Spur from 32a mcb

[Adam Smith]

Morning all,

As we know, you can run a spur on a ring final cct from the top of the 32a breaker. But can you run a single socket radial cct from a 32a breaker? essentially making it a spur but without the ring being present.

Thanks

 

[richy3333]

OK, I'm making an assumption on cable size as you don't give enough info - assuming 2.5mm T&E on a 32A protective device; so does the 32AMCB/RCBO protect your 2.5mm spur?

Or would you like to expand your question?

 

[Simonslimline]

There was a thread talking all about this recently. I think it was someones findings on an EICR, not what they had installed.

 

[Andy78]

I take this is a theoretical question as there would be no good reason to do this in practice.

Would a dedicated circuit be subject to design calcs ?
would a spur from a ring be subject to design calcs ?
would your calcs let you design a radial circuit in 2.5mm cable with a 32A OCPD ?

 

[Hawkmoon]

I take this is a theoretical question as there would be no good reason to do this in practice.

Would a dedicated circuit be subject to design calcs ?
would a spur from a ring be subject to design calcs ?
would your calcs let you design a radial circuit in 2.5mm cable with a 32A OCPD ?

One reason you may consider doing this is if the expected load on circuit (twin socket) is 20A or more.

By design the 2.5mm may be ok but in reality most would install with a more 'conventional' mcb/cable matching.

 

[Andy78]

One reason you may consider doing this is if the expected load on circuit (twin socket) is 20A or more.

By design the 2.5mm may be ok but in reality most would install with more a 'conventional' mcb/cable matching.

Err... would it ?

And if more than 20A was expected at a double socket, other circuit arrangements would probably be a better solution.

 

[Hawkmoon]

Err... would it ?

And if more than 20A was expected at a double socket, other circuit arrangements would probably be a better solution.

Yes, very likely it would - In what area(s) do you think it is unlikely to comply?

 

[Andy78]

Yes, very likely it would - It in what areas do you think it is unlikely to comply?

Well you are talking about 2.5mm T+E yes ? which has an upper rating of 27A in perfect installation conditions, and protecting it with a 32A OCPD on a radial circuit ? and it being okay by design calcs ?

is that right or am I missing something ?

 

[Paul.M]

Appendix 15:- An unfused spur should feed one single or one twin socket-outlet only........ An unfused spur may be connected to the origin of the circuit in the distribution board.

So yes you can. If there is a spare way in the board just fit a 20A mcb for that one socket.

 

[Simonslimline]

The previous argument was that the double socket cant have more than 26amps load due to the fuses in the plug tops.

 

[Hawkmoon]

Well you are talking about 2.5mm T+E yes ? which has an upper rating of 27A in perfect installation conditions, and protecting it with a 32A OCPD on a radial circuit ? and it being okay by design calcs ?

is that right or am I missing something ?

This has been discussed in another thread quite recently but the main points are:


Overload protection is provided by the twin socket (assuming that's what the OP is installing) 2 x 13A

Thermal constraints are met by the 1.5mm CPC


Even if compliant I still would not install a 2.5mm in this way because an electrician carrying out an EICR in future may wrongly record this as a defect - and to the customer, my competence would be in question...can't have that lol!

 

[Hawkmoon]

Appendix 15:- An unfused spur should feed one single or one twin socket-outlet only........ An unfused spur may be connected to the origin of the circuit in the distribution board.

So yes you can. If there is a spare way in the board just fit a 20A mcb for that one socket.

Not appropriate if expected demand greater.

 

[stevethesparks]

Personally, I would protect such a circuit with a 20 Amp mcb/rcbo. I can see where the OP is coming from as it would be a similar scenario connected from a ring being backed up with a 32 Amp mcb. What the OP hasn't taken into consideration is that with a ring present, firstly the spur from it would be unlikely to be of any considerable distance, whereas a radial may be, and also where a ring is concerned diversity would come also into play lessening the likelihood of 26 Amps being required from a lone twin SSO.

 

[Andy78]

This has been discussed in another thread quite recently but the main points are:


Overload protection is provided by the twin socket (assuming that's what the OP is installing) 2 x 13A

Thermal constraints are met by the 1.5mm CPC


Even if compliant I still would not install a 2.5mm in this way because an electrician carrying out an EICR in future may wrongly record this as a defect - and to the customer, my competence would be in question...can't have that lol!

I'm not saying it would be. I'm saying that making it a circuit of it's own you have to stick to design calcs, and design calcs won't let you protect a 2.5mm radial with a 32A OCPD. You could say it's a fixed load incapable of drawing any more than 26A and is therefore not subject to necessary overload protection..... but I wouldn't.

I think we are on the same page anyway, lol.

 

[Andy78]

Not appropriate if expected demand greater.

In this case a 26A draw would take over an hour to trip a 20A MCB. If the usage demand was such, then other circuit arrangments would be considered, like a dedicated circuit for each appliance or 25A overload protection.

 

[Hawkmoon]

I'm not saying it would be. I'm saying that making it a circuit of it's own you have to stick to design calcs, and design calcs won't let you protect a 2.5mm radial with a 32A OCPD. You could say it's a fixed load incapable of drawing any more than 26A and is therefore not subject to necessary overload protection..... but I wouldn't.

I think we are on the same page anyway, lol.

Course we are! -

Bear in mind overload protection can be at any point in a circuit...design calcs will permit.

I would not install this (for the reason I gave previously) but I also would not record it as a defect in an EICR.

 

[Adam Smith]

Thank you for all of the replies.

It is currently a theoretical question. The reason for not just replacing the breaker is that the board is one this nasty fisher price looking affairs (think CED but more flimsy and more flash moulding left overs hanging off it!) so I am having trouble finding a suitable breaker to put in.

The circuit will be around 12 metres from the board and will be supplying one FCU which will feed a 3kw immersion heater.

 

[Megavolt]

Thank you for all of the replies.

It is currently a theoretical question. The reason for not just replacing the breaker is that the board is one this nasty fisher price looking affairs (think CED but more flimsy and more flash moulding left overs hanging off it!) so I am having trouble finding a suitable breaker to put in.

The circuit will be around 12 metres from the board and will be supplying one FCU which will feed a 3kw immersion heater.

Why not run 4mm in ?

 

[Octopus]

You could use 4mm or 6mm T&E and everything would be in order!

 

[GLENNSPARK]

this is about good circuit design...rather than just the OPD....

there would be nothing to stop someone tapping off that socket outlet...thus creating a radial with multiple outlets fed in a bit of 2.5 twin...

hmm....

this is about radials V rings again really...isn`t it...

again:

you use whats appropriate for the job...

 

[HandySparks]

...there would be nothing to stop someone tapping off that socket outlet...thus creating a radial with multiple outlets fed in a bit of 2.5 twin...

What, like a spur off a spur on a 32A ring final? Better start putting them on a 20A breaker 'just in case'.

 

[Hawkmoon]

You could use 4mm or 6mm T&E and everything would be in order!

Arguably everything would be 'in order' with the 2.5mm! -

But then we would not be complying with domestic install 'norms'

 

[GLENNSPARK]

What, like a spur off a spur on a 32A ring final? Better start putting them on a 20A breaker 'just in case'.

a bit like that...yes

 

[Engineer54]

If the OP is talking about connecting this 2.5mm T&E into a 32A MCB supplying a ring circuit, and that cable, as has been stated is intended to connect a 3KW immersion heater via a switched DP FCU, then i can't see a problem as far as compliance is concerned. It's a fixed load with a BS 3162 fuse to protect the cord, that cannot overload and will be protected against short circuit by the 32A MCB protective device...

That's not to say, it's how i would supply an immersion heater!! ...lol!!