Cable selection for granny flat supply

[sidekick]

Have been asked to use 10mm swa to supply Granny flat with following spec from mcb on house csu.
Think should really be 16mmswa from separate switch in house. Am I missing something.

22 down lightslus 2 outside lights
10 2g sockets and 1 2g outside socket
electric oven
electric hob
electric shower
x 2 extractors
x 2 smoke detectors
3kw water heater
and x 3 poss 4 electric panel heaters

Quite a lot for 10mmswa. will first check with DNO to see if additional supply required and if not will supply separate feed from house via separate isolation switch.

 

[Guest111]

I would put it on it's own feed rather than one on the house CU however before deciding on a cable size I'd need to know a little more I think, things such as length of run for one,at the end of the day you are the electrician,work it this way they tell you what they think they want then you tell them what they need.Simple.

 

[Badged01]

As above but plus what is the expected loading of the new circuit ..... basic stuff for the design before you can even consider cable selection!

 

[sidekick]

well said Phil. Thanks.
Just my first glance query on there request as going to look at job this morning.

 

[sidekick]

Yep. Jumping the gun a little. Should find out spec of shower, hob and panel heaters etc this morning. At first request just didn't think 10mm swa from mcb was correct way to do it. Will calc today

 

[Octopus]

I'd be thinking that 16mm would be a minimum starting point as it'll be extremely hard and costly to replace it later if it needs upgrading.

 

[Guest111]

I'd be thinking that 16mm would be a minimum starting point as it'll be extremely hard and costly to replace it later if it needs upgrading.

At least,a lot of those big showers take a fair loading,could be a decent load especially when the "granny" is in the shower whilst she's got her tea in the oven. Don't you just love the term "granny flat" lol.

 

[Octopus]

At least,a lot of those big showers take a fair loading,could be a decent load especially when the "granny" is in the shower whilst she's got her tea in the oven. Don't you just love the term "granny flat" lol.

I always wonder about these threads where the supply size appears to have been decided. I'd also take it off Henley blocks rather than the house Cu and stick a metal clad SF in too.

 

[Engineer54]

First off, you need to assess what the realistic Max Demand load is going to be on this Granny Flat and then assess if the existing DNO supply is of a suitable size, or will an upgrade be needed, or a separate DNO metered supply need to be brought in to the Granny Flat....
I must say, 10 X double outlets as well as 4 wall panel heaters points along with 22 downlights, not forgetting the presence of the higher current drawing loads, sounds like a larger than average Granny Flat to me...

I never like to see minimal sized sub-main cable sizes installed, so my choice in this instance would be a 3 core 16mm SWA.

 

[Guest111]

Some useful advice so far

 

[sidekick]

Thankyou all for advice.
Been to look at job today and not happy.
Already 10mm swa running from 40a mcb in house to ip65 rated consumer unit on side of summer house. (hence request for me to use 10mm swa)
Hot tub and lights on this board and wanting to run swa from there to granny flat which is actually log cabin.
8.5kw shower
7.5kw hob
3.5kw oven
3kw water heater
11 socket ring
24 lights (all be it 22 led spots)
1kw sanitary pump etc
500w bathroom heater
At least 80 to 90a after summer house.

Would say definate no to 10mm and leap frog from summer house ???

 

[NickD]

What's the DNO supply situation?

 

[sidekick]

What's the DNO supply situation?

Almost sure TN-S but need to cut back of kitchen cupboard out to confirm on next visit.

 

[NickD]

Almost sure TN-S but need to cut back of kitchen cupboard out to confirm on next visit.

Sorry, I meant supply capacity. Should have been clearer.

 

[sidekick]

Sorry, I meant supply capacity. Should have been clearer.

Do you mean the DNO fuse. Label 100a but led to believe more often than not usually 80a inside carrier