Power to a shed

[adam.h]

Ok, so doing a job and the customer keeps wanting more and more stuff done as time goes on.
Which is a good thing I guess

Anyhoo, they are now wanting a brick shed with lights and a socket. I am already going to intall a new seperate cu for the extension as they don't want the old one touched in case there is more work to be done.
But would the best option be to run some 2core swa from the new cu on a 32A breaker (would this need to be on the RCD side?) then have another cu in the garage for the lights and the socket.

With voltage drop on 50m I calculated 2.5 would be sufficient with a max load avialable being 31.4amps on the cable, more than enough for lights and 1 socket.

Then earth swa at new cu end and leave unconnected at garage cu, then install earth rod for earth in garage.

 

[pushrod]

Ok, so doing a job and the customer keeps wanting more and more stuff done as time goes on.
Which is a good thing I guess

Anyhoo, they are now wanting a brick shed with lights and a socket. I am already going to intall a new seperate cu for the extension as they don't want the old one touched in case there is more work to be done.
1) But would the best option be to run some 2core swa from the new cu on a 32A breaker (would this need to be on the RCD side?) then have another cu in the garage for the lights and the socket.

2)With voltage drop on 50m I calculated 2.5 would be sufficient with a max load avialable being 31.4amps on the cable, more than enough for lights and 1 socket.

3) Then earth swa at new cu end and leave unconnected at garage cu, then install earth rod for earth in garage.

Just a beginner but here's my 2p's worth
1) If you put it on the rcd side at main CU the customer will have a long walk back from the shed to reset it, although it would protect the swa , though you are suggesting protecting it from the shed end - if you had 2 RCDs you would have probs with discrimination.
A small 2 way, RCD protected garage CU might be the way to go with 6A and 16A mcbs - giving you an Ib of 22A. How are you installing your swa - burying your armoured cable derates it by .9 but shouldn't affect it much in this case. (see no3)

2) Not sure where you get these figures from? using an Ib of 22 then i get a volt drop of 19.8V which is more than 5% and so too much. What figure are you using for mV/A/M ?- i took it to be 18 (table 4D4B)

3) be very interested to hear what others say about this as when i talk to people i get different answer depending on who i am talking to The most recent (from a lecturer) was that the armouring should always be earthed at both ends regardless of what else is going on eg an internal core being used as the cpc (obviously not applying in your case) The answer to this could affect your decision for 1).

hope this is of some use.

 

[adam.h]

This is how I worked it out......

Design current Ib is 10A

In 16A - being the breaker at the supply end (house cu)

Ca = 1 (at 20deg)
Ci = N/A as it wont be in any insulation
Cg = N/A as it wont be grouped with any other cables
Cc = 0.9 as it will be buried

So It = 16 / (1 x 0.9) = 17.7

Iz = 29A (2.5 swa - 18 mV/A/m from tables 4D4A and 4D4B

Vd = 18 x 10 x 50 / 1000 = 9V which is a 3.9% drop

Ib <= In <= Iz is true.

I would use a 6A and 10A breaker in the shed for the socket and lights. Which is how you have come up with Ib being 22.

But I though the design current would be what you expected to be your max current draw? hummm.....

 

[pushrod]

Hi Adam, agree with your actual maths but still not quite sure why you would use 10A as Ib for your volts drop calc if you have both a 6A and 10A mcb for the lights and sockets (i was guessing 6A and 16A mcbs that why i mentioned 22 earlier) , would you not have to use 16 for your Ib?

The other thing is if you are limiting the sockets to 10A how are going to cope with someone plugging in a big 3KW load (kettle, heater, garden machinery etc) do you have to label the sockets or use round pins??

edit : thinking about it i would say your Ib was the the max load of your lighting circuit probs about 1A (unless you have fluorescent tubes which could have a bigger in rush) and the value of the breaker on your sockets because that load is variable.

 

[adam.h]

I think you are quite right. Will do it for 22, cheers buddy.

 

[pushrod]

Adam, problem with a design current of 22 is you will have the big volts drop which would mean you having to up the size of your swa and a big increase in cost for you on going up to 50 metres of 4mm swa.

If they are only having a couple of low energy bulbs for lighting then the design current is neglible for them (about .1A) and if you fused the sockets to 13Amps then your volts drop is bang on 5% with the 2.5mm - could save a lot of money on expensive cable - just a thought

I suppose really you need to speak to the customer and find what they intend using the sockets for and see if they want the cost and flexibility of the 4mm swa.

 

[adam.h]

I was treating the main breaker form the supple as a spur so was a bit messed up. But sort of on the right track. There will only be a few lights and the socket is just for a lawn mower.
They have the old swa from the old garage that was removed so if thats still long enough i'll be using that, if not will work through and see if 2.5 is enough.