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SWA Sub Mains

Postby littletopping » Wed Sep 08, 2010 1:03 am

Just attempting to complete my self build for xmas and find I need to run a sub mains from meter position to new mains location more central in the building. The distance is approx 21m and the SWA will travel through a single storey roofspace before entering under the first floor of the main building for some 5m to new consumer unit position. Having checked the various SWA size calc sites I find they are not consistant with their findings as they varey between 10mm and 16mm, an opinion would be of assistance please re cable size. I am unsure also if 2core cable would be sufficient while using the outer sheath for the earth conductor as no part of the cable run is in danger of damage. Terminating the SWA at the C unit is not a problem but I presume some kind of metalclad switched fused isolator is required at the start end to terminate the cable and the feeding meter tails. Assistance greatly appreciated please.
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Postby ericmark » Wed Sep 08, 2010 4:14 pm

What size is incoming fuse. If 100A then volt drop around 5.88v which is close to 6.9v maximum and clipped direct it would need to be thermosetting 90 deg cable for 70 deg only rated at 89A clipped direct and method D is more likely so 78A max.
This is not a DIY job and unless you understand how to calculate cable sizes then you should not be doing it.
I would expect you will need 25mm cable.
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Postby sparx » Wed Sep 08, 2010 5:03 pm

presuming you are doing the electrical work under LABC 'Part-p' as part of self build and assuming also sufficient electrical ability to avoid danger.

(that's my bum flanged)! 8)

First thing is tails limited to no more than 3 metres from meter to isolator, also as sub main will require over current protection you will need to put in a local switch fuse.

10mm2 XLPE type SWA is only rated at 85A so if used you would need to fit a sw-fuse with an 80A fuse max thereby limiting your supply a little (assuming REC fuse to be 100A ).

PVC SWA is only rated @ 67A for a 10mm2 so would have to be fused at 60A!

So only real choice is between XLPE SWA either 10 or 16mm2.

However in reality you have to also take into account volt drop and EFLI
(earth fault loop impedance) which will impact on all circuit Zs readings.

Taking 16mm2 over 21 metres assuming say 50A max load gives:
2.9mV X 21m X 50A / 1000= 3.045Volts dropped at the incomming terminals of your Consumer unit.

Since allowence for lighting is 3% max from supply this only leaves
3.9v.drop for any circuit, not a lot.

Next is EFLI: using same cable will add around 0.05 ohms assuming use of 3rd core for earth, using swa sheath as well will reduce this a little but it all adds to the Zs readings.

Sorry if a little verbose but in summery, forget anything less than 16mm2

I would in fact use 25mm2 since any losses in the cable (heat due to resistance) you will be paying for daily, the cost of the cable difference will eventually be well repaid,

regards Sparx
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