How to work out the total design load to find my swa cable size

[Ted Curly]

Hi all I am having trouble trying to work out the total load and swa cable size.
I have 10 circuits in a outhouse 40 meters from the mains power supply 1 x lighting with 10 lights, 1 x lighting with 10 lights.
1 x 2.5mm radial for 2 heaters, 1 x 2.5mm radial for heater. 3 x 2.5mm socket rings with 3 sockets on each ring, 1 x water heater radial for 2 instantaneous hot water... I have calculated the lights at 100w per light x 20= 2kw, the heaters are around 3kw x3 =9kw.
Water heaters 3kw each=6kw... The problem is the sockets as I'm not sure what's going to be plugged in apart from a kettle. working them out at 13amp x 230v gives me nearly 3kw per socket x9 = 27kw which seems unrealistic... Adding all that up before diversity is 44kw which seems way ott for that. Thanks all

 

[Pete999]

Hi all I am having trouble trying to work out the total load and swa cable size.
I have 10 circuits in a outhouse 40 meters from the mains power supply 1 x lighting with 10 lights, 1 x lighting with 10 lights.
1 x 2.5mm radial for 2 heaters, 1 x 2.5mm radial for heater. 3 x 2.5mm socket rings with 3 sockets on each ring, 1 x water heater radial for 2 instantaneous hot water... I have calculated the lights at 100w per light x 20= 2kw, the heaters are around 3kw x3 =9kw.
Water heaters 3kw each=6kw... The problem is the sockets as I'm not sure what's going to be plugged in apart from a kettle. working them out at 13amp x 230v gives me nearly 3kw per socket x9 = 27kw which seems unrealistic... Adding all that up before diversity is 44kw which seems way ott for that. Thanks all

Are you applying the diversity calculations as described I APPENDIX A in the OSG Table A2

 

[Spoon]

As @Pete999 says, you need to add diversity to your calcs. again, as above, page 123 & 124 of the OSG.

 

[DefyG]

Adding all that up before diversity is 44kw which seems way ott for that.

And for the incoming supply!
Yep as previously said(#2&#3), apply some diversity.

 

[Strima]

What is the building being used for?

What is its construction?

Is it thermally sealed?

Have you considered LED lighting?

Heaters and kettles are not on constantly and will cycle through heating and cooling.

Unless you know how the building will be used then you cannot work out your maximum demand correctly. Constant loads of 3Kw per socket will have long term negative effects on them.

 

[Ted Curly]

Cheers guys working it out now the building is an 50sq meter shed 40meters from the supply to be used as waiting room and a treatment room for nails, facials ect, I am working the socket rings out at 100% of first ring and 50%remaining two rings for diversity so 32a x 3 rings =96a 100% of first leaves me 64a then 50%of this is 32a for the sockets with diversity the sockets work out to 64, the heaters are thermostatic control and feed off their own spurs from separate radials

 

[Spoon]

3 x 2.5mm socket rings with 3 sockets on each ring

Why do you require 3 circuits for 9 sockets?
What are being plugged into these sockets?

 

[Ted Curly]

I have separated them into 3rings to allow for each room to have dedicated breakers as not to lose the whole building if fault occurs and rings to future proof the installation as usage may change over time, silly I know to worry about future use

 

[Pete999]

I have separated them into 3rings to allow for each room to have dedicated breakers as not to lose the whole building if fault occurs and rings to future proof the installation as usage may change over time, silly I know to worry about future use

Use 2.5mm2 / 20Amp Radials in that case, a Ring final for 3 sockets is plain daft.

 

[Ted Curly]

Yes this is very true my original plan was to have the ring wired but leave the returning legs unconnected and make it 3 16a radials, and have the legs there if ever needed in the stud wall

 

[Ted Curly]

I have worked out the total ib of the circuit at 169.88amps and with added diversity came up with 120.36amp,,,, but if I do the 3 sockets rings as 20amp radials diversity will come in at 96.36amp..

 

[Pete999]

Yes this is very true my original plan was to have the ring wired but leave the returning legs unconnected and make it 3 16a radials, and have the legs there if ever needed in the stud wall

Not a good idea Ted, how did you intend leaving the end of the leg?

 

[Ted Curly]

The walls are stud walls, the sockets will be in fast fix boxes, the 3 return legs can be kept in floating in the wall not connected at either end as it is my uncle I feel it would end up beening a ring and more sockets added eventually knowing him.

 

[Pete999]

The walls are stud walls, the sockets will be in fast fix boxes, the 3 return legs can be kept in floating in the wall not connected at either end as it is my uncle I feel it would end up beening a ring and more sockets added eventually knowing him.

20 Amp Radial Circuits is the way to go Mate. Leaving dead newly installed cables, could cause confusion.

 

[The Ghost]

silly I know to worry about future use

Required not silly!

 

[Spoon]

Look at this logically.
You have three 3kW heaters. Two instant water heaters at 3kW each.
I'm presuming that the heaters are going to be thermostatically controlled and you will have these on a timer that will come on first thing in the morning. So that the place is warm. No one else will be in then. No one wants to walk into a place to have their nails done and it's cold.
By the time people are in, the place should be up to temperature. So that just leaves the heaters ticking over, hardly using anything. Also your two instant heaters, sockets and lights.
As you have not told me that are going to be plugged into the sockets I'm going to guess its not going to be more than 3kW.
Use LEDs for lights.

So the max demand you will most probably have is first thing in the morning, when all 3 heaters kick in. 9kW... Say 40A

 

[Ted Curly]

With the 20amp radials diversity will be at 96.36a = 22kw, the swa at 40meters run will have to be 16mm minimum for that much as the run is so long, the lighting all on leds, and the sockets will have a kettle, fridge, plug on lamp and few small face machines. At the house end the swa can either be placed into the existing board or on its on sub supply I just worried about earthing then as would it not require a delayed rcd at the house end, was considering a tt as its so far from house.

 

[Des 56]

What is your thinking regarding a time delayed Rcd for armoured cable ?
What earthing system is at the supply end to make that so ?

 

[davesparks]

With the 20amp radials diversity will be at 96.36a = 22kw, the swa at 40meters run will have to be 16mm minimum for that much as the run is so long, the lighting all on leds, and the sockets will have a kettle, fridge, plug on lamp and few small face machines. At the house end the swa can either be placed into the existing board or on its on sub supply I just worried about earthing then as would it not require a delayed rcd at the house end, was considering a tt as its so far from house.

Why does the submain need RCD protection?
Why does the distance matter in the decision to create a new TT earth?

 

[Ted Curly]

Hi the earthing arrangement is TN-C-S and 100a supply, I was thinking time delayed if was going into existing board as not to trip the house, or just Henley block and have its own sub board non rcd at house end and having a tt at outbuilding end