Neutral as Current Carrying Conductor

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jonny1982

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CA
I'm a little unclear of the exception to neutral counting as a current carrying conductor when derating. It says only when the neutral is carrying only the unbalanced load.....etc. What exactly does this mean.

Ex.: Adding a panel for a video game store off an existing meter stack. Does the feed from the meter stack to the 200 amp panel qualify for the this rule? Let's say it is 3 phase, 120/208 power. It's in an emt conduit. There are 3 hots, a neutral, and a ground. Are there 4 current carrying conductors in that pipe, or just 3?

Thanks
 

Dennis Alwon

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I guess it depends. If the panel feeds all the video equipment then I am guess that the loads are nonlinear. In this case the neutrals would be counted.

Normally a feeder with 3 hots and a neutral would not have 4 current carrying conductor but rather it would have 3 current carrying conductor. If the loads are nonlinear like those from fluorescents and computers, etc then you count the neutral as a current carrying conductor.
 

jonny1982

Member
Location
CA
I guess it depends. If the panel feeds all the video equipment then I am guess that the loads are nonlinear. In this case the neutrals would be counted.

Normally a feeder with 3 hots and a neutral would not have 4 current carrying conductor but rather it would have 3 current carrying conductor. If the loads are nonlinear like those from fluorescents and computers, etc then you count the neutral as a current carrying conductor.

So a feed for a lighting panel would most likely count the neutral as a ccc?
 

Dennis Alwon

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So a feed for a lighting panel would most likely count the neutral as a ccc?
Yes, if the lighting load was nonlinear. If they were incandescents lights then it would not count. It would also have to be the majority of the loads being nonlinear to force us to count the neutral.
 

mbrooke

Batteries Included
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United States
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Technician
I would count the neutral. If you know the loads are very harmonic you can always upsize the neutral. They even make AC cable for that.
 

infinity

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Location
New Jersey
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Here's a list of when the neutral is a CCC by system type:

Neutral Conductors:
Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC:

3?- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception, *if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.

1?- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.
 
Here's a list of when the neutral is a CCC by system type:

So if you have a 120/208 3 phase wye system. If you have all 3 hots along with a neutral for each, does this count as 6 current carrying conductors? And basically if your working in an office building almost everything is going to be non-liner loads...computers, florescent lighting, etc...
 
I am personally tired of this question! I really want the correct answer. Everyone has differing opinions on this. I have not found a common answer that everyone agrees with.
:rant::rant::rant:
 

charlie b

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Lockport, IL
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I am personally tired of this question! I really want the correct answer. Everyone has differing opinions on this. I have not found a common answer that everyone agrees with.
OK fine. Here is an answer that is correct, and for which there are no differing opinions, and that everyone will agree with. Are you ready? The answer is NEC 310.15(B)(5). :happyyes:

 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
150311-1116 EDT

wireman74:

Use logic. If you have X hots and for each hot it has its own neutral, then for each hot there are two current carrying conductors. Thus, a total of 2*X current carrying conductors.

If you have a multiphase system with 1 neutral and X hots, and if the character of the loads is such that no neutral current flows, then the total number of current carrying conductors is X.

If the loads produce neutral current, then you need to know the magnitude of the neutral current to decide the size of the neutral conductor, and the neutral conductor will have to be counted as a current carrying conductor. You could have more power dissipation in the neutral conductor than in any one of the hot conductors, and thus the neutral conductor might be equivalent to more than one current carrying conductor. The temperature rise in a conduit is a function of the total power dissipation within that conduit.

.
 

infinity

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New Jersey
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I am personally tired of this question! I really want the correct answer. Everyone has differing opinions on this. I have not found a common answer that everyone agrees with.
:rant::rant::rant:

The answer was in post #5, read it again. :)
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
First you need to understand the basic principal, which everyone should be able to agree upon, then you need to understand the factors that are opinion based and thus not subject to easy agreement.

1) The neutral circuit conductor will in general always carry some current. If the neutral is run, even if it isn't connected to a load, there will be _some_ current flow.

2) For purposes of calculating the derating of multiple conductors in a conduit, there are some situations where you do not need to _count_ the neutral. It is still present, and still carrying some current, but you don't need to count it when applying this rule. Rather than thinking 'I don't have to count the _neutral_, it might be easier to think 'I get to subtract 1 from the number of current carrying conductors in these special situations.'

The situations where you don't need to count the neutral (or get to subtract 1 from the total of current carrying conductors) are when you have a balanced set of 'hot' conductors and the net neutral current can balance out to zero when all of the 'hot' conductors are equally loaded.

If you look at the table that infinity posted, you will see that for a 3 phase 4 wire circuit you have 4 conductors but only count 3, and for a single phase 3 wire circuit you have 3 conductors but only count 2.

3) The situation that breaks the balance issue is when you have 'non-linear loads'. These are loads which can cause harmonic currents to flow. The problem here is that even when the voltages are perfectly balanced and properly out of phase, harmonics can be _in phase_. Even when the 'fundamental' currents balance on the neutral, the harmonic currents can add up. If you have significant non-linear loading, then lots of neutral current will flow even with a perfectly balanced load, so you can't get a 'discount' for a perfectly balanced system.

Just what constitutes a significant enough non-linear load is somewhat open to discussion, so you won't get a firm answer.

-Jon
 

jtinge

Senior Member
Location
Hampton, VA
Occupation
Sr. Elec. Engr
Here's a list of when the neutral is a CCC by system type:

Neutral Conductors:
Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC:

3?- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
[highlight]C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception, *if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.[/highlight]

1?- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.​

Rob, I saw similar information from MH (EC&M Oct 2011) but he stated [highlight]"The neutral conductor of a 4-wire, 3-phase, 120/208V or 277/480V wye-connected system is considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) if more than 50% of the neutral load consists of nonlinear loads [310.15(B)(5)(c)]."[/highlight] This seems different than what you described above.

Can you clarify what you mean by "if the current is more that 50% nonlinear"?

I think what is meant and the way I interpret the requirement is "if more than 50% of the loads, connected to the neutral, generate non-linear currents, then the neutral must be considered a CCC.

Is this a correct interpretation? I don't have the code with me, is the 50% threshold in the code? If not, what is the basis for the more than 50% value?
 

Dennis Alwon

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Retired Electrical Contractor

Rob, I saw similar information from MH (EC&M Oct 2011) but he stated [highlight]"The neutral conductor of a 4-wire, 3-phase, 120/208V or 277/480V wye-connected system is considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) if more than 50% of the neutral load consists of nonlinear loads [310.15(B)(5)(c)]."[/highlight] This seems different than what you described above.

Can you clarify what you mean by "if the current is more that 50% nonlinear"?

I think what is meant and the way I interpret the requirement is "if more than 50% of the loads, connected to the neutral, generate non-linear currents, then the neutral must be considered a CCC.

Is this a correct interpretation? I don't have the code with me, is the 50% threshold in the code? If not, what is the basis for the more than 50% value?

What is written by Rob and mike holt are the same.

Article 310.15(B)(5)(c) mentions the major portion of the load. Thus any amount over 50% is considered a major portion of the load, IMO
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
Unlike wireman74, I don't see where can be much difference of opinion on Rob's "chart". That pretty well sums it up.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Neutral Conductors:
Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC:

3?- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
[highlight]C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception, *if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.[/highlight]

1?- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.​

Rob, I saw similar information from MH (EC&M Oct 2011) but he stated [highlight]"The neutral conductor of a 4-wire, 3-phase, 120/208V or 277/480V wye-connected system is considered a current-carrying conductor for conductor ampacity adjustment of 310.15(B)(3)(a) if more than 50% of the neutral load consists of nonlinear loads [310.15(B)(5)(c)]."[/highlight] This seems different than what you described above.

Can you clarify what you mean by "if the current is more that 50% nonlinear"?

I think what is meant and the way I interpret the requirement is "if more than 50% of the loads, connected to the neutral, generate non-linear currents, then the neutral must be considered a CCC.

Is this a correct interpretation? I don't have the code with me, is the 50% threshold in the code? If not, what is the basis for the more than 50% value?

Although worded slightly differently they are the same. If you have a major portion of the load that is non-linear, (more than 50%) then it counts as a CCC.
 

jtinge

Senior Member
Location
Hampton, VA
Occupation
Sr. Elec. Engr
Although worded slightly differently they are the same. If you have a major portion of the load that is non-linear, (more than 50%) then it counts as a CCC.

Thanks for the clarification. Is more than 50% stated in the code or generally accepted practice?
 

kwired

Electron manager
Location
NE Nebraska
Thanks for the clarification. Is more than 50% stated in the code or generally accepted practice?
Dennis answered that in post 14. It does not specifically say 50% but general meanings of words means a majority must be at least 51%, though when rounding off it is generally accepted to round halves UP to next whole number.
 

GoldDigger

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Placerville, CA, USA
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Dennis answered that in post 14. It does not specifically say 50% but general meanings of words means a majority must be at least 51%, though when rounding off it is generally accepted to round halves UP to next whole number.

I can still see a problem applying that.
A single load with 50 percent THD would qualify.
But would 10 out of 20 loads, each with 5 percent THD (strictly speaking still non linear) also qualify.
Is it loads that aggegated together have more non linear than linear current or just more wattage of loads that are at all non linear than wattage of purely linear loads?
 

kwired

Electron manager
Location
NE Nebraska
I can still see a problem applying that.
A single load with 50 percent THD would qualify.
But would 10 out of 20 loads, each with 5 percent THD (strictly speaking still non linear) also qualify.
Is it loads that aggegated together have more non linear than linear current or just more wattage of loads that are at all non linear than wattage of purely linear loads?

I see your problem. My opinion - take the total current of the neutral conductor in question and determine what percentage of it is from non linear loads, but I will consider other approach at this so fire away if you have one.
 
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