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

 

[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

 

[Strima]

Waiting room sockets tend to get very little use, I would just got with 20 amp radials, I wouldn't expect more than a couple of amps on each, higher load would only be short duration for a couple of minutes whilst the kettle is on.

 

[richy3333]

40 metres, TNCS, exporting the earth - ‘tin hat on’

 

[davesparks]

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

I'll ask again, why does the submain need RCD protection?
How would a time delayed RCD prevent something in the house tripping?
Why does it need a TT arrangement at the outbuilding? What is preventing you from using, what is presumably, a perfectly good TNCS earth?

 

[Ted Curly]

I'll ask again, why does the submain need RCD protection?
How would a time delayed RCD prevent something in the house tripping?
Why does it need a TT arrangement at the outbuilding? What is preventing you from using, what is presumably, a perfectly good TNCS earth?

Hi apologies, I just assumed discrimination if it was in the main existing board, the sub board would not need RCD, the tt I was just learnt many years ago if the out building was some distance from the house to install tt as there is no extraneous parts made from wood and plastic so no need for pme, sorry guys if to old skool. I do think it is best to use the existing TNCS.

 

[Lucien Nunes]

120.36amp,,,, but if I do the 3 sockets rings as 20amp radials diversity will come in at 96.36amp..

This makes no sense. The load is determined by what people plug in, not by how you wire the sockets. What makes even less sense is the total consumption. Think about the heat you get from 120A (almost all electrical power gets converted to heat before it leaves the room)
120 x 230 = 27.6kW. Unless they leave the hot taps running, turn the heaters to max, keep refilling the kettle and take seven extra fanheaters in there they are not going to use 120A. The people would overheat even if the cables don't. If the installed heaters are adequate, the RFCs are going to see a couple of amps total except when the kettle is on.

Also, 2kW lighting? You can't buy GLS lamps any more

 

[shaun1]

To give you some comparison, I sometimes provide lighting and power for fashion shows. We have a supply run in to a room with at least 20 stations doing hair styling and another 20 doing makeup/nails etc. Each of those stations has a mirror with around 8 lighbulbs around it (now LED bulbs) and the hair stylists are given another 13A socket each. They all bring a hair dryer and straighters/curlers and countless other styling 'tools'. We also power 3/4 x 3kw space heaters on constantly. Think we also fed a couple of Irons and steamers as well. They have a short space of time to turn the models round so are working constantly for an hour or so.

We supply all that from a 125A 3ph supply and from memory it rarely gets above 50/60A per phase. It definitely didnt get anywhere near the limit.

 

[DefyG]

This is being taken off an existing single phase domestic supply of 100A!
Just how much spare capacity do you have or think you have? Probably nowhere near the figures of 96A and upwards being banded around!

 

[Baddegg]

Unless they leave the hot taps running, turn the heaters to max, keep refilling the kettle and take seven extra fanheaters in there they are not going to use 120A. The people would overheat even if the cables don't.

This is my teenage daughters alone in my house!

 

[Deleted account]

I think you need to contact your DNO to arrange a new 100A supply.

 

[Ted Curly]

This makes no sense. The load is determined by what people plug in, not by how you wire the sockets. What makes even less sense is the total consumption. Think about the heat you get from 120A (almost all electrical power gets converted to heat before it leaves the room)
120 x 230 = 27.6kW. Unless they leave the hot taps running, turn the heaters to max, keep refilling the kettle and take seven extra fanheaters in there they are not going to use 120A. The people would overheat even if the cables don't. If the installed heaters are adequate, the RFCs are going to see a couple of amps total except when the kettle is on.

Also, 2kW lighting? You can't buy GLS lamps any more

Hi I was just following the OSG Table A2 for diversity on the circuits, the sockets as I don't know what is going to be plugged in I used the breaker rating(what other option without knowing) , the lights I have assumed 100w max per fitting I know this is now old and we all use leds, the swa mains to supply the circuits will have to be min 16mm as 40 meter run, I am right to think thst the iz of the 16mm swa 3core is 94amp clipped direct, thst will be a problem if my ib is 96amp,, I need to re work the figures to find realistic figures for lighting and sockets as they won't be drawing that much Im being to cautious as I have to present my figures to my gaffer and Building control Friday. Thanks again all