Adjustment factors 310.15(B)(3)(a)

[ThreeTenFifteen]

Looking for a simple answer and explanation.

Using a 75kva xfmr, 3 wire delta (480v) to 4 wire wye (120/208). Do the secondary conductors need to be adjusted? Does the neutral not count as a current carrying conductor? Not sure if I need to go off the 90 degree column and multiply my conductor ampacity by 0.8 to size my feeders.

 

[infinity]

Unless you're feeding something that has a greater than 50% harmonic load the answer is no the neutral does not need to be counted as a CCC and therefore an adjustment factor does not need to be applied.

Welcome to the Forum.

 

[augie47]

You might take note that 240.4 "next size up" rule does not apply to transformer secondary conductors. If you are using a 250 amp secondary over-current device, the conductors must be rated at least 250 amp.

 

[Carultch]

Looking for a simple answer and explanation.

Using a 75kva xfmr, 3 wire delta (480v) to 4 wire wye (120/208). Do the secondary conductors need to be adjusted? Does the neutral not count as a current carrying conductor? Not sure if I need to go off the 90 degree column and multiply my conductor ampacity by 0.8 to size my feeders.

Neutral most often does not count. Even if it carries a significant imbalance, it still need not count. The total heat among imbalanced loads, never exceeds the heat generated in the wiring if all three phase conductors carried the full load.

Neutral counts if:
1. it is a mandatory part of the return path of the current, even with balanced loads. An immediately obvious example, is circuits that are connected line-to-neutral, where current has no other option. Another not-so-obvious example, if you derive a 120/208V "open wye" single phase circuit from a 120/208V three phase wye grid, and serve line-to-neutral loads. Even if you have balanced loads between both of the lines, the line-to-neutral voltages of the lines are not equal and opposite like they are on split phase. The currents driven by these voltages don't add up to zero on the neutral conductor, even if they are balacned.

2. if harmonic loads are significant, that the neutral current is enough to generate significant heat. In particular, harmonics that are a multiple of 180 Hz on a three phase wye, or multiples of 120 Hz on a split phase grid. These accumulate on the neutral, rather than cancel as what we expect with the fundamental frequency part of the load.

 

[paulengr]

Neutral most often does not count. Even if it carries a significant imbalance, it still need not count. The total heat among imbalanced loads, never exceeds the heat generated in the wiring if all three phase conductors carried the full load.

Neutral counts if:
1. it is a mandatory part of the return path of the current, even with balanced loads. An immediately obvious example, is circuits that are connected line-to-neutral, where current has no other option. Another not-so-obvious example, if you derive a 120/208V "open wye" single phase circuit from a 120/208V three phase wye grid, and serve line-to-neutral loads. Even if you have balanced loads between both of the lines, the line-to-neutral voltages of the lines are not equal and opposite like they are on split phase. The currents driven by these voltages don't add up to zero on the neutral conductor, even if they are balacned.

2. if harmonic loads are significant, that the neutral current is enough to generate significant heat. In particular, harmonics that are a multiple of 180 Hz on a three phase wye, or multiples of 120 Hz on a split phase grid. These accumulate on the neutral, rather than cancel as what we expect with the fundamental frequency part of the load.

Proof of this last part?

 

[winnie]

Proof of this last part?

If you assume _balanced_ harmonic loading, meaning that the waveform on each phase is the same simply displaced in time by 120 degrees relative to the fundamental period, then you find that each harmonic component K with frequency K*fundamental has a phase displacement of K*120. This means that for 3rd harmonic the phase displacement is 360 degrees. Thus fundamental components of the current balance out on the neutral, but triplen (3rd and multiple of 3) harmonics add on the neutral.

Of course this is an approximation. But you can see examples where there are many line-neutral rectifier loads and the neutral current is greater than expected if harmonics are not considered.

-Jon