215.4 feeders with common neutral conductor

[mrmuneeb]

hi fellas,

as mentioned in the title, I am having difficulty understanding ;

"Up to three sets of 3-wire feeders shall be permitted to utilize a common neutral."

does it mean if I have 6 generators each of three wire, then I should put 2 conductors of neutral?

please support with an example.

thanks

 

[winnie]

This article talks about feeders, not about generators. A 'common neutral' is not a requirement (and often does not make sense from a design perspective), it is simply something you are _permitted_ to do if the circumstances make sense.

In some limited circumstances, you are permitted to use a single conductor as the neutral for several feeder circuits, sort of the opposite of using conductors in parallel. So you might have 2 separate 3 phase breakers each feeding a panelboard with a separate set of 3 ungrounded conductors, but sharing a single neutral conductor.

-Jon

 

[jaggedben]

I have a question about sizing such a neutral...

Shouldn't 220.61 be modified so it's a little different at the bolded part?

... The maximum unbalanced load shall be the
maximum net calculated load between the neutral
conductor and any one ungrounded conductor.

Or am I missing something? It seems that a common neutral should be sized for the load between the neutral and all ungrounded conductors of the same leg/phase. No?

 

[jumper]

I have a question about sizing such a neutral...

Shouldn't 220.61 be modified so it's a little different at the bolded part?



Or am I missing something? It seems that a common neutral should be sized for the load between the neutral and all ungrounded conductors of the same leg/phase. No?

No. That could produce a number of what current the neutral is carrying at a given point in time, but may not be the maximum current for which the neutral should be sized for.

 

[jaggedben]

No. That could produce a number of what current the neutral is carrying at a given point in time, but may not be the maximum current for which the neutral should be sized for.

I'm not following your reply. My concern is that 220.61 refers to one ungrounded conductor, but if you have common neutral there are multiple ungrounded conductors on the same phase so the neutral current might be higher, no?

If I change my wording to "maximum net calculated load between the neutral conductor and ungrounded conductors of the same phase/leg" does that clarify? I just left out the extra verbiage because I was lazy, sorry.

Am I still missing something?

 

[Smart $]

...
Am I still missing something?

No. It is an oversight that should be addressed...

 

[jumper]

I'm not following your reply. My concern is that 220.61 refers to one ungrounded conductor, but if you have common neutral there are multiple ungrounded conductors on the same phase so the neutral current might be higher, no?

If I change my wording to "maximum net calculated load between the neutral conductor and ungrounded conductors of the same phase/leg" does that clarify? I just left out the extra verbiage because I was lazy, sorry.

Am I still missing something?

Okay gotcha. My answer is essentially the same. The sum amperage of all the ungrounded conductors of any given phase would be considered as one conductor IMO. Similar to sizing a bonding jumper or an GEC in a parallel install.

Size of Largest
Ungrounded Conductor or
Equivalent Area for
Parallel Conductors

But yes a little clarification would not hurt.

 

[GoldDigger]

Similar to sizing a bonding jumper or an GEC in a parallel install.

Size of Largest
Ungrounded Conductor or
Equivalent Area for
Parallel Conductors

But yes a little clarification would not hurt.

Especially given that in the described situation the same-phase conductors in different feeders are not, strictly speaking, in parallel. They just share a common return path through the single neutral wire.

I think we agree on what it should say, just maybe differing on how important it is to fix it.