Discuss How to work out the total design load to find my swa cable size in the Electrical Forum area at ElectriciansForums.net

Ted Curly

EF Member
Messages
24
Location
Birmingham
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

Forum Mentor
Messages
21,810
Location
Northampton
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

Forum Mentor
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7,166
Location
Lancashire
As @Pete999 says, you need to add diversity to your calcs. again, as above, page 123 & 124 of the OSG.
 

Strima

Electrician's Arms
Messages
3,525
Location
St Neots
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.
 
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Ted Curly

EF Member
Messages
24
Location
Birmingham
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
 
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Ted Curly

EF Member
Messages
24
Location
Birmingham
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

Forum Mentor
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21,810
Location
Northampton
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.
 
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Ted Curly

EF Member
Messages
24
Location
Birmingham
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
 
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Ted Curly

EF Member
Messages
24
Location
Birmingham
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

Forum Mentor
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21,810
Location
Northampton
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?
 
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Ted Curly

EF Member
Messages
24
Location
Birmingham
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

Forum Mentor
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21,810
Location
Northampton
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.
 

Spoon

Forum Mentor
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7,166
Location
Lancashire
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
 
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Ted Curly

EF Member
Messages
24
Location
Birmingham
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

Forum Mentor
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7,375
Location
Gliese 581C
What is your thinking regarding a time delayed Rcd for armoured cable ?
What earthing system is at the supply end to make that so ?
 
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davesparks

Forum Mentor
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12,803
Location
guildford
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?
 
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Ted Curly

EF Member
Messages
24
Location
Birmingham
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

Electrician's Arms
Messages
3,525
Location
St Neots
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.
 

davesparks

Forum Mentor
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12,803
Location
guildford
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?
 
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Ted Curly

EF Member
Messages
24
Location
Birmingham
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

Respected Member
Messages
2,822
Location
London / Tallinn
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

Regular EF Member
Messages
248
Location
United Kingdom
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

Regular EF Member
Messages
236
Location
South East
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!
 
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Ted Curly

EF Member
Messages
24
Location
Birmingham
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
 

Paignton pete

Regular EF Member
Messages
779
Location
Over the rainbow
What is the total load after diversity in the house at present?

The reason I ask is if it’s 80amps then you only have 20 amps left for the outbuilding. Will this be enough?
 

Baddegg

Still simmering Ken, not boiled yet buddy x
Electrician's Arms
Messages
1,282
Location
Portsmouth
Hi the main, supply is 100a in the house this is to power an outbuilding
He knows that ted he’s saying that if the upfront fuse is 100a and is already serving the house you’d need to deduct that usage (after diversity before you could think about using it on the outbuilding as well,
Diversity is more than just the 40% rule have a good think about how realistically the circuits will be loaded, for instance when you say 20 lights over 2 circuits you are unlikely to get near 100w total over both circuits with leds let alone 100w for one light ;)
 

Matthewd29

Regular EF Member
Messages
1,192
Location
Belfast
Although 96 amps sounds like an awful lot for what your talking about, presuming the swa is buried in a duct or ground or whatever I can't work out how you got 16mm
 
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Ted Curly

EF Member
Messages
24
Location
Birmingham
What is the total load after diversity in the house at present?

The reason I ask is if it’s 80amps then you only have 20 amps left for the outbuilding. Will this be enough?
Will check the house 2mo as I not to sure, I have wired bigger extensions with a load more fittings and never had a problem before, just put of as this is a 40meter run my gaffer told me to use 10mm swa but as I know this is wrong and have my own 18th edition I am liable if it hits the fan so I want to be exact sure of everything to the tee my nick name used to be health and safe ted. 96amp ib seems a lot for 20 led spots, 9sockets, 3 heaters and to water heaters instantaneous. That is like 22kw
 

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