Are my neutrals undersized

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I have ten parallel feeds from utility owned xmfr going to 277/480 3phase 4000A MDP underground. Each phase (A,B,C PHASE) is 500kcmil while the neutrals are all 350kcmil. Article 310.4 states that the wire must be of the same size. According to my understanding the neutrals are undersized and could this cause a triplen effect to the third, sixth, ninth etc. I no that the 500"s are not enough due that they are rated for only 3800A. Will tackle that problem later. Primary concern are the neutrals. If they are undersized what is the long term effect or concerns?
 

Dennis Alwon

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I don't believe 310.4 states what you are thinking. I believe it states that each neutral must be the same size characteristic, etc. Otherwise you would need the egc to be 500 KCM also.

I must say that I don't install parallel conductors so I may be missing something.

The neutral will depend on the load as to whether it is too small. My guess is that it is fine.
 

ed downey

Senior Member
Location
Missouri
310.4 is just saying that whatever size Grounded conductor you have installed per 230.42(C) you need to install the same size in each of the parallel feeders.

Therefore the Grounded conductor does not have to be the same size as the Phase Conductors.

-Ed
 

don_resqcapt19

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justanelectrician said:
please read 310.4 nec2005. it states "be the same size circular mil". am i reading the code right that all ten parallel neutrals are undersized?
Take another look at 310.4.
310.4 ... Conductors of one phase, polarity, neutral, or grounded circuit conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, or grounded circuit conductor to achieve balance. ...
You need to do or see the load calculation for the service to know if the grounded conductor is the corrrect size.
 

charlie b

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Take a look at 220.61. It gives you an opportunity to size the neutral wire differently than the ungrounded wire.
 

beanland

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Vancouver, WA
Ampacity

Ampacity

Wouldn't ten 500kcmil copper be good for 4300A IF the wire, lugs and conduit are rated for 90C?

As long as each of the ten conduits has a 350kcmil neutral, the smaller neutral is fine, normal, and acceptable.

Are the wires form the transformer being installed for ownership by the utility? If so, their sizing may be smaller than NEC. Utilities size the wire to the transformer which is often 1/3 to 1/2 the main rating. This is legal because the wire is outside the building and owned and maintained by the utility which is not under NEC.
 

don_resqcapt19

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beanland said:
Wouldn't ten 500kcmil copper be good for 4300A IF the wire, lugs and conduit are rated for 90C?
The equipment, not just the lugs, would also have to be rated for 90?C and that is rare.
 

jim dungar

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don_resqcapt19 said:
The equipment, not just the lugs, would also have to be rated for 90?C and that is rare.

To expand on Don's comment.
I know of no UL listed general equipment rated for cables sized based on the 90?C columns. Some equipment (i.e. 100% rated) requires 90?C insulation, but it still requires the conductors to be sized based on the 75?C column. However, there is industrial switchgear (often called drawout gear) that allows conductors sized per the 90?C column
 

acrwc10

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CA
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For what it's worth, is changing your screen name and asking essentially the same question over and over going to get you a the answer you want ?

discusted electrician said:
Sorry if this is lengthy but it is neccessary. I have several problems but will tackle them one at a time (all electrical and not emotional or personal).I have a 1500 kva 3phase xmfr that feeds a 4000A MDP. Their are 10 sets of parallel feeders from the xmfr to the MDP. The primary feeders are 500kcmil w/ a 350kcmil grounding conductor (neutral) with NO grounded conductor in each conduit (PVC). The MDP is not connected to a ground grid system but is bonded to building steel w/ a 3/0 copper. The NEC 2005 Table 250.122 shows that the neutral is undersized (4000A) and 310.4 also states the grounded conductor " must be the same size in circular mill". Is it mandatory that all service entrance conductors (parallel feeds) have a grounded conductor included and is the neutral undersized according to my understanding? Also, I have been experiencing severe problems with equipment that rely upon electronic boards that are constantly frying ( motors with variable speed drive VSD). Since much of the metal raceway that runs throughout the facilty do not have grounded circuits included in the branch feeders, but rely solely upon the metal raceway as grounding, could this be causing transient voltages to be applied back on to the PC board and blowing circuits. The facility is 30 years old and with 8 overhead cranes running along a rail system 12 hours a day, much of the counduit has vibrated loose from the connector, coupling, walll etc. thus loosing it's continuity and ground fault protection. Any help is greatly appreciated and thank-you.
 

acrwc10

Master Code Professional
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CA
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Building inspector
Let me add, it sounds like there is NOTHING WRONG WITH THE UTILITY SERVICE LATERAL If you have a harmonics issue it is on your side of the service most likely in the lighting ( that in no way is 4000 amps ) or on the 120/208 side of YOUR xfmer. If you think there is a harmonics issue check the load on the neutrals with a true RMS amp meter, and check neutral to ground voltage this will tell the tale.
 

440doublebuck

Member
Location
Baytown, Tx
were the vsd's a recent addition? If so, and vsd's are anything like vfd's, vfd's(variable frequency drive) produce harmonic currents that can interfere with electronic equipment and machines. Like mentioned before maybe an isolation transformer or line reactor could be something to look into.
 

charlie

Senior Member
Location
Indianapolis
acrwc10 said:
For what it's worth, is changing your screen name and asking essentially the same question over and over going to get you a the answer you want ?

I think it was because 'discusted electrician' should have 'disgusted electrician' and with the negative connotation, he changed the screen name but still needed to get answers to his questions. Remember that he is still new to the forum and should be treated with the same respect as everyone else. :)
 
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