Neutral Conductor on 3 phase 4 Wire

[soxedace]

I work in Florida so using the 2014NEC i have a building apart of a shopping center that has 120/208 4wire 3phase.Iv calculated the load of the building to be 175A. This building is a hair salon having 3phase Hot water heater, the only 3 phase powered equipment. Now sizing the feeder conductor. For the 3 phase system. Is the neutral a current carrying conductor forcing me to take a 80%reduction in ampacity for 4 current carrying conductor?

 

[LarryFine]

Is the neutral a current carrying conductor forcing me to take a 80%reduction in ampacity for 4 current carrying conductor?

Welcome, and no.

 

[Dennis Alwon]

Here is the article

310.15(B)(5) Neutral Conductor.


(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit shall not be
required to be counted when applying the provisions of
310.15(B)(3)(a).


(b) In a 3-wire circuit consisting of two phase conductors
and the neutral conductor of a 4-wire, 3-phase, wye-connected
system, a common conductor carries approximately the same
current as the line-to-neutral load currents of the other
conductors and shall be counted when applying the provisions
of 310.15(B)(3)(a).


(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

 

[soxedace]

So I fall into the case A.

(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit shall not be
required to be counted when applying the provisions of
310.15(B)(3)(a).


now in (C) what are the verification steps in validating if the major portion of the load consists of nonlinear loads, harmonic
currents are present in the neutral conductor. I think I'm just missing the code referenced explanation.

 

[infinity]

So I fall into the case A.

(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit shall not be
required to be counted when applying the provisions of
310.15(B)(3)(a).


now in (C) what are the verification steps in validating if the major portion of the load consists of nonlinear loads, harmonic
currents are present in the neutral conductor. I think I'm just missing the code referenced explanation.

Majority of the load means more than 50%, in a shopping center it is not an issue.

 

[kwired]

So I fall into the case A.

(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit shall not be
required to be counted when applying the provisions of
310.15(B)(3)(a).


now in (C) what are the verification steps in validating if the major portion of the load consists of nonlinear loads, harmonic
currents are present in the neutral conductor. I think I'm just missing the code referenced explanation.

About the only time C applies is in a data center or something similarly loaded.

You may have a lot of items that are non linear loads, but are they a majority of the overall load? Good chance your three phase water heater and any HVAC loads are linear loads (or don't utilize the neutral if they are non linear) and are a larger percent of total load than those non linear line to neutral loads.

 

[soxedace]

_{I would say 10% is lighting LED and others. 25% is water heating 3phase 12kw 35% is AC and 20% is small outlets for hair dry like appliances.
now these are all approximate... I guess I need to research determining harmonic currents and non-linear/ linear loading.}

 

[kwired]

I think a lot of places will let you run 2 sizes smaller neutral than ungrounded if not an obvious high amount of non linear load and not two phases +neutral of a wye system without looking much further into actual load calculations. If you want to run a smaller neutral than that they may want more justification for doing so, there are times it is easily justified.

 

[JoeStillman]

Are LED drivers a non-linear load? Back in the bad-old-days of fluorescent and HID ballasts, neutrals were seeing the dreaded triplen harmonics.

 

[kwired]

Are LED drivers a non-linear load? Back in the bad-old-days of fluorescent and HID ballasts, neutrals were seeing the dreaded triplen harmonics.

I don't see why not. But it takes a lot of them to become a majority of the load to be non linear load and have to be concerned on harmonics adding up on the neutral conductor. If lighting is a majority of your load then maybe this is something you pay closer attention to.

 

[charlie b]

My suggestion is that you forgo any allowable demand factors. It is likely that all workstations will be using power at the same time.