Elec.. Guide.. Build.. Regs Table 4.1.2a - Is this a cooker mistake?

[CDB]

Hi,

I hope my abbreviated title caught someone's attention and makes some sort of sense.

On page 58 of the Electricians Guide to the Building Regulations there are some standard circuits listed. In particular I'm looking at the cooker entry on page 59 and have a couple of questions:

1) Why is it permitted to have a length of 50m when it includes a socket outlet when page 64 (4.4) specifies that it must be on an RCD if it has a socket outlet? Should that not be NP like the other tables?

2) Why does this circuit specify 6mm when its possible to have a 4mm radial circuit rated a 32A for socket outlets. Is this where diversity calculations make things that are otherwise impossible possible?

Thanks for your time and I look forward to reading the answers.

Chris

 

[SparkyChick]

Hi Chris,

1) Length is normally governed by maximum allowed voltage drop which on a power circuit is 5% (so for U0=230v, thats about 11.5v maximum permissible drop). As for the RCD, if the cable is installed in a manner such that it is afforded mechanical protection in walls or partitions (from damage by nails, screws etc.) that includes earthing (i.e. steel conduit) or is installed at a depth of 50mm or more, RCD protection can be omitted. However, since socket outlets require RCD protection, if the cooker control unit has a socket, then the circuit must be provided with RCD protection to comply with the regulations.

2) I'm not going to comment too much on this at the moment, but personally I wouldn't put 4mm on the load side of a 32A breaker except in the case of a ring final circuit where I was concerned about voltage drop. It's maximum CCC according to table 4D5 is 37A and that's clipped direct. I don't believe that is enough headroom in the cable for fault conditions.

Hope that helps.

 

[davesparks]

It's the great British tradition of hanging on to things from the past. The imperial equivalent of 6mm was always used for cooler circuits and so it has stuck that 6mm is used for cooler circuits regardless of whether 4mm is adequate or not.

 

[davesparks]

2) I'm not going to comment too much on this at the moment, but personally I wouldn't put 4mm on the load side of a 32A breaker except in the case of a ring final circuit where I was concerned about voltage drop. It's maximum CCC according to table 4D5 is 37A and that's clipped direct. I don't believe that is enough headroom in the cable for fault conditions.

Hope that helps.

What headroom for fault conditions does it need? What calculation do you use to work it out?
And what do you know that the peopke who wrote the ccc tables don't know?

 

[CDB]

1) Length is normally governed by maximum allowed voltage drop which on a power circuit is 5% (so for U0=230v, thats about 11.5v maximum permissible drop). As for the RCD, if the cable is installed in a manner such that it is afforded mechanical protection in walls or partitions (from damage by nails, screws etc.) that includes earthing (i.e. steel conduit) or is installed at a depth of 50mm or more, RCD protection can be omitted. However, since socket outlets require RCD protection, if the cooker control unit has a socket, then the circuit must be provided with RCD protection to comply with the regulations.

Attempt 2 - last one got lost / tab crashed.

Ok, I get all the RCD vs length stuff but why does table 4.1.2.a suggest that a cooker, hob and socket circuit is OK without an RCD as long as its under 50m when both tables b and c and the section on cookers say not. I say this is a mistake in the book. Am I right?

Still mulling over 4mm vs 6mm

 

[Ian1981]

Cooker circuits can be calculated using diversity.
First 10 amps plus 30% of the reminder and add 5 amps if ccu incorporates a socket outlet.
With this method of calculation you'll often find that 6mm is oversized for what you require and that 4mm is more than adequate.
Some oven circuits you can even get away with a 2.5mm on a 20amp protective device.
It's always worth noting that you may also want to install a bigger cooker in the future so may want to install a bigger sized cable like 6mm
Usually cookers upto 15kw are adequate on a 32 amp protective device with diversity applied.

 

[Ian1981]

Attempt 2 - last one got lost / tab crashed.

Ok, I get all the RCD vs length stuff but why does table 4.1.2.a suggest that a cooker, hob and socket circuit is OK without an RCD as long as its under 50m when both tables b and c and the section on cookers say not. I say this is a mistake in the book. Am I right?

Still mulling over 4mm vs 6mm

I don't have the building regs book however max lengths of cables will depend upon the max voltage drop allowed and also if the max zs for the circuit is too high due to excessive length of the conductors
Volt drop is Ib x m/v/a/m x length/ 1000
Or in the case of a radial with 5% max volt drop
11.5 x 1000/ Ib x m/v/a/m

 

[Wilko]

Attempt 2 - last one got lost / tab crashed.

Ok, I get all the RCD vs length stuff but why does table 4.1.2.a suggest that a cooker, hob and socket circuit is OK without an RCD as long as its under 50m when both tables b and c and the section on cookers say not. I say this is a mistake in the book. Am I right?

Hi - I agree, should be NP without an RCD

 

[SparkyChick]

Now I've had time to read the table (got interrupted when writing my original reply), I agree, without an RCD it should be NP in the table because they state it has a socket outlet.

What headroom for fault conditions does it need? What calculation do you use to work it out?
And what do you know that the peopke who wrote the ccc tables don't know?

I used the basic Ib (load) should be less than or equal to In (OCPD rating) should be less than or equal to Iz (cable rating). So, given In is stated potentially as being 32A, Iz needs to be greater than 32A. For 4mm the only installation method for which that condition is met is clipped direct, for the others it ranges between 17.5A and 27A.

As for the question at the end if you look carefully, that section does include the words "but personally" as in my personal choice. I wouldn't even begin to think I have anywhere near the level of knowledge the people who produce those tables do, nor do I believe I proclaimed such a thing.

I looked at table 4D5 saw the current ratings for the various installation methods and the majority of them wouldn't comply with the rules in section 433.1.1 (the same rules that yield the basic Ib<=In<=Iz I used). Having then read the tables in the electricians guide to the building regs, it does state it's a minimum CSA requirement, which if clipped direct 4mm could satisfy, but any other installation method it would not based purely on the aforementioned rules. Obviously, that isn't even beginning to factor in cable resistance and voltage drop calculations, which could have an impact and force the use of a larger cable in order to meet minimum Zs and maximum voltage drop requirements. If this was a fixed load, I might be more ameanable to using 4mm, but it's not... the question pertains to a radial power circuit which could have any number of sockets or outlets on it and consequently who knows what connected to it.

Anyhow, regardless of anything else, if I want to elect to use a larger cable, that's my choice as the designer as I'm the one who has to be happy to stick their name on the certificate, not the people who write the CCC tables. If this is wrong, then why not provide a worked example to clarify so that others may learn from my error and so avoid scorn in the future?

 

[davesparks]

Now I've had time to read the table (got interrupted when writing my original reply), I agree, without an RCD it should be NP in the table because they state it has a socket outlet.



I used the basic Ib (load) should be less than or equal to In (OCPD rating) should be less than or equal to Iz (cable rating). So, given In is stated potentially as being 32A, Iz needs to be greater than 32A. For 4mm the only installation method for which that condition is met is clipped direct, for the others it ranges between 17.5A and 27A.

As for the question at the end if you look carefully, that section does include the words "but personally" as in my personal choice. I wouldn't even begin to think I have anywhere near the level of knowledge the people who produce those tables do, nor do I believe I proclaimed such a thing.

I looked at table 4D5 saw the current ratings for the various installation methods and the majority of them wouldn't comply with the rules in section 433.1.1 (the same rules that yield the basic Ib<=In<=Iz I used). Having then read the tables in the electricians guide to the building regs, it does state it's a minimum CSA requirement, which if clipped direct 4mm could satisfy, but any other installation method it would not based purely on the aforementioned rules. Obviously, that isn't even beginning to factor in cable resistance and voltage drop calculations, which could have an impact and force the use of a larger cable in order to meet minimum Zs and maximum voltage drop requirements. If this was a fixed load, I might be more ameanable to using 4mm, but it's not... the question pertains to a radial power circuit which could have any number of sockets or outlets on it and consequently who knows what connected to it.

Anyhow, regardless of anything else, if I want to elect to use a larger cable, that's my choice as the designer as I'm the one who has to be happy to stick their name on the certificate, not the people who write the CCC tables. If this is wrong, then why not provide a worked example to clarify so that others may learn from my error and so avoid scorn in the future?

It should be less than OR equal to.
You've completely missed out the rating factors, which can reduce the required ccc of the cable. For example if the ambient temperature is less than 30C then the temperature factor is greater than 1 and so reduces the required ccc of the cable.
You stated a ccc of 37A in your post and that you wouldn't connect that to the load side of a 32A ocpd.

Installing unnecessarily over-sized cables is a waste of time and resources.

 

[SparkyChick]

Unfortunately I got interrupted by visitors whilst writing it and I agree as it stands the final part is wrong because it should include a statement to the effect of "if the installation method is anything other than clipped direct".

Even adjusting for ambient at 25c (rating factor is 1.03). That gives ratings of (method=rating):-

100 = 27.81A, 101 = 22.66A, 102 = 27.81A, 103 = 18.025A, C = 38.11A, A = 26.78A

So unless I've made a mistake and misunderstood, it's still not big enough to be used if the installation method is anything other than clipped direct. I don't know what the rating factor is if ambient is less than 25c as I can't find it in the BYB.

In the example given of the radial power circuit with socket outlets on a 32A MCB, because you can't know the load, I'd err on the side of caution, but even then 6mm isn't necessarily big enough if the installation method happens to be 101 or 103 and A is right on the money (just slightly over if you adjust for ambient).

Or have I completely missed the point?

 

[Ian1981]

Diversity is applied for cooker circuits so you probably won't get anywhere near 32amp unless it's a 15kw cooker with socket combined in the ccu

 

[SparkyChick]

Yeah, but this part of the question relates to the suggestion in the sparks guide to the building regs that you can wire a radial socket circuit in 4mm and stick it on the end of a 32A breaker.

 

[Ian1981]

Yeah, but this part of the question relates to the suggestion in the sparks guide to the building regs that you can wire a radial socket circuit in 4mm and stick it on the end of a 32A breaker.

My mistake.......
Carry on

 

[SparkyChick]

The thing that gets me about diversity for a cooker is the fact that someone could rock up and turn everything one all at once (say 4 rings, 2 ovens) and plug a kettle in and boil a cuppa at the same time and for quite some time, that thing is going to be drawing it's maximum power.

So how do you reconcile that?