Calculating Cable Size for Ring Circuit

[appspark]

Hi All,
I'm needing to calculate the cable size for a long ring main. Cable type will be SWA/LSF.
The ring is feeding 5no Twin Sockets, on a 32A Type B RCBO, kicker is the length out and back is 300m.
Using cable calculation tools can i half the length, 150m, half the current, 16A and allow 20% volt drop?
the reason for the 20% is that i've done a bit of hunting and when manually calculating you get R1+R2/4 as you have parallel paths.
Using the calculation above i can use a 6mm which seems reasonable, but if i allow the full lot then i need a 50mm, which obviously cant be right.

Just looking for a nudge in the right direction if anyone can help?

Many Thanks

 

[Spoon]

Just out of curiosity, why SWA for wiring the sockets?
How to you terminate the armour?
If it is 6mm then you will have fun putting 2 of them into each socket.

 

[Pete999]

Hi All,
I'm needing to calculate the cable size for a long ring main. Cable type will be SWA/LSF.
The ring is feeding 5no Twin Sockets, on a 32A Type B RCBO, kicker is the length out and back is 300m.
Using cable calculation tools can i half the length, 150m, half the current, 16A and allow 20% volt drop?
the reason for the 20% is that i've done a bit of hunting and when manually calculating you get R1+R2/4 as you have parallel paths.
Using the calculation above i can use a 6mm which seems reasonable, but if i allow the full lot then i need a 50mm, which obviously cant be right.

Just looking for a nudge in the right direction if anyone can help?

Many Thanks

What's stopping you running a Sub Main, and fitting a small consumers unit closer to the site of the 5 twin sockets, and then install the RFC as normal using 2.mm2 or install a 32A Radial final circuit using 4mm2?

 

[appspark]

Thanks for the replies.
It needs to be in SWA as it's for a new football stadium.
I can run a small DB closer to the location but each run will still be 150 there and back so i'd still need to run the calcs.
The plan to terminate the armour would be to mount the socket on a 6x6 joint box which the armour is glanded into. 6mm will be a pain but at worst it'll be joint the 6mm and out to the socket in 4mm.

 

[Pete999]

Thanks for the replies.
It needs to be in SWA as it's for a new football stadium.
I can run a small DB closer to the location but each run will still be 150 there and back so i'd still need to run the calcs.
The plan to terminate the armour would be to mount the socket on a 6x6 joint box which the armour is glanded into. 6mm will be a pain but at worst it'll be joint the 6mm and out to the socket in 4mm.

Why there and back? if you ran a sub main it would be a 230V SP&N supply, that's the cable size you need to calculate. Not going to give you step by step instructions, as the Forum frowns upon that, your best bet would be to get an Electrician to do the design, install and test on completion.

 

[Spoon]

It needs to be in SWA as it's for a new football stadium

Don't quite understand where these sockets are going. Where about in the stadium are these being fitted? Most prob me being thick.
What are the sockets used for?

 

[appspark]

I went for a ring rather than a radial as then I can allow the higher volt drop without having to up the cable size too much. It may be a surprise but i am a sparky, i'm fine and happy doing cable calcs on radials but was looking for a bit of help on the ring side of things.

The sockets are just for general use, cleaning/washing equipment. also to add the possibility of food stalls within the stands at a later date if they require. To be honest I think they're asking for power ina location so they can butcher it at a later date without having to run in new cables.

 

[Pete999]

I went for a ring rather than a radial as then I can allow the higher volt drop without having to up the cable size too much. It may be a surprise but i am a sparky, i'm fine and happy doing cable calcs on radials but was looking for a bit of help on the ring side of things.

The sockets are just for general use, cleaning/washing equipment. also to add the possibility of food stalls within the stands at a later date if they require. To be honest I think they're asking for power ina location so they can butcher it at a later date without having to run in new cables.

Have you not considered my suggestion?

 

[appspark]

Have you not considered my suggestion?

I have but I cant fit small CU's all over the stand, and if i tried to do it in one run its 150m so therefore a 25mm SWA is needed, which obviously inst viable for a few sockets.
Maybe didn't explain it better before but in one stand they are asking for 40 sockets, 8 circuits, 5 sockets each. Wouldn't be keen sticking 8 consumer units in a football stand, you know what'd turn into.

Thanks

 

[Wilko]

Hi - I suggest to get the load fully tied down first (a written spec) as food stalls may fully load each outlet with water boilers and cookers etc. If so a domestic rfc isn't really going to cut the mustard, I reckon.

 

[Pete999]

I have but I cant fit small CU's all over the stand, and if i tried to do it in one run its 150m so therefore a 25mm SWA is needed, which obviously inst viable for a few sockets.
Maybe didn't explain it better before but in one stand they are asking for 40 sockets, 8 circuits, 5 sockets each. Wouldn't be keen sticking 8 consumer units in a football stand, you know what'd turn into.

Thanks

Your original post mentioned 5 sockets on a RFC now it's 40 sockets across 8 RFCs 1 consumers unit would d0, 8 RFCs wired from that CU, don't mean to be disrespectful Mate, but are you sure you are suitably experience to do this type of work?

 

[Deleted member 26818]

Making off SWA into sockets is not a big deal.
Just make the gland off into 20mm couplers.

 

[appspark]

Hi - I suggest to get the load fully tied down first (a written spec) as food stalls may fully load each outlet with water boilers and cookers etc. If so a domestic rfc isn't really going to cut the mustard, I reckon.

To be honest at best i think they're thinking more hotdog warmer etc than full setup, otherwise in that case i'd run a 63A supply to an isolator.
The unfortunate part of this job is that its design and build so they wont commit themselves to a spec. At the end of the day if they try and pull much more than 32A after i'm gone the breaker will go anyway.

 

[Deleted member 26818]

What CSA do you get if you calculate for a radial?

 

[appspark]

Your original post mentioned 5 sockets on a RFC now it's 40 sockets across 8 RFCs 1 consumers unit would d0, 8 RFCs wired from that CU, don't mean to be disrespectful Mate, but are you sure you are suitably experience to do this type of work?

I did say i didn't think i'd explained it very well Pete. the problem is that yes i will have 8 RFC doing 40 total sockets, I still have the issue that an RFC is still going to be 300m on the longest.
I'm more than capable of doing the work i was only looking for a nudge in the direction of how to calculate cable size on a RFC, i dint ask anyone to do it for me or even pass me a link to a page, just a nudge in the right direction.

 

[appspark]

What CSA do you get if you calculate for a radial?

Its like 25mm or something daft, 6mm is ok for sockets, 25mm is pushing it

 

[Pete999]

I did say i didn't think i'd explained it very well Pete. the problem is that yes i will have 8 RFC doing 40 total sockets, I still have the issue that an RFC is still going to be 300m on the longest.
I'm more than capable of doing the work i was only looking for a nudge in the direction of how to calculate cable size on a RFC, i dint ask anyone to do it for me or even pass me a link to a page, just a nudge in the right direction.

I think to be able to draw any conclusions Mate we would need a more detailed schedule, supply information, details of the load requirements etc, to avoid any bad feelings I'm going to retire from this thread.

 

[Pete999]

I did say i didn't think i'd explained it very well Pete. the problem is that yes i will have 8 RFC doing 40 total sockets, I still have the issue that an RFC is still going to be 300m on the longest.
I'm more than capable of doing the work i was only looking for a nudge in the direction of how to calculate cable size on a RFC, i dint ask anyone to do it for me or even pass me a link to a page, just a nudge in the right direction.

I think to be able to draw any conclusions Mate we would need a more detailed schedule, supply information, details of the load requirements etc, you mentioned that the client may need to add things at a later stage, or that's how I read your first post, this type of installation method, of suggesting that the best way forward is to install some sockets, and see what transpires at a later date is not the best way forward, I take it that this is not a Premiership Club you are working for, more than likely a lower tier outfit.
I think your best second bet would be to tie the management down to what they actually require now and future proof for the future, and base your cable calculations on the results supplied by them, it maybe that RFCs are not the ideal answer to their needs, there will be a lot of other requirements that will need consideration, RCD Protection, earthing methods, which would make your request for a "nudge in the right direction" for sizing a suitable cable for a RFC pale into insignificance, a proper design is required Mate. it will save the AFC you are working for money in the long run, with that I will exit this thread, as I have gotten myself in trouble before saying what I think and speaking my mind, good luck with your project whatever you decide.

 

[Spoon]

Again, I'm probably misreading this but, am I correct in that they have asked you to design these 8 RFC (40 sockets) but are not willing to clarify what is being plugged into them?

 

[appspark]

I think to be able to draw any conclusions Mate we would need a more detailed schedule, supply information, details of the load requirements etc, to avoid any bad feelings I'm going to retire from this thread.

No problem, thanks for your input.

 

[Deleted member 26818]

That length of run, you’re looking at 10mm2.
Could make off into adaptable boxes and then spur off in 2.5mm

 

[appspark]

Again, I'm probably misreading this but, am I correct in that they have asked you to design these 8 RFC (40 sockets) but are not willing to clarify what is being plugged into them?

Pretty much, but at least they know they wont get more than 32A from the circuit. i think the idea is much like a cleaners circuit, you know that only 1 or 2 will actually be in use at a time.

 

[appspark]

That length of run, you’re looking at 10mm2.
Could make off into adaptable boxes and then spur off in 2.5mm

Okidokie, thanks for the help. I was planning on using a 6x6 JB and coming out of there into the socket so I'm along the same lines of thinking as you.

Many Thanks

 

[Pete999]

Okidokie, thanks for the help. I was planning on using a 6x6 JB and coming out of there into the socket so I'm along the same lines of thinking as you.

Many Thanks

Not much help really, but you are welcome

 

[Strima]

Is three phase available?

Could you run a supply to a point then supply temprary rubber bricks for 16 amp distribution?

At a guess these are for food style outlets such as burger vans which mainly run off 16 amp commandos.

 

[appspark]

Is three phase available?

Could you run a supply to a point then supply temprary rubber bricks for 16 amp distribution?

At a guess these are for food style outlets such as burger vans which mainly run off 16 amp commandos.

yeah my main incomer is 400A Switchboard, i've then got a 3ph DB in each stand.
It's just sockets just now with the view to maybe sticking a guy with a hot dog roller machine in the corner for the bigger matches. it wouldnt ever be on the scale of a burger van rocking up and plugging in.
I've not even approached the subject yet with them over if they want to sockets locked so punters don't start charging their phone or fire up a 3 bar Electric fire for the winter games haha.

 

[Spoon]

I've not even approached the subject yet with them over if they want to sockets locked so punters don't start charging their phone or fire up a 3 bar Electric fire for the winter games haha

So, where about in the stadium are these sockets being fitted?
If the general public can get to them while at the stadium I would consider fitting isolators to each circuit.

 

[Lucien Nunes]

You are asking about cable sizes without giving us particulars of the length of each leg and the intended loading, which are two of the prime factors. The position of the sockets around the length of the ring will affect the voltage drop and you cannot simply apply a standard formula unless the load is evenly distributed along the length.

If all the sockets are clustered together with two long, nearly equal legs back to the board, then the voltage drop will be similar to a radial made of a cable of twice the CSA. E.g. if your ring is made of 6mm² running 100m to the first point, then 5m between each point and the next, then 100m back to the board, you effectively have a 12mm² radial with the same voltage drop and CCC. I would instead use a radial of 16mm², terminated in an adaptable box to bring it down to 4mm² and run that along the sockets as a radial.

If instead the sockets are distributed along the length of the cable, then the ring or two radials would be a better bet. With an evenly distributed load, the drop in a ring is the same if you consider the middle point broken, making it two radials of half the total length. Then, each radial has a load equivalent to half the total, at a point halfway along. In this case it doesn't matter whether the circuit is a ring or radials, the drop is equal. Since the voltage drop is probably going to dictate the cable size instead of the CCC, you might be better off with double the number of radials.

 

[westward10]

I find it difficult to believe a new football stadium does not have strategically placed distribution boards for lighting and power.

 

[Marvo]

Information about the load and future requirements sounds thin on the ground and will have a profound effect on the cabling schedule. I think given the logistics of running SWA cabling around the stands of a stadium for trading stalls I'd be suggesting to the customer they invest in getting a consultant in to do a proper spec.

 

[Dillb]

Blah blah blah

 

[Dillb]

I'd be suggesting to the customer they invest in getting a consultant in to do a proper spec.

They have, heres the address for them.


ElectriciansForums.co.uk | Free Electrical Advice - https://www.electriciansforums.co.uk/

 

[Toneyz]

Just out of curiosity, why SWA for wiring the sockets?
How to you terminate the armour?
If it is 6mm then you will have fun putting 2 of them into each socket.

It would depend upon the make, but 6mm would be the limit for termination in box and terminals

 

[Pete999]

Run a suitably sized sub main, in a roughly central position in the stadium, fit a suitably sized distribution board and wire your radials/RFC s from there, still can't understand why you are saying the sub main will need to be sized there and back.

 

[DefyG]

a 25mm SWA is needed, which obviously inst viable for a few sockets.

Why not, I once installed a 35sq cable for 1No 13A double socket (Fridge and kettle) to a fishing/hunting lodge over approx. 200m.
The owner thought less hassle than continually getting fuel for a generator! So eventually paid for itself.

 

[driverman]

Possibly run more than 1 sub circuit. 3 phase or 1 phase Sub circuits to be suitably sized for known load plus spare capacity for unknown (with -in reason) load. I would keep all the final sub circuits to a sensible distance. Possibly use 16 amp sockets for caterers to plug into. As mentioned before, I would get a spec to say to what is required. You may leave yourself wide open later on being sued, if not adequately designed.