Unbalanced current

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jwoody

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For the purpose of counting "current carrying conductors" in a raceway, per NEC 310.15(B)(3)(a): When does the neutral carry only "the unbalanced current from other conductors of the same circuit" (310.15(B)(5)? I would guess that this applies to 2 or 3 pole dedicated equipment that requires a neutral. Any help is greatly appreciated.
 
This may help:


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
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does not have to be dedicated equipment. Could anything that is a Multi-wire branch circuit.

example
If you have a two pole breaker on a multi-wire circuit. Feeding a bed room on one phase other feeds another bed room on other phase.

1: If Both circuit are being used then the neutral current is canceled out, If phase a has 15 amp, phase B has 10 then Current on Neutral is only 5 amps. Still the heat output of the circuit is 15 amps.

2: If only Phase A is Using 15 amps and nothing in Phase B is on then you still have 15 amps of heat output its, Its just now on the Neutral instead of Phase B/ and neutral like example 1

3: if Phase A is using 15 amps And phase B is also Using 15 amps. Then you have not current on the Nuretral.
Still 15 amps of Heat output.

I hope I said that so it's understandable.

I ran out of time on editing my first post***
 
If they equipment is Nonlinear loads like infinity posted then you have to count them. Since they won't cancel out the current on the Neutral.They will actually have more current on the nuetral in waves(additive). If I remember correctly.
 
AC\DC said:
example
If you have a two pole breaker on a multi-wire circuit. Feeding a bed room on one phase other feeds another bed room on other phase.

1: If Both circuit are being used then the neutral current is canceled out, If phase a has 15 amp, phase B has 10 then Current on Neutral is only 5 amps. Still the heat output of the circuit is 15 amps.

2: If only Phase A is Using 15 amps and nothing in Phase B is on then you still have 15 amps of heat output its, Its just now on the Neutral instead of Phase B/ and neutral like example 1

3: if Phase A is using 15 amps And phase B is also Using 15 amps. Then you have not current on the Nuretral.
Still 15 amps of Heat output.
Click to expand...
This only applies to a 120/240, 1Ø system. In #3 with a 3Ø, 208Y/120 system the neutral current would be 15 amps.
 
Correct unless unless it was a three phase multi-wire circuit then it still applies.


I don’t work a lot on 3 phase so I assume he talking about single phase
 
jwoody said:
For the purpose of counting "current carrying conductors" in a raceway, per NEC 310.15(B)(3)(a): When does the neutral carry only "the unbalanced current from other conductors of the same circuit" (310.15(B)(5)? I would guess that this applies to 2 or 3 pole dedicated equipment that requires a neutral. Any help is greatly appreciated.
Click to expand...
Here is a simplistic example dedicated equipment.
A 240V appliance,
It has a couple of 120V loads, a clock, timer, lights etc. All one the one leg of the 240V. As well as its straight 240V load.
Now say you total load is 20A, your 240V loads are 15A and your 120V loads are 5A.
Your neutral will be expected to carry the 5A from the 120V load that is above the balanced 240V load. (The Unbalanced Current)

Now here's an example of how you might experience a MWBC (typical 2 hot 1 neutral receptacle loads) best described when it goes wrong. (Unintentionally created MWBC)
A 120V circuit
Someone splices neutral from 2 or more circuits together
Turn off power from one circuit the 2nd circuit remains energized, a load is applied to second circuit
The neutral of the de-energize circuit will still carry the split of the neutral current associated with the energized circuit (parallel neutrals)
Now a properly configured MWBC
2 loads one 15A the other 10A
If loads are present on both phases of the of the MWBC (series) some of the current used of the one side of the circuit will feed through to the other side (nature of alternating current) reducing current to the 5A difference. Current through one load into shared neutral then paralleled (like the multiple neutral above) into the other load and returning. (Alernating current) (15A - 10A =5A)
At any other time if only one load is applied (say the 15A) there is no means for the current of that load to return on the opposing hot leg so full amount of that current will return on the neutral (15A - 0A = 15A)

Now another faulty installation issue.
MWBC both on same phase.
One 15A load and one 10A (parallel) creates a cumulative of 25A because there is no alternating of the current flow (both on same phase to cancel some of that load) the neutral will have to carry the full load of both circuit or full 25A, overloading the normal conductor sized for the 15A presumed maximum load. (15A + 10A =25A)
 
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