neutral cable sizing for underground distribution

[JLuc]

Hi all,

I will be involved in underground distribution (25kV,solidly grounded) in future projects.

I started reading on the subject I still have interrogations

one of them is:

How do you size the neutral cable when going underground from an overhead line?

One of our client has an underground part on its distribution system: typically they have:

-3x 1conductor, xlpe, concentric neutral, #1/0, (phase)
-1x #1/0 RW90 (neutral)

all in the same conduit (100mm).

Question is: If I need, say 750MCM, phase conductor, how will I size the neutral conductor? (750MCM neutral conductor?)

Other question: Is the neutral conductor necessary since the 3 concentric neutrals on each phase are able to carry the neutral current anyway?

Thanks for your help

JL

 

[davidbeach]

If you have a concentric neutral - which is not the same thing as a copper foil shield - then there is no need for a separate neutral. Use 33% concentric neutral for three-phase circuits and use 100% concentric neutral otherwise.

 

[JLuc]

If I don't have concentric neutral, how will I size the neutral conductor? Does it have to be the same size than the phase conductor? ( I would say yes)

Thanks

JL

 

[davidbeach]

What is the load on the neutral? If the load being fed is essentially all 3-phase you don't need a full neutral. On the other hand if you have lots of single phase load you will want to get close to a full sized neutral. No harm in an over sized neutral, standard configurations have a definite advantage.