what size cable is best?

[craig20102010]

So im looking to wire in a microwave oven and a actual oven as well. im going to put eaach of these on their own circuit breaker for ease of seperation and isolation incase of any problems.

They are both rated at 16amps which is really low i thought but i have checked and its correct if anyone wants to double check my results the oven im looking to install is a Electrolux model no. EOC5651CAX 3.5kw which works out too 15amps

The distance they will be from the main fuse box will be around 5 meters in a straight line. roughly about 7 meters given corners etc its a fairly large kitchen.

I will be having a new fuse box fitted with MCBs and RCDs installed with plenty of capacity.

Id just like to know what size cable you would reccommend for each appliance?

Iv thought about a 20amp cable but given the distance and being that some of the cable will be behind a wall i was thinking maybe do i need a 25amp cable with a 16amp MCB or 20amp MCB? or does it need to be a 25amp possibly a 32amp with a 16 or 20amp mcb. The reason im thinking a 20amp MCB is because 16 is right on the limit of what its rated at or is that whats its suggesting is the best fuse size for that unit?

Thanks in advanced.

Craig

 

[ban-all-sheds]

• http://www.google.co.uk/search?q=cable+installation+methods+17th+edition
  
  
• //www.diynot.com/wiki/Electrics:MCBtoCable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• http://web.archive.org/web/20070327014113/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  
• http://www.batt.co.uk/upload/files/4d5.pdf
  
  The references above which are in italics are out of date. The principles are still valid, and you can absolutely learn valid things from them, but you will need to refer to the latest edition of the Wiring Regulations to get the correct data.
  

Not trying to be funny, but YOU are taking on the role of designer, so YOU have to be able to successfully show your local Building Control department that YOU can comply with Part P when you apply for Building Regulations approval, so YOU have to actually be able to do, and actually do, the design work, not us, as YOU have to be able to sign this:

I being the person responsible for the design of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the design hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to 2011 except for the departures, if any, detailed as follows:

 

[deadshort]

Hi, 2.5mm


DS

 

[craig20102010]

Erm yeh thanks for that. Im fully aware.

I was planning on getting it signed off by a qualified electrician who has part P after its finshed which will prove its been installed correctly and more importantly, safe.

Im trying to save my self money as you can probably understand and im more then competent at fitting wiring with the correct guidance. Although iv met some 'skilled tradesmen' who cant do what there paid to do.

I dont trust most tradesmen to work on anything i own as they seem to bodge things as they get complacent so i want to make sure its done right in the first place. They may have the knowledge, which is what im asking for now but when its comes down to fitting it, the end result is little to be desired. Dont get me wrong not all tradesmen are like this but its these few that give the good ones a bad name.

Im an Engineer by trade electrical and mechanical. Im confident of the correct cable to use but i would just like things confirming by people who do it every day. If i feel something is out of my cabability i would leave it for someone who is trained.

Any advice is good advice so thank you for yours.

Ill have a read up on the links youve posted

 

[craig20102010]

Hi, 2.5mm


DS

Thanks bud. i thought this would be capable and within the range i would require.

I just didnt want to fit anything to thick as wider cable is harder to work with. no point in overkilling it.

I have thought about installing 4mm cable just incase i decide to fit a larger capacity oven, which may draw a larger current. So i guess as long as i know 2.5mm is the minimum i can install ill be fine.

So would a 20amp MCB be the best fuse for the job since that cable is rated at 23amps? or 16amp since the oven only uses a maximum of 15amps.

This isnt too much off a big issue since when the fuse box is fitted this can be changed at any time between 16 and 20

 

[deadshort]

Erm, sorry i stupidly read the title of the post and assumed that was a question.


One of the bodge it tradesmen will be along in a second to interpret your actual enquiry, sir.


Regards,

DS

 

[craig20102010]

Erm, sorry i stupidly read the title of the post and assumed that was a question.


One of the bodge it tradesmen will be along in a second to interpret your actual enquiry, sir.


Regards,

DS

Cheers. ill wait on then.

 

[AndyPRK]

As you say you maybe better off fitting 4mm or 6mm for the oven, in case in the future you want to add an electric hob.

The cable rating depends on length and if its fitted in insulation (i.e doesn't allow the cable to dissipate heat)

But yes a 20A for 2.5mm will be fine, unless the oven instructions say otherwise.

 

[craig20102010]

As you say you maybe better off fitting 4mm or 6mm for the oven, in case in the future you want to add an electric hob.

The cable rating depends on length and if its fitted in insulation (i.e doesn't allow the cable to dissipate heat)

But yes a 20A for 2.5mm will be fine, unless the oven instructions say otherwise.

well its always going to be an oven as theres no room for a hob and the hob is at the other end of the kitchen where the extractor is and a large 6mm cable. So literally the most this could ever be would be a double oven.

At the minute its just a single oven and ill have a second cable for a microwave which also has a reccommended rating of 16amp.

Cable will be routed behind a plasterboard wall and then under the floor boards between ground and first floor. So there should be plenty of ventilation for the cable to stay cool.

I may increase the oven one to a 4mm just to be safe and due to the distance its got to travel and incase of future upgrades to a double oven.

 

[ericmark]

Iv thought about a 20amp cable but given the distance and being that some of the cable will be behind a wall i was thinking maybe do i need a 25amp cable with a 16amp MCB or 20amp MCB? or does it need to be a 25amp possibly a 32amp with a 16 or 20amp mcb. The reason im thinking a 20amp MCB is because 16 is right on the limit of what its rated at or is that whats its suggesting is the best fuse size for that unit?

That's the big error. Only flex which should be visible along it's length has a fixed amps rating. One cable can have a range of 13.5A to 27A according to how installed.

In the main we install cables for cookers that can take 32A which will in the main serve most cooking appliances. However with any FIXED appliance the manufacturer can and does state the minimum and maximum supply clearly the minimum is so it will not trip and the maximum is to do with cables inside the appliance if there is a fault you want supply automatic removed rather than appliance become live or go on fire.

There is a big question when an appliance arrives with a 13A plug can you cut it off. The manufacturer has fused the appliance and cutting off plug means you have removed that protection. It is OK if you replace the fuse using for example a FCU but not if you omit the fuse.

So in the main 6mm² cable on a B32 MCB is used but you have to verify that will be OK with your installation with your appliances. I would be surprised to find a microwave oven that required over 13A so I would suggest you get out the paper works and read it all again.

 

[ericmark]

Cable will be routed behind a plasterboard wall and then under the floor boards between ground and first floor.

That does not sound right. Just can't see how cable can be within the safe zones when going from plasterboard wall to under the floor? Not saying it can't but you do need to look at the zones and see it does comply.

 

[craig20102010]

So in the main 6mm² cable on a B32 MCB is used but you have to verify that will be OK with your installation with your appliances. I would be surprised to find a microwave oven that required over 13A so I would suggest you get out the paper works and read it all again.

your right about the microwave. The reason i got that mixed up, I was originally looking at a Combination oven which did require alot more power then a conventional microwave. As i was trying to find ways to have a second oven as well as a microwave without buying a double oven as i want the Pyrolytic cleaning and they dont appear to do a double oven with that feature.

So this rules out needing a larger cable for the microwave anyway.

That does not sound right. Just can't see how cable can be within the safe zones when going from plasterboard wall to under the floor? Not saying it can't but you do need to look at the zones and see it does comply..

Surely this is just the same as your average plug socket? All my plugs sockets are behind a sheet of plasterboard, which then trace up the wall and then goes under the floor boards in the bedrooms as they link the rest of the sockets together on the ring.


If this is the case anyway what size cable would you recommend? or would you go straight away for the 6mm² cable to power the oven?

Reading through the instructions it says the Minimum cable size to use would be 2.5mm² and the minimum size fuse is 15amp and the max is 20amp so i guess thats my answer for the fuse size. Can it be a 16amp breaker or is this too close to the 15amp fuse rating and will it damage the MCB. This is what i read on one of them links that was posted earlier.

 

[deadshort]

Hi, 2.5mm


DS[/quote

 

[JohnW2]

That does not sound right. Just can't see how cable can be within the safe zones when going from plasterboard wall to under the floor? Not saying it can't but you do need to look at the zones and see it does comply..

Surely this is just the same as your average plug socket? All my plugs sockets are behind a sheet of plasterboard, which then trace up the wall and then goes under the floor boards in the bedrooms as they link the rest of the sockets together on the ring.

The point is that a sockets circuit, not surprisingly, has several (visible) sockets (and sometimes FCUs etc.), each of which creates horizontal and vertical 'safe zones in which 'concealed' cables (e.g. behind plasterboard) are allowed to travel. With circuits for cooking appliances, there are not usually going to be be any visible accessories (like sockets etc.) between the CU and the vicinity of the appliance - which means that it can be difficult to find safe zones for any of the cable which is 'in a wall', since very close to corners or ceilings are about the only 'safe zones' you are likely to have. An electrician would obviously understand all this. Similarly with lighting circuits - wires 'in walls' are then most commonly travelling to/from wall switches, and they can therefore travel in the 'safe zones' created by the switches.

Kind Regards, John

 

[craig20102010]

but surely having a switch at the oven point would be the same as having a socket on the wall? Just the socket is now an isolation switch for the oven. Its no different apart from a higher current but still protected by a MCB.

You know there's an Isolation switch there so therefore there must be a cable travelling upwards (which is what i plan to do to keep it the same as the sockets)

This too would show the direction of travel of the cable or that there indeed was a cable in the wall leading to this oven switch.

I can see what your saying in the fact that when an oven is fitted the wiring going to the oven is concealed also behind the wall and there's no way of telling where abouts that cable is within the wall but only maybe if you dont have an isolation switch, which im sure your meant to have.

I plan on having tall tower units with an oven in the middle of one. That whole wall will be covered and concealed with tower units. Ill also have a oven switch in side one of the other tower units so that i am able to isolate the circuit. Your still going to have a cable coming out of the wall, which will be coming out of a junction box. For me the Junction box on the back of the wall is a dead give away that there's a cable there and it makes sense to keep it travelling in the same direction as the rest of the sockets do, which is vertical to the socket/junction box/oven switch on the wall.

Im sure when they build new houses and they have oven switches on the wall, the cable runs behind the plasterboard exactly the same as a 13amp socket.

Maybe im missing the point sorry, but i can almost guarantee if i was to employ an electrician to do this job he would do exactly the same as what i plan on doing, Which is the logical thing. In fact i asked an electrician what route he would take to wire in the oven in my kitchen and he said straight up the wall and across the landing back into the CU. So i cant see how this isnt going to be within a "safe zone" Its going straight up a wall under the floor then it doesnt resurface from the flooring untill its above the cupboard where the CU lies. Same as an electric shower. The only difference with a shower, is it goes up the plasterboard wall, where there will be no sockets or plugs or any signs, then into the loft and back down and into the shower. So no different again. Id say the oven is alot safer to be able to tell where a cable lies then what an electric shower would be.


Thanks for all of your advice anyway.