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# Calculating Cable Size for Ring Circuit

Discuss Calculating Cable Size for Ring Circuit in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

#### appspark

##### Regular EF Member
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

Forum Mentor
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

Forum Mentor
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

##### Regular EF Member
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

Forum Mentor
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

Forum Mentor
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

##### Regular EF Member
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

Forum Mentor
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

##### Regular EF Member
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

Electrician's Arms
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

Forum Mentor
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?

#### spinlondon

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

#### appspark

##### Regular EF Member
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.

#### spinlondon

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

#### appspark

##### Regular EF Member
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.

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