC's.

 

[iceworm]

Okay.
Under what conditions or design criteria could the neutral could be associated with either or both feeder circuits?

 

[Smart $]

Okay.
Under what conditions or design criteria could the neutral could be associated with either or both feeder circuits?

Two feeder circuits ?
Scenario A: one 3? 3W, one 3? 4W
Scenario B: two 3? 3W + Common N

For latter scenario, see 215.4.

 

[don_resqcapt19]

Okay.
Under what conditions or design criteria could the neutral could be associated with either or both feeder circuits?

I don't really know where or why you would use a common grounded conductor with multiple feeders, but it is specifically permitted in 215.4.

 

[fmtjfw]

It may be a flawed question.

The possible 3-phase feeders include
(DCG) xxxV delta, corner grounded -- no neutral
(DNH) xxxV delta center grounded w/wild leg. -- balanced current neutral
(DNG) xxxV delta ungrounded -- no neutral
(Y) xxx/xxxY Y -- balanced current neutral




.....(DCG).....(DNH).....(DNG).....(Y)
(DCG)..--........6.........--......6
(DNH)..6.........7.........6.......7
(DNG)..--........6.........--......6
(Y)....6.........7.........6.......7


4 cases (--) have no neutrals and are excluded from possible answers
8 cases (6) have a single neutral carrying only balanced current & need not be counted
4 cases (7) have two neutrals, so a shared neutral which needs to be counted.

 

[Smart $]

It may be a flawed question.

The possible 3-phase feeders include
(DCG) xxxV delta, corner grounded -- no neutral
(DNH) xxxV delta center grounded w/wild leg. -- balanced current neutral
(DNG) xxxV delta ungrounded -- no neutral
(Y) xxx/xxxY Y -- balanced current neutral




.....(DCG).....(DNH).....(DNG).....(Y)
(DCG)..--........6.........--......6
(DNH)..6.........7.........6.......7
(DNG)..--........6.........--......6
(Y)....6.........7.........6.......7


4 cases (--) have no neutrals and are excluded from possible answers
8 cases (6) have a single neutral carrying only balanced current & need not be counted
4 cases (7) have two neutrals, so a shared neutral which needs to be counted.

The question states two 3? feeders... automatically 6 CCC no matter the system configuration.

No matter what the source configuration(s) is(are), whether the neutral is a common neutral or not cannot be resolved with the provided information. Therefore, given you can only choose one of the answers, the only absolutely correct answer is 7.

 

[jim dungar]

I don't really know where or why you would use a common grounded conductor with multiple feeders, but it is specifically permitted in 215.4.

A left over from what used to be done with column width panelboards? Individual neutrals where marshalled in a pull box and a single combined neutral was run with the branches.

 

[iceworm]

A left over from what used to be done with column width panelboards? Individual neutrals where marshalled in a pull box and a single combined neutral was run with the branches.

Yep - 215.4 does say that. I have no idea what it is left over from. No physics I know of that would make this worth while. I was hoping you guys knew.

I would not think it would be column width panelboards - the neutral marshalling would be for branch circuits - wouldn't it?

ice

 

[Smart $]

...

I would not think it would be column width panelboards - the neutral marshalling would be for branch circuits - wouldn't it?

Not necessarily. Other than one or more ocpd's at the load end of conductors, there is nothing else that distinguishes a branch circuit from a feeder circuit (other than a common neutral under current Code).

 

[iceworm]

Not necessarily. Other than one or more ocpd's at the load end of conductors, there is nothing else that distinguishes a branch circuit from a feeder circuit (other than a common neutral under current Code).

Okay - isn't that enough.

 

[Smart $]

Okay - isn't that enough.

Well, isn't it a possibility that a column width panelboard supplies only feeders, or a combination of feeders and branch circuits, rather than just branch circuits?

 

[iceworm]

$S -
I wasn't looking for guesses. I am looking for someome that knows why

 

[electrofelon]

What is that newish code about neutrals, when I read the question I thought about it. Striking a blank.......something about cant use a common neutral for multiple circuits? Only for branch circuits but not feeders?