Breaker size selection? - Help!

claret73 · Jun 16, 2009

Hi Guys
Quickie that is concerning me for my L2 Exam...
I obviously do not know the ins & Outs of the final installation, but certain rumblings from tutors and other lads mark out this scenario...

I know we have a 3Phase board feeding a Consumer Unit via 3 core, 2.5mmsq SWA.
The Main switch in the CU is 100A & at most we'll get a Power circuit & lighting. So assuming MCBs total my Max thinking of 38A (6A Lighting, 32A Ring) would my breaker selection at TPN Board be 40A? & so on if circuit was a radial. Am I missing bringing into play max demand of cable of 2.5?

I don't do any 3Phase at work & has been quite patchy for me at college...been trying to read & find out but my head is spinning...
Cheers if any pointers on this
Andy

Jono Pashley · Jun 16, 2009

To use 2.5mm 3 core XLPE SWA you would have to use a 30A cartridge fuse or a 20A MCB.

Richard Andrews · Jun 16, 2009

claret73 said:
Hi Guys
Quickie that is concerning me for my L2 Exam...
I obviously do not know the ins & Outs of the final installation, but certain rumblings from tutors and other lads mark out this scenario...

I know we have a 3Phase board feeding a Consumer Unit via 3 core, 2.5mmsq SWA.
The Main switch in the CU is 100A & at most we'll get a Power circuit & lighting. So assuming MCBs total my Max thinking of 38A (6A Lighting, 32A Ring) would my breaker selection at TPN Board be 40A? & so on if circuit was a radial. Am I missing bringing into play max demand of cable of 2.5?

I don't do any 3Phase at work & has been quite patchy for me at college...been trying to read & find out but my head is spinning...
Cheers if any pointers on this
Andy

What my old lecturer said was 'a professor knows it all - an acedemic knows where to look it up!'
Do a google search for swa/cable size/load/voltage drop per metre.
Remember a ring hasn't got to be 32A - for garden offices I've installed 20A Ring/6A lighting circuits (bearing in mind the load of a heater and computer equipment can be accomodated by 20amps). This way you can protect the whole circuit with a 32A MCB - I'm sorta certain the last time I looked such up the load capacity on the 2.5mm SWA was 34Amps...
Determine the load requirements and work backwards - this way you won't ever end up installing a 6mm T&E cable for a customer supplied shower which requires a 10mm cable!!!!
I hope such helps you.
Good luck.

Jono Pashley · Jun 16, 2009

It has to be greater or equal to In which has to be greater or equal to Ib

It - cable current carrying capacity(may be Iz if the capacity has been corrected)
In - Protective Device Rating
Ib - Design Current

I only get a rating of 31A for 2.5 3 core XLPE SWA

scotsparky · Jun 16, 2009

Sound aboot right jonno as its a 3 core swa then have a look at the regs which will tell you volt drop

R!and Rn Isnt as problem as both conductors the same

claret73 · Jun 17, 2009

Cheers Guys
Obviously I'm aware of the cable calcs, but doesn't help me too much not knowing the loading requirements off the sockets. Anyhow, did the paperwork last night...

TPN Board all rigged so just have to concentate on the singlephase CU and circuits.
They have specced a 32A MCB in the TPN DB. This will be fed via 2.5 XLPE/PVC SWA 3core. 3x Circuits in CU:
1/Ring final - 3 Twin SO with a Twin SO Spur (no known loads)
2/ 1KW Load fed via a 13A DP FCU
3/ Lighting circuit, 1 lamp via 2way switching

Not knowing the socket demands, I chose a 32A MCB for the ring, a 6A for lighting & a 16A for FCU...
I know I need & WILL look at the regs book for the selection of the MCB in the TPN, but am confused or confusing myself as to how this calc was done to attain the TPN breaker...
Cheers anyhow for your responses.
Always appreciated.