Code for sharing neutral

[Jpflex]

I came across a code a while back about NEC not allowing circuits to share a neutral, (multi wire branch circuits may be exempt) but I don’t remember details on standard wiring sharing neutral.

I have a simple 2 gang box with 2 regular switches for outside door light and one inside door light. Will I need two white 14-2 romex lines or can I just use one line and pigtail power and neutral from this one line.

Junction boxes typically have one line and splice into parallel using one line and one neutral from home run, so what is this code about no longer being able to share neutral?

 

[wwhitney]

Junction boxes typically have one line and splice into parallel using one line and one neutral from home run, so what is this code about no longer being able to share neutral?

If the home run just has one ungrounded conductor and one neutral, there is no neutral sharing going on. There's just one branch circuit.

What is disallowed is for the home run to be, say, (2) 20A ungrounded conductors on different breakers (possibly on the same phase) and a 40A common neutral conductor. You'd instead need (4) 20A conductors for two separate 2 wire branch circuits. Or (3) 20A conductors if you want to run an MWBC and comply with the MWBC rules.

Cheers, Wayne

 

[readydave8]

There is no new code prohibiting sharing neutral, there are newer codes requiring simultaneous disconnect, and marking or grouping

 

[wwhitney]

There is no new code prohibiting sharing neutral, there are newer codes requiring simultaneous disconnect, and marking or grouping

Depends on your point of view--200.4 (now A) is new in the 2011 NEC. : - )

Interestingly, as 200.4(A) refers to a neutral conductor rather than a grounded conductor, on a corner grounded delta you can still use a double size grounded conductor for 2 branch circuits.

Cheers, Wayne

 

[readydave8]

New to me may include anything originating in this century

 

[Another C10]

To me it seems some codes are just way over and beyond, all I know is if 2 or 3 circuits share a neutral just make sure each circuit is on a separate phase. loading up on a shared neutral using 2 or 3 similar phases will overheat the neutral, unless upsizing it, but that's never a good practice.

 

[infinity]

Depends on your point of view--200.4 (now A) is new in the 2011 NEC. : - )

Interestingly, as 200.4(A) refers to a neutral conductor rather than a grounded conductor, on a corner grounded delta you can still use a double size grounded conductor for 2 branch circuits.

Cheers, Wayne

I think that the corner grounded Delta was intentionally left out of this section. Can you think of any scenario where you would use two branch circuits with an oversized neutral with this system?

 

[wwhitney]

I think that the corner grounded Delta was intentionally left out of this section. Can you think of any scenario where you would use two branch circuits with an oversized neutral with this system?

The first idea that comes to mind is relocating an existing panel to a new location, and using the old cabinet as a large junction box. If the voltage system has a neutral, you have to extend each branch circuit neutral separately to the new location, per 200.4(A). But on a corner grounded delta you could tie all the grounded conductors together and extend them as one large conductor.

Cheers, Wayne

 

[Another C10]

Will I need two white 14-2 romex lines

If that 2 gang has (2) 14-2 w/g .. coming from the panel, then just use one cable ( being 1 hot and 1 neutral ), pigtail off the hot for both switches , connect the neutrals of the switch legs to the 1 neutral that's it .. Unless you need 2 dedicated circuits for two separate switches. .. you should know that being a residential/commercial sparky ..

 

[Jpflex]

If that 2 gang has (2) 14-2 w/g .. coming from the panel, then just use one cable ( being 1 hot and 1 neutral ), pigtail off the hot for both switches , connect the neutrals of the switch legs to the 1 neutral that's it .. Unless you need 2 dedicated circuits for two separate switches. .. you should know that being a residential/commercial sparky ..

I was aware of all you mentioned, however this post was regarding confusing to a new code or rules changing how things used to be done. Such as neutrals now having to be included in boxes for future electronic switch installations

 

[GoldDigger]

Depends on your point of view--200.4 (now A) is new in the 2011 NEC. : - )

Interestingly, as 200.4(A) refers to a neutral conductor rather than a grounded conductor, on a corner grounded delta you can still use a double size grounded conductor for 2 branch circuits.

Cheers, Wayne

That would be two branch circuits off the same hot line?
If the two circuits came from opposite ungrounded lines and had equal sized OCPD, the maximum neutral current would be equal to rather than double the ungrounded line current.

 

[GoldDigger]

I was aware of all you mentioned, however this post was regarding confusing to a new code or rules changing how things used to be done. Such as neutrals now having to be included in boxes for future electronic switch installations

I think, even if not clearly stated, that the required neutral in a switch box must be properly associated with at least one of the branch circuits present in that box.

 

[wwhitney]

If the two circuits came from opposite ungrounded lines and had equal sized OCPD, the maximum neutral current would be equal to rather than double the ungrounded line current.

Wouldn't that be sqrt(3) instead of equal? Because the currents are 60 degrees out of phase, not 120? This is a corner grounded delta, no neutral.

Cheers, Wayne

 

[GoldDigger]

Wouldn't that be sqrt(3) instead of equal? Because the currents are 60 degrees out of phase, not 120? This is a corner ground.

I do not think so. lmagine a thee phase wye with equal current on all three lines and an ungrounded neutral. The current in the neutral will be zero. So we cannot tell the difference between that wye and a delta. Current I in both ungrounded legs therefore must lead to current I in the grounded leg.
The line-to-line (phase) currents are still 120 degrees out of phase, even though the coils are drawn as a triangle with 60 degree angles. A sixty degree phase shift is just the combination of a 120 degree phase shift and a 180 degree phase shift.

 

[LarryFine]

Imagine a fully-loaded wye with 20a on each phase. 0a neutral current.

Now, deduct 5a from one phase. Neutral current increases by that 5a.

Remove that phase's entire current. Neutral current is now the full 20a.

 

[wwhitney]

I do not think so. lmagine a thee phase wye with equal current on all three lines and an ungrounded neutral. The current in the neutral will be zero. So we cannot tell the difference between that wye and a delta. Current I in both ungrounded legs therefore must lead to current I in the grounded leg.

But in that example if the 3 legs are A-B-C (with C grounded for our corner grounded delta, not sure if that's the standard terminological choice) you would have (say) 3 equal loads A-B, B-C, and A-C. The line currents would be sqrt(3) times the load currents.

While I was speaking of using (2) single phase branch circuits (without specifying which ungrounded legs are used), and you brought up the subcase of one A-C and one B-C. In which case with 2 equal loads, the line current in C would be sqrt(3) times the line current in A or B.

Anyway, the simplest example of 200.4(A) is using two single phase branch circuits on the same ungrounded leg, sharing a double size grounded conductor. Prohibited if the grounded conductor is a neutral of the supply system, but apparently not prohibited when it isn't.

Cheers, Wayne

 

[GoldDigger]

I see my problem, Wayne, thanks. I forgot that since there is no OCPD on the grounded conductor the maximum grounded conductor current does not come when there is a balanced three phase load.

 

[wwhitney]

I see my problem, Wayne, thanks. I forgot that since there is no OCPD on the grounded conductor the maximum grounded conductor current does not come when there is a balanced three phase load.

That brings to mind the following scenario:

Say you have a 240V corner grounded delta (A, B, C, and C grounded) with just OCPD on the ungrounded phases (single phase i.e. two bus panel). You run a 3-wire circuit (A, B, C) to supply (3) 10A resistive non-continuous 2-wire loads in a delta configuration. The line currents are 17.3A, so you use 20A ampacity wires and 20A OCPD.

Now if the A-B load is off or fails open, then the grounded conductor is no longer protected against overload. The A-C and B-C heaters could draw double current (20A) without tripping their OCPD or exceeding the ampacity of the A or B conductors, and the grounded C conductor would then see 34.6A.

220.61(A) Exception has a requirement for a 140% factor for calculating the neutral load from the unbalanced load on a 2-phase feeder. Perhaps it should be extended to cover the grounded conductor of a corner grounded delta with a 170% factor.

Cheers, Wayne