Reduced Neutral Sizing

[GreyBeard]

Hi, just want to check something. I am on a facility which has a 4 core 70mm non armoured feed from the transformer, coming into the DB at the other end it is now a 4 core 25mm non armoured cable (obviously it has a joint somewhere) and the earth conductor is from an unknown source, but both of these cables have reduced neutrals and the later is less than 50% the phase conductors.

I am planning to change the cables, but just wondering if anyone has seen reduced neutrals that small before?

 

[James]

You need to figure out where the transformer feed goes to
could it be a distribution board or LV metering and fuse?

 

[Rockingit]

N conductor at 50% of phase conductors is 'OK' although it does assume a balanced load and little neutral current, also assumes a sensible lack of neutral harmonics - so conductor size/s at each point can be excused. However as @James points out, logically there has to be a reason for the reduction in sizes so before working out if your neutral is up to the job you first need to know the entire story - if it's as simple as a joint and reduction in ongoing csa then clearly this isn't acceptable regardless of discussion on Neutral sizing. If you need a reminder, In = sqrt (Ia^2+Ib^2+Ic^2) - (IaIb+IaIc+IbIc)

 

[GreyBeard]

You need to figure out where the transformer feed goes to
could it be a distribution board or LV metering and fuse?

You need to figure out where the transformer feed goes to
could it be a distribution board or LV metering and fuse?

It goes to a DB from the transformer, but as I say it starts off as 70 and ends up as 25, so there must be a joint somewhere underground

 

[GreyBeard]

N conductor at 50% of phase conductors is 'OK' although it does assume a balanced load and little neutral current, also assumes a sensible lack of neutral harmonics - so conductor size/s at each point can be excused. However as @James points out, logically there has to be a reason for the reduction in sizes so before working out if your neutral is up to the job you first need to know the entire story - if it's as simple as a joint and reduction in ongoing csa then clearly this isn't acceptable regardless of discussion on Neutral sizing. If you need a reminder, In = sqrt (Ia^2+Ib^2+Ic^2) - (IaIb+IaIc+IbIc)

Thanks, thats was what I thought, just needing confirmation. The max is 160A, 200A & 140A and Neutral 130A, ave are 45A, 80A & 40A with 37 on the Neutral., so its not balanced, that is not to say that this was not so when it was installed. Some of these supply and distribution cables have been in for up to 40 years.

I have managed to update about 1/3 of the camp in the last 18 months, but due to budget constraints I can not do everything straight away.