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.

On Thursday, 22 June 2017 11:17:18 UTC+1, wrote:

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.

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


NT

On 22/06/2017 11:17, wrote:

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.

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

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?

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 he

http://wiki.diyfaq.org.uk/index.php/...ricity_outside

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

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.

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?

Which submain are we talking about?

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

see: http://wiki.diyfaq.org.uk/index.php/Earthing_Types

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.

http://wiki.diyfaq.org.uk/index.php/...ide#CPC_sizing

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 he

http://wiki.diyfaq.org.uk/index.php/...our_as_a_C PC

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.


--
Cheers,

John.

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On 22/06/2017 11:17, wrote:
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.

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


--
Adam