Radial Circuit

[dpgball]

I have a radial circuit connecting 4 sockets in my kitchen. The cable from the CU is 4mm and then there are 4 sockets connected one after the other using 2.5mm twin and earth cable. Then there is another socket which is connected as a spur from the second socket in the line also using 2.5mm cable. (I hope this makes sense)

The circuit is protected by a 20A MCB. The length of the initial 4mm cable in the circuit is about 8 metres and the next 4 sockets are connected about 1m apart.

Does this sound safe? Any help would be appreciated. I am trying to avoid calling in an electrician but will be willing to if needed.

 

[ban-all-sheds]

Perfectly safe. Before you move out, you should document the fact that part of the circuit is wired in 2.5mm², lest some future occupant loads the circuit heavily, keeps tripping the breaker, and ups it to 25A on the basis of the 4mm² he can see at the CU...

 

[comms]

So long as you don't go any higher than 20amp fuse/mcb rating it will be ok.
Think about changing the 2.5 to 4mm between the sockets, you could then change fuse/mcb to 30/32amp.

 

[dpgball]

Your help is much appreciated. Am I right in thinking that as I am using an MCB rather than a fuse, if the circuit were to overload then the MCB would trip straight away before the circuit cable would overheat? Therefore in theory the cable would not cause a fire if overloaded.

 

[ban-all-sheds]

That's right. Currently ( ) the rating of the breaker is below that of the cable, which is the right way round.

See this //www.diynot.com/forums/viewtopic.php?p=81696#81696
for more info.

 

[Spark123]

An MCB will not necessarily trip immediately as the result of an overload. An MCB (roughly) has a fusing factor of 1.45. This means that a current of 1.45*20A is required to operate the MCB within 1 hour. This fusing factor is already taken into consideration by the regs in the cable selection tables, so the cable is adequate as to not overheat. If using a rewirable BS3036 fuse which has a fusing factor of 2, a de-rating factor of 0.725 needs to be applied when selecting a cable.

 

[Tickle]

I have a similar circuit in my kitchen, it has 4mm from the CU to the cooker/socket combination box, then 2.5mm cable leading to a double socket then the same again to another double. the MCB in the CU is a 40A.

From what I have read in previous posts this MCB sounds very wrong!!!!!!

 

[ColJack]

5 years old post you resurected there.. well done..

however back to your post..

yes it's dodgy..

the 2.5mm should be protected by nothing bigger than a 27A fuse, and that's assuming that it's clipped direct and no other de-rating factors..

if you only had one single or double socket on the 2.5mm² then it's allowable as the current would be limited to 26A max due to the fuses in the plugs..

in general a 2.5mm radial is usually on a 20A fuse or breaker..

the 4mm however really shouldn't be on the 40A breaker either since it's max capacity is 37A...

 

[Tickle]

i have been looking at double socket fronts, there seems to be different kinds (single pole, double pole).

Which would I need to extend my radial round my kitchen.

 

[ban-all-sheds]

You need the sort that your electrician will bring with him.