extending a 45A curcuit

[davey_duck_breath]

Hello any advice / thoughts would be appreciated:

I have a 45a cooker switch with socket on the wall behind the cooker but I want to either move it or do away with it completely. I now have a gas hob and oven on a 13amp plug witch are both plugged into a socket spurred off a Cooker Connection Unit. The cooker switch is above the connection unit.
What I propose to do is use a 30A junction box (can't find any 45a ones) and bury it in the plaster thus doing away with the switch. I will also spur off this to feed the extractor.

Is it a legal requirement to have an accessible switch for the cooker?
(My socket is under the worktop behind drawers - not really accesible easily)

If i do need an accessible switch I could put one in a different place to the existing one but would then need to use 2 30amp junction boxes burried in the plaster and some extra cable (probably just 2.4mm).

I can't really see the point in keeping the circuit 45amp as everything is on 13amp plugs anyway.

Any thoughts?

 

[ban-all-sheds]

These topics have all been covered extensively here - do some searches.

In summary

1) You may not use a 30A JB on a 45A circuit

2) You may not bury JBs in plaster

3) A socket spurred off a CCU is probably connected using a cable that is too small - have you checked it?

4) If you add another leg to this radial to supply the extractor, don't forget to use an FCU

5) Yes you must have an accessible isolation switch within 2m of the appliance

6) Your socket under a worktop behind drawers is not correctly located - it cannot serve as an accessible means of isolation, and it should have an isolation switch itself above the worktop which is accessible

7) You should not mix smaller cable sizes, e.g. 2.5mm (I assume that's what you meant) on a circuit wired with larger sizes without fusing down. You might no longer need a 45A MCB in the CU for what you'll be powering on the circuit, but a future occupant might.

 

[plugwash]

7) You should not mix smaller cable sizes, e.g. 2.5mm (I assume that's what you meant) on a circuit wired with larger sizes without fusing down. You might no longer need a 45A MCB in the CU for what you'll be powering on the circuit, but a future occupant might.

is this actually a reg or is it just considered good practice

 

[ban-all-sheds]

433-02-01

 

[plugwash]

any chance you could tell me exactly what that says?

 

[ban-all-sheds]

It's something like "protective devices shall be provided to break an overload current flowing in the circuit conductors before such a current could cause a temperature rise detrimental to insulation, joints, terminations, or the surroundings of the conductors. Circuits shall be so designed that a small overload of long duration is unlikely to occur".

 

[dingbat]

"protective devices shall be provided to break an overload current flowing in the circuit conductors before such a current could cause a temperature rise detrimental to insulation, joints, terminations, or the surroundings of the conductors. Circuits shall be so designed that a small overload of long duration is unlikely to occur".

That's actually 433-01-01, but 433-02-01 adds useful detail about nominal setting of protective devices.

(Sorry to be pedantic, but, hey, when in Rome...)

 

[plugwash]

ok thats now what i was asking

i was asknig if their was any regulation that prohibited stuff like

20A breaker--existing 10mm cable--socket--new 2.5mm cable--socket

 

[ban-all-sheds]

That's actually 433-01-01, but 433-02-01 adds useful detail about nominal setting of protective devices.

So it is.

I'll go back and edit my post of 21:16 and make you look a twit.....

 

[ban-all-sheds]

ok thats now what i was asking

i was asknig if their was any regulation that prohibited stuff like

20A breaker--existing 10mm cable--socket--new 2.5mm cable--socket

Ah - I see.

NAFAIK.

But best practice certainly comes into play, in case one day someone thinks "Oh look - 10mm cable, I'll put that on a 50A breaker and use it for my new cooker"

 

[ban-all-sheds]

433-02-01 adds useful detail about nominal setting of protective devices.

FYI, y'all - protective devices have 3 values of interest here:

In: its nominal rating (e.g. 32A)
I1: its non-fusing/non-tripping current. For a Type B MCB this is 1.13In
I2: its fusing/tripping current. For a Type B MCB this is 1.45In, and it must trip within 1 hour

So if we have a circuit whose design current (i.e. the current that it is expected to carry in normal use) is Ib, then this should not be greater than the rating of the protective device:

Ib <= In

If Iz is the current-carrying capacity of the cable for continuous service allowing for the way that it has been installed, then this must be no less than the rating of the device:

In <= Iz

Finally, 1.45 times the current carrying capacity Iz of the cable must be no less than I2, the current causing effective operation of the device:

1.45Iz >= I2

So Ib <= In <= Iz

and

I2 <= 1.45Iz.


Or if you prefer pictures:

Running an MCB at or near its limit for long periods will cause it to overheat, and eventually weaken. But you shouldn't do that - look at the diagram, Ib is lower than all the rest.

Wonder if I should post this in the For Reference section?

 

[dingbat]

I'll go back and edit my post of 21:16 and make you look a twit.....

Hey, I'm quite capable of doing that all by myself!!

 

[davey_duck_breath]

Thanks for your answers folks thats already a big help, I have a couple of further points to clarify though:

ok so the socket spured off the CCU is via 2.5mm cable - is this not ok given that there is a socket on the end? just like a normal spur? or a spur to an FCU?

if i cant bury a JB in the plaster maybe i could remove the existing 6/10mm cable from the wall back down to the CCU and extend from there. Whats the best way to join 45A cable? I Guess I could use another CCU as it will be hidden. (i need to extend up to the switch above the work top and then back down again)

And yes the supply up to the extractor will have an FCU on the end, is 2.5mm ok for this spur?

 

[davey_duck_breath]

this was a duplicate post. just deleted.

 

[ban-all-sheds]

ok so the socket spured off the CCU is via 2.5mm cable - is this not ok given that there is a socket on the end? just like a normal spur? or a spur to an FCU?

It isn't quite the same. Yes, there is the same argument that because there is a socket, the fuse in the plug will prevent the appliance from overloading the 2.5mm cable, but with a ring main, the In of the device (30/32A) is not a lot greater than the Iz of the cable (up to 27A, depending on how it's installed), and it's considered OK - i.e. a fault would be cleared before the cable was damaged. In your case you've got a 45A breaker on that circuit, which is a lot more than the capacity of the cable. If it's not against the regs it's a Bad Idea™.

if i cant bury a JB in the plaster maybe i could remove the existing 6/10mm cable from the wall back down to the CCU and extend from there. Whats the best way to join 45A cable? I Guess I could use another CCU as it will be hidden. (i need to extend up to the switch above the work top and then back down again)

The best way is not to have any joins at all, but if you must, then proper crimps are usually regarded as the best way, followed by a suitably rated JB. I must admit I can't visualise what you need to do - you currently have a cable coming from the CU to a cooker control unit (i.e. a switch, maybe with a socket as well) above the worktop, which then runs to a cooker outlet plate mounted beneath the worktop. What needs to change and why?

And yes the supply up to the extractor will have an FCU on the end, is 2.5mm ok for this spur?

Same argument as above regarding 2.5mm. Can you not spur off a socket circuit?

I'm surprised plugwash hasn't chipped in yet suggesting you put a mini CU in place of the CCU, to correctly and safely distribute the lower-rated circuits that you want.....