SkillsUSA preliminary contest Industral Motors Controls Written test Q7

[don_resqcapt19]

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.

The column width panels boards have their own rule permitting the use of a "common" neutral. 300.3(B)(4)

 

[don_resqcapt19]

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?

200.4(A) Installation. Neutral conductors shall not be used for more than one branch circuit, for more than one multiwire branch circuit, or for more than one set of ungrounded feeder conductors unless specifically permitted elsewhere in this Code.

This came out of a proposal that I submitted. The code was trying to use the specific permissions, as found in 251.4 and 225.7(B), as a prohibition of using a common neutral for other applications. A specific permission to do something, in no way tells you that you can't do something else.

 

[Smart $]

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

... 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.

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?

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

Running a common neutral use to be allowed for branch circuits, too. It is an electrically sound principal to reduce the number of wires in a conduit. Never questioned it from that perspective. When you consider it only carries unbalanced current, it's not much different than running a single EGC for multiple sets of circuit conductors.

Where ocpd is involved, such as on the ungrounded side of the circuits, branching at a reduced size (thus ampacity) without ocpd would be considered taps. When you consider that with no ocpd on the neutral side of circuits, all are common back to the source at some point anyway... just a matter of sizing appropriately before and after branching. So what does it matter if the branching doesn't occur at the same points as the ungrounded side of circuits.

Wasn't too long ago it became prohibited in general for branch circuits, as Don noted...

This came out of a proposal that I submitted. The code was trying to use the specific permissions, as found in 251.4 and 225.7(B), as a prohibition of using a common neutral for other applications. A specific permission to do something, in no way tells you that you can't do something else.

 

[iceworm]

I'm being a lously physics student - don't know why.

See the attachmnet. This is what I am hearing.

I understand what 215.4 says. I just don't understand the physics of where and why one might use this.

 

[Smart $]

I'm being a lously physics student - don't know why.

See the attachmnet. This is what I am hearing.

I understand what 215.4 says. I just don't understand the physics of where and why one might use this.

Consider multiple feeder circuits with very little maximum unbalanced neutral current. Code requires neutral to not be sized smaller than the minimum size EGC for the circuit. With a common neutral being permitted, you can combine the calculated maximum unbalanced neutral currents to reduce number and total cross-sectional area of conductors (and quite likely cost).

Where all individual circuit neutrals must be larger than the minimum EGC, it doesn't provide any benefit.

 

[fmtjfw]

Modular office furniture ("cubes") used to use super neutrals a lot. Typical unit would have 3 120V single-phase circuits with one circuit with its own hot and neutral (for the computer) and two hots with a super neutral for lights and anything else. The hots would be #12 and the super neutral would be #10 as I recall.

 

[iceworm]

Modular office furniture ("cubes") used to use super neutrals a lot. Typical unit would have 3 120V single-phase circuits with one circuit with its own hot and neutral (for the computer) and two hots with a super neutral for lights and anything else. The hots would be #12 and the super neutral would be #10 as I recall.

And none of these are feeders - correct?

 

[GoldDigger]

And none of these are feeders - correct?

Absolutely correct.
Groups of adjacent cubicles would be on one branch circuit (multipole).
The panel would be labelled.
Lights
Lights
Lights
Receptacles
Receptacles
Cubes
Cubes
Cubes
Cubes
Cubes
Cubes.

 

[iceworm]

Absolutely correct.
Groups of adjacent cubicles would be on one branch circuit (multipole). ....

So this would not have anything to do with 215.5?

 

[GoldDigger]

So this would not have anything to do with 215.5?

The circuits terminating at receptacles in the cubicles are just branch circuits. There may be feedthrough conductors passing through one set of cubes in the way to others, but there are no panels or in fact any OCPD in the modular equipment. Each circuit originates at a panel somewhere and comes through floor or down a power pole.
Feeders? Not any.

 

[don_resqcapt19]

The office cubicles would not be using a "common neutral" as talked about in 215.4. Each circuit has its own neutral.

 

[iceworm]

The office cubicles would not be using a "common neutral" as talked about in 215.4. Each circuit has its own neutral.

So, so far there are no known scenerios where the 215.4 would be used?

ice

 

[Smart $]

So, so far there are no known scenerios where the 215.4 would be used?

ice

Did you miss my last post?

 

[iceworm]

Did you miss my last post?

No, I didn't miss it. And I should not have ignored it.

I'm just guessing that this was put in the code for a particular reason - and this isn't it.

I suspect this was a special interest addition. It is in the 96 code, so it was before then.

Just curious to know what it might have been. It does not feel like we got it.

ice

 

[don_resqcapt19]

215.4 was in the 78 code, the oldest I have around. I have no idea why it was put into the code and I have never seen it used.

 

[iceworm]

me neither. curiouser and curiouser. Is this where the queen of hearts executes me before the trial

 

[fmtjfw]

What is the number of CCCs in a conduit containing a grounding wire, two 3-phase feeders branch circuits and a neutral with linear loads?

What is the number of CCCs in a conduit containing a grounding wire, two 3-phase feeders and a neutral associated with one feeder with linear loads?

 

[don_resqcapt19]

The first one still doesn't really tell me if there are one or two neutrals, the second one does.

 

[Smart $]

The first one still doesn't really tell me if there are one or two neutrals, the second one does.

Does it matter if loads are linear? [No]

BTW "and a neutral" means one, being it is after "branch circuits", IMO. If it were before, then it would be two.

 

[kwired]

So, so far there are no known scenerios where the 215.4 would be used?

ice

Doesn't mean you couldn't run two feeders in a common raceway and use only one common neutral from what I can see. All conductors have to be in the same racway - this will result in needing to derate for number of current carrying conductors in the raceway and possible need to increase conductor sizes.