NEC 310.15 E neutral conductor

Jpflex

Jpflex

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Victorville
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Electrician commercial and residential
NEC 310.15 E provides 3 scenarios where a neutral is to be considered current carrying. This is especially useful for adjusting a conductors ampacity when bundled such as in conduit with more than 3 current carrying conductors.

The code says:

1) a neutral that carrys only the unballace current from the same circuit is NOT counted

2) in a 3 phase, 4 wire WYE system, the neutral is counted

3) in a 3 phase, 4 wire WYE system, serving nonlinear loads, the neutral is counted


HOWEVER NEC code point 3) DOES NOT NEED TO BE HERE BECAUSE NEC ALREADY SAID ALL 4 WIRE Y SYSTEMS WITH A NEUTRAL ARE TO HAVE NEUTRALS COUNTED REGARDLESS OF LOAD TYPE

Second, why do we not see single or "split phase" systems included here. A single phase 2 wire with ground circuit (other than a multi-wire branch circuit) can be shown with a clamp on ammeter to have equal current in the grounded neutral as compared to the ungrounded wire

If for example, wires are ran in a conduit for two 120V receptacles supplied from a single phase system (1 hot and 1 neutral per receptacle) , why is NEC not listing this as a neutral to be counted as current carrying. (Excluding MWBC)?
 
#2 doesn't say what you posted, it says

(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).
Click to expand...
 
Using the formular below figure it with 10 amps on three phase conductors and then with just two.

Nuetral = √(a² + b² + c² - ab - bc - ac)
 
roger said:
#2 doesn't say what you posted, it says
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Where is (b), i dont see this in my 2023 code book.

3) does say what i posted. Without copying word for word, it is saying that 4 wire 3 phase Y systems shall have its neutral counted if serving non linrar loads
 
roger said:
Using the formular below figure it with 10 amps on three phase conductors and then with just two.

Nuetral = √(a² + b² + c² - ab - bc - ac)
Click to expand...
Ok ill check with c2 and ac eliminated for single phase
 
Jpflex said:
Number 2 was not word for word but implys only using 2/3 phases
Click to expand...
Yes a MWBC with two ungrounded and one neutral conductor would be three CCC's from a 3 phase Wye system. Three ungrounded and a neutral would also be 3 CCC's from the same system.
 
2 hots and a neutral supplied from a wye - neutral is a CCC

3 hots and a neutral supplied from a wye when there are non-linear loads - neutral is a CCC

3 hots and a neutral and linear loads - neutral is *not* a CCC
 
2 and 3 may represent similar scenatios

2) 2/3 phases and count neutral from a y system supplying single phase loads from a MWBC

3) 4 wire, 3 phase y circuit count neutral when supplying non linear loads. Non linear loads utilizing neutral can be taken from 1/3 phases and neutral just like above

Otherwise 3 phase loads typically use 3/3 phases but not neutral unless for unballance current return
 
infinity said:
Yes a MWBC with two ungrounded and one neutral conductor would be three CCC's from a 3 phase Wye system. Three ungrounded and a neutral would also be 3 CCC's from the same system.
Click to expand...
Wait i thought the neutal in a MWBC was not counted because it carries only the unballance. Does this only apply to single phase?
 
Jpflex said:
Wait i thought the neutal in a MWBC was not counted because it carries only the unballance. Does this only apply to single phase?
Click to expand...
Look below:


Neutral Conductors:
Click to expand...

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
Click to expand...
 
Jpflex said:
Wait i thought the neutal in a MWBC was not counted because it carries only the unballance. Does this only apply to single phase?
Click to expand...
You're not reading what's being posted. Notice when members are saying two ungrounded conductors.
 
jaggedben said:
2 hots and a neutral supplied from a wye - neutral is a CCC

3 hots and a neutral supplied from a wye when there are non-linear loads - neutral is a CCC

3 hots and a neutral and linear loads - neutral is *not* a CCC
Click to expand...
Ok but is 310.15 E 1 referring specificslly to MWBC on single phase systems without simply saying this?

1) a neutral conductor that carries only the unballanced current from other conductirs of the same circuit shall NOT be requited to be counted as current carrying

My concern is when to count neutrals on single phase systems. To my understanding, a MWBC in this sitiation does not have to count neutral

But a single phase with 2 circuits with 2 individual hots and 2 indipendent grounds would?
 
Jpflex said:
If for example, wires are ran in a conduit for two 120V receptacles supplied from a single phase system (1 hot and 1 neutral per receptacle) , why is NEC not listing this as a neutral to be counted as current carrying. (Excluding MWBC)?
Click to expand...
It does say that 4 CCCs are counted, it's just that 310.15(E)(1) is not as clear as it could be. When the text refers to "unbalanced current from other conductors of the same circuit," that plural means there is more than one other conductor. A 2-wire circuit only has one other conductor, so 310.15(E)(1) never applies to a 2-wire circuit.

Really what 310.15(E)(1) and (2) should say is that you don't count the neutral when your circuit is a "full boat." Meaning some number of ungrounded conductors plus a neutral conductor, and the sum of the voltages of the ungrounded conductors (relative to the neutral) is zero.

Any single conductor in a collection of multiple conductors comprising a circuit will carry the unbalance current of all the other conductors. It's just that when that voltage sum criterion is satisfied, and currents are in phase with voltage, the unbalance current on the neutral will be 0 when the ungrounded conductors are balanced.

Cheers, Wayne
 
Jpflex said:
Wait i thought the neutal in a MWBC was not counted because it carries only the unballance. Does this only apply to single phase?
Click to expand...
In a single phase MWBC, aka "split phase", the neutral only carries the unbalanced load. But when using 2 out of 3 phases with a neutral, the neutral will carry almost as much current as the most loaded phase.
 
infinity said:
Look below:
Click to expand...

CoolWill said:
In a single phase MWBC, aka "split phase", the neutral only carries the unbalanced load. But when using 2 out of 3 phases with a neutral, the neutral will carry almost as much current as the most loaded phase.
Click to expand...
So if a neutral carries nearly as much current as the 2/3 phase line currents of a 3 phase system (assuming Y) then i would expect that you could not use a MWBC sharing the sane neutral because neutral current would be double here?

Each circuit would have to be supplied with its own neutral?
 
Jpflex said:
To my understanding, a MWBC in this sitiation does not have to count neutral

But a single phase with 2 circuits with 2 individual hots and 2 indipendent grounds would?
Click to expand...
You've got it.

1743706162306.png

1743706088465.png
 
Jpflex said:
So if a neutral carries nearly as much current as the 2/3 phase line currents of a 3 phase system (assuming Y) then i would expect that you could not use a MWBC sharing the sane neutral because neutral current would be double here?

Each circuit would have to be supplied with its own neutral?
Click to expand...
Read post #11 and perform the calculation I gave you in post #3. This should not be above your capabilities
 
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