Neutral cable size

Consider there being a load imbalance across the three phases. What would the maximum neutral current be? The answer to this question dictates the minimum conductor size.

Is the neutral not a conductor should it not be the same size as the other service wires. Neutral just being a name assigned to it as it's potential is at ground.

Shucks, I've seen engineers specify double the size of the phase conductors for neutral conductors, claiming it's needed to reduce harmonics in the supply transformer.

I'm an electrical engineer, and I'm not ignorant in power transmission and distribution, but I never have been able to convince myself that large neutral conductors will do anythinng to reduce harmonics. Maybe someone else who reads this post can enlighten me and you.

Bill; the harmonics are from the loads, they generate additional current & heat, not the transformer, years ago the neutral insulation in 12/5 SO flexible cords used in computer applications dried up & crumbled from the heat, thus the super neutral was born, typically the next wire size larger is used. perry

Note taken . . . I understand the reason for the up-sized neutral conductor, but it's not, as electrical engineers have told me (and I'm one too), to reduce harmonics in supply transformers.

Guest,

Depending on the installation method for the cables, I also am surprised at the arrangement. Sizing a neutral at 60% the ampacity of the phase conductors is not uncommon. The others posting to this correctly point to the need to consider the unbalanced load and the need for safely carrying the harmonic currents imposed by non-linear line-neutral loads such as computers or electronic lighting ballasts.

If the phase cables are in separate conduits then the neutrals should be divided in the same manner, with 5 such cables sized each at 60% of the ampacity of the phase conductors. Then each set of phase conductors would have a neutral in the same conduit. Even if the conduits are non-metallic and proper methods are used to avoid magnetic induction where the conduits enter enclosures, the method you mention should have all sets of parallel conductors exposed to identically the same conditions (same cable type, insulation type, length, etc. for each set of conductors, as well as same number of conductors in each set).

--JMM

Although neutral in 3 phase star connection which is the case in Generator is used to provide zero potential point, but due to unbalanced load neutral also has to carry the unbalanced load current (vector difference of the 3 phase current).

The size of neutral as per standard is half of the phase conductor but as far as depending upon the unbalanced current calculation, appropriate size has to be used.

IEC 60947-1, IEC 60439 - 1 suggese the size of conductor depending upon current

unbalance current can be calcuated as per

Iunbalaced = Ir + Iy + Ib

positive, negative and zero sequence parameters has to taken in to account.

my email id is divyesh_om@yahoo.co.in

for more details on fault calculation contact on email

Hi i am electrical technician as my experience with electrical cabling .

the Neutral cable size just depend to Unbalanced load .so some where for reduce the cost for cabling they are using 1/2 size for neutral in three phase line .

but in this case if they cannot balanced the load they must to use TNCS ground configiration for solve the neutral cable with 1/2 size for protect the Neutral wire .

Mohammd

mohammadahmad.jafari@hotmail.com