Neutral Conductors:
Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC:
3?- 208Y/120 or 480Y/277 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
[highlight]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 an exception, *if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.[/highlight]
1?- 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.
Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.
Rob, I saw similar information from MH (EC&M Oct 2011) but he stated [highlight]"The neutral conductor of a 4-wire, 3-phase, 120/208V or 277/480V wye-connected system is considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) if more than 50% of the neutral load consists of nonlinear loads [310.15(B)(5)(c)]."[/highlight] This seems different than what you described above.
Can you clarify what you mean by "if the current is more that 50% nonlinear"?
I think what is meant and the way I interpret the requirement is "if more than 50% of the loads, connected to the neutral, generate non-linear currents, then the neutral must be considered a CCC.
Is this a correct interpretation? I don't have the code with me, is the 50% threshold in the code? If not, what is the basis for the more than 50% value?