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A very long electrical supply to garden shed



 
 
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  #1  
Old August 25th 05, 11:13 AM
Senior Member
 
First recorded activity by DIYBanter: Aug 2005
Posts: 242
Red face A very long electrical supply to garden shed

I am planning to run an electrical supply to my shed and wish to get some advice before calling in an electrician, so as to avoid him baffling me with requirements and fleecing me of my hard earned cash!

The supply is to come from the CU in my garage, run surfaced fixed around the garage, through the wall, around the kitchen, through the wall and up the garden to the shed. The length of run through the house will be 10m and then 45m up the garden (armoured cable I assume buried in a duct 150mm below ground).

It will be supplying 3 No. strip double fluorescent lights, 3 No. internal double sockets, 1 No. external double socket and a water-butt pump (only used occasionally though). The maximum amount of things I will be using at the same time would be a couple of battery chargers, a circ saw and the lights.

Questions:

1. Can he utilise a spare 32A MCB running through a 80mA RCD on my CU in the garage?

2. If not, would is required?

3. What size of cable would be required on this 55m run with, what I assume will be, a huge voltage drop?

4. Can he install a switch within the kitchen to turn off the supply when I'm not in the shed? (This is to avoid the local thieving chavs nicking my juice!)

5. Can he install T&E through the house and changing at a weatherproof junction box on the external kitchen wall, to save on the cost of using Armoured all the way?

6. Will he insist on a separate CU in the garage running from the meter? (More money though to get electricity board involved)

7. Is it necessary to have a CU with separate breakers in the shed to control/isolate all the different components?

Any advice would be welcome to avoid being taken for a ride.
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  #2  
Old August 25th 05, 01:17 PM
Christian McArdle
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1. Can he utilise a spare 32A MCB running through a 80mA RCD on my CU
in the garage?


Yes.

3. What size of cable would be required on this 55m run with, what I
assume will be, a huge voltage drop?


Indeed. And you should really allow voltage drop between the garage and the
main house CU as well (unless the garage CU is located at the service
point). Also, there is voltage drop within the shed installation.

Assuming that there is no additional voltage drop to take into consideration
and allowing 10m within the shed, 10mm SWA is indicated for the full 32A
load.

If you design for a lower load, such as 20A, which will fulfill all your
needs, you're on the margins of being able to use 6mm.

4. Can he install a switch within the kitchen to turn off the supply
when I'm not in the shed? (This is to avoid the local thieving chavs
nicking my juice!)


Yes. Use a 45A DP switch for this. Often sold as cooker or shower switches.

5. Can he install T&E through the house and changing at a weatherproof
junction box on the external kitchen wall, to save on the cost of using
Armoured all the way?


Yes.

6. Will he insist on a separate CU in the garage running from the
meter? (More money though to get electricity board involved)


No.

7. Is it necessary to have a CU with separate breakers in the shed to
control/isolate all the different components?


No, as it runs from an RCD circuit anyway, but you might as well. There's a
Volex "garage" unit, which has an RCD and a couple of MCBs in a waterproof
box. If you don't have one, the lighting circuit must come off an FCU, as
32A would be too much without further protection.

Any advice would be welcome to avoid being taken for a ride.


It may be inappropriate to export the house earth this distance. You may
need a TT earthing system (earth rod) and RCD, with the house earth (i.e.
SWA armour) isolated at the shed consumer unit.

Christian.



  #3  
Old August 25th 05, 02:31 PM
Mr Fizzion
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On Thu, 25 Aug 2005 10:13:08 +0100, Cordless Crazy
wrote:

3. What size of cable would be required on this 55m run with, what I
assume will be, a huge voltage drop?


Voltage drop is given by V=IR where I is the current in amps and R is
the resistance of the wires in ohms. The resistance of the wires is
found from the resistivity of copper - 1.68*10^-8 ohm metres: R =
(length in metres * resitivity) / cross-sectional area of the wire in
square metres.

Mr F.

  #4  
Old August 25th 05, 04:16 PM
Ian_m
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"Mr Fizzion" wrote in message
...
On Thu, 25 Aug 2005 10:13:08 +0100, Cordless Crazy
wrote:

3. What size of cable would be required on this 55m run with, what I
assume will be, a huge voltage drop?


Voltage drop is given by V=IR where I is the current in amps and R is
the resistance of the wires in ohms. The resistance of the wires is
found from the resistivity of copper - 1.68*10^-8 ohm metres: R =
(length in metres * resitivity) / cross-sectional area of the wire in
square metres.

You can use the voltage drop calculator here to find the size of wire you
need.
http://www.tlc-direct.co.uk/Technica...ltageDrop.html

My Dad ran 6mm for 50m to his garden shed to power lights and power tools.
Wire got there in a combination of tacked onto wall and burried underground.
Came from a small CU (holding 63A RCD and 2 MCB's, on for garage) next to
main CU (wired up using 10mm wires) so that you could isolate the supply
easily and if there was a fault would not affect any of the house ring
mains. Ran to another CU in shed with 63A switch and split into plugs and
lighting via two MCB's. Not too sure what was done with earth but I think an
earth rod was used.


  #5  
Old August 25th 05, 04:30 PM
Will Dean
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"Cordless Crazy" wrote in message
...


(armoured cable I assume buried
in a duct 150mm below ground).


You don't need to put armoured cable in a duct. It's designed for direct
burial.

Will


  #6  
Old August 25th 05, 07:03 PM
John Blessing
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"Mr Fizzion" wrote in message
...
On Thu, 25 Aug 2005 10:13:08 +0100, Cordless Crazy
wrote:

3. What size of cable would be required on this 55m run with, what I
assume will be, a huge voltage drop?


Voltage drop is given by V=IR where I is the current in amps and R is
the resistance of the wires in ohms. The resistance of the wires is
found from the resistivity of copper - 1.68*10^-8 ohm metres: R =
(length in metres * resitivity) / cross-sectional area of the wire in
square metres.

Mr F.


Isn't that formula valid only for DC not AC current?

--
John Blessing



  #7  
Old August 25th 05, 09:57 PM
[email protected]
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SFAIUI (and that's only a little) Pirelli specify a maximum
surrounding (ambient) temperature for their SWA (around 5deg C IIRC) &
that it should be surrounded by something that will take the heat away.
See
http://www.pirelli.co.uk/en_GB/cables_systems/energy/catalogue_pdf/lv...
IIRC Pirelli specify depth min 450mm & surrounding the cable in soft
sand (which should be moist, or at least not liable to dry out).

Modern SWA is not laid in ducts normally - presumably you'd have to
derate it to take account of the reduced heat conduction. How much all
that mattters in your case depends on whether and by how much the cable
is overrated for your full load & what proportion of the time it is
carrying full load amps (ie the duty cycle).

HTH

  #8  
Old August 26th 05, 06:26 AM
John Rumm
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It will be supplying 3 No. strip double fluorescent lights, 3 No.

3A ish

internal double sockets, 1 No. external double socket and a water-butt


20A ought to be plenty

pump (only used occasionally though). The maximum amount of things I
will be using at the same time would be a couple of battery chargers, a
circ saw and the lights.



In fact 20A would do the whol building just fine then.

Questions:

1. Can he utilise a spare 32A MCB running through a 80mA RCD on my CU
in the garage?


Yes, and sort of. Do you mean 80A RCD or 80mA Trip RCD? The former is a
common total current rating for some RCDs, the latter would be an "odd"
trip threshold (30mA or 100mA being far more common). For sockets that
may reasonably be used for powering appliacnes outside, then you really
want the sockets protected by a RCD with a 30mA trip current. (the total
current carrying capacity is not relevant here).

3. What size of cable would be required on this 55m run with, what I
assume will be, a huge voltage drop?


6/10mm SWA for 20/32A circuit.

4. Can he install a switch within the kitchen to turn off the supply
when I'm not in the shed? (This is to avoid the local thieving chavs
nicking my juice!)


Yup

5. Can he install T&E through the house and changing at a weatherproof
junction box on the external kitchen wall, to save on the cost of using
Armoured all the way?


Yup

6. Will he insist on a separate CU in the garage running from the
meter? (More money though to get electricity board involved)


Noe, and whay would that involve the electricity board anyway?

7. Is it necessary to have a CU with separate breakers in the shed to
control/isolate all the different components?


It is adviseable - at least to separate the ligting circuit from the
sockets. Also see below.

Any advice would be welcome to avoid being taken for a ride.


Since you want to use a circular saw in the shed you ought to pay close
attention to what happens under fault conditions. When you accidentally
snag your cable and end up with the saw kicking back at the same time as
it trips the RCD, do you really want to be plunged into darkness?

The solution may be to run the cable from a non RCD protected supply
[1], and then include a RCBO for the power circuit in the shed CU. That
way a fault on the socket circuit will not kill the lights.

[1] You did not mention what type of earthing you currently have in the
house, there are two particular implications that spring to mind:
Firstly you may *need* an RCD at the head end for protection of the SWA
and shed CU from phase to earth faults. In which case using a time
delayed one would be stongly advised so that the downstream on can
discriminate socket circuit faults and trip first before the upstream
one looses your lights. At the distance you are talking about you will
probably want to make the shed a TT install anyway, however if you have
PME setup in the house, you may find it very difficult to meet the
standards required in the shed to export the house earth anyway and be
forced to go TT.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
  #9  
Old August 26th 05, 10:54 AM
Mr Fizzion
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On Thu, 25 Aug 2005 17:03:11 GMT, "John Blessing"
wrote:

"Mr Fizzion" wrote in message
.. .
On Thu, 25 Aug 2005 10:13:08 +0100, Cordless Crazy
wrote:

3. What size of cable would be required on this 55m run with, what I
assume will be, a huge voltage drop?


Voltage drop is given by V=IR where I is the current in amps and R is
the resistance of the wires in ohms. The resistance of the wires is
found from the resistivity of copper - 1.68*10^-8 ohm metres: R =
(length in metres * resitivity) / cross-sectional area of the wire in
square metres.

Mr F.


Isn't that formula valid only for DC not AC current?


If you are considering a section of cable then it's valid for AC too
unless the cable is tightly coiled into an inductor.

You do however get the "skin effect" with AC where all the current is
concentrated in the outside of the cable and none of it flows through
the central core.

The skin depth in copper at 50Hz is around 10mm, which I believe in
practice means that there isn't any point in having copper cable whose
radius of cross section is larger than 10mm. (but many years since I
studied that kind of thing!)

Mr F.

 




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