Cable size-disagreement

[Paul505]

A new sub-main 3-phase supply to be installed on a large commercial site.....to supply a new food outlet (small building)
Sub main to be wired in SWA
Length of run 160 meters
Max load 65-70amps per phase (information supplied by catering company) no diversity has been applied by them.Also no allowance is required for any future expansion.

Not going to go into the full details.... but a disagreement exsits about the size of cable to use.
Would like to see what anyone on here would come up with.....

 

[Sintra]

What method is the SWA to be installed?

 

[Paul505]

Buried.....
Thanks for the reply Sintra

 

[scotsparky]

due to size power factor is requiered. However as none is givenn i have used the Impedancce rating (z) on the appendiz 4 chats.

assuming 90deg thermosessing SWa

I recon 75mm should do as that covers volt drop as well for sub mains distribution.

Watch opuit for your K factor if your using your SWA as an earth.

 

[Paul505]

35mm and 70mm have been suggested....90 thermosetting
On impedance rating (z) does that incorperate a poor power factor if none is given?
what volt drop did you get on the 50mm?
5-core cable used....regarding earth
Thanks Scottsparky

 

[scotsparky]

changed the cable size sorry.

AS its a sub main cable i use 1% volt drop as it allows the other 4% so be used on the final circuits allowing smaller cables

remember your derating factors for burried cables.

if you lookm at appendix 4 then on the larger cables you will see three colums
r,x,z

r = Resistive
x = reactive
z = impedance

as you dont know what it is supplying use the cable impedance figure.

 

[Paul505]

All the final circuits are within 10 mtrs of the DB....which shouldnt create too much of a problem with cable sizes on the distribution side.
As the sub main cable is the most expensive part and is practically the end of the circuit,the v drop allowance can be close to the max of 5%....surely?


Thanks for the replys




cable method has changed slightly
20% buired
80% clipped direct

 

[scotsparky]

be care full with that rememer Volt drop on lighting is only 3% appendix 12 table 12A page 358 in the regs.
As 20 is burried you have to use that derating figure.