310.15(B)(5) 2011 NEC (to count it or not to count)

[wagnikon]

In a 4 wire 3 phase circuit consisting of fluorescent fixtures (non linear loads) the neutral is counted. 310.15(B)(5)(c)

What if it is just a single 120v or 277v circuit feeding all fluorescent fixtures? Does the neutral count or does that fall under 310(B)(5)(a)?

Regardless of if you count the neutral or not in a single 120v or 277v circuit consisting of all non linear loads, is the neutral actually carrying current due to non linear loads?

 

[augie47]

If it's just a two wire circuit, 120v or 277 volt + neutral, the neutral always counts regardless of the type load. All the current that is delivered by the phase conductor has to return on the neutral conductor as it has no other path.

 

[jumper]

In a 4 wire 3 phase circuit consisting of fluorescent fixtures (non linear loads) the neutral is counted. 310.15(B)(5)(c)

What if it is just a single 120v or 277v circuit feeding all fluorescent fixtures? Does the neutral count or does that fall under 310(B)(5)(a)?

Regardless of if you count the neutral or not in a single 120v or 277v circuit consisting of all non linear loads, is the neutral actually carrying current due to non linear loads?

From another member:

Here's some examples of when to count the neutral as a CCC:

208Y/120 volt system-different circuit types:

A)- 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B)- 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C)- 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A)- A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B)- In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C)- In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with one exception, *if the current is more than 50% nonlinear then the neutral would count as a CCC.

120/240 volt system-different circuit types:

D)- 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E)- 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's

Notes:
D)- A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E)- In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.

Regardless of if you count the neutral or not in a single 120v or 277v circuit consisting of all non linear loads, is the neutral actually carrying current due to non linear loads?

No, the neutral carries current to complete the circuit ,series in this case.

 

[GoldDigger]

In a 4 wire 3 phase circuit consisting of fluorescent fixtures (non linear loads) the neutral is counted. 310.15(B)(5)(c)

What if it is just a single 120v or 277v circuit feeding all fluorescent fixtures? Does the neutral count or does that fall under 310(B)(5)(a)?

Regardless of if you count the neutral or not in a single 120v or 277v circuit consisting of all non linear loads, is the neutral actually carrying current due to non linear loads?

If you are not connecting a multi-wire-branch-circuit (MWBC) in which you are already counting all of the phase conductors (whether used or not) then you generally must count the neutral.
Even if you have a balanced load on an MWBC, the neutral will carry current when non-linear loads are present. In particular triplen harmonics of all three phases will add on the neutral of a wye system.

If you are only counting one phase conductor, say for a single phase 120V circuit, then you also have to count the neutral since it is carrying return current.
If you are counting both phase conductors for a 208V branch to a line-to-line lighting load, then there is no current carrying neutral to be counted.
If you have two single phase wye loads on different phases, then you have to count both phase wires and the neutral, even for linear loads, since the two phase currents will not cancel in the neutral.

The counting process for the neutral is very simple if you analyze the magnitude of the wire currents compared to the OCPD trip points. But there are also some situations where you can be totally assured that only one of two wires will be carrying current at any given time (travelers in a 3-way, for example) and yet the Code seems to require you to count them both.

 

[wagnikon]

Perfect. Thank you.