When to count the neutral as current carrying

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DaleW

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My question is: Is the neutral required to be counted
as a current carrying conductor where there is 12 dedicated circuits (1 phase, 1 neutral and ground for each) supplying a 30A 120V twist lock receptacle for Data Center servers? Should Table 310.15 B (4)(a) be used or
310.15 B (4)(c)?
 
DaleW said:
12 dedicated circuits (1 phase, 1 neutral and ground for each) supplying a 30A 120V twist lock receptacle
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I assume there are receptacles, 12 of them.

For any given receptacle, the current that goes to the receptacle is equal to the current returning from the receptacle.

Just as the current that is in the "hot" (no pun intended) conductor heats that conductor, the same current (equal in magnitude) heats the neutral.

Neither (4)(a) or (4)(c) apply.
 
Your 12 circuits contain 24 circuit carrying conductors since both the ungrounded and grounded conductors carry an equal amount of current.
 
Bob NH said:
There is an old but still applicable paper on the effects of connecting 120 Volt power supplies as are used in computer systems. The effect goes beyond the load in the neutral conductors, and goes all the way back to the transformers.

http://www.itic.org/archives/articles/19870601/three_phase_power_source_overloading_caused_by_small_computers_and_electronic_office_equipment.php
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That was interesting Bob, thanks.

I found this quote interesting (bold print and underline by me):
"6. Where a shared neutral conductor for a 208Y/120 volt system must be used for multiple phases, use a neutral conductor having at least 1.73 times the ampacity of the phase conductors. A convenient way to accomplish this is to use paralleled neutral conductors, each having the same ampacity as the phase conductors."
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Wasn't there a discussion about this not long ago? Wasn't the consensuses that it was not OK?
 
Thank you all for the replies. I understand all of the replies, however my real question arose when sizing the conduit and applying the derating percentages of Table310.15 (B)(2)(a) for the application. 310.15(B)(4)(a) states the neutral is not counted when applying the table
and 310.15 (B)(4)(c) states I do count if a major portion of the load consists of non linear loads. Because I was using a dedicated circuit for each receptacle, does (c)
still apply, requiring me to count the neutral as a current carrying conductor for derating and conduit fill....
 
The conductor that you are calling a neutral counts as a current carrying conductor in your installation.
Most would call that conductor that you are referring to as a grounded conductor not a neutral, as it is carrying full current, not imbalance as a neutral would.
 
DaleW,

Sometimes we miss the thing right in front of us.

Unbalance current.

The current in the grounded conductors in your pipe (that you are thinking of) can only be in that ungrounded conductor. Part of the current is not going into a second hot, resulting in the remainder, the unbalance, going into the "neutral".

You are still not understanding my original reply:
al hildenbrand said:
For any given receptacle, the current that goes to the receptacle is equal to the current returning from the receptacle.

Just as the current that is in the "hot" (no pun intended) conductor heats that conductor, the same current (equal in magnitude) heats the neutral.

Neither (4)(a) or (4)(c) apply.
Click to expand...

(4)(a) and (4)(c) apply when there are two or more hots from different phases sharing a single neutral.
 
DaleW,

al makes the essential point: the rules about not counting a neutral as a current carrying conductor can only come into play in circuits where the neutral is _shared_, and then in only some cases.

For example, a 'full boat' (3 hots from each of the three phases, single shared neutral) supplying linear loads. In this case, current flowing in each hot leg gets balanced by the current flowing in the other legs; if all three circuits are equally loaded, then no current actually flows on the neutral. If you try to calculate out the case of 'maximum heating', you will find that only 3 of the 4 conductors will be carrying current.

In the circuits that you describe, you have no sharing of the 'neutral's, and thus no reduction in the number of current carrying conductors.

Another note: you must always count everything that goes into the pipe for purpose of conduit fill. Equipment grounding conductors don't carry current and never count for derating, but they certainly take up space in the conduit. Similarly, neutrals that are not counted as current carrying still take up space in the conduit and count for conduit fill.

In your case, all the conductors count for derating.

-Jon
 
Just for referance:
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 wires and the neutral 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 shall therefore be considered a current-carrying conductor.
Click to expand...

Not listed there is what you have, (2-wire circuits) both current carrying. (24 @ 45%)
 
hardworkingstiff said:
That was interesting Bob, thanks.

I found this quote interesting (bold print and underline by me):


Wasn't there a discussion about this not long ago? Wasn't the consensuses that it was not OK?
Click to expand...

If I recall, the post was on tie-ing multiple, small gauge neutral wires together in junction boxes (violation). For paralleling they have to be 1/0 or larger.

I used to think the oversize neutral was just theoretical until I saw an old panel that had the insulation melted off the neutrals- the phase conductors were fine, just old. The building had been converted to a data center.
 
"What did you come up with for conduit fill and wire size for these 12 30a circuits?"

This is probably a cost-driven solution.

Alternatives:
1. He could run 5 #10s (3 ungrounded, 1 grounded, and 1 EGC) in each of four 1/2" RNC or EMT, and split off at the ends to each of the three final circuits. Derating would be to 80% with the neutral.

2. If the calculated load doesn't exceed 28 Amps per circuit, then he could run #10 AWG; 6 phase conductors, 2 neutrals, and a ground in each of two 3/4" PVC Sch 40 or 3/4" EMT. The 8 CC Conductors would be derated to 70% of the 90C ampacity of 40 amps = 28 amps. The OCPD could still be 30 Amps using the "next size breaker" rule. The individual circuits could be branched off at a final box where each circuit goes to the individual circuits.

3. Rather than running 12 circuits in a conduit, do a feeder load calculation and run the appropriate conductors to a subpanel located where the 12 circuits branch off. Then run the 12 individual circuits as required.
 
Back to the original question. The 12 dedicated circuits are 120V. 120V circuits use the white neutral wire to carry current. So I would count the white neutral as a current-carrying wire. If it were 208/240V circuits the current-carrying conductors would be the black, red and/or blue wires. The white would be a true neutral. In that case I wouldn't count the white wire. But in either case, you always have atleast 2 current carrying-conductors in every circuit. So with 12 dedicated circuits you would have a minimum of 24 current-carrying conductors. Since the circuits are dedicated they all need to have their own white neutral. If the circuits were not dedicated you would share the neutral between the three phases. So NOT dedicated would go (black, red, blue, white) X 4 = 16 conductors in the raceway. Dedicated would go (black, white, red, white, blue, white) X 4 = 24 conductors in the raceway.
 
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