Only if it passes the adiabatic equation for the fault current protection utilising the 32A MCB, tho I wouldn't expect it to.
lighting spur from a ring main/socket
- Thread starter kevsterdiy
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I take it we are talking twin and earth, the CPC in the 1.5mm cable may be a bit small (1mm), I wouldn't consider fitting anything below 2.5mm with 1.5mm CPC to a ring circuit.
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I can see everyone's view point here, as an outsider looking if if you will.
From what I have been told as a novice (don't take anything i say as correct just a view point) would be to use 2.5.
In terms of derating to 20amps MCB because of using 2.5 on the spur I can see why people would suggest 4mm, avoiding any risk of derating/chance of overloading a spur from the ring - the ring side of the FCU fuse.
But if you look as said above would the load on this radial spur exceed the fuse in the FCU, before it blows?
It seems the most simple method would be to fit the new socket or FCU as part of the ring/extending the ring with 2.5, then the accessory from it with its correct rated wire and fuse?
Sorry if I've opened a can of worms!
From what I have been told as a novice (don't take anything i say as correct just a view point) would be to use 2.5.
In terms of derating to 20amps MCB because of using 2.5 on the spur I can see why people would suggest 4mm, avoiding any risk of derating/chance of overloading a spur from the ring - the ring side of the FCU fuse.
But if you look as said above would the load on this radial spur exceed the fuse in the FCU, before it blows?
It seems the most simple method would be to fit the new socket or FCU as part of the ring/extending the ring with 2.5, then the accessory from it with its correct rated wire and fuse?
Sorry if I've opened a can of worms!
Neither would I. I've not done any calcs, but I'd guess that if the 1.5/1 run was short enough it might comply. Not good practice, however.
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It is more to do with the fault current which can flow through the cable and the time it takes to disconnect. With an MCB the lower the fault current the better, ensuring at least enough current is flowing to operate the MCB in the given time which is where your Zs comes into it.
doing spurs in 2.5 is no problem surely?
as said above it is whether the cable can take the fault current.if you cut the ring half way round the circuit its still 2.5 on a 32a cct is it not?
the only problem is when people take spurs from sockets to feed other sockets and overload the 2.5 feeding the new outlet but for a spur controlling a light its no problem.
i understand the 4mm principle but its overkill
that said,the best way is still to take 2 2.5 cables to the spur and crimp one onto the existing leg thus building it into the ring.i dont understand why more people dont do this as it usually only takes another twenty minutes!
as said above it is whether the cable can take the fault current.if you cut the ring half way round the circuit its still 2.5 on a 32a cct is it not?
the only problem is when people take spurs from sockets to feed other sockets and overload the 2.5 feeding the new outlet but for a spur controlling a light its no problem.
i understand the 4mm principle but its overkill
that said,the best way is still to take 2 2.5 cables to the spur and crimp one onto the existing leg thus building it into the ring.i dont understand why more people dont do this as it usually only takes another twenty minutes!
Of course, all this would be academic if we simply discontinued the flawed practice of installing ring circuits.
Given that most sparks don't know how to test rings and have no idea of the fallacious reasoning behind them, it seems nonsensical to allow them. Existing rings will be around for many years to come, so they should be understood, but for new installs why bother?
All this talk of 4 sq mm? Well, yes, by all means wire radial circuits in 4 sq mm if it makes you feel better (a radial, by the way, is different from a spur, CJ) but with a bit of thought there really is no need; 20A 2.5 sq mm radials will serve 50 sq metres.
Think installation design. Think a greater number of less complicated circuits...
...just think.
Given that most sparks don't know how to test rings and have no idea of the fallacious reasoning behind them, it seems nonsensical to allow them. Existing rings will be around for many years to come, so they should be understood, but for new installs why bother?
All this talk of 4 sq mm? Well, yes, by all means wire radial circuits in 4 sq mm if it makes you feel better (a radial, by the way, is different from a spur, CJ) but with a bit of thought there really is no need; 20A 2.5 sq mm radials will serve 50 sq metres.
Think installation design. Think a greater number of less complicated circuits...
...just think.
think a bigger breaker board that I don't have room for...
every room on it's own radial..
2 in the kitchen as well as the cooker...
how exactly is a ring flawed?
I can see your point if your house is very linear and long then a radial is more practical..
but with most houses being square(ish) then a ring that takes power around the periphery of the house and back to it's starting point is surely more practical?
I am aware of the difference between a spur and a radial..
however.. if i had 3 double sockets wired in a ring in the cupboard where the consumer unit is and then take a spur off of one socket and run it to the back of the property to the coal house. is it still just a spur since the total run of it would be longer than that of the ring it's attatched to?
every room on it's own radial..
2 in the kitchen as well as the cooker...
how exactly is a ring flawed?
I can see your point if your house is very linear and long then a radial is more practical..
but with most houses being square(ish) then a ring that takes power around the periphery of the house and back to it's starting point is surely more practical?
I am aware of the difference between a spur and a radial..
however.. if i had 3 double sockets wired in a ring in the cupboard where the consumer unit is and then take a spur off of one socket and run it to the back of the property to the coal house. is it still just a spur since the total run of it would be longer than that of the ring it's attatched to?
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So to sum up for anyone reading this thread asking the same question as I;
1) 2.5mm spur from a 2.5ring main to socket or FCU is ok?
2) 4mm spur from a 2.5ring main to socket or FCU is ok?
3) To extend/add into a 2.5mm ring a socket or FCU with 2.5mm is ok?
1) 2.5mm spur from a 2.5ring main to socket or FCU is ok?
2) 4mm spur from a 2.5ring main to socket or FCU is ok?
3) To extend/add into a 2.5mm ring a socket or FCU with 2.5mm is ok?
all of the above are acceptable..
best method is 3 but 1 and 2 are just as good.
best method is 3 but 1 and 2 are just as good.
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