Another electrical question: sub main wiring

[nomail]

The large'ish 1930s house I moved to a few weeks ago has some "interesting" wiring that I need to sort out over the months ahead. The incomer is in the (attached) garage. There's a Henley block that splits the tails into 4. One of these goes to a 16 way plastic CU in the garage, two go to 4 way DBs (to sheds, elec shower and elsewhere) in the garage, and 1 goes (via an isolator) to a sub-main that goes to an 8 way (wire-fused) CU in the house for the sockets. One of the MCBs in the 14 way board feeds a second 8 way (wire-fused) CU in the house for the lights. The plan is to rationalise this mess by replacing all the garage CUs with a single 21 way MK sentry metal CU and to replace the two ancient 8 way wylex CUs with a single 16 way metal CU (and a single sub-main). My questions are about the sub-main and shed wiring.

The sheds each have small CUs for lighting and a few sockets, each shed has an earth rod. My guess is the load in each shed will never exceed

20A so I plan to run a cable to each shed from a non-RCD'd MCB in the 21 way CU. The local shed CUs will have RCDs. Any problems with that?

I plan to feed the sub-main to the 16 way CU from an isolator connected to the meter tails. Any problems with that?

I need to investigate a bit more but I think the sub-main cable is currently 16mm2 T&E with a 4mm2 grn/yel wired in parallel. How do I calculate the necessary size of CPC for the sub-main?

As an aside, I think I've only got one run of rubber insulated cable to replace but I was surprised to find some disconnected lead covered cable in a shed, together with some original 1930s switches and sockets.

[tabbypurr]

There are collectors that would like those. Or you could refit them on 12v or 5v, or both if 3 pin.

NT

[John Rumm]

Yup a common feature of most older places! ;-)

That's fine, although you will need to take care in routing the incomer to the shed so as not to fall foul of the unprotected cable rules (i.e. that non RCD cables should be >= 50mm below the surface, or visible, or protected by earthed conductive sleeving.

More details on outside stuff here:

Sounds ok. Often the electricity distributor will have rules about the maximum distance you can take the supply after their cutout without additional isolation / protection. So having your own isolator (and possibly fuse) close to the origin solves those issues.

Which submain are we talking about?

Also what earthing system does the house have at the incomer?

see:

In general, if you are not attempting to export and equipotential zone along with your earth (typically TN-C-S head end and exported earth), then the CPC size will be what you would normally expect for a circuit, and you can use the adiabatic check to verify the CPC is adequate.

Some extra complexity comes into play if you have a SWA submain, and you are using the armour wires as your main and only CPC. Then you need to know the total cross sectional area of the armour (data you can only get from either the cable manufacturer or some BS docs - the tables in BS7671 don't normally tell you), and you also need to make an allowance to convert that CSA into a "copper equivalent" area to allow for higher impedance of the steel.

Details for the common SWA cables are included here:

[ARW]

Does the house have an earth rod or does the electric co supply you with an earth?