Derating feeders.

[Jkr3497]

When exactly do you derate feeder amperage on a 3 pase 4 wire system due to having more than 3 current carrying conductors? The book seems to be kind of vague on the issue. When do you count the neutral (grounded conductor) as a current carrying conductor. I guess I just can't interprete the code properly.

 

[Dennis Alwon]

Here is something written by Trevor, one of our mods



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.

 

[eHunter]

Dennis, would the exception also apply to E, if the load current is more than 50% nonlinear ?

 

[jumper]

Dennis, would the exception also apply to E, if the load current is more than 50% nonlinear ?

If I am reading your question correctly, no. Harmonics are not a factor in 120/240 1P systems for the neutral.

310.15(B)(5)(c) On a 4-wire, 3-phase wye circuit where the major
portion of the load consists of nonlinear loads, harmonic currents
are present in the neutral conductor; the neutral conductor
shall therefore be considered a current-carrying conductor.

 

[kwired]

When exactly do you derate feeder amperage on a 3 pase 4 wire system due to having more than 3 current carrying conductors? The book seems to be kind of vague on the issue. When do you count the neutral (grounded conductor) as a current carrying conductor. I guess I just can't interprete the code properly.

As has been mentioned - only when there is high harmonic producing loads does the neutral need to be counted as a current carrying conductor. Otherwise with linear loads, the heat produced by the conductors in the raceway will never exceed what is produced by three conductors. If all three phases have equal current the neutral is zero. (three conductors are producing heat) If two phases are loaded with line to neutral loads and third phase has no current - you still have three conductors producing heat and the fourth is not. This is true for wye systems because the neutral carrying current from two phases will carry roughly same current as the two phases if those two currents are the same, but neutral will carry zero current with three balanced phases.