Hello,

I would like to run some 6mm^2 T&E to the garage or rather get a Part
P person to do it for me

My set-up is meter switch fuse CU.

I presume I change this to:
meter switch fuse henley blocks cu (house) and cu (garage)

I've never used henley blocks before; is there any advantage to using
two SP blocks or one DP block?

The cable will exit the house under the stairs into the garage (no
underground or overhead runs required) but because it will exit at
about 1.5m above floor level, it will be at a height that it might get
knocked by people or cars, so I think I should protect it in conduit.
Are all conduits created equal or are some better than others? How
thick is 6mm^2 T&E, does it require "fat" conduit? of course, IIRC
conduit will reduce the max load the wire can carry.

Thanks.

Fred wrote:
Hello,

I would like to run some 6mm^2 T&E to the garage or rather get a Part
P person to do it for me

My set-up is meter switch fuse CU.

I presume I change this to:
meter switch fuse henley blocks cu (house) and cu (garage)

I've never used henley blocks before; is there any advantage to using
two SP blocks or one DP block?

The cable will exit the house under the stairs into the garage (no
underground or overhead runs required) but because it will exit at
about 1.5m above floor level, it will be at a height that it might get
knocked by people or cars, so I think I should protect it in conduit.
Are all conduits created equal or are some better than others? How
thick is 6mm^2 T&E, does it require "fat" conduit? of course, IIRC
conduit will reduce the max load the wire can carry.

Thanks.

Do you you have a spare way in your consumer unit? If so run the garage
power from there, if not fit a small consumer unit with a single way and
run the power from there. What are your earth arrangements and what
are you wanting to run off this cable? Photos of the arrangement would help.

On Wed, 19 Mar 2008 19:34:09 +0000, James Salisbury
wrote:


Do you you have a spare way in your consumer unit?

Yes but...

If so run the garage power from there

The CU is RCD'ed and I thought it was bad practice to run your
garage/outdoor supply from an RCD'ed unit because if the RCD trips
because of the lawn mower, the whole house goes dark.

if not fit a small consumer unit with a single way and run the power from there.

I was going to fit a small CU like you describe but I thought I should
site that in the garage not the house?

What are your earth arrangements

I will have to check the faq for the correct terminology

what are you wanting to run off this cable?

Garage light, outdoor PIR light, freezer, lawnmower, drill, saw, etc.

Fred wrote:

My set-up is meter switch fuse CU.

Let's call that switch-fuse 1 (SF1). Presumably it's there because the
house CU is some distance from the meter.

I presume I change this to: meter switch fuse henley blocks cu
(house) and cu (garage)

I've never used henley blocks before; is there any advantage to using
two SP blocks or one DP block?

You might not need the Henley blocks(s). If the output terminals in SF1
are big enough to accommodate two outgoing cables then you could split
at that point without the need for any kind of junction box - a tidier
arrangement. If you do need to split after SF1 remember that Henley
blocks can only be used with single sheathed cable (6181Y), unless
mounted in some sort of enclosure. (I'd use a DP block, unless there's
any good reason for preferring 2xSP ones.)

The cable will exit the house under the stairs [...]

Whoa! "The cable" here is the 6 mm^2 that will run to the a second CU
in the garage? Before that you will need a 2nd switch-fuse (SF2) to
'fuse down' to (say) 30 A for the distribution circuit going to the
garage CU. A 1-way Wylex unit fitted with a 30 A BS 1361 cartridge
would be a suitable choice for this. Without SF2 your cable will only
be protected by the fuse in SF1, which is presumably 60 or 100 A for the
house.

[...] into the garage (no underground or overhead runs required) but
because it will exit at about 1.5m above floor level, it will be at a
height that it might get knocked by people or cars, so I think I
should protect it in conduit.

OK. You haven't said anything about what circuits and equipment are
going into the garage, but it might make sense to do the whole of the
garage sub-installation in conduit. If all the wiring is enclosed you
could wire in 'singles' which is much easier to use in conduit than T&E.

Are all conduits created equal or are some better than others?

There are two grades of PVC conduit, standard and heavy gauge
(most people use the latter), and there's steel. HG PVC would be fine
for a domestic garage, is fairly cheap, is quite easy to work with and
will give a tidy-looking job. Steel is obviously tougher, but involves
more of a learning curve to master the necessary skills to use it and
you'd have to hire bending and threading tools. Use PVC!

How thick is 6mm^2 T&E, does it require "fat" conduit? of course,
IIRC conduit will reduce the max load the wire can carry.

Sizes and ratings here
http://www.wiki.diyfaq.org.uk/index....es#Cable_Sizes

6 mm^2 T&E will go through 25 mm PVC conduit OK for simple runs with
only one or two bends. Don't even think about trying to use 20 mm.

Your earthing arrangements? Remember that 6 mm^2 T&E only has a 2.5
earth (CPC), which may be too small. You may need to consider running a
separate 6 mm^2 earth for the distribution circuit.

HTH
--
Andy

Fred wrote:
Hello,

I would like to run some 6mm^2 T&E to the garage or rather get a Part
P person to do it for me

My set-up is meter switch fuse CU.

I presume I change this to:
meter switch fuse henley blocks cu (house) and cu (garage)


It sounds like this has a fair amount of relevant content for this
situation:

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



--
Cheers,

John.

/================================================== ===============\
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| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Fred wrote:

what are you wanting to run off this cable?

Garage light, outdoor PIR light, freezer, lawnmower, drill, saw, etc.

You may want to consider a non RCD protected (or dedicated RCD/RCBO) for
the freezer. No point losing the contents when the PIR lamp gets damp!


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Wed, 19 Mar 2008 20:32:37 +0000, Andy Wade
wrote:
[with some snipped]
Fred wrote:

My set-up is meter switch fuse CU.

Let's call that switch-fuse 1 (SF1). Presumably it's there because the
house CU is some distance from the meter.

Yes.

If the output terminals in SF1
are big enough to accommodate two outgoing cables then you could split
at that point without the need for any kind of junction box - a tidier
arrangement.

Connect the CU tails and the T&E into the switch fuse if it will fit?
I had not thought of that.

If you do need to split after SF1 remember that Henley
blocks can only be used with single sheathed cable (6181Y)

As you can tell, I have only just started to think about this. I had
forgotten that one cable will be T&E not a "tail" cable (I presume
that 6181Y means a tail).

Whoa! "The cable" here is the 6 mm^2 that will run to the a second CU
in the garage? Before that you will need a 2nd switch-fuse (SF2) to
'fuse down' to (say) 30 A for the distribution circuit going to the
garage CU. Without SF2 your cable will only be protected by the fuse in SF1, which is presumably 60 or 100 A

Whoa indeed. Another refinement that I had missed in my initial
thoughts. I presume I could run single sheathed cable from the Henley
block to SF2 and the T&E from the other side of SF2 which would solve
my tail to T&E transition problem. I must confess I have not seen 30A
switch fuses; I thought they were 80A or 100A. I will have to pop to a
electrical trade counter.

There are two grades of PVC conduit, standard and heavy gauge
(most people use the latter), and there's steel. HG PVC would be fine
for a domestic garage, is fairly cheap, is quite easy to work with and
will give a tidy-looking job. Steel is obviously tougher, but involves
more of a learning curve to master the necessary skills to use it and
you'd have to hire bending and threading tools. Use PVC!

6 mm^2 T&E will go through 25 mm PVC conduit OK for simple runs with
only one or two bends. Don't even think about trying to use 20 mm.

25mm HG plastic it is then.

Your earthing arrangements? Remember that 6 mm^2 T&E only has a 2.5
earth (CPC), which may be too small. You may need to consider running a
separate 6 mm^2 earth for the distribution circuit.

I will have to check this. Thanks.

Fred wrote:

Connect the CU tails and the T&E into the switch fuse if it will fit?
I had not thought of that.

Not the 6 mm^2 T&E directly (into SF1) since you're still fused at 60 or
100 A at that point - but you might get 16 mm^2 single sheathed or T&E
in, for the short run to SF2. It depends on what make and size SF1 is,
but 100 A stuff often has 50 mm^2 terminal capacity, which would be
sufficient. Check out the actual available wiring space, as well as
terminal capacity.

(I presume that 6181Y means a tail).

Yes "meter tail type cable" readily available in cut lengths in 16 and
25 mm^2 for tails etc., but also available in the full range of sizes:
http://www.tlc-direct.co.uk/Main_Ind...New/index.html

Whoa indeed. Another refinement that I had missed in my initial
thoughts. I presume I could run single sheathed cable from the Henley
block to SF2 and the T&E from the other side of SF2 which would solve
my tail to T&E transition problem.

Exactly so.

I must confess I have not seen 30A switch fuses; I thought they were
80A or 100A. I will have to pop to a electrical trade counter.

It's a 1-way consumer unit really, e.g.:
http://www.tlc-direct.co.uk/Products/WY106.html
(but fit a 30 A cartridge fuse, not an MCB, as that will give you better
downstream fault discrimination).

--
Andy

On Thu, 20 Mar 2008 11:11:07 +0000, Andy Wade
wrote:

Not the 6 mm^2 T&E directly (into SF1) since you're still fused at 60 or
100 A at that point

That's the second time I've made that mistake. I won't make it again.

(I presume that 6181Y means a tail).

Yes "meter tail type cable" readily available in cut lengths in 16 and
25 mm^2 for tails etc.

Thanks. Where can I learn what all the codes, e.g. 6181Y mean?

IIRC 16mm^2 is good for 80A and 25mm^2 is good for 100A. The fuse at
the meter is 100A so I hope the tails to SF1 are 25mm^2. The switch
fuse is rated 80A and the CU is rated 63A so I think the cable from
SF1 to the CU is only 16mm^2.

Since SF1 is rated at 80A would I be right to use 16mm^2 to SF2?


http://www.tlc-direct.co.uk/Products/WY106.html
(but fit a 30 A cartridge fuse, not an MCB, as that will give you better
downstream fault discrimination).

I've had a look at TLC and the Wylex range seem to be the only ones
that can be used with either fuses or MCBS. The other products do not
mention fuses. The price seems reasonable enough too. There is also a
plastic version a little bit cheaper. Is there an advantage to using
the metal or the plastic version? I guess metal is stronger so takes a
few knocks but on the other hand, doesn't it pose a problem in that it
must be earthed?

Thanks.

Fred wrote:

I've had a look at TLC and the Wylex range seem to be the only ones
that can be used with either fuses or MCBS. The other products do not
mention fuses. The price seems reasonable enough too. There is also a

Hager also do HRC fuse carriers for their consumer units. They also fit
Contactum and MK enclosures.

http://www.tlc-direct.co.uk/Main_Ind...ers/index.html

(and they do come with a fuse fitted (still not found a use for the
spare I ordered! ;-))

plastic version a little bit cheaper. Is there an advantage to using
the metal or the plastic version? I guess metal is stronger so takes a
few knocks but on the other hand, doesn't it pose a problem in that it
must be earthed?

Assuming your not TT[1], then either should be ok. Earthing the metal
ones is not difficult - they usually have the earth bus bar riveted to
the metal of the box anyway, so it is hard to avoid.

[1] With TT installs its better (IIRC a requirement) to use insulated
CUs since it lowers the possibility of a fault to earth occurring on the
up stream side of your RCD, where the earth fault loop impedance will
probably be too high to sink enough current to open the main cutout.


--
Cheers,

John.

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|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Fred wrote:
On Thu, 20 Mar 2008 11:11:07 +0000, Andy Wade
wrote:

Thanks. Where can I learn what all the codes, e.g. 6181Y mean?

Actually it should be 6181YH these days, the final H indicating the new
(harmonised) colours. These UK-specific codes originated in the cable
manufacturers' association (CMA). They don't seem to have a website, or
maybe don't even exist any more under that name. I found a couple of
lists on the web though:

http://www.voltimum.co.uk/news/1577/...l?fullsize=yes
and
http://www.pxcables.com/products.html

IIRC 16mm^2 is good for 80A and 25mm^2 is good for 100A.

More or less - the clipped direct ratings for a single-phase circuit are
87 and 114 amps respectively.

The fuse at the meter is 100A so I hope the tails to SF1 are 25mm^2.
The switch fuse is rated 80A and the CU is rated 63A so I think the
cable from SF1 to the CU is only 16mm^2.

Sounds OK.

Since SF1 is rated at 80A would I be right to use 16mm^2 to SF2?

16 will be fine. Actually you could probably use a smaller size since
the downstream fuse in SF2 provides overload protection and the 80 A
fuse upstream only has to provide s/c fault protection - but using 16
mm^2 will avoid the need for an adiabatic heating calculation.

Is there an advantage to using the metal or the plastic version? I
guess metal is stronger so takes a few knocks but on the other hand,
doesn't it pose a problem in that it must be earthed?

What John Rumm said. Also the metal type will have round knockouts, so
will be more suitable if you're going down the all-conduit road.
Plastic is more suited to surface-wired T&E, where you can trim the
cable openings to fit round the cables. Also look at MEM as another
manufacturer offering HBC fuse carriers.

--
Andy

On Wed, 19 Mar 2008 23:58:33 +0000, John Rumm
wrote:

You may want to consider a non RCD protected (or dedicated RCD/RCBO) for
the freezer. No point losing the contents when the PIR lamp gets damp!

Thanks. That's a good point. I thought all the regs. demanded RCD
protection now though?

I was going to get a 2-way garage CU but I think I now need a 3-way:
lights, sockets, and freezer socket.

I don't know much about RCBOs. I think they are the way to go: at
least they only switch off the faulty circuit rather than the whole
system when there is a fault, but OTOH don't they just switch the live
whereas a RCD's CU isolates both L&N when tripped?

Fred wrote:

On Wed, 19 Mar 2008 23:58:33 +0000, John Rumm
wrote:

You may want to consider a non RCD protected (or dedicated RCD/RCBO) for
the freezer. No point losing the contents when the PIR lamp gets damp!

Thanks. That's a good point. I thought all the regs. demanded RCD
protection now though?

The 17th edtn is not mandatory yet... you could also protect the circuit
cable such that it meets the requirements for a non RCD protected
circuit under the 17th edition.

However the simplest solution is a RCBO for that circuit. This does
include a RCD - but it is dedicated to that circuit and hence unaffected
by anything that may go wrong on other circuits.

I was going to get a 2-way garage CU but I think I now need a 3-way:
lights, sockets, and freezer socket.

I don't know much about RCBOs. I think they are the way to go: at
least they only switch off the faulty circuit rather than the whole
system when there is a fault, but OTOH don't they just switch the live
whereas a RCD's CU isolates both L&N when tripped?

No they switch both live and neutral - the neutral of the circuit has to
pass through them as well as the live (makes sense when you think how
they work). So they switch both. They normally hav a flying lead that
attaches them to the neutral bus bar in the CU. The circuit live and
neutral then attach to the RCBO.


--
Cheers,

John.

/================================================== ===============\
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| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:

No they switch both live and neutral

Not necessarily, and most are single-pole switched.

- the neutral of the circuit has to pass through them as well as the
live (makes sense when you think how they work). So they switch
both.

That's a non-sequitur. Just because the neutral passes through doesn't
necessarily mean it's switched.

Both single- and double-pole switched RCBOs are available. Generally
speaking 2-module width ones are DP and 1-module width ones are SP,
although that rule might not hold universally. Manufacturers'
literature should make the situation clear though.

--
Andy

Andy Wade wrote:
John Rumm wrote:

No they switch both live and neutral

Not necessarily, and most are single-pole switched.

Apologies for making an assumption then. The only ones I had examined
closely were two pole switched devices - but that was a while ago, and I
don't recall the actual brand.

- the neutral of the circuit has to pass through them as well as the
live (makes sense when you think how they work). So they switch
both.

That's a non-sequitur. Just because the neutral passes through doesn't
necessarily mean it's switched.

No indeed, very true.

Both single- and double-pole switched RCBOs are available. Generally
speaking 2-module width ones are DP and 1-module width ones are SP,
although that rule might not hold universally. Manufacturers'
literature should make the situation clear though.

The ones I looked at were the two module ones.

(although going back to the OPs question, even a single pole RCBO solves
the discrimination issue for the freezer, and single pole RCD protection
is still a significant advance on none (for situations that warrant it)).


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:

(although going back to the OPs question, even a single pole RCBO solves
the discrimination issue for the freezer, and single pole RCD protection
is still a significant advance on none (for situations that warrant it)).

Yes.

More ideas (sponsored by MK) at
http://mk.learninglounge.com/resources/sott17esv.html

--
Andy

On Tue, 01 Apr 2008 01:27:22 +0100, Andy Wade
wrote:

Both single- and double-pole switched RCBOs are available. Generally
speaking 2-module width ones are DP and 1-module width ones are SP,
although that rule might not hold universally


What are the disadvantages of a single pole RCBO over a double pole
one? Surely the important thing is that the live is cut in an
emergency. What are the advantages of isolating the neutral too? The
only one I can think of is if the Cu is wired back-to-front and I
would hope anyone wiring a Cu would know enough not to do that!

Thanks.

Sorry to add to my own post but previously I was thinking of buying on
of those "garage CUs" with two MCBS; one for sockets and one for
lighting. I wasn't sure whether to "upgrade" to a 4-way CU and have
one for lighting, one for sockets (a 32A ring?) and one for the
freezer (possibly a 16A radial?). Bearing in mind that it would be
supplied by 30A fused 6mm^2 T&E would this be sensible? Or is there a
danger that someone might come along in the future, see a 4-way CU,
fill the other MCB space and add to the rings and overload it all?

Thanks.

On Wed, 02 Apr 2008 19:07:07 +0100, Owain
wrote:

I've forgotten what you're supplying but why don't you put in a 6 or
even 8 way consumer unit with plenty of spare space, and supply at 45A
with 10mm T&E? The additional cost now is marginal, and the spare
capacity could come in very usefuy later

Thanks; that's worth thinking about. Would 10mm^2 fit into 25mm
conduit?

I was interested in RCBOs until I saw they are about £40 each! At that
price I think it would be cheaper to buy two RCD'ed CUs and fit them
side by side in the garage: one for the external lights and sockets
and the other for the freezer so that if the RCD trips on CU1, CU2
supplies power to the freezer uninterrupted. Is this technically and
legally possible?

In the future I would like to add a motorised door to the garage.
There is no other door into the garage so if the RCD trips I will have
to wind the door by hand! Would it be better to put the RCD outside
the garage, i.e. under the stairs? Notwithstanding the freezer
problem, why can't I have the RCD inside the house, rather than inside
the garage? The advantage of that would be that the whole 6mm^2 (or
10mm^2) cable run would be protected from house to garage.

On Mon, 07 Apr 2008 13:24:12 GMT, Fred
wrote:

In the future I would like to add a motorised door to the garage.
There is no other door into the garage so if the RCD trips I will have
to wind the door by hand! Would it be better to put the RCD outside
the garage, i.e. under the stairs? Notwithstanding the freezer
problem, why can't I have the RCD inside the house, rather than inside
the garage? The advantage of that would be that the whole 6mm^2 (or
10mm^2) cable run would be protected from house to garage.

To try and answer my own question: is the downside to locating the RCD
in the house that in the event of a trip you will have to check two
separate CUs: one in the house and one in the garage? OTOH if I went
with plan A and just had a fuse in the house, I suppose I would have
to still visit both boxes to see whether the fuse had blown or an MCB
tripped.

I suppose that I also need to measure the length of the cabel run to
make sure that if the RCD is in the house, all the disconnect times
are still within limits?

Fred wrote:
On Mon, 07 Apr 2008 13:24:12 GMT, Fred
wrote:

In the future I would like to add a motorised door to the garage.
There is no other door into the garage so if the RCD trips I will have
to wind the door by hand! Would it be better to put the RCD outside
the garage, i.e. under the stairs? Notwithstanding the freezer
problem, why can't I have the RCD inside the house, rather than inside
the garage? The advantage of that would be that the whole 6mm^2 (or
10mm^2) cable run would be protected from house to garage.

To try and answer my own question: is the downside to locating the RCD
in the house that in the event of a trip you will have to check two
separate CUs: one in the house and one in the garage? OTOH if I went
with plan A and just had a fuse in the house, I suppose I would have
to still visit both boxes to see whether the fuse had blown or an MCB
tripped.

With the RCD in the house, you make it difficult to avoid having the
feed to the freezer sharing a RCD with other devices, hence the
implications of a trip are worse.

The fuse in the house is also highly unlikely to trip if sized
appropriately for the submain.

I suppose that I also need to measure the length of the cabel run to
make sure that if the RCD is in the house, all the disconnect times
are still within limits?

The only time is it likely to be a factor is if your earth loop
impedance is marginal in the garage. In which case you would be better
off making the garage a TT install in its own right.

Can't remember if you have posted the type of earthing in your house, or
the length of the run to the garage. If you let us know those it should
be easy enough to give a worst case estimate.


--
Cheers,

John.

/================================================== ===============\
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|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Thu, 10 Apr 2008 02:11:54 +0100, John Rumm
wrote:

With the RCD in the house, you make it difficult to avoid having the
feed to the freezer sharing a RCD with other devices, hence the
implications of a trip are worse.

Thanks. It seems that there are pros and cons to each way.

The advantage of having the RCD in the garage is that you can wire to
keep the freezer on if something else trips. However, I wonder about
moving the freezer inside when I get round to decorating the kitchen
and hbow often do RCDs trip anyway?

The advantages of having the RCD in the house a
1 the whole cable is RCD protected in case of accident
2 if there is a trip, I would not have to manually wind up the garage
door to reset the RCD... of course, that assumes I am not in he
garage when it happens. If I were in the garage with the door shut
when it tripped, that would be different again!

The fuse in the house is also highly unlikely to trip if sized
appropriately for the submain.

The fuse would be 30A for 6mm^2T&E but I would connect that to a 2-way
CU in the garage with smaller MCBs, only the garage CU would not have
a RCD, so the garage circuits would still be fused appropriately.

I suppose that I also need to measure the length of the cabel run to
make sure that if the RCD is in the house, all the disconnect times
are still within limits?

The only time is it likely to be a factor is if your earth loop
impedance is marginal in the garage. In which case you would be better
off making the garage a TT install in its own right.

I can't remember how long the cablet o the garage would be exactly but
definitely less than 10 metres. It's just I like the idea of that
cable being RCD protected in case someone knocks/drills/drives into
the conduit carrying it. How resilient is heavy duty plastic conduit
anyway?

Thanks again.

Fred wrote:
On Thu, 10 Apr 2008 02:11:54 +0100, John Rumm
wrote:

With the RCD in the house, you make it difficult to avoid having the
feed to the freezer sharing a RCD with other devices, hence the
implications of a trip are worse.

Thanks. It seems that there are pros and cons to each way.

The advantage of having the RCD in the garage is that you can wire to
keep the freezer on if something else trips. However, I wonder about
moving the freezer inside when I get round to decorating the kitchen
and hbow often do RCDs trip anyway?

On a properly installed setup without any faulty appliances - almost
never. But you can't always rely on the no faulty appliances bit.

The advantages of having the RCD in the house a
1 the whole cable is RCD protected in case of accident

If you want this (or need it due to poor earth fault loop impedance)
then the solution is to include a 100mA trip RCD with time delay at the
head end as well as the more sensitive RCD at the destination. That way
you have earth fault protection for the cable[1] and personal shock
protection in the garage.

[1] Note that if you use SWA with earthed armour, you have this anyway
in most cases with TN head ends.

2 if there is a trip, I would not have to manually wind up the garage
door to reset the RCD... of course, that assumes I am not in he
garage when it happens. If I were in the garage with the door shut
when it tripped, that would be different again!

In the garage RCD but not protecting the door motor would seem to be the
optimum arrangement.

The fuse in the house is also highly unlikely to trip if sized
appropriately for the submain.

The fuse would be 30A for 6mm^2T&E but I would connect that to a 2-way
CU in the garage with smaller MCBs, only the garage CU would not have
a RCD, so the garage circuits would still be fused appropriately.

With 6mm^2 you could fuse at 40A if required. Ought to leave ample power
available for anything you are likely to do in the garage.

I suppose that I also need to measure the length of the cabel run to
make sure that if the RCD is in the house, all the disconnect times
are still within limits?
The only time is it likely to be a factor is if your earth loop
impedance is marginal in the garage. In which case you would be better
off making the garage a TT install in its own right.

I can't remember how long the cablet o the garage would be exactly but
definitely less than 10 metres. It's just I like the idea of that
cable being RCD protected in case someone knocks/drills/drives into
the conduit carrying it. How resilient is heavy duty plastic conduit
anyway?

At 10m, even with a TN-S head end you really don't have a problem. The
Live to earth round trip resistance of 6mm^2 is 10.49 mOhm/m so about
0.1 ohms total. Add the worst case assumption for your main earth
impedance of 0.8 ohms and you get 0.9 total. Lets say you nail right
through the end of the cable, you will get a fault current of 230/0.9 =
255A. Referring to fig 3.3 in BS7671 (16th) that shows a time of 0.4
sec to open a 32A fuse, and probably under 1 sec for a 40A one.

Finally a check to ensure the CPC in the cable will last at least a
second and so long enough during fault conditions - we need s mm^2 of
CPC where :

s = sqrt( 255^2 x 1 ) / 115 = 2.2 mm^2

Your cable will probably have a 2.5 mm^2 CPC, so not a problem there. In
reality this is a pessimistic calculation since your external fault loop
impedance is probably better than 0.8 ohms and hence the fault current
bigger (which will dramatically shorten the time to clear the fault and
hence the size of CPC required will go down)

For more on this, have a read of:

http://wiki.diyfaq.org.uk/index.php?...n ection_time

The heavy gague plastic conduit is reasonably tough - clomping it with a
hammer is unlikely to damage the cable inside. You could drill through
it if it was buried in plaster.



--
Cheers,

John.

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