Upsized Neutral ?

[dduffee260]

We were having a small study group yeserday. One person asked why you could not pull a #6 Neutral along with 3 -black #12s, 3-red #12s and 3-blue #12s. Besides the derating factor along with the general cost that 1- #6 is more expensive than 3- #12s. Where in the code book does it not allow it?

 

[iwire]

One person asked why you could not pull a #6 Neutral along with 3 -black #12s, 3-red #12s and 3-blue #12s. ........ Where in the code book does it not allow it?

IMO the NEC does not prohibit that at all.

As a matter of fact it is specifically allowed by 225.7(B) for outside branch circuits and feeders.

The fact that it is specifically allowed by 225.7(B) leads one to think that there must be a section prohibiting it for inside wiring. However I can not find any general prohibition against it.

IMO it would almost always cost more to install 9 circuits using one large neutral so it is probably not an issue that really comes up.

 

[lowryder88h]

We were having a small study group yeserday. One person asked why you could not pull a #6 Neutral along with 3 -black #12s, 3-red #12s and 3-blue #12s. Besides the derating factor along with the general cost that 1- #6 is more expensive than 3- #12s. Where in the code book does it not allow it?

260, there are wire companies out there w/ made up bundled conductros w/ what they call super neutral w/ the other conductors derated for the amount of conductors included. I've used it on a number of jobs, intial cost is more but w/ the savings on labor (set-up, identification, & etc. ) it was well worth it.

 

[iwire]

Super Neutral cable is not generally used as 260 is talking about.

A typical super neutral cable will have 12 AWG ungrounded with a 10 AWG grounded conductor or 10s with an 8 AWG grounded conductor. These cables are used for non-linear loads and sometimes up sized again for distances.

However you will only have one 20 amp circuit from each phase connected to the super neutral.

260 is describing three 20 amp circuits per phase connected to a single common neutral.

Here is a single phase example of using a common neutral for many circuits.

 

[dduffee260]

I have seen some information about the super neutral set ups. My thinking was that it was for harmonics.

Like I said, the main reason cost wise would be primarily that a larger neutral would be more expensive than the smaller ones as far as materials. I think the labor savings though for the "marrying" process for the conductor bundles would be more savings on the labor to make up for the wire difference.

I have to admit though, I never see it. It would be confusing to some to look into a panel and see the different wire sizes going into a conduit. Honestly I don't know if I could get used it. But as they say, dinosaurs get old and go extinct.

 

[racerdave3]

Super Neutral cable is not generally used as 260 is talking about.

A typical super neutral cable will have 12 AWG ungrounded with a 10 AWG grounded conductor or 10s with an 8 AWG grounded conductor. These cables are used for non-linear loads and sometimes up sized again for distances.

However you will only have one 20 amp circuit from each phase connected to the super neutral.

260 is describing three 20 amp circuits per phase connected to a single common neutral.

Here is a single phase example of using a common neutral for many circuits.

Bob, I understand what you have shown in the illustration you posted, but my question is how do you calculate the load on the neutral in such an application with so many circuits connected? We all know that when sharing a neutral on multiple circuits that are installed on different phases, the neutral will only carry the imbalace of the separate loads. So in an instance where there are so many circuits connected to a shared neutral, would you do the same calculation, yet size the neutral in groups of 2? Meaning in this case where there were 8 circuits sharing a single neutral and say all 8 breakers were 20 amps, would the neutral be sized to carry the maximum possible load of 80 amps? Along the same lines of sharing neutrals, on a 3 phase panel, if you were to share a neutral with 3 single phase loads (all on different phases) how would one calculate the imbalance that would be carried by the common neutral? It is not as clean and neat as simply sharing the neutral between 2 circuits.

 

[racerdave3]

I have seen some information about the super neutral set ups. My thinking was that it was for harmonics.

Like I said, the main reason cost wise would be primarily that a larger neutral would be more expensive than the smaller ones as far as materials. I think the labor savings though for the "marrying" process for the conductor bundles would be more savings on the labor to make up for the wire difference.

I have to admit though, I never see it. It would be confusing to some to look into a panel and see the different wire sizes going into a conduit. Honestly I don't know if I could get used it. But as they say, dinosaurs get old and go extinct.

DDuffee, you are correct that the super neutral is used because of the havoc that harmonics plays on circuits that supply non-linear loads. The harmonics will really distort the common sine wave and in essance create "dirty power" that will be reflected in an increase in neutral current.

 

[cpal]

UL requirements

UL requirements

I believe UL has a standard ( I'm not at work so I do not have the numbers)that requires that each grounded conductor be capable of disconnect at the panel board to ensure maintenance safety while servicing a circuit (UL67??) for the grounding conductors requires that they stay in tact.

In the circuit Bob posted if one circuit is shut off and that single neutral is disconnected per product standards. there could be circuit unbalance causing abnormal voltages across certain loads and creating damage. also any one between the grounded circuit conductor and ground or grounded point would experience a shock potential, (at least)



408.41 Grounded Conductor Terminations.
Each grounded conductor shall terminate within the panelboard in an individual terminal that is not also used for another conductor.

 

[iwire]

Charlie I don't see any of what you mentioned prohibiting the circuit I posted a diagram of.

In fact that diagram is from the 2002 NEC Handbook.

 

[hmspe]

The neutral would need to be sized for the maximum current it could see. For any multi-leg system the worst case neutral current is when all the breakers on one leg are fully loaded and all breakers on all other legs are at zero load. For the sketch Bob posted, and assuming all the breakers were 20 amp, the worst case neutral load would be 4 X 20A = 80A.

Three phase is the same. Add up the breaker ampacities on each leg, counting only the breakers supplying loads with a neutral connection, then size the neutral for the highest leg's total.



Martin

 

[cpal]

Charlie I don't see any of what you mentioned prohibiting the circuit I posted a diagram of.

In fact that diagram is from the 2002 NEC Handbook.

Bob
I agree that the text of the NEC is not violated by the diagram you posted.

But I do believe that the UL standard (which I can not find) which drove the requirement for one neutral per terminal was developed for maintenance procedures. I still believe that the common neutral to that many ungrounded conductors if removed from a panel board neutral bar could present the conditions I stated in the previous post.

That being said without the UL standard (which I can not recall the number of ) I suggest there is an argument to limit the application depicted in the diagram.

 

[iwire]

But I do believe that the UL standard (which I can not find) which drove the requirement for one neutral per terminal was developed for maintenance procedures. I still believe that the common neutral to that many ungrounded conductors if removed from a panel board neutral bar could present the conditions I stated in the previous post.

Charlie, you lost me here.

How does the circuit I depicted behave any differently than a standard multi wire branch circuit in regards to the neutral?

One neutral conductor per terminal is not the same as one neutral per circuit.

225.7(B) specifically allows this circuit.

 

[iwire]

Meaning in this case where there were 8 circuits sharing a single neutral and say all 8 breakers were 20 amps, would the neutral be sized to carry the maximum possible load of 80 amps?

Yes, that is also pretty much exactly what we find in 225.7(B).

 

[cpal]

Charlie, you lost me here.

How does the circuit I depicted behave any differently than a standard multi wire branch circuit in regards to the neutral?

One neutral conductor per terminal is not the same as one neutral per circuit.

225.7(B) specifically allows this circuit.

I'm not haveing a problem with your interpertation of the Code. I agree with you.

I'm haveing a problem with my lack of memory and an inability to access research material.

I can not formulate a resonable argument with out info from the UL standard. Not that a argument would hold water from a Code enforcemnt prespective.