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Old February 7th 19, 02:24 PM posted to uk.d-i-y
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Default Question about sub-main sizing

My TNC-S supply enters in the attached garage, where the tails are split
to a CU (in the garage, feeding: the kitchen, cooker, utility and a few
other bits) and a 60A switched fuse which feeds a 16mm2 sub-main.
The sub-main is about 20m long and connects to an ancient fuse board
that supplies the house radial and ring outlets. A second fuse board in
the same location is connected to one of the MCBs in the CU via a 2.5mm2
cable and supplies power to most of the house lights.
My plan *has* been to fit a new CU close to the garage and extend all
the cables back to it, but that requires a lot of disruption. I'm now
wondering whether I should simply replace the two wylex boards with a
single new (RCBO) CU in their present position and feed it from the
sub-main.
From some quick checks the existing 16mm2 will be OK at 20m for volts
drop if I protect it with a 50A MCB. However, from a quick look (more
poking around required) the only earth connection seems to be the CPC in
the 16mm2 T&E so that will need improving and the disruption involved
won't be much less than extending all the circuits, although it will be
less work.
What do the relevant panel experts think?

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Old February 7th 19, 05:57 PM posted to uk.d-i-y
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First recorded activity by DIYBanter: Jul 2006
Posts: 22,632
Default Question about sub-main sizing

On 07/02/2019 14:24, wrote:

My TNC-S supply enters in the attached garage, where the tails are split
to a CU (in the garage, feeding: the kitchen, cooker, utility and a few
other bits) and a 60A switched fuse which feeds a 16mm2 sub-main.
The sub-main is about 20m long and connects to an ancient fuse board
that supplies the house radial and ring outlets. A second fuse board in
the same location is connected to one of the MCBs in the CU via a 2.5mm2
cable and supplies power to most of the house lights.
My plan *has* been to fit a new CU close to the garage and extend all
the cables back to it, but that requires a lot of disruption. I'm now
wondering whether I should simply replace the two wylex boards with a
single new (RCBO) CU in their present position and feed it from the
sub-main.


Yup, that sounds like a less disruptive option.

From some quick checks the existing 16mm2 will be OK at 20m for volts
drop if I protect it with a 50A MCB. However, from a quick look (more
poking around required) the only earth connection seems to be the CPC in
the 16mm2 T&E so that will need improving and the disruption involved
won't be much less than extending all the circuits, although it will be
less work.


Since its a PME head end, the sub main will need to extend the main
equipotential zone to the new CU. So its earth will not only need to
meet the requirements of being a functional earth, but also a main
bonding conductor between supply and CU. That means you need 10mm^2 of
copper or equal or better amount of copper equivalent via some other means.

What do the relevant panel experts think?


You could either run a separate main bonding conductor (10mm^2
green/yellow single), or upgrade the T&E to say a 3 core SWA, and
parallel the armour with one of the cores.

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

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

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

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


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
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Old February 7th 19, 06:38 PM posted to uk.d-i-y
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First recorded activity by DIYBanter: Dec 2017
Posts: 396
Default Question about sub-main sizing

On 07/02/2019 17:57, John Rumm wrote:
On 07/02/2019 14:24, wrote:

My TNC-S supply enters in the attached garage, where the tails are
split to a CU (in the garage, feeding: the kitchen, cooker, utility
and a few other bits) and a 60A switched fuse which feeds a 16mm2
sub-main.
The sub-main is about 20m long and connects to an ancient fuse board
that supplies the house radial and ring outlets. A second fuse board
in the same location is connected to one of the MCBs in the CU via a
2.5mm2 cable and supplies power to most of the house lights.
My plan *has* been to fit a new CU close to the garage and extend all
the cables back to it, but that requires a lot of disruption. I'm now
wondering whether I should simply replace the two wylex boards with a
single new (RCBO) CU in their present position and feed it from the
sub-main.


Yup, that sounds like a less disruptive option.

*From some quick checks the existing 16mm2 will be OK at 20m for volts
drop if I protect it with a 50A MCB. However, from a quick look (more
poking around required) the only earth connection seems to be the CPC
in the 16mm2 T&E so that will need improving and the disruption
involved won't be much less than extending all the circuits, although
it will be less work.


Since its a PME head end, the sub main will need to extend the main
equipotential zone to the new CU. So its earth will not only need to
meet the requirements of being a functional earth, but also a main
bonding conductor between supply and CU. That means you need 10mm^2 of
copper or equal or better amount of copper equivalent via some other means.

What do the relevant panel experts think?


You could either run a separate main bonding conductor (10mm^2
green/yellow single), or upgrade the T&E to say a 3 core SWA, and
parallel the armour with one of the cores.

.... snipped
Thanks John, that was what I thought.
The existing 16mm2 T&E runs between joists (mostly just resting on the
ceiling, rather than being clipped) from the garage to the fuse boards
so it's going to be a (necessary) pain to lift carpets and boards to run
the green/yellow, but it would be more of a pain to replace it with SWA
- why would I consider doing that?

I haven't been able to find the rules for sub-main protection. Is it
permissible to run it from a 50A DP MCB in the garage CU, or does have
to be from a separate DP fused/switch from a Henley block?

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Old February 7th 19, 06:50 PM posted to uk.d-i-y
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Posts: 180
Default Question about sub-main sizing

I've not got tables to hand but check your earth loop impedance against disconnection times for the 40A mcb
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Old February 7th 19, 07:17 PM posted to uk.d-i-y
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Posts: 396
Default Question about sub-main sizing

On 07/02/2019 18:50, Cynic wrote:
I've not got tables to hand but check your earth loop impedance against disconnection times for the 40A mcb

The sub-main earth loop impedance (20m of 10mm2 and 20m of 16mm2) should
be about 60mOhm (37mOhm + 23mOhm) so about 0.4 ohms total (TN-C-S) -
about 600A fault current. A type C MCB needs 10x rated current, so 500A
for a 50A MCB. Unless I've boobed it should be OK.


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Old February 7th 19, 11:01 PM posted to uk.d-i-y
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First recorded activity by DIYBanter: Jul 2006
Posts: 22,632
Default Question about sub-main sizing

On 07/02/2019 18:38, wrote:
On 07/02/2019 17:57, John Rumm wrote:
On 07/02/2019 14:24,
wrote:

My TNC-S supply enters in the attached garage, where the tails are
split to a CU (in the garage, feeding: the kitchen, cooker, utility
and a few other bits) and a 60A switched fuse which feeds a 16mm2
sub-main.
The sub-main is about 20m long and connects to an ancient fuse board
that supplies the house radial and ring outlets. A second fuse board
in the same location is connected to one of the MCBs in the CU via a
2.5mm2 cable and supplies power to most of the house lights.
My plan *has* been to fit a new CU close to the garage and extend all
the cables back to it, but that requires a lot of disruption. I'm now
wondering whether I should simply replace the two wylex boards with a
single new (RCBO) CU in their present position and feed it from the
sub-main.


Yup, that sounds like a less disruptive option.

*From some quick checks the existing 16mm2 will be OK at 20m for
volts drop if I protect it with a 50A MCB. However, from a quick look
(more poking around required) the only earth connection seems to be
the CPC in the 16mm2 T&E so that will need improving and the
disruption involved won't be much less than extending all the
circuits, although it will be less work.


Since its a PME head end, the sub main will need to extend the main
equipotential zone to the new CU. So its earth will not only need to
meet the requirements of being a functional earth, but also a main
bonding conductor between supply and CU. That means you need 10mm^2 of
copper or equal or better amount of copper equivalent via some other
means.

What do the relevant panel experts think?


You could either run a separate main bonding conductor (10mm^2
green/yellow single), or upgrade the T&E to say a 3 core SWA, and
parallel the armour with one of the cores.

... snipped
Thanks John, that was what I thought.
The existing 16mm2 T&E runs between joists (mostly just resting on the
ceiling, rather than being clipped) from the garage to the fuse boards
so it's going to be a (necessary) pain to lift carpets and boards to run
the green/yellow, but it would be more of a pain to replace it with SWA
- why would I consider doing that?


You could use the T&E as a draw wire to pull in the single SWA that
would give you both power, earthing, and EQ bonding.

I haven't been able to find the rules for sub-main protection. Is it
permissible to run it from a 50A DP MCB in the garage CU, or does have
to be from a separate DP fused/switch from a Henley block?


No you can run from a MCB without any problem. Its commonly done.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd -
http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
  #7   Report Post  
Old February 7th 19, 11:36 PM posted to uk.d-i-y
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First recorded activity by DIYBanter: Dec 2017
Posts: 396
Default Question about sub-main sizing

On 07/02/2019 23:01, John Rumm wrote:
On 07/02/2019 18:38, wrote:
On 07/02/2019 17:57, John Rumm wrote:
On 07/02/2019 14:24,
wrote:

My TNC-S supply enters in the attached garage, where the tails are
split to a CU (in the garage, feeding: the kitchen, cooker, utility
and a few other bits) and a 60A switched fuse which feeds a 16mm2
sub-main.
The sub-main is about 20m long and connects to an ancient fuse board
that supplies the house radial and ring outlets. A second fuse board
in the same location is connected to one of the MCBs in the CU via a
2.5mm2 cable and supplies power to most of the house lights.
My plan *has* been to fit a new CU close to the garage and extend
all the cables back to it, but that requires a lot of disruption.
I'm now wondering whether I should simply replace the two wylex
boards with a single new (RCBO) CU in their present position and
feed it from the sub-main.

Yup, that sounds like a less disruptive option.

*From some quick checks the existing 16mm2 will be OK at 20m for
volts drop if I protect it with a 50A MCB. However, from a quick
look (more poking around required) the only earth connection seems
to be the CPC in the 16mm2 T&E so that will need improving and the
disruption involved won't be much less than extending all the
circuits, although it will be less work.

Since its a PME head end, the sub main will need to extend the main
equipotential zone to the new CU. So its earth will not only need to
meet the requirements of being a functional earth, but also a main
bonding conductor between supply and CU. That means you need 10mm^2
of copper or equal or better amount of copper equivalent via some
other means.

What do the relevant panel experts think?

You could either run a separate main bonding conductor (10mm^2
green/yellow single), or upgrade the T&E to say a 3 core SWA, and
parallel the armour with one of the cores.

... snipped
Thanks John, that was what I thought.
The existing 16mm2 T&E runs between joists (mostly just resting on the
ceiling, rather than being clipped) from the garage to the fuse boards
so it's going to be a (necessary) pain to lift carpets and boards to
run the green/yellow, but it would be more of a pain to replace it
with SWA - why would I consider doing that?


You could use the T&E as a draw wire to pull in the single SWA that
would give you both power, earthing, and EQ bonding.

Yes, but it's not going to be simple to draw SWA through joist cut-outs.
Why should I consider replacing the existing T&E with SWA rather than
simply adding a 10mm2 or 16mm2 green/yellow?

I haven't been able to find the rules for sub-main protection. Is it
permissible to run it from a 50A DP MCB in the garage CU, or does have
to be from a separate DP fused/switch from a Henley block?


No you can run from a MCB without any problem. Its commonly done.

Is SP permissible, or does it need to be DP?
  #8   Report Post  
Old February 7th 19, 11:59 PM posted to uk.d-i-y
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First recorded activity by DIYBanter: Jul 2006
Posts: 22,632
Default Question about sub-main sizing

On 07/02/2019 23:36, wrote:
On 07/02/2019 23:01, John Rumm wrote:
On 07/02/2019 18:38,
wrote:
On 07/02/2019 17:57, John Rumm wrote:
On 07/02/2019 14:24,
wrote:

My TNC-S supply enters in the attached garage, where the tails are
split to a CU (in the garage, feeding: the kitchen, cooker, utility
and a few other bits) and a 60A switched fuse which feeds a 16mm2
sub-main.
The sub-main is about 20m long and connects to an ancient fuse
board that supplies the house radial and ring outlets. A second
fuse board in the same location is connected to one of the MCBs in
the CU via a 2.5mm2 cable and supplies power to most of the house
lights.
My plan *has* been to fit a new CU close to the garage and extend
all the cables back to it, but that requires a lot of disruption.
I'm now wondering whether I should simply replace the two wylex
boards with a single new (RCBO) CU in their present position and
feed it from the sub-main.

Yup, that sounds like a less disruptive option.

*From some quick checks the existing 16mm2 will be OK at 20m for
volts drop if I protect it with a 50A MCB. However, from a quick
look (more poking around required) the only earth connection seems
to be the CPC in the 16mm2 T&E so that will need improving and the
disruption involved won't be much less than extending all the
circuits, although it will be less work.

Since its a PME head end, the sub main will need to extend the main
equipotential zone to the new CU. So its earth will not only need to
meet the requirements of being a functional earth, but also a main
bonding conductor between supply and CU. That means you need 10mm^2
of copper or equal or better amount of copper equivalent via some
other means.

What do the relevant panel experts think?

You could either run a separate main bonding conductor (10mm^2
green/yellow single), or upgrade the T&E to say a 3 core SWA, and
parallel the armour with one of the cores.

... snipped
Thanks John, that was what I thought.
The existing 16mm2 T&E runs between joists (mostly just resting on
the ceiling, rather than being clipped) from the garage to the fuse
boards so it's going to be a (necessary) pain to lift carpets and
boards to run the green/yellow, but it would be more of a pain to
replace it with SWA - why would I consider doing that?


You could use the T&E as a draw wire to pull in the single SWA that
would give you both power, earthing, and EQ bonding.

Yes, but it's not going to be simple to draw SWA through joist cut-outs.
Why should I consider replacing the existing T&E with SWA rather than
simply adding a 10mm2 or 16mm2 green/yellow?


You can do whichever is easiest and meets the required spec. Sometimes
adding a wire can be more difficult that pulling in a new one. However
much will depend on the detail.

I haven't been able to find the rules for sub-main protection. Is it
permissible to run it from a 50A DP MCB in the garage CU, or does
have to be from a separate DP fused/switch from a Henley block?


No you can run from a MCB without any problem. Its commonly done.


Is SP permissible, or does it need to be DP?


MCBs are typically SP, but you will have DP switching at the destination
CU, and also at the source CU via the main switches, so the switching on
the circuit protection does not matter as such so long as it can break
the circuit in the even of a fault.



--
Cheers,

John.

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