Neutral derating

[erock1]

So the debate is, is a line to neutral a current carrying conductor. The paragraphs in sec. 310.15 5 a and b are a little unclear. Could some one clear this up for me.

 

[kwired]

I assume you are talking about 310.15(B)(5).

The key here that isn't directly stated is to consider how much heat is contributed to the raceway or cable containing the conductors in question.

If you have a simple 120 volt 2 wire circuit - each conductor carries same amount of current - each conductor produces similar amount of heat within raceway or cable. If you have a 120/240 volt three wire multiwire branch circuit - the neutral only carries unbalance of the two "hot conductors". No matter how you distribute the load here you never have a situation that creates more heat then if that load were on two of the three conductors.

If A = 10A and B = 10A then N = 0A
If A = 5A and B = 15 A then N = 10A
If A = 20 and B = 0 then N=20

Either way you still have same heating within the raceway or cable - just is introduced in different places

Three phase wye systems however must have all three phases balancing across a neutral to cancel total heating effects, that is why a two wires and neutral considers the neutral to be current carrying in that case - as it doesn't carry the imbalance

 

[erock1]

I guess what I'm trying to figure out is for just a single circuit, like you stated if they is 20 amp on a phase and 0 on b phase the neutral has 20 amp of current. That being said then 1 hot and 1 neutral will always have the same current on it, meaning that I would have to derate the neutral on a single circuit.

 

[Smart $]

I guess what I'm trying to figure out is for just a single circuit, like you stated if they is 20 amp on a phase and 0 on b phase the neutral has 20 amp of current. That being said then 1 hot and 1 neutral will always have the same current on it, meaning that I would have to derate the neutral on a single circuit.

In typical US wiring, there are only three general scenarios where the neutral DOES NOT count as a current carrying conductor:

1. a 120/240V 1Ø 3W service, feeder, or multiwire branch circuit*,
2. a 208Y/120V 3Ø 4W service, feeder, or multiwire branch circuit, and
3. a 480Y/277V 3Ø 4W service, feeder, or multiwire branch circuit.

*Includes that portion of a 240/120V 3Ø 4W high-leg delta system.


The neutral of branch circuits cannot be reduced in ampacity compared to the ungrounded conductors... so you just match the same parameters as the ungrounded conductors, whether the neutral is counted as a current carrying conductor or not.

 

[Carultch]

So the debate is, is a line to neutral a current carrying conductor. The paragraphs in sec. 310.15 5 a and b are a little unclear. Could some one clear this up for me.

The way I think of it, is if neutral is a mandatory part of the return path of current, it counts. Examples: single phase/2-wire, single phase/3-wire when drawn from 2-phases of a 3-phase WYE system, corner grounded 3-phase systems.

If the neutral is simply there in the event that you do have imbalances on the phases, then as long as loads are linear, no matter what the balancing, it doesn't count. In otherwords, if you can transmit the power without any current on the neutral, pretending that you could balance the phases. Examples: the standard residential single phase/3-wire Edison system, and any 3-phase WYE system. The catch is that loads need to be linear, so that harmonics don't significantly accumulate on the neutral. The idea is that the total heat in all 4 wires when unbalanced will not exceed the heat in 3 balanced wires only.

If the "major portion of the loads" is nonlinear, harmonic currents (particularly those that are multiples of 3 of the fundamental) accumulate on the neutral and cause it to count. The big question here, is what metric do I use to determine just how nonlinear it has to be? In otherwords, what do I look for in my O-scope?

 

[Dennis Alwon]

I guess what I'm trying to figure out is for just a single circuit, like you stated if they is 20 amp on a phase and 0 on b phase the neutral has 20 amp of current. That being said then 1 hot and 1 neutral will always have the same current on it, meaning that I would have to derate the neutral on a single circuit.

I think you mean that you would count the neutral as a current carrying conductor. Yes you would.

 

[Smart $]

I guess what I'm trying to figure out is for just a single circuit, like you stated if they is 20 amp on a phase and 0 on b phase the neutral has 20 amp of current. That being said then 1 hot and 1 neutral will always have the same current on it, meaning that I would have to derate the neutral on a single circuit.

I think you mean that you would count the neutral as a current carrying conductor. Yes you would.

The way he wrote that makes it hard to determine exactly what he means. If you go by the context of the first sentence for the second... he still has a 3-wire circuit, and the neutral would not count even though it has 20A of current on it...because the b phase conductor has zero, and it does count. The reason for the neutral not counting is that you never have more than an arithmetic sum of 40A current on all three wires.

 

[Tony S]

Physics, not code.

For 1Ph Line = Neutral

For 3Ph I_n = √((I_a²+I_b²+I_c²)-((I_a*I_b)+(I_a*I_c)+(I_b*I_c)))

 

[iwire]

Physics, not code.

For 1Ph Line = Neutral

For 3Ph I_n = √((I_a²+I_b²+I_c²)-((I_a*I_b)+(I_a*I_c)+(I_b*I_c)))

Regardless of that formula there are times the NEC does not consider a neutral current carrying even if it is carrying current.

It is a code question.

 

[infinity]

This may help:

Neutral Conductors:
Here's some examples of when to count and not count the neutral as a current
carrying conductor or CCC:
3Ø- 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.
1Ø- 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 Meier 2014

 

[Smart $]

...For 1Ph Line = Neutral...

That is not true for a 1Ø 3-wire circuit.

 

[kwired]

I guess what I'm trying to figure out is for just a single circuit, like you stated if they is 20 amp on a phase and 0 on b phase the neutral has 20 amp of current. That being said then 1 hot and 1 neutral will always have the same current on it, meaning that I would have to derate the neutral on a single circuit.

You need to look at a bigger picture here. As I said before it is all about the heat developed within the raceway or cable.

If you have 20 amp on A, zero on B then neutral is carrying an imbalance of 20. The raceway still has two conductors carrying 20 amps each.

If you have 20 amp on A, 20 on B then neutral imbalance is now zero - still only two conductors in the raceway carrying 20 amps.

If you have uneven load on A and B then the neutral is only carrying the unbalance - heating effects within the raceway are still only equal to what two conductors would produce because one "hot" will be higher, the other will be lower and the neutral is the imbalance.

Pull two hots and a corresponding neutral for each in the raceway and now all of them are always contributing heat when carrying a load - there is no balancing effect on a common neutral anymore.

Improperly connect the MWBC, example both "hots" on line A, and you don't have a balancing neutral instead if you had 20 amps on each "hot" conductor then the neutral would be carrying 40 amps, and adding third conductor heating to the raceway.

 

[roger]

I guess what I'm trying to figure out is for just a single circuit, like you stated if they is 20 amp on a phase and 0 on b phase the neutral has 20 amp of current. That being said then 1 hot and 1 neutral will always have the same current on it, meaning that I would have to derate the neutral on a single circuit.

Here are some simple illustrations that may help.

A two wire circuit

A true neutral

Currents adding together

Roger