Cable supply for a building

[ians]

To many accounting queries on this forum...so lets get an electrical discussion going.

I need to install a 400 amp supply in a building ...miles away from the transformer....not really but hey... it sounds better.

We are going to upgrade the meter panel and fit a 400 amp circuit breaker in the panel with a private meter.

We going to install 2 x 110 mm pvc ducts under the road (distance 25 m)

up the side of the building exposed to direct sunlight for 4 m

onto a 200 mm MD cable tray which will run 100 m on the ceiling with the 2 cables space the width of the tray apart.

turn into the unit 6 m along the wall

then down into the unit another 10 m

into a floor standing 400 amp sub panel

is it gona be 2 x 95 mm x 4 core ecc or 2 x 120 mm ecc

a wild guess would say 2 x 95 mm should do it ...but because of the distance maybe the 120 mm sounds more likely ...but the fact that in is running most of the way on an open cable tray spaced ...we could get away with it.


the existing installation already has a 2 x 95 mm x 4 core cables ...but i dont believe they are suitable for 400 amps considering they run in a duct under ground 130 m 500 mm below the surface....they should be derated to 300 amps sounds about right.

what do you think?

[GCE]

Hi Ians
I would go at least 120sqmm and install 160mm sleeves and think of creating manholes instead of bends.Manhole at least 1m x 1m on top and bigger underneath .
I would also try and come up on the inside of the building if possible and keep it out of the sunlight

If you look at the regulations there are so many factors that need to be taken into account and the balancing of phases is critical. We have had 200 amp constant on a 95sqmm cable where a meter past through sunlight and the cable heated up to such an extent that we had to double up.
The diversity factor will also play a role and if you are only going to peak 400 amp now and then and the majority of time run at 200 amp you could change the thought process

The regs say take the worst case scenario which would be the 4m in sunlight - Clause 6.2.12 allows for direct solar radiation and gives a derating factor of 0.65 on 95sqmm for coastal.( table 6.19 pg 130)

With the derating factor it suddenly looks like 120sqmm could be to small - If you could get the cable out of direct sunlight and just use a temperature derate at say 50 degrees , on the 120sqmm at 0.71 you will just make it.( Table 6.10 pg117)


6.2.8.2 If the ambient temperature varies along the cable route, the
correction factor selected shall be the one appropriate to the part of the route
where the most adverse conditions are created. From table 6.10, select the
correction factor appropriate to the ambient temperature and to the type of
cable

[ians]

I decided to contact a local cable supplier and hear what they have to say...an interesting point was mentioned ...the sleeves...heat dissipation seems to be the key factor...direct in the ground 95 mm ...in a sleeve 120 maybe even 150 was mentioned...open and spaced out of direct sunlight ...70 mm was mentioned.

so far it sounds like a 95 mm ...mounted on a 200 mm cable tray spaced more than 25 mm it could be capable of 230 amps per cable ...with a little correction factor for the short sleeve across the parking lot...it should be within spec...also considering the load with be on and off due to heaters switching on/off and motors electronically controlled for max performance...the chance of 80 - 100 % full load for extended periods is highly unlikely.

[GCE]

Hi Ians

Would be interested to know what the end decision was on the cable sizing from the supplier

[ians]

The 23 m of duct will will determine if the cable is a 120 mm or a 95 mm ...I dont think people realise how much heat affects a calculation...even if it is only for a short section of the run...i am considering running the cables in separate larger ducts to reduce the heat...it all boils down to the heat dissipation.