4 mm Cooker circuit 32amp MCB

[ianphilip007]

just visited a customer who requires a 7.3kw hob connected to replace existing gas hob. Currently has a 2.3kw single oven connected to a 4mm T+E cooker radial (ref method c) Protected by a 32amp MCB

i advised that for the hob to be installed the cooker radial would have to be rewired using 6mm T+E as the 4mm was undersized for the load. but having done the calculations and thought about it I'm not quite sure any thoughts.

7.3kw + 2.3kw = Toal Max Load of 9.6KW = 41.7amps

Diversity applied(no socket at cooker switch) 10A + 30% remainder 10+9.51 = 19.51

with diversity applied Ib <In<It

can't find any reference in the regs to a 4mm cooker radial protected via 32amp MCB as the standard is 6mm/32mcb. Not standard circuit arrangement but as 4mmT+E can carry 37 Amps (ref C) I'm thinking that the additional load may be added.

 

[Plonker 3]

The regs wont tell you what needs wiring in what size/ type of cable. What it will do is give you the figures so you can do the cable calcs as you have done.

 

[cobrauk]

Ref C (clipped direct) max amps 37amps table 4D5

 

[ianphilip007]

yes 4mm is 37 amps ref method c (clipped direct) As i thought, 6mm may not be required as the calculations show that the existing 4mm cable is protected and can carry the load (diversity applied) no real difference to a 6mm cable being able to feed 15kw.

i just thought or assumed that 6mm was the minimum cable size allowed for a hob/oven appears i was wrong

 

[SPARTYKUS]

A good question to ask is, do you understand why this will be okay, and why the diversity calculation allows for it....?

 

[Leesparkykent]

It's Nice to see for once that some one has actually taken the time to do the calculations themselves and not just come on here and ask a question that is basically asking someone else to work it all out for them.

 

[ianphilip007]

i was just unsure about the 4mm cable size for a cooker radial. for some reason thought there was a minimum of 6mm. but the cable calcs prove there shouldn't be any problem with a 4mm on a 32amp mob with the required loading

 

[ianphilip007]

A good question to ask is, do you understand why this will be okay, and why the diversity calculation allows for it....?

The diversity calculation allows for it because the hob and its elements will not be drawing the stated power ratings at all times and the same goes for the single oven. therefore the actual current draw with diversity applied is much lower than the stated max power ratings of both appliances. (10amps +30% of remainder +5amps(if socket on CCU) the formula used to calculate diversity. therefore a 4mm cable which can safely carry 37amps (ref method c) protected by a 32amp MCB
satisfys ib<in<It when diversity is applied.

as i posted earlier the standard circuit of 6mm t+e on a 32 amp MCB can supply up to 15kw load applying diversity of course. on reflection the situation described in my first post is no different electrically. i just assumed wrongly that it had to be a 6mm cable as a matter of course.

 

[telectrix]

will be fine on 4mm. one reason 6mm is more otfen used is that you're more likely to have some on the van.

 

[Essex_]

will be fine on 4mm. one reason 6mm is more otfen used is that you're more likely to have some on the van.

I would hazard a guess that most don't understand how to do the calculations and just work on a 1.5 = 6/10A. 2.5 radial = 16/20A. 2.5 ring = 32A. Cooker = 6mm & 40A.

4mm radial. Your pulling my leg. No such thing.

Well done IP007 for working it out for yourself.

 

[Deleted account]

Just thought I'd throw this one in.

once got involved in a lengthy debate with several sparks over diversity in oven/hob.

2 were adamant that you use diversity for the breaker size but not the cable size.

another 2 were adamant that you use diversity for breaker and cable size.

since this debate I've been asking other sparkles there opinions and the answers I get are one or the other or in one case I got "just use 6mm"

me. Well I always thought it was for cable as well as breaker, but not to sure now. As a result I have since erred on the side of caution and used cable suitable for full load without diversity and the breaker with diversity. If anything I have future proofed it.

i cannot pin it down in the regs as it depends on interpretation.

can anyone clarify.

 

[Essex_]

Just thought I'd throw this one in.

once got involved in a lengthy debate with several sparks over diversity in oven/hob.

2 were adamant that you use diversity for the breaker size but not the cable size.

another 2 were adamant that you use diversity for breaker and cable size.

since this debate I've been asking other sparkles there opinions and the answers I get are one or the other or in one case I got "just use 6mm"

me. Well I always thought it was for cable as well as breaker, but not to sure now. As a result I have since erred on the side of caution and used cable suitable for full load without diversity and the breaker with diversity. If anything I have future proofed it.

i cannot pin it down in the regs as it depends on interpretation.

can anyone clarify.

Diversity will guide you towards the cable size. Then the MCB will be guided by the cable size.

 

[telectrix]

you,us diversity to calculate the load current. from that you select a suitable cable size, using installation method and volt drop calcs. then you select your OCPD according to Ib<In<Iz. simplessssss.

 

[Leesparkykent]

Your calculations will tell you what size OCPD you should be selecting, and then the cable will be selected from CCC, reference method, volt drop, etc.

edit: posted same time as telectrix.

 

[telectrix]

snap....