De-Rate: Greater than three conductors or phases

[ctrane]

Suppose Panel A feeds Panel B, and the voltage is 208/120/3 Phase, Y-Connected. Now, if you send the three phases plus neutral to feed Panel B, and Panel B is mostly made up of 120 non-linear circuits, does the neutral count as a conductor? Must the wire in the conduit feeding Panel B de-rated? Or, does the de-rate only happen when you have more than three "going in the same direction"? Meaning, the neutral current is traveling in opposite direction of the phase conductors.

 

[Pierre C Belarge]

Take a look at 310.15(B)(4), which it sounds like you may have.

 

[ctrane]

Is there any kind of ratio that you can apply to a panel and deem the neutral a current carrying conductor? In the example I used, something like if the ratio of (120V circuits)/(1phase and 3phase) > some number, than consider the neutral a conductor?

 

[infinity]

Is there any kind of ratio that you can apply to a panel and deem the neutral a current carrying conductor? In the example I used, something like if the ratio of (120V circuits)/(1phase and 3phase) > some number, than consider the neutral a conductor?

The NEC uses the words "major portion" of the load being non-linear to describe when the neutral counts as a CCC in a 208Y/120 volt system. If the non-linear loads were greater than 50% of the total load than the neutral would count as a CCC. 50% or less, it does not count.

310.15(B)(4)(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 shall therefore be considered a current-carrying conductor.

 

[George Stolz]

Well, it sounds as though 310.15(B)(4)(b)&(c) describe your situation to a tee. So, you will be well advised (if not obliged) to consider the neutral a CCC.

Lowering the estimated load on that neutral will not achieve any breaks in terms of derating; your ungrounded conductors will need to be upsized right along with the neutral.

It sounds as though you're willing to kick the ungrounded conductors up a notch, but want to keep the neutral small. Why?

Edit to correct reference