Cable size query

[terryjw]

Installing seperate oven and hob. I have 6mm2 cable from consumer unit to the cooker control unit, then will run 6mm2 cable from control unit to connector box. The mcb in the consumer unit is a Hager, MT140, B40 ( is that 40amp?). My query is as follows.

The installation guide for the double cooker says to use 4mm2 pvc cable. The oven is rated at 4.8Kw.

The installation guide for the electric hob says to use 2.5mm2 cable. The hob is rated at 6.0Kw.

So, can I substitute 6mm2 cable for the oven, and can I run both these cables from the same connector box?

Thanks

Terry

 

[securespark]

Do a seach on diversity for cookers, using 30% & 10A as keywords.

 

[terryjw]

OK, so if I apply diversity, I get:-

4800+6000 = 10800/240 = 45A so 10A +30% of remainder = 20.5A

Have I understood right?

So can I use 2.5mm2 cable for both units, providing the oven is protected by a fcu? Or stick to 6mm2 for the oven with fcu?


Terry

 

[Pensdown]

Or stick to 6mm2 for the oven with fcu?


Terry

That's a good way of doing it.

 

[terryjw]

Ok, thanks for that Pensdown,

So, can I wire the hob straight from the ccu, and do away with the connector box? Will a 6mm2 and a 2.5mm2 be ok to wire together at the ccu?

[ccu]---------2.5mm2---------[hob]
l
l------------6mm2..............[fcu]------------6mm2--------------[oven]

Terry

 

[Pensdown]

You're almost there. Use the Cooker CCU as it is now to feed the oven (in 6mm cable) and install a local spur fed from the live side of the cooker control unit for the hob.

 

[terryjw]

Right,

So its ccu with 6mm2 to oven.

install a local spur fed from the live side of the cooker control unit for the hob.

OK, what cable size here, and does it feed into fcu?, and how do I install the spur, seeing as 2 6mm cables will not go into the terminal.

Thanks agin, and sorry to keep asking, I want to make sure I get it right.

Terry

 

[bernardgreen]

[ccu]---------2.5mm2---------[hob]

Not good if the hob has to be isolated in an emergency.

[ccu]---------2.5mm2---------[fcu]---------[hob]

means you would not have to go to the consumer unit to shut of the power to the hob

 

[Pensdown]

Let's start again.

Installing seperate oven and hob. I have 6mm2 cable from consumer unit to the cooker control unit, then will run 6mm2 cable from control unit to connector box.

Just to confirm, as you already have a 6mm feed to your old cooker from a 40amp B type MCB you can use that to feed you new oven via the cooker control unit and outlet wall plate. You could use 4mm cable from the cooker control unit to the cooker but if it's already wired in 6mm I would use that.

The installation guide for the electric hob says to use 2.5mm2 cable. The hob is rated at 6.0Kw.

Even with diversity, I would not connect the hob to a 13amp fused connection unit, I would use a 20amp double pole switch. You can feed this from either the live the load side of the cooker control unit in 2.5mm cable as the load is fixed and after diversity it's below the rating of the cable. If you feed it from the load side your cooker control unit will act as an isolator for both oven and hob.

 

[terryjw]

OK, think I might understand.

[consumer unit].....6mm2......[cooker switch]....[plate]....6mm2......[oven]
l
l
2.5mm2
l
[20A pole switch]...2.5mm2....[hob]

Is that it?

If so, do I bring the two cables from the cooker switch or put a suitable junction box between the cooker switch and the outlet plate?

Thanks again for your patience and taking time to answer.

Terry

 

[Pensdown]

You feed the 20amp double switch with 2.5mm cable directly from the L & N load terminals in the cooker control unit. You don't need any junction boxes.

Please make sure you fully understand what you're doing before you start because I'm getting a little twitchy this end.

 

[securespark]

I don't understand why you are using a 20A switch for a mid-twenties load?

I know diversity can be applied to the whole circuit, but you can't use diversity to "lower" the rating of switching devices.

The switch should be suited to the flc of the load it switches.

 

[terryjw]

Please make sure you fully understand what you're doing before you start because I'm getting a little twitchy this end.

Do not worry Pensdown, I am just double checking here. Just that I have read so many different solutions to this type of query. It makes sense what you've written.

I don't understand why you are using a 20A switch for a mid-twenties load?

and

The switch should be suited to the flc of the load it switches.

So, I use a higher switch?


Terry

 

[Pensdown]

IIRC diversity is used to calculate the design current of a circuit. Once you have the design current don't you then size the cable, switch gear and protective device accordingly?