Is a 4mm spur ok for a ring main.

[JONOXNAM]

Hi, ref fig 15A in app 15 (7671amm1) would it be OK to spur 2x2G BS1363 socket outlets if the spur is run in 4mm? i.e. is figure 15A definitive for ring configurations?

I ask as a customer wants to have an alteration completed now with a view in a years time to a rewire where a radial 4mm will replace the 2.5 ring that the alteration is to be fed from.

 

[malcolmsanford]

Got to remember that except for appendix 1 all the appendices are formative not normative. So I doubt that you would be getting an early morning knock on your door from the Ring final circuit police if you did as you say.

There could be an unbalancing issue if someone decide to plug 4 3kw heaters in those 2 sockets, but the same could be said if you did the same to the first 2 sockets on a normal ring.

By using the 4mm cable you comply with reg 433.1.103 and 433.1.1 concerning the Iz

 

[Deleted member 9648]

Depending on anticipated use you could run your spur through a 13a FCU in order to fully comply with current ring requirements.....that said I wouldnt have any issues with Malcolms excellent advise.

 

[JONOXNAM]

Got to remember that except for appendix 1 all the appendices are formative not normative. So I doubt that you would be getting an early morning knock on your door from the Ring final circuit police if you did as you say.

There could be an unbalancing issue if someone decide to plug 4 3kw heaters in those 2 sockets, but the same could be said if you did the same to the first 2 sockets on a normal ring.

By using the 4mm cable you comply with reg 433.1.103 and 433.1.1 concerning the Iz

Many thanks, I did think of a potential issue - connection of different size conductors so I will be using a wago type connector so each conductor will be terminated into its own connector.

 

[DurhamSparky]

fit 4mm and your fine...!

 

[GT1]

Increasing the csa not so much an issue as decreasing it !

 

[Deleted member 26818]

Regulation 433.1.103 to my mind, just describes how a 'standard' RFC can be designed to meet the requirements of Section 433.
Appendix 15, shows different options.
I don't believe that it is a requirement that all RFCs must comply with.

 

[nick611]

i think a 4mm spur is fine they will carp about balancing, but this is bollox. 4mm =36A ring main 32A job done. balanced ring main no such thing

 

[nick611]

fit 4mm and your fine...!

durham sparky at last someone who see,s a 4mm spur from a ring main is ok and dose,nt say you can,t spur off a spur and dose,nt resort to balancing as their last resort. i salut you.

 

[Engineer54]

durham sparky at last someone who see,s a 4mm spur from a ring main is ok and dose,nt say you can,t spur off a spur and dose,nt resort to balancing as their last resort. i salut you.

Don't dismiss the balancing aspect of a RFC out of hand!! You need to think a little more deeply before calling it a load of bollox!! It is a parallel circuit, but with a big difference, it's not supplying an end load, the loads are being drawn throughout each of it's 2 legs. If one leg is grossly loaded, that leg can become very warm....

In this case, if this 4mm spur on spur is being used in a kitchen or utility room for high current drawing appliances, it's not going to be a very good idea is it?? The closer to the first RFC socket on one of the RFC legs this spur is being taken from, the more affect it will have on that legs smaller cable. Fine for occasional light to medium loads but i'd be a lot more sceptical if these sockets were being installed for known heavy loads....

 

[PC Electrics]

The 4mm is protected fine by the 32A mcb.

I would only echo the concerns regarding the balance on the existing ring. If the 4mm spur is likely to be heavily loaded (eg a utility area) and is connected into the ring towards one end, then that end of the ring in just 2.5 will also be heavily loaded.
Many people (including many "real sparks") believe that the current in a ring travels 50/50 round the legs. This is not so, the current in each section is inversely proportional to the resistance of each leg, so the closer the load to one end, the higher the current in that end.

 

[Engineer54]

The 4mm is protected fine by the 32A mcb.

I would only echo the concerns regarding the balance on the existing ring. If the 4mm spur is likely to be heavily loaded (eg a utility area) and is connected into the ring towards one end, then that end of the ring in just 2.5 will also be heavily loaded.
Many people (including many "real sparks") believe that the current in a ring travels 50/50 round the legs. This is not so, the current in each section is inversely proportional to the resistance of each leg, so the closer the load to one end, the higher the current in that end.

My point exactly, nothing wrong with the protection factor of the ring or the 4mm spurred sockets...

There are very good reasons for the restrictions and rules relating to spurs off of RFC's.... Yes they can be adapted, but a deal of thought needs to be applied when doing so!!

 

[JONOXNAM]

My point exactly, nothing wrong with the protection factor of the ring or the 4mm spurred sockets...

There are very good reasons for the restrictions and rules relating to spurs off of RFC's.... Yes they can be adapted, but a deal of thought needs to be applied when doing so!!

Thanks for the inputs. The spur will be feeding a laptop, lamp and the other plate is free for a hoover etc..

As I said the circuit will be made an A2 in about a year.

Great to hear people that understand that a ring leg can be overloaded, as I remember being told this on my 2330 and the need to run the out leg to the first socket then the in leg to the next socket etc vs running a big loop.... plus this can save you having a massive return unloaded leg...

I have heard comments at the wholesalers counter like 'well there is only 16A flowing in each half of the ring'...

 

[La Poste]

Great to hear people that understand that a ring leg can be overloaded, as I remember being told this on my 2330 and the need to run the out leg to the first socket then the in leg to the next socket etc vs running a big loop.

Hi there.

Could you explain bit more about what you mean by this?

Thanks.

 

[Deleted member 9648]

Thanks for the inputs. The spur will be feeding a laptop, lamp and the other plate is free for a hoover etc..

As I said the circuit will be made an A2 in about a year.

Great to hear people that understand that a ring leg can be overloaded, as I remember being told this on my 2330 and the need to run the out leg to the first socket then the in leg to the next socket etc vs running a big loop.... plus this can save you having a massive return unloaded leg...

I have heard comments at the wholesalers counter like 'well there is only 16A flowing in each half of the ring'...

It wont be a massive return unloaded leg.....wired in a big loop,(the normal way), the difference in resistance between the first socket and the rest of the loop will be a fraction of an ohm,therefore even in the circumstances you describe a considerable proportion of the load will take the long way round.
Has anyone yet come across a melted leg(s) of a ring due to imbalance?...I haven't in over 30yrs.