Is neutral a current carrying conductor?

Status
Not open for further replies.
M

Machiavelli999

Member
This seems to be an ongoing issue and source of confusion for many. I am in a fight right now with an inspector regarding this. I have a delta-wye, 112.5KVA, 3 phase transformer feeding a 400A rated panel with a 400A main breaker.

My secondary conductors between the panel and the transformer are 4#600KCM, #1/0G. The #600KCM conductors are rated at 420A assuming 75 degree rated terminations. The inspector is telling me that unless I can prove with a third party inspection that there is no current on the neutral, I have to derate my secondary conductors and upsize them to an absurd (2) x 4#300KCM.

The NEC seems pretty clear in Article 310.15(B)(4)(a) that the neutral 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)(2)(a). 310.15(B)(4)(c) does say that if the "major portion of the load consists on nonlinear loads" the neutral shall be considered a current carrying conductor. But my panel is not feeding a data center or a computer room. It's mostly pumps and motors.

I mean to fight this requirement and think that this inspector is confused. What do you guys think?
 
IMO, the conductor size is Code compliant ( the neutral is not a CCC)
 
Machiavelli999 said:
400A rated panel with a 400A main breaker.

My secondary conductors between the panel and the transformer are 4#600KCM, #1/0G. I have to derate my secondary conductors and upsize them to an absurd (2) x 4#300KCM.
Click to expand...

I thought 2 300mcm = 1 600???? I am confused once again:angel:
 
The inspector is incorrect. To qualify as a non-CCC it only needs to be carrying the imbalance between itself and the phase conductors unless your connected load was greater than 50% non-linear. Tell him to read 310.15(B)(4)(a)&(c) {2008} again.


310.15(B)(4) 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)(2)(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)(2)(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.
Click to expand...
 
Current Carrying or Counted?

Current Carrying or Counted?

I'm new here so please bear with me. Regarding post #6 and the referenced code 310.15(B)(4)(a)(b)and (c), this section is many times referred to as whether or not a neutral conductor is current carrying, when all three circumstances are stated as such. They are all current carrying conditions, but only in condition (a) is the neutral conductor not required to be counted.

It is in section (a) that the terminology seems to get a little non-descriptive. Could someone shed a little light with an interpretation please. Am I alone in this thought?
 
It seems like the inspector is confused by what is and is not a "non-linear loads" or he is in the wrong column, is he looking at the AL table?
 
Sir4bz said:
I'm new here so please bear with me. Regarding post #6 and the referenced code 310.15(B)(4)(a)(b)and (c), this section is many times referred to as whether or not a neutral conductor is current carrying, when all three circumstances are stated as such. They are all current carrying conditions, but only in condition (a) is the neutral conductor not required to be counted.

It is in section (a) that the terminology seems to get a little non-descriptive. Could someone shed a little light with an interpretation please. Am I alone in this thought?
Click to expand...

I'll take a stab at it. I'm sure others can explain it better but here goes.

A.) you seem to have that

B.) this is saying if you only run 3 conductors of a 4 wire system the neutral must be counted as a CCC. For example, you have a 3 ph system/panel and you run a single ph circuit from it (2 ungrounded, & 1 neutral) the neutral will carry approx. the same current as a line to neutral load. This is because the neutral is a common conductor and because you're not using the other phase, the neutral will carry current.

C.) If you know that a majority of the loads are non-linear (fluorescent lights, computers etc.) then you must count the neutral because of the harmonic currents that will be caused from the non-linear loads.

Hope this helps!
If I don't have this right, someone please jump in and correct me!
 
Sir4bz said:
I'm new here so please bear with me. Regarding post #6 and the referenced code 310.15(B)(4)(a)(b)and (c), this section is many times referred to as whether or not a neutral conductor is current carrying, when all three circumstances are stated as such. They are all current carrying conditions, but only in condition (a) is the neutral conductor not required to be counted.

It is in section (a) that the terminology seems to get a little non-descriptive. Could someone shed a little light with an interpretation please. Am I alone in this thought?
Click to expand...

Basically if it complies with (a) is not counted as a CCC. If it complies with (b) or (c) it is counted. So for (a) in a 3?, 4W feeder the neutral is only carrying the unbalanced current from the other ungrounded conductors.
 
Little Bill said:
I'll take a stab at it. I'm sure others can explain it better but here goes.

A.) you seem to have that

...

If I don't have this right, someone please jump in and correct me!
Click to expand...
You have it right.

Essentially, (b) and (c) are special instances of (a), where it is impossible for the neutral conductor current to be zero when the other circuit conductors are carrying current.
 
Last edited:
To count or Not to Count

To count or Not to Count

I agree with Infinity in regards to section (c) that according to the wording used, if the OP could prove loading to the inspector, the install could sell. I do however have arguments or points to consider concerning this section.

If we agree with section (b) that with a 1-Phase/3-wire circuit, a common conductor carries approximately the same current as the line–to-neutral load currents of other conductors. Then we would have to agree that adding a third Phase to this configuration would only increase heat and complete the loading of the neutral, thereby requiring our new configuration to be counted as current carrying as well.

Likewise in section (c) a 3-Phase/4-wire circuit must be counted where certain loads are considered, but it fails to mention a 1-Phase/3-wire circuit under the same conditions, which should most certainly be counted as well if we follow the example of section (b).

And in (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).

From the wording here, (a) would only appear to be applicable to 3-Phase/4-wire circuits, as section (b) cancels the possibility of a 1-Phase/3-wire circuit, and the plural would cancel out the possibility of a 1-Phase/2-wire circuit. But if this was the intention, wouldn’t it be easier to cancel section (c) and add it’s criteria as an exception to section (a)?

And just what are we to consider with the word unbalanced? How is it used? Are we to relate it to branch feeds that are grouped together? Or, maybe an unbalanced panel feed that’s within spec? Are we to relate it to inductance/capacitance in the circuit? Can’t a simple 1-Phase/2-wire circuit be unbalanced? It depends on how we are to understand unbalanced.

My thoughts… the plural in (a) is a mistake, and the circumstance that gives us the least loaded neutral and the least heat generated would be the 1-Phase/2-wire circuit. I always ask the AHJ before planning a project. Some agree… some don’t. But to say the least, I feel this section needs some clarity.
 
Last edited:
Sir4bz said:
I agree with Infinity in regards to section (c) that according to the wording used, if the OP could prove loading to the inspector, the install could sell. I do however have arguments or points to consider concerning this section.

If we agree with section (b) that with a 1-Phase/3-wire circuit, a common conductor carries approximately the same current as the line?to-neutral load currents of other conductors. Then we would have to agree that adding a third Phase to this configuration would only increase heat and complete the loading of the neutral, thereby requiring our new configuration to be counted as current carrying as well.
Click to expand...

First (b) does not apply to a single phase 3 wire circuit. It only applies to a circuit that consists of two ungrounded conductors and the grounded conductor from a 3 phase wye system.

If we assume that the conductors used for the ungrounded and grounded conductor are identical we can define the heat as I?R based on the current in each of the conductors. When we use two ungrounded conductors along with a single grounded conductor from a 3 phase system the current on all of the conductors will be the same. If we have 20 amps on each of the ungrounded conductors we will also have 20 amps on the grounded conductor. Each of the conductors will be producing 400R watts of heat for a total of 1200R watts of heat. If we add the third ungrounded conductor to the circuit and load it to 20 amps, we will have the three ungrounded conductors each producing 400R watts of heat, but there will be no current on the neutral and it will produce no heat. We have the same 1200R watts of heat with the 3 ungrounded conductors loaded to 20 amps as we have with two ungrounded conductors loaded to 20 amps.
Likewise in section (c) a 3-Phase/4-wire circuit must be counted where certain loads are considered, but it fails to mention a 1-Phase/3-wire circuit under the same conditions, which should most certainly be counted as well if we follow the example of section (b).
Click to expand...
This is a special case for non-linear loads. Even with all three phases loaded identically, there will still be current in the neutral and we will have all 4 conductors producing heat. The current in the neutral can be as much as 173% of the current in the ungrounded conductors when the load is totally non-linear. Note that this section only applies to 3 phase wye systems.

[/quote]And in (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).

From the wording here, (a) would only appear to be applicable to 3-Phase/4-wire circuits, as section (b) cancels the possibility of a 1-Phase/3-wire circuit, and the plural would cancel out the possibility of a 1-Phase/2-wire circuit. But if this was the intention, wouldn?t it be easier to cancel section (c) and add it?s criteria as an exception to section (a)?[/quote]
(a) applies to both single and 3 phase circuits.

[/quote]And just what are we to consider with the word unbalanced? How is it used? Are we to relate it to branch feeds that are grouped together? Or, maybe an unbalanced panel feed that?s within spec? [/quote]
If we are talking about a single phase circuit, we are talking about un-equal currents in the two ungrounded conductors that are associated with a common grounded conductor...a multi-wire circuit. If one ungrounded conductor has 10 amps on it and the second has 20 amps on it, the grounded conductor will have 10 amps on it (the unbalanced current). The heat on the the hot with 10 amps will be 100R, the heat on the neutral will be 100R and the heat on the hot with 20 amps will be 200R, for a total heat of 400R. If we put 20 amps on one hot and nothing on the second hot, then the grounded conductor will also have 20 amps on it. Each of the conductors will be producing 400R watts of heat for total of 800R watts. The same amount of heat is produced when you have both ungrounded conductors will 20 amps of load.

The same thing with 3 phase circuits where the ungrounded conductors are not equally loaded, the "unbalanced" current will flow in the grounded conductor.

Are we to relate it to inductance/capacitance in the circuit? Can?t a simple 1-Phase/2-wire circuit be unbalanced? It depends on how we are to understand unbalanced.
Click to expand...
No, a two wire circuit can not be unbalanced...both conductors will always have the same amount of current.

My thoughts? the plural in (a) is a mistake, and the circumstance that gives us the least loaded neutral and the least heat generated would be the 1-Phase/2-wire circuit.
Click to expand...
With a single phase multiwire circuit, the most heat is produced under two conditions, first, with one ungrounded conductor fully loaded and no load on the second ungrounded conductor, and second with both ungrounded conductors fully loaded. These two conditions produce equal amounts of heat. All other combinations of loading produce less heat.
 
Status
Not open for further replies.
Top