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.

 

[PC Electrics]

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.

The statement in bold is true as a stand alone statement. However, it isn't the absolute difference in resistance that determines the current split it is the ratio of resistance that matters.
In any ring if a load is present at a socket exactly one quarter the way round the ring then the load split will be three quarters to the short leg.

Fortunately in most domestic situations loads are transient and rarely continuous. So even a ring which is occasionally overloaded on one leg (by a drier for example) may show no signs of distress, because the overload is transient and occasional. Also the total lengths of domestic rings isn't usually that great these days because houses usually have at least two. And back in the 60s when it was common for houses to only have one ring, they were built with 7/029 cable which has a higher ccc than metric 2.5.

You're right, but that doesn't mean the potential effects of imbalance should be ignored. It is part of the design process and should be considered, if only to discount the effects.

 

[La Poste]

they were built with 7/029 cable which has a higher ccc than metric 2.5

Does 7/029 mean 7 strands each with 029 in csa?

 

[Engineer54]

The statement in bold is true as a stand alone statement. However, it isn't the absolute difference in resistance that determines the current split it is the ratio of resistance that matters.
In any ring if a load is present at a socket exactly one quarter the way round the ring then the load split will be three quarters to the short leg.

Fortunately in most domestic situations loads are transient and rarely continuous. So even a ring which is occasionally overloaded on one leg (by a drier for example) may show no signs of distress, because the overload is transient and occasional. Also the total lengths of domestic rings isn't usually that great these days because houses usually have at least two. And back in the 60s when it was common for houses to only have one ring, they were built with 7/029 cable which has a higher ccc than metric 2.5.

You're right, but that doesn't mean the potential effects of imbalance should be ignored. It is part of the design process and should be considered, if only to discount the effects.

I've seen plenty of badly designed loop RFC getting pretty warm on one leg of the ring back at the CU. One of the reasons why i always stipulate ''Staggered'' RFC's, balancing problems are reduced dramatically....

 

[Engineer54]

Does 7/029 mean 7 strands each with 029 in csa?

Yes!!!

 

[PC Electrics]

Does 7/029 mean 7 strands each with 029 in csa?

7 strands each 0.029 inch in diameter

 

[La Poste]

One of the reasons why i always stipulate ''Staggered'' RFC's, balancing problems are reduced dramatically....

Thanks for the advice, just one more question.

Could you please explain what you mean by staggered.

 

[La Poste]

From another thread, thanks.

I would imagine there are several given names for this type of wired ring circuit, but i was brought up with the term ''Staggered'' and most of the older sparks i've come accross, also called it by this name.
Probably ''leapfrog'' is another name for it too!! lol!!

Echo,

Basically, from the CU/DB you take the first pair of the ring to the first socket, then take the second pair to the second socket. From the first socket you take a pair to the third socket, then from the second socket you take a pair to the forth socket. and continue on until you link the final socket with the previous socket (omitted cpc, easier to say ''pair''...lol!!)

This configuration spreads the rings loading across the the whole ring circuit and not to just one leg. It eliminates unloaded long return runs back to the CU/DB. Can be used with multi-core cables, MICC, or more usually in conduit/trunking installations with single core wiring.... Not much call for using in domestic installations, because if ring designed properly there will no long unloaded return runs back to the CU.

 

[Engineer54]

Take both legs out of the CU, one to the first socket the other to the second socket. Then from the first socket to the third socket, again from the second socket to the fourth socket ...and so on and so on, until you link the last two sockets!! That's a Staggered ring circuit!! lol!!!

 

[PC Electrics]

Yes!!!

NO

it's diameter in thous of an inch NOT csa

see my post number 20

 

[Engineer54]

NO

it's diameter in thous of an inch NOT csa

see my post number 20

Oooooops your right!! Must pay more attention!! i'll write that out a 100 times!! lol!!!

 

[PC Electrics]

Oooooops your right!! Must pay more attention!! i'll write that out a 100 times!! lol!!!

Hey, we all miss a nail in the tyre occasionally!
Was just concerned to get it correct in case anyone tries to calculate the metric equivalent values from the information

 

[GLENNSPARK]

just a thought here.....if the spur in 4mm T&E was quite a long run from the ring.....would you not have a Zs problem here 4mm only having a 1.5 CPC n all that?....

 

[PC Electrics]

PS: 7/029 = 3mm csa and ccc=31A

 

[La Poste]

Not sure if this is right but will post anyway:

http://www.fultonindustries.com.au/pdf/references/CABLE%20SIZES.pdf

 

[PC Electrics]

the conversions in that table are not exact and it does not convert back the old imperial standard sizes.

make use of this: