Hob, Oven and Dishwasher Question

[bernithebiker]

Hi - I have a cable which is 2.5mm, with 3 +ves and 1 -ve (and 1 earth).

I need to connect up a hob, (6.3kW) an oven (3.3kW) and a dishwasher (unknown but estimate 2kW).

At the main board there are two 20amp circuits.

What I am thinking of doing is to take 1 of the +ve cables and make it into a -ve. In that way I will have 2 x 2.5mm pairs. I will connect the hob to 1 pair and the oven and dishwasher to the other.

I fully realise that I should have 4 or 6mm cable for the hob, but it would be extremely difficult to replace the existing cable. I figure that as long as only 3 of the 4 rings are drawing power at any one time, my max power will be around 4 to 5kW. And if all 4 should draw then the 20A fuse will blow, before any dangerous wire heating could occur.

Am I right (ish?!) Any comments? Thanks!

 

[bernithebiker]

...sorry, another idea came to me;

to combine the two 2.5mm pairs together and put the hob and the dishwasher, (8kWish) onto them.

In that way I have an 8kW load into what is effectively a 5mm cable. This is split when it arrives at the main board into 2 x 20A circuits. So I guess I have a kind of 40A protection? (Seeing as a total load of 40A would be split in 2).

Then, as for the oven (3.3kW), I will just have to chase a cable down from an existing socket above. The oven is supplied with a plug on it, so I am assuming it could be just plugged into any old socket. If I can make sure any kettle or toaster, etc is NOT plugged into the other socket on this circuit (protected by a 16A fuse at the main board) I should be OK.

I have a Karcher which takes 3000W and that can be plugged into any socket, so why not the oven, right?!

Help!

 

[Steve]

you are right about the hob cable being protected, but there is a risk someone might replace the MCB at a later date, not realising that the circuit is no longer protected. Out of interest, this is the only principle you have got right.

DO NOT combine the 2 cables in the way you explain.

3KW is the MAXIMUM for a 13A plug. 3.3kw is an awkward load, and i recommend you chop off the oven's plug and wire into a 13A FCU, since it came with a 13A plug.

And it is not ok to design circuits based on "making sure not other appliances aare plugged in" etc. Sounds like your kitchen need rewiring.

Hi - I have a cable which is 2.5mm, with 3 +ves and 1 -ve (and 1 earth).

this, and the suggestions you have put forward, do nothing for my confidence in your ability to do this work, and I STRONGLY RECOMMEND YOU GET SOMEONE IN WHO KNOWS WHAT THEY ARE DOING, FOR YOUR OWN SAFETY, AND THAT OF YOUR FAMILY.

Do you know about part p?

 

[bernithebiker]

Thanks for the reply which i really appreciate.

However, could you explain why I shouldn't put the two pairs together? As I see it, this gets me a 5mm wire on 2 x 20A circuits. (My main board is oldish and has fuses, not modern circuit breakers).

In fact, in this way I could put the oven (3.3kW) and the hob (6.3kW) onto these two pairs and I should be OK, both for cable size and fuse size.

Then I could put the dishwasher (2kW) onto one of the plug sockets.

(Thanks for the concern for my stupidity/safety - I will probably end up getting an electrician in anyway, but nice to understand what's going on)!!

 

[TrueBlue]

You could in theory twist the cables together and get 5mm/2, it's not really the done thing but it's not 'incorrect'. You cannot then split the cables into 2 mcbs. It's either 1 cable or 2 cables, not both. You fail to mention where the cpc is in all this planning, which is a bit concerning.

 

[racket]

Do you know about part p?

Here we go........another can of worms.

 

[Steve]

Do you know about part p?

Here we go........another can of worms.

racket, until the admin of this forum gets their act together and puts ban-all-shed's part p explanation at the top of the board with the stickies, this is the best way of letting DIYers - who visit the site and ask questions - know about part p. Not many people still know about this, and it can only be a good thing that they find out. I didn't intend a debate to start, I was merely informing bernithebiker of the legalities of this work.

berni, it is not acceptable to wire a circuit like that. It is not properly protected and would require stupidly high currents to flow before one of the fuses blew. It also does not meet the regs for isolation - requiring 2 switches to be broken / fuses to be pulled to isolate the circuit. regs require only one point of isolation for each circuit (i think). 5mm² is not a recognised cable size.

 

[bernithebiker]

er, sorry, but what is a cpc?

I see your point about having to pull two fuses/switches to break the supply, which I agree is less than ideal.

Although with 2 x 20A fuses (one on each circuit) I assume a load of >40A would pop one or both of the fuses.

Although 5mm is not a recognised size, is that a problem per se? (maybe when I come to resell the house?)

I guess I should be OK putting the dishwasher onto a spur of the socket as it is only 2kW. However if someone puts a kettle and toaster on the same socket I may pop the 16A fuse......(I believe this socket to be radial).

I realise that all this is less than perfect, but my only other solution is to run a new 6mm cable out 3 walls and through the garden to a new circuit breaker on the MCB.

Any help on this temporary solution is greatly appreciated.

Ta, Berni.

 

[TrueBlue]

Sorry, the cpc is the earth.

re the fuses. If you want a 40A fuse, it's just as easy to put in a 40A fuse surely. What your talking about with the 2 20A fuses will not 'add' up to 40A protection. Take my word for it, it just doesn't work like that.

 

[bernithebiker]

ok, don't worry, I do have an earth!! All well connected.

I would have thoguht the current would split evenly down the 2 circuits, no?

 

[TrueBlue]

ok, don't worry, I do have an earth!! All well connected.

I would have thoguht the current would split evenly down the 2 circuits, no?

If you split the cable into 2 fuses and one popped, the circuit would remain energised by 'diverting' through the other fuse. Unless that one popped too. Remember if you are using a 5mm/2 cable for your live, then your earth will need to be at least 3mm/2, where as you will only have a 1.5mm/2 one.

 

[bernithebiker]

If one 20A fuse pops, (due to say a 40A / 2 = 20A current) then all the load (40A) suddenly goes through the remaining 20A fuse so that would instantly pop too......(this is my old A level physics coming out here....!!)

Good point about the earth - I believe it is also 2.5mm, not 1.5, but I will check tomorrow (2nd home).

I could also, as you say, just use one 40A fuse, but I have yet to find a 40 or even a 30A that fits the holder...but I will continue to look.

Thanks very much for all your help - I will try my temporary solution whilst I book in an electrician to fit a 6mm cable with 30(?)A fuse/trip.

Watch this space to see if it works!

 

[Steve]

Fuses dont blow instantly at their rating. This is the problem here. Your 20A fuse will not blow at 20A straight away. It could probably pass 25A for an hour or 2, and actually blow instantly at something insane like 80A. So the fault current needed to blow both fuses (almost) instantly (if my figures are right, plucked out of thin air), is 160A, and by the time this much current has passed, damage may have occured to the installation. It probably will not meet disconnection times, and the CPC might melt before the fuses blow.

 

[bernithebiker]

OK I take your point. I was at the house today, and in fact it seems that the wire in question is actually more like a 4mm as it is definitely one size larger than some 2.5mm that I have just bought.

So adding the two pairs together (to avoid cable overheating issues) may actually be overkill.

How about this - use one pair for the hob (i.e. 4mm +ve, -ve and 20A fuse), one pair for the oven and dishwasher (i.e. 4mm on 20A fuse).

6kW for the hob, 5ish for the oven and dishwasher. If what you say re. the fuses is correct, then even if the hob pulls full power (about 24A) I should be OK, fusewise, as such a load would only be shortlived. Likewise for the oven and dishwasher (about 20A peak load).

In this way, I should be almost legal! I also intend to put individual switched fuses of either 10 or 16A inline with the oven and dishwasher for extra protection.

Thoughts?

 

[TrueBlue]

You're still not acounting for your earthing.