Hi Eric,
It would be 25m to the consumer unit from the garage, it's only 10m from the garage to the downstairs ring - so I guess the volt drop for a 13A spurred socket and a 5A light fitting would be fine.
A ring can be (When RCD protected to BS7671:2008) up to 106 meters long and taking a fused spur from the ring right next to the consumer unit would allow the spur to be nearly 50 meters but taken from the centre of a 106 meter ring it would be zero as the allowance has already been used up.
Unlikely it will be centre of a 106 meter ring but to work out the volt drop permitted you would need first to work out the volt drop at the point of the spur.
The problem is you can't read it direct one has to work it out from the impedance (resistance with AC circuit is called impedance) at the point where the spur is taken minus the impedance at the consumer unit. Or one could measure the resistance with a low ohm meter but with two conductors of unequal length that is not easy either.
So if for example the volt drop at the point where you are taking spur from was 9 volt then you could just manage the 10 meter run but that includes wiring used in the garage. So the impedance would need to be less than 1 ohm approx for the supply to be usable.
There is also the rule that any appliance over 2kW should have a dedicated supply so if the shed is considered as an appliance then it would need to be fused to 7A not 13A we come back to the electrician doing the work and how close to the rules he wants to go.
So you would need to find the electrician first and ask him what he would accept.
I understood, perhaps wrongly, that the steel conduit would need to be earthed.
Yes the steel conduit would need to be bonded and this leaves us with a problem. With SWA the plastic coating allows bonding one end only and using a TT supply. But with conduit we would need to bond both to house and the shed and this raises a problem where the house is fed from a TN supply.
I would be interested to hear what others have to say on this as an interesting problem which until your post I had not considered. I would think the house would have to be TT to be able to connect the conduit to the earth system without major erosion problems. The diodes could help but the real point is you should not really use un-coated conduit underground. I spend some time on pipe lines and the rapping of the pipe with an insulating material and the testing of the rap with the Spy detectors which used 2500 volts was a major part of the work. Even then there were still huge sacrificial anodes buried and a cathodic protection generator to stop erosion.
I would expect if you tried to move the cable in the conduit you will find it is solid. And if you dig up the conduit you will find huge holes in it. The failure of the conduit could damage the cable if the conduit was in concrete then not so much of a problem but in earth I would expect if you try to do an insulation resistance test and an earth continuity test it will fail.
Even if it does not fail then likely it will in the future. So I would say a lift and swap for SWA is required even if taking the supply from the original point. There is little to stop one cleating the SWA to the house wall so once you reach house it does not need to be below ground. Nor does it have to go inside the house. It can follow the wall around the house.
So I would renew with SWA cable and I think most would recommend the same.
