unbalanced neutral

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normbac

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Was reading about unbalanced neutral on previous posts and came across this quote from Augie
(The exception is where, on a three phase system, the major portion of the load is non-linear (electronics, ballasts, etc) you must still count the neutral.
On a normal run with 20 amp circuits and #12 THHN you have to exceed 9 ccc to create a problem, so a good rule of thumb is just keep your conduit fill to 9 wires (+ equip ground) and avoid the problem.)

Question #1 doesnt Table 310.15(B)(2)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable necessary when using 9 conductors?

Question #2 so if one neutral and three hots are used on ballast lighting from 3 different phases of a 277v system the neutral carries current?

Thanks for any clarity on this
 
normbac said:
Was reading about unbalanced neutral on previous posts and came across this quote from Augie
(The exception is where, on a three phase system, the major portion of the load is non-linear (electronics, ballasts, etc) you must still count the neutral.
On a normal run with 20 amp circuits and #12 THHN you have to exceed 9 ccc to create a problem, so a good rule of thumb is just keep your conduit fill to 9 wires (+ equip ground) and avoid the problem.)

Question #1 doesn't Table 310.15(B)(2)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable necessary when using 9 conductors?
Click to expand...

Yes, adjustments are required by the NEC when there are more than three CCC's in a in a raceway but that doesn't necessarily mean a larger wire is needed, start with # 12 conductors from the 90 deg column @ 30 amps and nine CCC's still leaves the # 12 at 21 amps

normbac said:
Question #2 so if one neutral and three hots are used on ballast lighting from 3 different phases of a 277v system the neutral carries current?
Click to expand...

Technically yes, but in reality there would probably never be enough additive current to hurt anything.

Roger
 
normbac, the point I was trying to make was the neutral on a 3 phase wye circuit must be counted as a ccc when the majority of the load in nonlinear.
Using #12 THHN, one could but 3 sets of MWBC (phase A-B-C & N) in a conduit for a total of 12 wires. If the load is linear, only the 9 phase conductors would count as ccc so the 30 amp conductors would be derated to 21 amps {70% of 30 per 310.15(B0(2)}. If however, the load is nonlinear then the neutrals are ccc and the #12s now have an ampacity of 15 amps. For that reason, some folks just use a rule of thumb not to install more than 9 #12s
 
augie47 said:
normbac, the point I was trying to make was the neutral on a 3 phase wye circuit must be counted as a ccc when the majority of the load in nonlinear.
Using #12 THHN, one could but 3 sets of MWBC (phase A-B-C & N) in a conduit for a total of 12 wires. If the load is linear, only the 9 phase conductors would count as ccc so the 30 amp conductors would be derated to 21 amps {70% of 30 per 310.15(B0(2)}. If however, the load is nonlinear then the neutrals are ccc and the #12s now have an ampacity of 15 amps. For that reason, some folks just use a rule of thumb not to install more than 9 #12s
Click to expand...
so if you were to take three different phases to feed three sets of 277 single phase ballast lights the shared neutral would count per nec
 
If a majority of the load was ballasts, then I would say Yes. The neutrals would have to be counted and you would be limited to (2) sets of 3 phase 4w MWBCs
 
In the OP's post we are looking at branch circuit neutrals.
250.66 is used for sizing grounding electrode conductors. It does come into play in that a SERVICE neutral can not be any smaller than the service grounding electrode as noted in 250.24.
Branch circuit neutrals are sized to the corresponding phase conductor and it's overcurrent device.
Service neutrals are sized per 220.22 with 250.66 being a minimum for service neutrals.
 
augie47 said:
In the OP's post we are looking at branch circuit neutrals.
250.66 is used for sizing grounding electrode conductors. It does come into play in that a SERVICE neutral can not be any smaller than the service grounding electrode as noted in 250.24.
Branch circuit neutrals are sized to the corresponding phase conductor and it's overcurrent device.
Service neutrals are sized per 220.22 with 250.66 being a minimum for service neutrals.
Click to expand...

I see the difference now. Thanks for clearing up. So for branch circuits is there a table for selecting, or should they always match the phase conductors.
 
In a lot of applications, and on most drawings that I've seen, neutral is sized same as phase conductors. However, if there are heavy harmonic distortions or other conditions in a special application, I've seen the neutral sized upto 200% of phase conductors.
 
PhaseShift said:
I see the difference now. Thanks for clearing up. So for branch circuits is there a table for selecting, or should they always match the phase conductors.
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No there is no such table. Sizing the neutral can vary depending on the load being served. For service equipment look at art. 250.24(C)(1).
 
Dennis Alwon said:
No there is no such table. Sizing the neutral can vary depending on the load being served. For service equipment look at art. 250.24(C)(1).
Click to expand...

So for service equipment the neutral size would be somewhere between the ungrounded conductor size, and those listed in 250.66. 250.24(C) references grounded conductor brought to equipment. How does this differ from an EGC, which is pertaining to 250.122? How does this section apply to the neutral?

I've seen some residential services where there was just the (2) ungrounded conductors and then a neutral/ground wire as (1) wire? Is this legal? Does it depend on where the ground rod is located?
 
PhaseShift said:
I've seen some residential services where there was just the (2) ungrounded conductors and then a neutral/ground wire as (1) wire? Is this legal? Does it depend on where the ground rod is located?
Click to expand...
From the utility transformer all the way to the main disconnect, one conductor is all there should be. It is a neutral, it is grounded, and can and should be used for bonding service-related enclosures.

The GEC may be connected anywhere along that conductor, from the point of attachment, to the meter enclosure (if allowed) to the service disconnect, whether within or ahead of the main panel.
 
LarryFine said:
From the utility transformer all the way to the main disconnect, one conductor is all there should be. It is a neutral, it is grounded, and can and should be used for bonding service-related enclosures.

The GEC may be connected anywhere along that conductor, from the point of attachment, to the meter enclosure (if allowed) to the service disconnect, whether within or ahead of the main panel.
Click to expand...

What if the grounding electrode, GEC, and system bonding jumper were located at the transformer? Then would'nt you need a 4th conductor as an EGC since the neutral and ground in the main panel would not be bonded?
 
On a SDS yes. Utility (service) transformers are handled differently.
 
augie47 said:
If a majority of the load was ballasts, then I would say Yes. The neutrals would have to be counted and you would be limited to (2) sets of 3 phase 4w MWBCs
Click to expand...
Ummm... depends on the type of ballast. If electronic, yes. So-called "magnetic", no.
 
augie47 said:
On a SDS yes. Utility (service) transformers are handled differently.
Click to expand...

So for a transformer with the GEC and system bonding jumper at the transformer then we would need at fourth EGC between transformer and panel correct? And if the GEC and system bonding jumper were at the panel then we would not need a fourth wire and only require (2) ungrounded phases and the neutral. Is this correct?

Do most or all of the utilities take the second approach above as you mentioned, and only need just the neutral?
 
PhaseShift said:
So for a transformer with the GEC and system bonding jumper at the transformer then we would need at fourth EGC between transformer and panel correct?
Click to expand...
This could only be an SDS... and yes you'd need a grounding conductor to the panel (therein isolated from the grounded [neutral] conductor), but it is called an Equipment Bonding Jumper.

PhaseShift said:
And if the GEC and system bonding jumper were at the panel then we would not need a fourth wire and only require (2) ungrounded phases and the neutral. Is this correct?
Click to expand...
For a service, yes. For an SDS, no... you'd still need an EBJ.

PhaseShift said:
Do most or all of the utilities take the second approach above as you mentioned, and only need just the neutral?
Click to expand...
AFAIK, yes.
 
The diagram that is confusing me is the one attached. These represent the two different cases where the grounding electrode and system bonding jumper are located at different locations for a SDS. In these diagrams what are the conductors that I have questioned? Are these the EBJ's that were referenced?
 
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