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Choosing cable size. Formula?



 
 
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  #1  
Old November 30th 06, 04:58 PM posted to uk.d-i-y
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Posts: 70
Default Choosing cable size. Formula?

Hi.

I want to run an armoured cable from the house to my new shed. It's a
total of 15m distance and it will power:

1 x 4' daylight tube 36w
2 x 150w pir floods 300w
1 x small freezer 60w?
4 x wall sockets (occasional battery charger, power tools etc)1000w?

Say a potential max load of approx 1500w.

w/v = a?

1500 / 240 = 6.25a.

Local supplier wants to sell me 4.0mm cable. Seems a bit OTT to me
for this. Surely 2.5 would be fine? Opinions?

Also, would you use a consumer unit in the shed to split the lighting
and sockets or be happy with a junctionbox? There will be a dedicated
fuse at the house end after all.

Thanks.

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  #2  
Old November 30th 06, 05:18 PM posted to uk.d-i-y
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Posts: 101
Default Choosing cable size. Formula?


"Mike Barnard" wrote in message
...
Hi.

I want to run an armoured cable from the house to my new shed. It's a
total of 15m distance and it will power:

1 x 4' daylight tube 36w
2 x 150w pir floods 300w
1 x small freezer 60w?
4 x wall sockets (occasional battery charger, power tools etc)1000w?

Say a potential max load of approx 1500w.

w/v = a?

1500 / 240 = 6.25a.

Local supplier wants to sell me 4.0mm cable. Seems a bit OTT to me
for this. Surely 2.5 would be fine? Opinions?

Also, would you use a consumer unit in the shed to split the lighting
and sockets or be happy with a junctionbox? There will be a dedicated
fuse at the house end after all.

Thanks.

--
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It is the starting current of the freezer that is significant and will be a
whole lot higher than 60w/240.
Even so 2.5mm should be OK.

Always fuse shed/workshop lights separately. It is vital that in the event
of a fuse blowing that you are not operating rotating machinery (albeit
slowing down) when the lights go out.
Fuse in the house 30A, two fuses/mcbs in the workshop 16amp and 6 amp.

Bob


  #3  
Old November 30th 06, 05:50 PM posted to uk.d-i-y
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Posts: 13
Default Choosing cable size. Formula?

Bob Minchin wrote:
Always fuse shed/workshop lights separately. It is vital that in the event
of a fuse blowing that you are not operating rotating machinery (albeit
slowing down) when the lights go out.


But... if the fuse to the lights blows, you'd be operating said
machinery in the dark at full speed.

Alex
  #4  
Old November 30th 06, 05:58 PM posted to uk.d-i-y
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Posts: 402
Default Choosing cable size. Formula?

Mike Barnard wrote:

Hi.

I want to run an armoured cable from the house to my new shed. It's a
total of 15m distance and it will power:

1 x 4' daylight tube 36w


x 1.8 if not power factor corrected, so say 65 VA.

2 x 150w pir floods 300w


300 VA, near enough, so 365 VA for lighting, say 2 amps, rounding up.

1 x small freezer 60w?


Optimistic, 150 W is more typical. Allow one amp (230 VA).

4 x wall sockets (occasional battery charger, power tools etc)1000w?


Several 13 A sockets on a radial circuit? The smallest I'd allow for
that is 16 amps. You might want to plug a fan heater in in the winter,
for example.

Say a potential max load of approx 1500w.
1500 / 240 = 6.25a.


Min. acceptable design current ~19 amps - call it 20 and use a 20 A fuse
or MCB in the house.

Local supplier wants to sell me 4.0mm cable. Seems a bit OTT to me
for this. Surely 2.5 would be fine? Opinions?


Voltage drop for 2.5 mm^2 at 70 deg. is 18 mV/A/m, i.e. 5.4 V (2.3%) for
20 A & 15 m - that seems OK and gives you 3.8 V to play with for the
shed wiring. Current rating won't be a problem - 2.5 XLPE SWA to BS
5467 derated for 70 deg. conductor temp is 28 A clipped direct (more
when buried).

So yes, 2.5 would be OK, provided your length figure is OK (cables
lengths often end up being more than you expect) and it's fed directly
from the house consumer unit.

4 mm^2 cable costs very little more and will give you some capacity in
reserve for the future.

Also, would you use a consumer unit in the shed to split the lighting
and sockets or be happy with a junctionbox? There will be a dedicated
fuse at the house end after all.


The simplest satisfactory method is to take the SWA cable straight to a
metalclad box (could be one of the socket boxes), and use one or more
switched fused connection units as lightswitches. Fuse your lights at 5
amps, either all on one circuit, or one for the fluorescent and
another for the floods. Sockets are protected by the fuse/MCB at the
house. This gives no fault discrimination, so the light goes out if
there's a fault - not recommended if you intend to use serious machine
tools or woodworking machinery.

Supply arrangements at the house end? Earthing arrangements? RCD
protection of the sockets is essential, where is that going?

--
Andy
  #5  
Old November 30th 06, 06:06 PM posted to uk.d-i-y
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Posts: 402
Default Choosing cable size. Formula?

Alex wrote:

But... if the fuse to the lights blows, you'd be operating said
machinery in the dark at full speed.


Far more likely is that the socket circuit trips for some reason, and
takes supply for lights with it, so you are in the dark as machinery is
slowing down. The design will get more elaborate to achieve full
discrimination. I got the impression this a just a small shed, not a
serious workshop.

--
Andy
  #6  
Old December 1st 06, 12:07 AM posted to uk.d-i-y
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Posts: 19,355
Default Choosing cable size. Formula?

In article ,
Mike Barnard wrote:
Local supplier wants to sell me 4.0mm cable. Seems a bit OTT to me
for this. Surely 2.5 would be fine? Opinions?


If 15 metres is the total run you're looking at 16 quid for 2.5mm 2 core,
21 for 4mm at TLC prices including VAT. The larger size can do no harm. ;-)

--
Is the hardness of the butter proportional to the softness of the bread?*

Dave Plowman London SW
To e-mail, change noise into sound.
  #7  
Old December 1st 06, 08:37 AM posted to uk.d-i-y
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Posts: 3,357
Default Choosing cable size. Formula?

On Thu, 30 Nov 2006 16:58:37 +0000 someone who may be Mike Barnard
wrote this:-

Also, would you use a consumer unit in the shed to split the lighting
and sockets or be happy with a junctionbox?


A main switch to switch off the installation in the shed is
advisable and can also be used to terminate the incoming cable. A
small consumer unit does this neatly and then all there will be are
light switches and sockets.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54
  #8  
Old December 1st 06, 03:25 PM posted to uk.d-i-y
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Posts: 70
Default Choosing cable size. Formula?

On Thu, 30 Nov 2006 16:58:37 +0000, Mike Barnard
wrote:

Thanks to all. I'll use 4.0mm and an rcd consumer unit as
reccomended.


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