I want to run power to a shed. I think my best option is to export
the house earth but I'd like a second opinion.

Summary of proposed installation

incomming supply to house - Ze 0.15 ohms, TN-S

I plan to take power from spare slot on 30mA RCD side of consumer
unit (20 A mcb should be more that enough).

Total length of cable run is around 20m. I plan to take a length of
4mm2 T&E to outside wall and then connect to 4mm2 SWA (roughly half
SWA half T&E).

Shed is wooden with concrete floor.

I don't intend to fit a cu in the shed, just fit a fcu (5A) for
lights and wire a few sockets directly to the incoming SWA as a
radial circuit (+ maybe a 20A DP switch as an isolator).

I've not yet checked the disconection times, voltage drop etc
although my feeling is that for this distance with a 20A mcb it
should be OK (don't have resistance/m data for swa to hand). Are
there any issues with exporting the earth through SWA armor (as
opposed to 3 core) assuming the earth loop impedance is sufficiently
low.

1) Anyone think I would be wise to install TT earthing (+ cu etc) in
the shed?

2)When I take the 4mm T&E through the exterior wall presumably it
needs to be enclosed in conduit (plastic/steel?) and taken to a
weather proof junction box IP???? where I can join it to the swa.

3)Are there any issues with exporting the earth through SWA armor (as
opposed to 3 core) assuming the earth loop impedance is sufficiently
low?

Thanks

Jim

Jim Ingram wrote:

I want to run power to a shed. I think my best option is to export
the house earth but I'd like a second opinion.

Summary of proposed installation

incomming supply to house - Ze 0.15 ohms, TN-S

I plan to take power from spare slot on 30mA RCD side of consumer
unit (20 A mcb should be more that enough).

Total length of cable run is around 20m. I plan to take a length of
4mm2 T&E to outside wall and then connect to 4mm2 SWA (roughly half
SWA half T&E).

Shed is wooden with concrete floor.

I don't intend to fit a cu in the shed, just fit a fcu (5A) for
lights and wire a few sockets directly to the incoming SWA as a
radial circuit (+ maybe a 20A DP switch as an isolator).

The usual problem of discriminating between circuits in the shed.

I've not yet checked the disconection times, voltage drop etc
although my feeling is that for this distance with a 20A mcb it
should be OK (don't have resistance/m data for swa to hand). Are

Voltage drop is 11 mV/A/m, or about 4.4 V in this case.

R per meter will be a factor of the conductor CSA rather than the cable
type as such.

If you were using 3 core SWA with the third core as CPC, then you get
9.22 mOhms/meter on 4mm sq or about 0.2 Ohms. So a total loop impedance
of 0.35 ohm. So a prospective fault current of over 650A. That ought to
open a 20A type B MCB pretty sharpish! (I expect Andy will be along
shortly to do the calc for using the screen in place of 3 core ;-)

there any issues with exporting the earth through SWA armor (as
opposed to 3 core) assuming the earth loop impedance is sufficiently
low.

Not really... need to make sure the gland is well done up, and stick a
boot over it to stop corrosion.

1) Anyone think I would be wise to install TT earthing (+ cu etc) in
the shed?

It would give you the option of taking the submain feed from the non RCD
size of the house CU, and then having split load in the shed. It also
saves you a trek back to the house when you trip the RCD.

2)When I take the 4mm T&E through the exterior wall presumably it
needs to be enclosed in conduit (plastic/steel?) and taken to a
weather proof junction box IP???? where I can join it to the swa.

If you go for the 3 core SWA then you could terminate the screen in a
metal (waterproof) box but continue the inner part through the wall to
an internal JB. Or if you don't have to have the SWA flat against the
wall you can take the whole thing through.

3)Are there any issues with exporting the earth through SWA armor (as
opposed to 3 core) assuming the earth loop impedance is sufficiently
low?

I refer the honerable gentleman to the previous answer ;-)

--
Cheers,

John.

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

"John Rumm" wrote in message
...
Jim Ingram wrote:

I want to run power to a shed. I think my best option is to
export
the house earth but I'd like a second opinion.

Summary of proposed installation

incomming supply to house - Ze 0.15 ohms, TN-S

I plan to take power from spare slot on 30mA RCD side of consumer
unit (20 A mcb should be more that enough).

Total length of cable run is around 20m. I plan to take a length
of
4mm2 T&E to outside wall and then connect to 4mm2 SWA (roughly
half
SWA half T&E).

Shed is wooden with concrete floor.

I don't intend to fit a cu in the shed, just fit a fcu (5A) for
lights and wire a few sockets directly to the incoming SWA as a
radial circuit (+ maybe a 20A DP switch as an isolator).

The usual problem of discriminating between circuits in the shed.

Do you think this will be a significant problem as i'm essentially
planning a single radial circuit with a fused 5A spur to the lights.
I'm not actually sure I care if a fault on the light circuit blows
the 5A fuse or trips the 20A mcb first. If I was planning anything
more elaborate I'd fit another cu.

One thing I did forget to ask was that I was intending to go from
4mm2 cable up to the shed and then to 2.5mm2 to sockets inside the
shed. Is it OK to mix different size cables on a circuit, provided
disconection times etc are met.


I've not yet checked the disconection times, voltage drop etc
although my feeling is that for this distance with a 20A mcb it
should be OK (don't have resistance/m data for swa to hand). Are

Voltage drop is 11 mV/A/m, or about 4.4 V in this case.

R per meter will be a factor of the conductor CSA rather than the
cable
type as such.

If you were using 3 core SWA with the third core as CPC, then you
get
9.22 mOhms/meter on 4mm sq or about 0.2 Ohms. So a total loop
impedance
of 0.35 ohm. So a prospective fault current of over 650A. That
ought to
open a 20A type B MCB pretty sharpish! (I expect Andy will be along
shortly to do the calc for using the screen in place of 3 core ;-)

I found some values on the net which worked out a bit higher than
yours (9mOhm/m looks like 6mm2 on the table I found), but still gave
a fault current of 500A.


there any issues with exporting the earth through SWA armor (as
opposed to 3 core) assuming the earth loop impedance is
sufficiently
low.

Not really... need to make sure the gland is well done up, and
stick a
boot over it to stop corrosion.

1) Anyone think I would be wise to install TT earthing (+ cu etc)
in
the shed?

It would give you the option of taking the submain feed from the
non RCD
size of the house CU, and then having split load in the shed. It
also
saves you a trek back to the house when you trip the RCD.

Surely I could do this anyway and take the house earth to the split
load cu in the shed?



2)When I take the 4mm T&E through the exterior wall presumably it
needs to be enclosed in conduit (plastic/steel?) and taken to a
weather proof junction box IP???? where I can join it to the swa.

If you go for the 3 core SWA then you could terminate the screen in
a
metal (waterproof) box but continue the inner part through the wall
to
an internal JB. Or if you don't have to have the SWA flat against
the
wall you can take the whole thing through.

Because of where the T&E cable will have to run - under floorboards
upstairs. I would rather join the cables outside (although I take
your point - probably a more reliable connection). The weatherproof
junction boxes I've seen are all plastic?

Thanks

Jim

Jim Ingram wrote:

I want to run power to a shed. I think my best option is to export
the house earth but I'd like a second opinion.

Summary of proposed installation

incomming supply to house - Ze 0.15 ohms, TN-S

I plan to take power from spare slot on 30mA RCD side of consumer
unit (20 A mcb should be more that enough).

Total length of cable run is around 20m. I plan to take a length of
4mm2 T&E to outside wall and then connect to 4mm2 SWA (roughly half
SWA half T&E).

OK so far - let's do some sums:

Design current (Ib) - assume 20A

Voltage drop: live conductors are 4mm^2 all the way, drop is 11 mV/A/m
[Table 4D2B] hence 4.4 V max. drop for Ib = 20A and a 20 m run. This is
under 2% and leaves you another 2% to play with for wiring within the
shed. All OK here.

Earth fault loop impedance (Zs):

(a) contribution from 10 m of 4mm^2 T&E with 1.5mm^2 CPC - total
resistance R1+R2 = 16.7 mohm/m [OSG Table 9A], hence contribution to Zs
is 0.17 ohm for 10m run;

(b) ditto for 10m of SWA (XLPE insulated to BS 5467): phase conductor
4.6 mohm/m, armour (~19mm^2) 7.9 mohm/m, total 12.5 mohm/m, say 0.13 ohm
total contribution to Zs for a 10m run.

(a) plus (b) gives about 0.3 ohm total contribution to Zs. Taking Ze as
0.8 ohm (the recommended design basis, rather than relying on your
measured value of 0.15 ohm) gives Zs ~ 1.1 ohm. This is a 'cold' value
by the way based on resistances at 20 deg. C. Consulting Table 2D of
the OSG for a B20A MCB tells us that the maximum measured (cold) Zs is
1.92 ohm, so you're miles in, and are not relying on your RCD for earth
fault clearance. (You could consider feeding from the non-RCD side of
the consumer unit and putting another RCD in the shed, for the sockets.)
With a Type C MCB we need Zs 0.96 ohm, so there's a potential
problem problem there in principle - although not in practice because of
your nice low Ze, which is most unlikely in practice to increase.

CPC adiabatic compliance - with MCB protection tripping in 0.1 s it's
hardly necessary to check this - but here goes anyway:

Worst case is with Zs = 1.1 ohm (see above), earth fault current is ~
230 V / 1.1 ohm - say 200 A (rounding down to allow for a bit of
heating). For the T&E cable with a copper CPC the min. CPC size is
sqrt(I^2*t)/k with k = 115, i.e. 200*sqrt(0.1)/115 which is about
0.55mm^2. The cable has a 1.5mm^2 CPC, so no problems here. Now for
the SWA armour: same calculation but with a k value of 46, giving a min.
CPC of ~1.4mm^2. As we have 19mm^2 of armour there's no problem here
either - this confirms that it's OK to use the armour as the CPC and
there's no need to use a 3-core SWA.

Shed is wooden with concrete floor.

Dry inside, no incoming metal service pipes coming out of the ground, no
intention regularly to use Class 1 appliances outdoors? - if all those
apply you're OK with the exported earth. If not, consider the TT option.

I don't intend to fit a cu in the shed, just fit a fcu (5A) for
lights and wire a few sockets directly to the incoming SWA as a
radial circuit (+ maybe a 20A DP switch as an isolator).

That's OK, provided you accept the inconvenience of losing both light
and power if anything trips the MCB or RCD in the house.

I've not yet checked the disconection times, voltage drop etc
although my feeling is that for this distance with a 20A mcb it
should be OK (don't have resistance/m data for swa to hand). Are
there any issues with exporting the earth through SWA armor (as
opposed to 3 core) assuming the earth loop impedance is sufficiently
low.

See calculations above.

1) Anyone think I would be wise to install TT earthing (+ cu etc) in
the shed?

Google for previous lengthy debates.

2)When I take the 4mm T&E through the exterior wall presumably it
needs to be enclosed in conduit (plastic/steel?) and taken to a
weather proof junction box IP???? where I can join it to the swa.

You don't need to protect the T&E in the wall, but you do need to be
sure that water can't get in to the IP66 (say) box through your cable
hole at the back.

3)Are there any issues with exporting the earth through SWA armor (as
opposed to 3 core) assuming the earth loop impedance is sufficiently
low?

That's not usually an issue with the smaller SWA sizes (greater ratio of
armour area to copper area). For larger sizes a separate CPC or third
core may become necessary.

--
Andy

"Andy Wade" wrote in message
...
Jim Ingram wrote:

I want to run power to a shed. I think my best option is to
export
the house earth but I'd like a second opinion.

Summary of proposed installation

incomming supply to house - Ze 0.15 ohms, TN-S

I plan to take power from spare slot on 30mA RCD side of consumer
unit (20 A mcb should be more that enough).

Total length of cable run is around 20m. I plan to take a length
of
4mm2 T&E to outside wall and then connect to 4mm2 SWA (roughly
half
SWA half T&E).

OK so far - let's do some sums:

Design current (Ib) - assume 20A

Voltage drop: live conductors are 4mm^2 all the way, drop is 11
mV/A/m
[Table 4D2B] hence 4.4 V max. drop for Ib = 20A and a 20 m run.
This is
under 2% and leaves you another 2% to play with for wiring within
the
shed. All OK here.

Earth fault loop impedance (Zs):

(a) contribution from 10 m of 4mm^2 T&E with 1.5mm^2 CPC - total
resistance R1+R2 = 16.7 mohm/m [OSG Table 9A], hence contribution
to Zs
is 0.17 ohm for 10m run;

(b) ditto for 10m of SWA (XLPE insulated to BS 5467): phase
conductor
4.6 mohm/m, armour (~19mm^2) 7.9 mohm/m, total 12.5 mohm/m, say
0.13 ohm
total contribution to Zs for a 10m run.

(a) plus (b) gives about 0.3 ohm total contribution to Zs. Taking
Ze as
0.8 ohm (the recommended design basis, rather than relying on your
measured value of 0.15 ohm) gives Zs ~ 1.1 ohm. This is a 'cold'
value
by the way based on resistances at 20 deg. C. Consulting Table 2D
of
the OSG for a B20A MCB tells us that the maximum measured (cold) Zs
is
1.92 ohm, so you're miles in, and are not relying on your RCD for
earth
fault clearance. (You could consider feeding from the non-RCD side
of
the consumer unit and putting another RCD in the shed, for the
sockets.)
With a Type C MCB we need Zs 0.96 ohm, so there's a potential
problem problem there in principle - although not in practice
because of
your nice low Ze, which is most unlikely in practice to increase.

CPC adiabatic compliance - with MCB protection tripping in 0.1 s
it's
hardly necessary to check this - but here goes anyway:

Worst case is with Zs = 1.1 ohm (see above), earth fault current is
~
230 V / 1.1 ohm - say 200 A (rounding down to allow for a bit of
heating). For the T&E cable with a copper CPC the min. CPC size is
sqrt(I^2*t)/k with k = 115, i.e. 200*sqrt(0.1)/115 which is about
0.55mm^2. The cable has a 1.5mm^2 CPC, so no problems here. Now
for
the SWA armour: same calculation but with a k value of 46, giving a
min.
CPC of ~1.4mm^2. As we have 19mm^2 of armour there's no problem
here
either - this confirms that it's OK to use the armour as the CPC
and
there's no need to use a 3-core SWA.

Shed is wooden with concrete floor.

Dry inside, no incoming metal service pipes coming out of the
ground, no
intention regularly to use Class 1 appliances outdoors? - if all
those
apply you're OK with the exported earth. If not, consider the TT
option.

I don't intend to fit a cu in the shed, just fit a fcu (5A) for
lights and wire a few sockets directly to the incoming SWA as a
radial circuit (+ maybe a 20A DP switch as an isolator).

That's OK, provided you accept the inconvenience of losing both
light
and power if anything trips the MCB or RCD in the house.

I've not yet checked the disconection times, voltage drop etc
although my feeling is that for this distance with a 20A mcb it
should be OK (don't have resistance/m data for swa to hand). Are
there any issues with exporting the earth through SWA armor (as
opposed to 3 core) assuming the earth loop impedance is
sufficiently
low.

See calculations above.

1) Anyone think I would be wise to install TT earthing (+ cu etc)
in
the shed?

Google for previous lengthy debates.

2)When I take the 4mm T&E through the exterior wall presumably it
needs to be enclosed in conduit (plastic/steel?) and taken to a
weather proof junction box IP???? where I can join it to the swa.

You don't need to protect the T&E in the wall, but you do need to
be
sure that water can't get in to the IP66 (say) box through your
cable
hole at the back.

3)Are there any issues with exporting the earth through SWA armor
(as
opposed to 3 core) assuming the earth loop impedance is
sufficiently
low?

That's not usually an issue with the smaller SWA sizes (greater
ratio of
armour area to copper area). For larger sizes a separate CPC or
third
core may become necessary.


Hi Andy,

Many thanks for such a detailed analysis.


Cheers
Jim

Andy Wade wrote:
Jim Ingram wrote:

I want to run power to a shed. I think my best option is to
export the house earth but I'd like a second opinion.

OK so far - let's do some sums:....

delurk

This post represents all that is best of uk.d-i-y. Sincere thanks to Andy,
and to all the experts & enthusiasts who share their wisdom here.

relurk

Jim Ingram wrote:

Do you think this will be a significant problem as i'm essentially
planning a single radial circuit with a fused 5A spur to the lights.
I'm not actually sure I care if a fault on the light circuit blows
the 5A fuse or trips the 20A mcb first. If I was planning anything
more elaborate I'd fit another cu.

It was more a case of if a fault on the socket circuit taking out the
lights was going to give you a problem. i.e. power tools can carry on
spinning even with not power and the lights off.

One thing I did forget to ask was that I was intending to go from
4mm2 cable up to the shed and then to 2.5mm2 to sockets inside the
shed. Is it OK to mix different size cables on a circuit, provided
disconection times etc are met.

It would be more usual to run a radial in 4mm T&E in this circumstance.
If you go to 2.5mm then you have to watch the cable load.

I found some values on the net which worked out a bit higher than
yours (9mOhm/m looks like 6mm2 on the table I found), but still gave
a fault current of 500A.

According to the OSG my figures are right for a 4mm sq phase with 4mm sq
CPC....

However thinking about it, my figurs are half wrong since you are not
going to have the 4mm sq CPC for the whole length - only the SWA bit.
The T&E will typically have a 2.5mm sq CPC which will raise R1+R2/m to
12.02 ohms. As you say though, it does not make much difference since
you still get a decent fault current.

1) Anyone think I would be wise to install TT earthing (+ cu etc)
the shed?

It would give you the option of taking the submain feed from the
non RCD
size of the house CU, and then having split load in the shed. It
also
saves you a trek back to the house when you trip the RCD.


Surely I could do this anyway and take the house earth to the split
load cu in the shed?

Yup, looks like it.

Because of where the T&E cable will have to run - under floorboards
upstairs. I would rather join the cables outside (although I take
your point - probably a more reliable connection). The weatherproof
junction boxes I've seen are all plastic?

You can use plastic - you will just need a earthing tag connection to
the gland to provide connection to it rather than relying on the box.

--
Cheers,

John.

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

John Rumm wrote:

The T&E will typically have a 2.5mm sq CPC which will raise R1+R2/m to

Just thought were are talking 4mm again not 6, so the CPC may only be
1.5mm (c. 16 ohms)

--
Cheers,

John.

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

John Rumm wrote:

Jim Ingram wrote:
One thing I did forget to ask was that I was intending to go from
4mm2 cable up to the shed and then to 2.5mm2 to sockets inside the
shed. Is it OK to mix different size cables on a circuit, provided
disconection times etc are met.

It would be more usual to run a radial in 4mm T&E in this circumstance.
If you go to 2.5mm then you have to watch the cable load.

There's no problem with 2.5mm^2 here - the whole circuit is only rated
at 20A. As I showed, with a B20A MCB and the 4mm^2 submain, there's 2%
voltage drop and about 0.8 of an ohm of Zs in hand - so you you could
run up to about 12m of 2.5 T&E before you hit any limit. At 20A (for a
double socket) you hit the voltage drop limit first. A branch in 2.5
feeding a single socket (13A) could go for 20m (still voltage-drop
limited). How big's the shed...?

--
Andy

Andy Wade wrote:

There's no problem with 2.5mm^2 here - the whole circuit is only rated
at 20A. As I showed, with a B20A MCB and the 4mm^2 submain, there's 2%

Yup true, I was forgetting he had a 20A MCB on it....

feeding a single socket (13A) could go for 20m (still voltage-drop
limited). How big's the shed...?

Based on my experiance of sheds, never big enough! ;-)

--
Cheers,

John.

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

John Rumm wrote:

Based on my experiance of sheds, never big enough! ;-)

This is, of course, a truism.

--
Andy