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

 

[Ted Curly]

Yes the gaffer is also a spark with 20 years in the job, my gaffer not interested in this project as its me uncles place for me auntie and there is no money involved, I just don't want to look a numbty if I get the swa cable size wrong and it's expensive and dangerous I will have to calculate again to get the ib down, like you say I will no where near be using 100w per light with leds in place

 

[Pete999]

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..

How did you apply the diversity?

 

[telectrix]

run supply cable overhead and if it gets warm,hang the washing out.

 

[Pete999]

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

This Gaffer you speak of, is he delegating the design of this project to you as an exercise to test your metal, or does he not know how to do it himself?
If it's the later then he should employ a designer to scope out the design, nothing wrong with getting you to attempt the design and correct your work if it's required, but the way it sounds us he is using you as a scapegoat, should the design element go ----- up.

 

[Spoon]

Selwyn Froggitt was the program he done before the gaffer.. Fred was his name in the gaffer

Yes mate. Thats how I always remember the bloke, from Selwyn Froggitt. I should have been more informative..

 

[Deleted account]

Yes the gaffer is also a spark with 20 years in the job, my gaffer not interested in this project as its me uncles place for me auntie and there is no money involved, I just don't want to look a numbty if I get the swa cable size wrong and it's expensive and dangerous I will have to calculate again to get the ib down, like you say I will no where near be using 100w per light with leds in place

That makes sense.

Good luck on the calcs.

Can you come back to us again once you’ve done it.

 

[westward10]

Allowing for 63A over 40m and a 2% volt drop, this allows an extra 1% for your lighting to comply with a 3% requirement you will need a minimum 25mm cable.

 

[Ted Curly]

How did you apply the diversity?

Hi I did diversity for the sockets 100% first circuit 50% of other two, I used the 20amp breaker rating as I gonna have to find out what is plugged in (unless there a better way) , the lights I worked out for 100w max a fitting I know thst well ott il have look at fitting and use bulb rating spose, then 66% for each light circuit diversity so did 10lights 100w per fitting = 1000/ 230 =4.34amp for each circuit I have two, the heaters (39.12 @ 3000/230 x3 )I took the first 10a of 1st then 50%remaininf two left me with 24.56a @ 3000/230 and the water instantaneous heater no allowance. Total diversity 96.36 way to high if calcs was more accurate regarding light bulb w and socket load would be lower

 

[Pete999]

Hi I did diversity for the sockets 100% first circuit 50% of other two, I used the 20amp breaker rating as I gonna have to find out what is plugged in (unless there a better way) , the lights I worked out for 100w max a fitting I know thst well ott il have look at fitting and use bulb rating spose, then 66% for each light circuit diversity so did 10lights 100w per fitting = 1000/ 230 =4.34amp for each circuit I have two, the heating I took the first 10% of 1st then 50%remaininf two left me with 24.56a and the water instantaneous heater no allowance

Thanks

 

[telectrix]

Yes mate. Thats how I always remember the bloke, from Selwyn Froggitt. I should have been more informative..

then he did Greengrass in Heartbeat.

 

[Spoon]

then he did Greengrass in Heartbeat.

Wasn't a fan of Heartbeat.

 

[davesparks]

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.

What is the reason you were given for converting to TT if its over a certain distance? And for that matter what value do you use for the distance at which you need to do this?
What do you mean by no extraneous parts therefore no need for PME?
None of this sounds 'old school' to me, more like nonsense!

 

[Lucien Nunes]

The process seems to have started in the wrong place, with a list of final circuits. It should go something like this:
1. What equipment is going to be used in the new building?
2. What is its total load?
3. What is the total load of the house?
4. Is the supply large enough for both?
5. If so, what cable is needed to feed the new building.
Then you can get to the details of RFCs or radials etc.

The fallacy at the moment is that you've chosen some final circuits without knowing the load, then tried to assess the load from the choice of circuits.

 

[Pete999]

Wasn't a fan of Heartbeat.

Still am, got loads of episodes to watch, all those lovely young ladies in 60s outfits what more could ask? no Spoon don't say it, I know exactly what you are thinking.

 

[Charlie_]

I’m sure I seen Bill Maynard in the Sweeney too.
Carry ons and he used to be in Steptoe