neutral doesn't count as current carrying ?

[Stevenfyeager]

Please correct me, I thought we count neutral conductors as current carrying when we adjust for amperage when in a conduit. Table 310.15 (B) (3) (a) (Adjustment for more than 3 CCC ) But 310.15 (5) (a) says not to count them. ?? Am I wrong ? Thanks !

 

[GoldDigger]

You count neutrals as current carrying when you have one hot and one neutral or two out of three 3 phase hots and a neutral. You do not count it when the neutral is part of a full MWBC and carries only the unbalanced current from the hot conductors.

The rationale is that whenever there is neutral current, one or more of the phase currents has been reduced in proportion, leaving the heat dissipation the same.

 

[infinity]

Here's a summary.

Understanding the Neutral Conductor

Dennis, here's a really basic lesson, and we'll start with direct current, because the fact that we use alternating current is not relevant to the basics: Picture a battery, like you'd find two of in a flashlight. You have a terminal at each end, with a difference in potential of 1.5 volts...

forums.mikeholt.com

 

[winnie]

A slightly different perspective: instead of asking if the _neutral_ is a CCC, instead ask 'given this set of conductors, how many CCC do I have'?

In a three phase wye system:
1 hot + 1 neutral + 1 EGC counts as 2 CCC
2 hot + 1 neutral + 1 EGC counts as 3 CCC
3 hot + 1 neutral + 1 EGC counts as 3 CCC

-Jon

 

[xptpcrewx]

2 hot + 1 neutral + 1 EGC counts as 3 CCC

It would be 2 CCC for single phase MWBC per post #2 and section 310.15(E)(1).

 

[DrSparks]

Please correct me, I thought we count neutral conductors as current carrying when we adjust for amperage when in a conduit. Table 310.15 (B) (3) (a) (Adjustment for more than 3 CCC ) But 310.15 (5) (a) says not to count them. ?? Am I wrong ? Thanks !

I always count it. You're supposed to when there are a majority of non linear loads, which is just about everything now days. LED lights, fluorescent ballasts, all electronic devices, computers. These devices all utilize switching power supplies that have harmonic currents that return to the circuit not in line with the primary phase of current. These currents have an additive effect that can overload a neutral.

Also, Google the term "super neutral," for more info on this.

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[winnie]

It would be 2 CCC for single phase MWBC per post #2 and section 310.15(E)(1).

I agree.

I headed my table 'for a 3 phase wye system' and was lazy about putting up the tables for single phase systems.

-jon

 

[electrofelon]

I always count it. You're supposed to when there are a majority of non linear loads, which is just about everything now days. LED lights, fluorescent ballasts, all electronic devices, computers. These devices all utilize switching power supplies that have harmonic currents that return to the circuit not in line with the primary phase of current. These currents have an additive effect that can overload a neutral.

Also, Google the term "super neutral," for more info on this.

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I think you are stuck back in the 90's. Its really not an issue now.

 

[DrSparks]

I think you are stuck back in the 90's. Its really not an issue now.

So are white wires magical now or did electronic devices implement third order filters or reactors?

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[electrofelon]

So are white wires magical now or did electronic devices implement third order filters or reactors?

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I am not an expert in power electronics, but I would guess the latter. All I know is Everytime I have measured the neutral load on balanced MWBC's serving "electronic" loads, I get a value close to zero.

 

[DrSparks]

I am not an expert in power electronics, but I would guess the latter. All I know is Everytime I have measured the neutral load on balanced MWBC's serving "electronic" loads, I get a value close to zero.

I know they use PF correction in data centers but not sure about LED drivers specifically. IDK, everything is always changing. I just got into the habit of running dedicated neutrals now and I'm used to it. I even get the white with colored tracers to eliminate the need for grouping.

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[electrofelon]

I know they use PF correction in data centers but not sure about LED drivers specifically. IDK, everything is always changing. I just got into the habit of running dedicated neutrals now and I'm used to it. I even get the white with colored tracers to eliminate the need for grouping.

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Although I have never seen or heard of an issue with this in modern times, I will say the NEC is rather dumb on this. They treat a load as just linear or nonlinear. Seems grossly over simplified to me as there is lots in between. In a perfect world perhaps everything would be required to have a THD value marked and there could be an accepted value where neutral currents become an issue for sizing and derating......or if modern power supplies are good enough they could just delete that stupid wording and requirement.

 

[paulengr]

NEC also has a “gotcha” clause referring to not only unbalanced currents but nonlinear currents. For instance I have clicked an 800 HP fan on a VFD with over 90 A of common mode current. It only had a ground but if it had a neutral, it would be 90 A even with balanced currents.

 

[winnie]

I'd like to see a motor/VFD combo that puts the common mode current the neutral.

In my experience there is no neutral circuit connection and the common mode current is via capacitive coupling to the grounded metal of the motor frame. Then this high frequency current goes everywhere in the connected grounded metal wrecking havoc with all the nearby electronics.

I guess if you built a motor with an electrostatic shield around the windings you could connect a specific circuit conductor to 'catch' this capacitive current. But even then I wouldn't want that current flowing on the neutral back to the supply transformer, I'd want to get it back to the VFD somehow.

Jon

 

[clarice]

How many current carrying conductors do I have in 3 phase when I run 3 120 volt circuits -
A - Black
B - Red
C- Blue
and 3 neutrals ( one for each phase)?

 

[electrofelon]

How many current carrying conductors do I have in 3 phase when I run 3 120 volt circuits -
A - Black
B - Red
C- Blue
and 3 neutrals ( one for each phase)?

6 CCC

 

[infinity]

How many current carrying conductors do I have in 3 phase when I run 3 120 volt circuits -
A - Black
B - Red
C- Blue
and 3 neutrals ( one for each phase)?

Read through the thread and tell us how many you think there are.

Please update your profile to include your occupation and location.

 

[electrofelon]

Rob has the better answer

 

[ggunn]

A slightly different perspective: instead of asking if the _neutral_ is a CCC, instead ask 'given this set of conductors, how many CCC do I have'?

In a three phase wye system:
1 hot + 1 neutral + 1 EGC counts as 2 CCC
2 hot + 1 neutral + 1 EGC counts as 3 CCC
3 hot + 1 neutral + 1 EGC counts as 3 CCC

-Jon

For three phase systems, yes.

 

[winnie]

For three phase systems, yes.

Yup, as stated at the top of the list.

Here is a more complete list:

In a single phase center tap system:
1 hot + 1 neutral + 1 EGC counts as 2 CCC
2 hot + 1 neutral + 1 EGC counts as 2 CCC (presuming proper selection of hots from different legs)
2 hot + 1 EGC counts as 2 CCC
2 hot + 2 neutral + 1 EGC counts as 4 CCC (separate neutral install)

In a three phase wye system
1 hot + 1 neutral + 1 EGC counts as 2 CCC
2 hot + 1 neutral + 1 EGC counts as 3 CCC (presuming proper selection of hots from different legs)
2 hot + 0 neutral + 1 EGC counts as 2 CCC
3 hot + 1 neutral + 1 EGC counts as 3 CCC (presuming proper selection of hots from different legs)
3 hot + 0 neutral + 1 EGC counts as 3 CCC
3 hot + 2 neutral + 1 EGC counts as 5 CCC (partial separate neutral install )
3 hot + 3 neutral + 1 EGC counts as 6 CCC (presuming proper selection of hots from different legs)

In a three phase corner grounded delta system
1 hot + 1 grounded conductor + 1 EGC counts as 2 CCC
2 hot + 0 grounded conductor + 1 EGC counts as 2 CCC
2 hot + 1 grounded conductor + 1 EGC counts as 3 CCC

The three phase high leg delta system is left as an exercise for the reader

-Jon