65 mtr run from main dwelling to bungalow at bottom of garden.

[BertUK]

just wondering how you guys would go about this, my idea is 25 mm tails from meter into henley block 25 mm to main board in house and 25 mm to 100A isolator for dwelling at bottom of garden,2 core 25mm swa and a 16mm earth run down to dwelling at bottom of garden to new split load 10 way dual rcd board,the dwelling will have a 10.5 kw shower and
2 ring mains and 2 lighting circuits plus some outside security lights and a smoke circuit.

 

[Guest123]

Ok, some rough sums.

10.5KW shower = 45A (no diversity)
2 x ring finals allowing 16A per circuit = 32A
2 x lighting circuits allowing 3A per circuit = 6A

Total load there of 83A, as I said a rough estimation on my part as I dont have all the info to hand. It's down to you to arrive at a more accurate figure using diversity.

So a load of 83A @65M using 25mm 2 core??? personally I'd use a 3 core cable to save having to run an additional CPC.

Total VD for the distribution circuit using the resistive component of a 25mm SWA (1.75) = 1.75 x 65 x 83 / 1000 = 9.44V.


This is below the 5% max VD value of 11.5V for circuits other than lighting, however as you are installing lighting in the bungalow you are restricted to the 3% max VD of 6.9V.

As I said if you apply a more accurate diversity calculation with all factors taken into consideration you may gain enough to use a 25mm SWA if not, a 35mm would be required IMO.

 

[yellowvanman]

Why use a 3 core or seperate earth - use 2-core SWA and put an earth stake in for the bungalow, don't have to worry about bonding water supply back to main house etc.

I appreciate shower doesn't have any diversity but how long will it be on for? Whilst the 3% voltage drop for lighting is a general target, you can ignore that if you know the lighting you install is immune to voltage fluctuations. Generally the 3% rule will apply to more complex lighting.

 

[Guest123]

I appreciate shower doesn't have any diversity but how long will it be on for? Whilst the 3% voltage drop for lighting is a general target, you can ignore that if you know the lighting you install is immune to voltage fluctuations. Generally the 3% rule will apply to more complex lighting.

As I said ROUGH figures on my part as we dont have sufficient info such as who will be living in the bungalow, duty cycle of appliances etc etc.


My post was more to evoke thought in the OP rather than hard & fast figures.

 

[yellowvanman]

My comment wasn't meant to critise.

 

[Spudnik]

Dont forget that if the sub main is not protected through the dwelling CU with a suitable MCB, then the sub must have a switchfuse within the first three meters.

Will also be easier to terminate the swa

 

[BertUK]

i could get away with 1 ring main and 1 lighting circuit as its a kitchen/lounge downstairs and bedroom/bathroom in the roof space.the only appliances in the house will be a tv in the lounge and a microwave and a kettle in the kitchen,upstairs will be the usual girly things of an hairdryer and straighteners.its not a huge property maybe 4 sockets in kitchen and 2 in lounge,a couple in the bedroom and maybe 6 lights,kitchen/lounge/top stairs/bottom stairs/bedroom and bathroom.

 

[Engineer54]

That's not a garden, ...That's a Field!! lol!!!

Go with the 3 core SWA cable Bert, ....forget about roding, it has the potential to cause you more problems than you need!!!

 

[malcolmsanford]

I know my mate the engineer and his dislike of the "rod". but in this case as the bungalow is 65 mteres away I would be definately looking at making this installation have it's own equipotential zone.

As with Yellowvanman I would be thinknig about a 2 core SWA and untilising the SWA as the CPC. If you do rod the bungalow then the CU earth bar would be that installations MET.

What is the house earthing system ? If it is TN then I would fit a Fuse Isolator/MCB enclosure to protect the SWA, a 60/50amap fuse for that install should be ample, in all honesty you could get away with a BS 88 50amp if your calcs show that 25mm is nearly on the threshold. You can even think about a 50amp MCB as well. Then disregard the TN at the garage CU.

If your house is already a TT then you need to RCD protect the SWA with a 100mA S type RCD for that SWA as the chances of you getting a good enough Zs for disconnection is slim.

At the Bungalow I would fit

2 X A3 20 amp radials

1 X 6amp light, I would also think about putting a couple of emergency lights in, as 65 mts from the house, and the lights fail can be disconcerting.

45amp shower

So 50/60amp supply would be fine. I would also use RCBO's rather than a split board, IMO much better.

Finally I would also run out telephone/data and perhaps TV, as once you have dug 65 mts your not going to want to dig it again.

 

[sparks1234]

i could get away with 1 ring main and 1 lighting circuit

Presumably your line of thinking is you can now calculate on only 1 ring and 1 lighting circuit. This is no critisism of yourself and I did harp on about this on a thread once before about maximum demand. How does this make sense?, just because you now decide to put less circuits in to "get round " regs, it doesn't make the load go away, all it does is put all the load on less circuits.
The method of calculating max load when it comes to circuits with lights, power and not a single fixed load is daft..........all in my own opinion...of course

 

[ezzzekiel]

scratch post my mistake

 

[mechelec]

You will have two properties fed from one 100A?? DNO cutout fuse?? What is the loading in the main dwelling are you sure it will take the additional load or would you be better with a new DNO supply and meter in the bungalow? As lenny calculated approx 83 A in bungalow this does not leave alot of capacity for the main dwelling. I may have missed something!!