Cooker circuit with junction box

[Mug-o-Latte]

I'm moving a 6mm cable for a cooker circuit that is already embedded in a wall, and don't want to have to run a new cable and chase out the wall again, but the existing cable won't reach the junction box.

Is there a law, or any common sense reason (or probably the other way round as I'm more concerned with the common sense reasons!), why there shouldn't be a junction box between the cooker switch and the consumer unit MCB? This would mean I just need to add an extra length of cable and a junction box (suitably rated of course).

 

[felix]

The rules say you must run an unbroken length of cable. Common sense says that's rubbish but --

If you don't make your connections properly in the junction box you will get hot spots. What common sense might not tell you is that the resistance of copper rises with temperature.

1) You get a hot spot.
2) Its resistance goes up.
3) The current is largely unchanged because it depends mostly on the load.
4) The hot spot drops more voltage.
5) It gets hotter.

This effect can lead to thermal runaway. I've seen it happen where copper bars carrying thousands of amps were bolted together but the bolts were loose or the surfaces weren't clean. I suspect it was for this reason that the rules were written.

Rules are for the guidance of wise men and the blind obedience of fools - and the total disregard of senior management. Which one are you?

 

[HandyJon]

Is there a law, or any common sense reason (or probably the other way round as I'm more concerned with the common sense reasons!), why there shouldn't be a junction box between the cooker switch and the consumer unit MCB? This would mean I just need to add an extra length of cable and a junction box (suitably rated of course).

If you do use a junction box, it must accessible so you can't hide it under plaster. You could make the existing cooker switch a blank with the junction inside it. If you really must remove the existing junction box, then the wires must be joined together with crimps. Either way getting an electrician out to do the work won't cost a bomb.

 

[Mug-o-Latte]

Soooo .... with a 6mm cable, and a 5.1Kw cooker, that says I have less AMPs through this circuit than my ring (if you add kettle, toaster, etc), BUT, I break the rules!



Where can I read this 'rule' that I am breaking?

And why do you get junction boxes rated at more than 20 AMP's?

And why do pigs fly?

 

[Mug-o-Latte]

If you do use a junction box, it must accessible so you can't hide it under plaster. You could make the existing cooker switch a blank with the junction inside it. If you really must remove the existing junction box, then the wires must be joined together with crimps. Either way getting an electrician out to do the work won't cost a bomb.

The junction box will be in the roof void (bungalow). The cooker switch is already in the wall, with 6mm going up into the roof void. Due to the need to move some wires around, I was planning a junction box with one 6mm wire in from the cooker switch, and another 6mm wire out to the CU.

 

[plugwash]

The rules say you must run an unbroken length of cable.

can you quote a reg saying that?

i sure as hell have never heared anyone mention it before.

 

[felix]

Sorry plugwash. I got it word of mouth from three independent sources. Admittedly one of these was a salesman in an electrical shop with a vested interest in selling cable. Since you're the second person to say it ain't true, I guess that cancels out the other two, both of whom claimed to be electricians. That suits me.

 

[towman]

Just one point, if you put in a switch does that not mean you got two lengths of cable anyway? And why would connections in a switch be any different to those in a junction box? And I can't believe the "Regs" say you can't have two switches in a cable?

 

[mapj1]

I am unaware of any regulation, certainly in my copy, that says a JB in 6mm is not allowed. You may have trouble finding a physically large enough junction box, and making it easily accessible, but that's another matter.
regards M.

And if you ever want to join really high current cables, and are paranoid about good contacts, you could do worse than mount a Henley block in a suitable box...
you then have a 100A junction box - and the terminals in Henleys are made for serious currents, and are double screwed.

 

[steve190]

So long as the Jb is accessible, if required, for an inspectio & test and is rated to carry the current then no reason not to use one.

 

[felix]

Thanks for all the advice. As you might have guessed, I'm not your average sparky. I'm an electronics engineer who spends his days around multi-megawatt magnetrons. I know all the circuit theory but wiring regulations are a weak spot. I really like this forum.