Calculating cable size

[mightyworthit]

I am looking at running a seperat sub metered consumer unit in a granny flat.
Ive found a cable size calculator on the TLC electrical supplies page
here.

http://www.tlc-direct.co.uk/Technic...230&length=35&submit=Calculate+Min+Cable+Size

as you can see (I hope), it looks as if I can runn 16mm SWA 2 or 3 core XLPE if my power consumption is no more than 16 KW.

How realistic is 16kw consumption for a 1 1/2 bed granny flat assuming 3 ring mains 2 lighting circuits. Small kitchen with electric cooker and tumble dryer washing machine etc. Gas CH and HW.

To calculate power consumption in this case do I just add up all my KW ratings of all appliances lights electrics etc or is there a magic formula or the like?

I would sorely like not to have to hunt down buy and install 25 mm SWA.
Thanks

 

[mightyworthit]

perhaps I should re -categorize this as a power consumption query rather than a cable sizing query.

 

[GaryMo]

Where is this supply going to be picked up from?
You say a separate sub metered supply, is this going to be connected to a standard 100A supply which already supplies a property?

 

[sparkyspike]

You need to allow 'diversity' to your current/power calculations. You really need electrical experience to do this but as a guide:

66% of total lighting load
100% of water heaters
100% of electric heating up to 10A worth. 50% of remainder
Allow 10A for cooker + 30% of remaining cooker load. Add 5A if you have a cooker socket outlet.
100% of largest ring
40% of remaining outlets

Add it all up to get your max. demand, then work out your submain fuse. Then your cable needs to carry the current of the FUSE plus you need to apply factors such as length of cable and method of cable installation.

It looks like the 16mm cable you're looking at is good for a 63A switchfuse.

 

[sparkyspike]

By the way, if you need 25mm cable then get it. Don't try and squeeze your calcualtions to accomodate a smaller and cheaper cable. 25mm 3 core is readily available.

 

[Optician]

3 ring circuits in a granny flat? how big is it?

 

[ban-all-sheds]

as you can see (I hope), it looks as if I can runn 16mm SWA 2 or 3 core XLPE if my power consumption is no more than 16 KW.

1) Try it again with PVC SWA, because you almost certainly won't be able to run the cable at 90º.

2) Think hard about whether you should be doing this job given your lack of knowledge.