Max size mcb for 4mm radial?

GreySquirrel

what would be the max size mcb for 4mm radial?

2011

GreySquirrel wrote: »

what would be the max size mcb for 4mm radial?

Socket circuit?? I would not go beyond 20A, but from memory I think a 25A is permitted.

GreySquirrel

2011 wrote: »

Socket circuit?? I would not go beyond 20A, but from memory I think a 25A is permitted.

Yup, sockets. Was looking at the charts in the rules. for a cable under a suspended floor, current carrying capacity 32 or 30 Amps

2011

GreySquirrel wrote: »

Yup, sockets. Was looking at the charts in the rules. for a cable under a suspended floor, current carrying capacity 32 or 30 Amps

As I'm sure you know current carrying capacity should always be of a higher value that the protective device.

I would generally design a socket circuit to use a 16 or 20A MCB or RCBO.

Is there a specific reason that you require a larger MCB?

GreySquirrel

2011 wrote: »

As I'm sure you know current carrying capacity should always be of a higher value that the protective device.

I would generally design a socket circuit to use a 16 or 20A MCB or RCBO.

Is there a specific reason that you require a larger MCB?

There is a long run of 4mm to some of the kitchen sockets. It was originally connected to an electric CH boiler. The cable was there but I had removed the mcb before checking the size of the mcb

There is also a 2.5 mm running to kitchen. I think that was for the timer to the boiler. If I use the 2.5mm for a radial socket circuit what size mcb for that?

The boiler is now longer used. It was only used for for a few months. If anyone wants it, it's availabe. Its a trianco azteck.

GreySquirrel

GreySquirrel wrote: »

Yup, sockets. Was looking at the charts in the rules. for a cable under a suspended floor, current carrying capacity 32 or 30 Amps

The pictures in the rules look like smilies

2011

GreySquirrel wrote: »

There is a long run of 4mm to some of the kitchen sockets. It was originally connected to an electric CH boiler. The cable was there but I had removed the mcb before checking the size of the mcb

That is not really a problem. Boilers consume very little power. I think you will find that a spur fused at 3A will be more than enough for the average boiler.
The fact that you don't know what size MCB was on it is pretty irrelevant.

If you are satisfied that the cable is a 2.5 or greater I would suggest that you use a 20A B type MCB (fed from an RCD) or use a 20A type B RCBO.

There is also a 2.5 mm running to kitchen. I think that was for the timer to the boiler. If I use the 2.5mm for a radial socket circuit what size mcb for that?

Same as above

slavetothegrind

its an electric boiler 2011, must be a low power one on a long 4sq run?

might take that off your hands op, will pm if it suits the job i have in mind.

2011

slavetothegrind wrote: »

its an electric boiler 2011

Yes, it would seem that you are correct.
I misread the OP's post.

It would seem that I wrongly assumed that the boiler was gas or oil, but required an electrical supply to operate. However it now appears that this unit may have an electric element.
It changes nothing though.
My advice remains the same.

GreySquirrel

slavetothegrind wrote: »

its an electric boiler 2011, must be a low power one on a long 4sq run?

might take that off your hands op, will pm if it suits the job i have in mind.

Yup. it's electric powered but no longer connected. I have the manual somewhere. I'll have a look later.

GreySquirrel

2011 wrote: »

Yes, it would seem that you are correct.
I misread the OP's post.

It would seem that I wrongly assumed that the boiler was gas or oil, but required an electrical supply to operate. However it now appears that this unit may have an electric element.
It changes nothing though.
My advice remains the same.

I have a 20 amp mcb that i can put in the 4mm radial circuit

GreySquirrel

GreySquirrel wrote: »

Yup, sockets. Was looking at the charts in the rules. for a cable under a suspended floor, current carrying capacity 32 or 30 Amps

If 2.5mm and 4mm take same max mcb, what's the point of 4mm?

2011

GreySquirrel wrote: »

If 2.5mm and 4mm take same max mcb, what's the point of 4mm?

Well first of all in this case the 4sq. is already installed.

Also 4sq. will have reduced volt drop and earth fault loop impedance. This becomes more important on longer runs.

2011

In industrail installations 4sq. would be normal for socket circuits for phase, neutral and earth with each circuit fed from a dedicated 20A type C RCBO.

GreySquirrel

2011 wrote: »

Well first of all in this case the 4sq. is already installed.

Also 4sq. will have reduced volt drop and earth fault loop impedance. This becomes more important on longer runs.

okily dokily

the 4mm is already there, about a ten metre run. the 2.5 mm is there but not connected. it's a longer run to what was my ma's bedroom. It is about 15/20 metres long.

Also I'm trying to identify exactly which sockets in the kitchen are on the 4mm radial. I have one of those gadgets like a plug with little lamps that light up when you plug it in. there doesn't seem to be a logical arrangement of sockets. Also some of the sockets are on a ring circuit. I'm presuming that cos the mcb is 32 amp with two cables running out

maxfresh

GreySquirrel wrote: »

okily dokily

the 4mm is already there, about a ten metre run. the 2.5 mm is there but not connected. it's a longer run to what was my ma's bedroom. It is about 15/20 metres long.

Also I'm trying to identify exactly which sockets in the kitchen are on the 4mm radial. I have one of those gadgets like a plug with little lamps that light up when you plug it in. there doesn't seem to be a logical arrangement of sockets. Also some of the sockets are on a ring circuit. I'm presuming that cos the mcb is 32 amp with two cables running out

I'd switch off all the mcbs ,then switch on one at a time and use your socket tester ,then put little sticker on socket saying what mcb its fed from.

your right about the ring cct ,two phase conductors from a single mcb 32 amp

2011

maxfresh wrote: »

your right about the ring cct ,two phase conductors from a single mcb 32 amp

Thats they way they were done alright. No longer permitted though.

GreySquirrel

Sorry for the delay and thanks for the info.

maxfresh

2011 wrote: »

Thats they way they were done alright. No longer permitted though.

ye in new regs no ring cct in kitchen