Multiple phase wires in circuit breakers

[Sparkysystem]

I would greatly appreciate the forum members views on the following, as I would like to give customers accurate advice when finding an unusual number of phase wires in a circuit breaker:

3 phase wires can sometimes be found connected to a circuit breaker on a domestic consumer unit. One way within BS 7671 that this can occur is a spur connected to the ring final circuit at the Consumer unit. P149 OSG.

This does not say fused spur, just spur, so I would expect the maximum of one double socket on this spur. Having said this, if a non-fused spur is allowed, then I see no reason why a fused spur could not also be connected at the consumer unit circuit breaker.

Question 1. Can a fused spur be connected to a ring final circuit circuit breaker at the consumer unit?

Question 2. Are there any other situations where more than 2 phase wires can be connected to a ring final circuit's circuit breaker within BS 7671 regulations?

Question 3. Two or more radial circuits providing lighting can sometimes be found connected to a circuit breaker in a domestic consumer unit. There is clearly the problem of nuisance tripping but is this method of connection contrary to BS 7671?

Kind Regards.

 

[andy]

Question 1. Can a fused spur be connected to a ring final circuit circuit breaker at the consumer unit?


yes

Question 2. Are there any other situations where more than 2 phase wires can be connected to a ring final circuit's circuit breaker within BS 7671 regulations?

you should only take 1 spur from 1 point. so that a max of 3 wires. but ive seen 2 or more spurs from the same point many times

Question 3. Two or more radial circuits providing lighting can sometimes be found connected to a circuit breaker in a domestic consumer unit. There is clearly the problem of nuisance tripping but is this method of connection contrary to BS 7671?


designed properly it wont trip unless there is a fault. there is nothin wrong with 2 radials form 1 MCB, providing the cable is rated greater than the MCB. the 2nd radial can be a branch of the other radial, and connected at the CU rather than a light etc

 

[Lectrician]

Agree fully.

 

[plugwash]

you should only take 1 spur from 1 point.

this is something i hear a lot but i've never seen anyone quote a reg that backs it up.

 

[Sparkysystem]

Page 149 of OSG states:

The total number of fused spurs is unlimited but the number of non-fused spurs does not exceed the total number socket-outlets and items of stationary equipment.

A non-fused spur feeds only one single or one twin socket-outlet or one permanently connected equipment. Such a spur is connected to a circuit at the terminals of socket-outlets or at junction boxes or at the origin of the circuit in the distribution board.

This is as close as BS 7671 gets, which is not ruling out 2 spurs connected to a ring final circuit socket-outlet.

However, the MK Catalogue shows that their double socket-outlets are only designed to have 3 x 2.5mm sq wires in each terminal. This means if you use a socket-outlet designed like this and you put 4 wires in you would contravene BS7671 by exceeding the design limits of the socket-outlet.

 

[ban-all-sheds]

Question 3. Two or more radial circuits providing lighting can sometimes be found connected to a circuit breaker in a domestic consumer unit. There is clearly the problem of nuisance tripping but is this method of connection contrary to BS 7671?


designed properly it wont trip unless there is a fault. there is nothin wrong with 2 radials form 1 MCB, providing the cable is rated greater than the MCB. the 2nd radial can be a branch of the other radial, and connected at the CU rather than a light etc

You could argue that that contravenes 314-01-01....

 

[andy]

Question 3. Two or more radial circuits providing lighting can sometimes be found connected to a circuit breaker in a domestic consumer unit. There is clearly the problem of nuisance tripping but is this method of connection contrary to BS 7671?


designed properly it wont trip unless there is a fault. there is nothin wrong with 2 radials form 1 MCB, providing the cable is rated greater than the MCB. the 2nd radial can be a branch of the other radial, and connected at the CU rather than a light etc

You could argue that that contravenes 314-01-01....

and what is 314-01-01? (dont have regs around)

 

[Qedelec]

Regulation 314-01-01 requires that every installation be divided into circuits as necessary, not only to avoid danger but to minimize inconvenience in the event of a fault.

 

[Sparkysystem]

Reg 314-01-01 Every installation shall be divided into circuits as necessary to:

(i) avoid danger and minimize inconvenience in the event of a fault, and
(ii) facilitate safe operation, inspection, testing and maintenance.

IMHO

I feel Andy is right if the installation is designed, erected and documented correctly. For instance, you would have no more lights on the 2 radials than you would have on the one.

Having said that, it may be true that often when we come across the 2 radials, they are not designed, erected and documented correctly, and in those cases ban-all-sheds is likely to be correct.

 

[plugwash]

minimize is one of those nasty words in the regs, taken literally it would mean that you would have to put every device on a seperate cuircuit.

 

[andy]

minimize is one of those nasty words in the regs, taken literally it would mean that you would have to put every device on a seperate cuircuit.

and split accross the 3 phases incase 1 phase goes down, and have numerous independant supplies to the house, incase 1 substation goes down...

 

[plugwash]

exactly which leaves the question of where do you draw the line.

i'd personally say that following the spirit of that rule multiple wires running back to the CU should be seperate cuircuits unless there is a good reason for them not to be (e.g. CU full or interconnection of some sort between the wires later).

btw why are split load CUs considered acceptable by so many when they are a blatent violation of the discrimination rules.

 

[andy]

btw why are split load CUs considered acceptable by so many when they are a blatent violation of the discrimination rules.

Price.

 

[Sparkysystem]

Following on from Andy’s guidance concerning lighting radials and the fact that it is the terminal design capacity, not directly BS7671, that determines the number of wires in a terminal, the following seems true:

In a domestic consumer unit, with a single 20 Amp circuit breaker supplying socket-outlets covering a total area of less than 50m sq and with correct design, erection and documentation, there can be two or more 2.5mm sq wires forming 2 or more radials from this single circuit breaker to feed the sockets.

A single 32Amp circuit breaker can supply a ring final circuit covering 50m sq and also supply a 4mm sq radial circuit for socket-outlets covering 25m sq. There would be two 2.5mm sq wires and one 4mm sq wire coming from the single circuit breaker.

The terminal design capacity of an MK Sentry circuit breaker is 35mm sq.

This outcome from our thread is not what I was expecting but it appears accurate.

My interest in this is that when doing Periodic Inspections and when ensuring existing circuits are safe before extending them, I want to give customers accurate information. I feel Plugwash is correct for ‘best practice’ and would follow his line unless there is very good reason to deviate.

Many thanks to all who have contributed and please let me know if I have misunderstood the situation.

 

[andy]

In a domestic consumer unit, with a single 20 Amp circuit breaker supplying socket-outlets covering a total area of less than 50m sq and with correct design, erection and documentation, there can be two or more 2.5mm sq wires forming 2 or more radials from this single circuit breaker to feed the sockets.

radials are in length, not floor space served.

A single 32Amp circuit breaker can supply a ring final circuit covering 50m sq and also supply a 4mm sq radial circuit for socket-outlets covering 25m sq. There would be two 2.5mm sq wires and one 4mm sq wire coming from the single circuit breaker.

you shouldnt really mix a ring and radial on the same MCB

The terminal design capacity of an MK Sentry circuit breaker is 35mm sq.

that mostly applies to 1 cable entering the MCB. it doesnt mean you can have 35 1mm² cables going into it