wiring of a ring main? why is it 2.5mm cable? and the regs?

[bob1234]

Hello,

I am wondering as to why a ring main is wired in 2.5 mm flat twin when in the regs book it can only take 21amps?? and it is protected 32amp? how does that work why is 2.5mm cable used for it and a 32amp breaker? how do they work that out? and also i am just wondering in the regs book on page 249 what is this used for ?? with the chart at the side of the page

thanks,

 

[SparkyNinja]

A ring main can be wired in 2.5mm cable and be protected by a 32amp mcb becuase it is wired in 'ring' so the load on the circuit can theoretically be considered to be on a 5mm size conductor.

It was designed years ago when copper was expensive, so using 4mm and 6mm radials was ousted by the introduction of the ring allowing for 2.5mm meaning less copper would be needed.

It's important to remember that the 32amp is only if the ring remains. Any spurs need to be fused down if they are to carry more than a socket/twin socket, as potentially the single 2.5mm being spurred will not carry 32amps as you have noted.



It has been discussed here a few times that we believe the ring circuit will one day be written out of the regs book as it is considered that if a fault occurs on the ring (open circuit) it will still work, and the cable involved will potentially become 2 x radials and so overload may occur on the cabling.

 

[bob1234]

ah right then ye,

and why does is it also allow you to use 4mm and 2.5mm for when you wire a radial what the difference why can they just use 2.5mm?

And so basically they are just splitting the load over so the cable will be able to carry 21 x 2 =42 amps, so that why you can use a 32 amps mcb is that right? so why not put a smaller breaker than 32amps on it then instead?

 

[SparkyNinja]

and why does is it also allow you to use 4mm and 2.5mm for when you wire a radial what the difference why can they just use 2.5mm?

Depends on the demand of the circuit to be installed. The bigger the size of the conductor, the bigger the demand can be.

And so basically they are just splitting the load over so the cable will be able to carry 21 x 2 =42 amps, so that why you can use a 32 amps mcb is that right? so why not put a smaller breaker than 32amps on it then instead?

Why would you install a smaller mcb if it's not necessary? All loads on the ring will be individually fused down.

 

[bob1234]

ah right,

so i am just getting confused because the the maxium a 2.5mm cable can take is 21amps so i thought you could get away with a smaller breaker,

so i take it would be wrong to wire a ringmain in 4mm cable and put it on a 32amp breaker is that right?

 

[SparkyNinja]

so i take it would be wrong to wire a ringmain in 4mm cable and put it on a 32amp breaker is that right?

Well no, but not necessary and would cost you more in cable. The reason it isn't wrong is because it's a larger cable and so no dangers would occur from overload of the cabling.
Depending on the sockets also, 2 x 4mm may be awkward to terminate in the back etc..

It's not wrong... it's not necessary.

 

[bob1234]

wouldnt 4mm cable affect the tripping times of the mcb though?

 

[SparkyNinja]

in what way?

 

[Goldberg]

so i take it would be wrong to wire a ringmain in 4mm cable and put it on a 32amp breaker is that right?

Yes - it would be wrong because you'd have to be crazy to do that, and you should therefore have all tools and sharp objects taken away from you.

 

[RF Lighting]

Unless of course 4.0mm² is required due to volts drop or derating factors, and then of course, it would be exactly the right cable to use.

 

[Goldberg]

OK OK, you got me.

 

[Adam_151]

Unless of course 4.0mm² is required due to volts drop or derating factors, and then of course, it would be exactly the right cable to use.

Theres a certain knack to folding the cables back into shallow dado trunking boxes when you have 2x4mm pvc/pvc T+E though

 

[RF Lighting]

Curl it up to the middle and back on itself

 

[ban-all-sheds]

OK OK, you got me.

Doesn't that depend on whether you meant crazy to use 4mm² for a ring , or crazy to install a ring if it meant you needed 4mm²?

You need 6mm² for a ring final if it's Ref Method 103....

 

[ban-all-sheds]

bob1234, looks like you could do with getting a copy of this: