6mm cooker cable

[Iona222]

If this protected by a 40amp rcbo is this unacceptable.

Does the cooker kw not effect rcbo size?

 

[pc1966]

Cable overload protection also depends on the installation method. Here, probably on dodgy territory.

Cable fault protection comes down to the circuit characteristics and if that meets the adiabatic limit for a given cable. Here, almost certainly.

The cooker rating does come in to it. If the absolute max of the cooker (plus any 13A socket) is less than, say, 32A limit for your typical 6mm in the majority of install methods, then all you need is fault protection. But more generally cookers will exceed that max value and you get away with it due to diversity - that typical usage never has all on max for any length of time, but you cannot use diversity for overload protection (only for circuit sizing) as it is possible to overload, so your RCBO must protect against that scenario.

TL;DR 40A is on the high side unless cable is known to be freely ventilated on whole length. Check OSG for methods and corresponding ratings.

 

[Iona222]

Cable overload protection also depends on the installation method. Here, probably on dodgy territory.

Cable fault protection comes down to the circuit characteristics and if that meets the adiabatic limit for a given cable. Here, almost certainly.

The cooker rating does come in to it. If the absolute max of the cooker (plus any 13A socket) is less than, say, 32A limit for your typical 6mm in the majority of install methods, then all you need is fault protection. But more generally cookers will exceed that max value and you get away with it due to diversity - that typical usage never has all on max for any length of time, but you cannot use diversity for overload protection (only for circuit sizing) as it is possible to overload, so your RCBO must protect against that scenario.

TL;DR 40A is on the high side unless cable is known to be freely ventilated on whole length. Check OSG for methods and corresponding ratings.

Do you not take it fact the size of the cooker and if there's a socket on the cooker point as well mate. Im just trying to learn this.

So your saying if after calculation of the amps added 10amp for the socket if its over 32amp? Then what?

 

[telectrix]

cooker diversity says:

10A + 30% of remainder (+5A if socket integral).

e.g . 10kW total rating = 42A. so

10A + (30% of 32A) = 10 + 9.6 = 19.6A. add the 5A for socket if needed. = 24.6A.

32A MCB/RCBO and 6.0mm is good unless loads of insulation.

fault protection is a differemt kettle of kippers.

 

[Iona222]

cooker diversity says:

10A + 30% of remainder (+5A if socket integral).

e.g . 10kW total rating = 42A. so

10A + (30% of 32A) = 10 + 9.6 = 19.6A. add the 5A for socket if needed. = 24.6A.

32A MCB/RCBO and 6.0mm is good unless loads of insulation.

fault protection is a differemt kettle of kippers.

So if there was no socket you would stick that on a 20amp?

 

[pc1966]

So if there was no socket you would stick that on a 20amp?

Generally no. But you could if pushed and there was some bizarre reason to justify do so.

Usually the use of diversity load calculations comes in when you get big cookers/hobs with lots of separate heating elements. For diversity to be "reasonable" you should have several separate elements so it is typically the case they are not on all at once, and once running not at full power so they are all cycling on their thermostats.

For the majority of electrical design work you would stick to the "standard circuits" outlined in the OSG Table 7.1 as it saves you the effort of doing the detailed calculations, and risk of getting them wrong. So for most common cases like lights, sockets, or modest "fixed loads" you go with the common standard option unless there is a special case.

• So lights are usually 6A B-curve MCB on 1mm.
• Sockets are usually a RFC with 32A B-curve MCB and 2.5mm.
• A few sockets, and localised, then the RFC does not make sense so you use a radial either 20A/2.5mm or 32A/4mm, in most cases.
• Simple fixed loads, always a radial, table gives you sizes to choose based on load current.
• For a cooker/hob (so "fixed" total but independent elements) then typically you do your diversity calcs for domestic cookers and if less than 32A then you go with 32A/6mm in most cases.

The OSG table has the max lengths and acceptable installation methods in all cases. Generally I would always use the column 8 max length (unless I knew Zs was higher or had exceptional justification for using the "RCD 30mA" columns).

You are quite right to question and think!

While the OSG "standard circuit" will cover you for the majority of cases, you will encounter a special case from time to time and then you need to go back to the theory and do your calculations fully to see what combination will work. But going to a smaller than typical circuit is unusual, and likely to present problems in the future if a new cooker is fitted and then, no matter what the manufacturers say, there will be a lot of cases connected up and problem following. Worst case is some numpty changing the MCB without checking the cable size!

So basically this is the long-winded version of what @telectrix said!

 

[pc1966]

The OSG Appendix A has the various diversity guidance to use for different situations (e.g. commercial kitchen) but it is far from obvious just how.

The IET's "Electrical Installation Design Guide" has some worked examples, so if you can afford it I would suggest getting a copy as it helps illustrate a lot of these examples.

 

[Iona222]

The OSG Appendix A has the various diversity guidance to use for different situations (e.g. commercial kitchen) but it is far from obvious just how.

The IET's "Electrical Installation Design Guide" has some worked examples, so if you can afford it I would suggest getting a copy as it helps illustrate a lot of these examples.

Thanks mate, everything is easy enough just cooker and showers, but ill get this book tomorrow may come in handy.

 

[DPG]

Thanks mate, everything is easy enough just cooker and showers, but ill get this book tomorrow may come in handy.

Didn't you say you were doing EICRs?

 

[Iona222]

Didn't you say you were doing EICRs?

I've got the 2391-52 18th edition. I'm nearly completed level 3. I'm just wanting to become a good electrician that does things by the book mate.

 

[DPG]

I've got the 2391-52 18th edition. I'm nearly completed level 3. I'm just wanting to become a good electrician that does things by the book mate.

Sorry, I might have got it wrong. I thought you were carrying out condition reports.

 

[Iona222]

Sorry, I might have got it wrong. I thought you were carrying out condition reports.

No need to panic yourself mate.