3 Phase Power Supply Cable

Does BS 7671 say anything about it?

Hi

Thanks for replying BS7671 is the 17th edition that I have been reading and I cannot find anything in it.

Regards

James Brown

I'm surprised that there wouldn't be anything. There might be some criteria for arrangement and spacing between separate cables, as well as moisture resistance in the sand bed. Some capacitance issues could also apply.

Read again. I've got the page in front of me.

In my opinion both BS 5467 and BS6724 recommend AWA (Non magnetic Aluminium wire for single cores cable].Only for 2 and more cores SWA.

It is not recommended to run a single-core cable in a steel conduit also, since the eddy current induced in the conduit will heat the conduit and will raise the cable temperature in turn.

He will be runing the 3 single cores together in the metallic conduit, therefore the currents will cancel each other regarding the eddy currents.

You should not run single cables in steel conduit but if you install all 3 phase and neutral in the conduit you will not have the problem mentioned in post#4

Alternately you can use 2 runs of smaller size SWA cable to Manage the installation difficulties. You will have to apply corrective derating factors which you will find in a good manual or from cable manufacturer. You cannot use unarmoured cables directly buried in sand bed, but have to provide some mechanical protection like cable duct etc against inadvertent damage. You can of course run all 3 phase unarmoured wires and neutral in the same metal cable tray if required. This input is subject to conforming to your local codes with usual disclaimers.

300mm2 cable is suitable to carry 400 A, but again it depends the voltage drop due to the length. In that case you have to calculate the voltage drop and select 2 runs of cable.

Dear Mr.JamesBrown,

If you run single core (instead of 3 core) Inductance problem is to be tackled, that is why 3 core - which will annel the Inductance - is used.

You can think of 2 runs or 3 runs of 3 core (or 3.5 core if Neutral is required) for deciding the cable size of each run, which needs de-rating factor etc. is to be accounted.

DHAYANANDHAN.S

As I already said: no such thing SWA[Steel Wire Armoured] single core cable but only AWA[Aluminum Wire Armoured] single core cable and no interdiction -except as mentioned above-to use it in a duct or directly buried.

Some problems will arise as: armour losses-by eddy current and by circulating current if the armour is grounded[earthed] at both ends.If it is grounded in a single point another problem will be the build-up induced voltage. In a three-phase cable it is not such a problem.