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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

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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

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

Strima

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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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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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Spoon

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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?
 
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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

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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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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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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

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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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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

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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

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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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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

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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

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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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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

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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

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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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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

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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 :(
 
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.
 
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

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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!
 
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  • #30
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

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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

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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

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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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  • #35
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
 

Paignton pete

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Didn’t realise you where working on someone else’s house. I thought you where working on your own property.

Is your boss an electrician?

Does he know about loading and cable calcs.

You quite rightly point out it’s your liability if your the qualified supervisor, not his.
If the property does not have a big enough supply, tell him and the customer.

Your profile is limited so I don’t know the extent of your experience or qualifications.
 
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  • #37
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
The 16mm swa is clipped direct 3core, 94amps current carrying capacity is what I found out from the technical information from the wholesaler when contacted them, as I couldn't find it in the books and online was getting different amounts. In your knowledge what would you say the current carrying capacity of 16mm swa 3core clipped direct and buried is.
 

Matthewd29

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The 16mm swa is clipped direct 3core, 94amps current carrying capacity is what I found out from the technical information from the wholesaler when contacted them, as I couldn't find it in the books and online was getting different amounts. In your knowledge what would you say the current carrying capacity of 16mm swa 3core clipped direct and buried is.
Clipped direct is fine I was only taking a guess at buried
 
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  • #39
Didn’t realise you where working on someone else’s house. I thought you where working on your own property.

Is your boss an electrician?

Does he know about loading and cable calcs.

You quite rightly point out it’s your liability if your the qualified supervisor, not his.
If the property does not have a big enough supply, tell him and the customer.

Your profile is limited so I don’t know the extent of your experience or qualifications.
Hi yes it is my uncles house I started out on my own in November after 15years with the council doing house bashing and so on electrically for them and have just ended up subbing out to a company with 6 of us since then and get left to it completely,
I have 15 years experience in wiring and have my level 2-3 +my 18 edition and my testing and inspection +part p and a hnc in construction, I know my job just not may council houses required lavish outhouses to be fitted by us requiring 40meter runs of swa, I am new to forums and appreciate all the views from follow sparks and the vast knowledge you bring from each sector of r trade thanks guys.
 

Paignton pete

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Haven’t got my book to hand but it’s not 94amps

Minimum 25mm would be required if it’s 94amps, however if it is 94 amps then you’ve got 6 amps left for the main house.

Is that enough?

At a guess 16 mm SWA is about 70-75 amps( guess).

If buried this will drop to maybe 63-68 amps( again a guess).

It’s all in the big blue book. You should be able to get these figures for yourself and work it out.

Do not use my figures, they may be wrong.

Don’t forget to do voltage drop on cable size and pfc.
 
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Paignton pete

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Is this your personal job and the gaffer you normally work for recommended you use 10 mm SWA? Or is this not your job and your working for a gaffer?

Is the gaffer an electrician?
 

Charlie_

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“Is the gaffer an electrician?”

No I think he was the boss of a small engineering company in Yorkshire
 
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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

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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?
 

Pete999

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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 Boobs up.
 

Spoon

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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.. :laughing:
 

Paignton pete

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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.
 
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.
 
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  • #53
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
 
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Pete999

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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
 

davesparks

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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

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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

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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.
 
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  • #61
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!
Hi it was just the way was taught many moons ago regarding tt and outbuildings ,,, and the no extraneous parts was meant to indicate for this instance the out building is made of wood no metal, and if used tt would be no pme, but dnt worry about tt I going to use the tncs earth, sorry about the way I type not very good at typing on these small phones and new to forums cheers guys.. Also I will have update on what equipment is going to be used tomorrow so can make better assumption of ib of sockets,, sorry so late to reply had 12 hour day and at it again in 8
 
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  • #62
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.
I was hoping it wouldn't come to thst size the 40m swa will be clipped direct ,the lighting circuits max length run will be around 10meters,, 16mm swa Vd is around 2.6 mV/a/m
 
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  • #63
I was hoping it wouldn't come to thst size the 40meters run of swa will be clipped direct ,the lighting circuits max length run will be around 10meters,, 16mm swa Vd is around 2.6 mV/a/m
 

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