13.7kw cooker on 2.5mm T&E

[JohnnyG]

Someone has asked me to connect up their shiny new cooker to the existing point which is fed by 2.5mm T&E, about 8-9m of it clipped direct, & on a B16a mcb.
I've agreed to do it only if he lets me replace the whole run (which will include ripping up floorboards etc.) with 6mm & a 32a mcb as by my calculations, even after diversity, 2.5mm is unacceptable as I *think* the max it can take is 27a and that'd need a 25a breaker which is cutting it fine.
The bloke who bought it is adamant that the existing cable will do for some reason but I'm not going to connect it up as I'm adamant it's completely unsuitable.
He's a PA to a very wealthy lady whom he bought it on behalf of and I have my feelings that he's just trying to save face after cocking up on his purchase.
Have I missed something in my calculations?
Would any of you connect it up to the 2.5mm?
If I'm wrong and it is suitable I'd gladly tell him so

 

[Taylortwocities]

No point connecting to the 2.5mm if its protected by a 16A breaker! It will only pop on a very regular basis.
25A breakers only available from a very limited selection of manufacturers.
How long is the run, don't forget volt drop too!

Why are you bothering with 6mm? Better to go to 10mm, it won't cost much more and it will future proof future cooker choices.

I trust you'll be putting in an RCBO to protect this [new] circuit and do the necessary re Part P, etc etc....

 

[JohnnyG]

We both agreed the 16a would need changing so didn't really bother mentioning that.
Volt drop was included in my calcs - 9m or so.
Fair point, if I do do it I'll put 10mm in.
It's RCD protected already and will be notified if I do it.

Would you use the 2.5mm?

 

[martinxxxxxx]

What is the rating of the cooker in amps or KWs?

What does the cooker instruction manual say?

It is against the regulations to have a circuit that is slightly overloaded. Someone is going to ask which reg and I will have to go to the van and get the red book.

If he is adamant, maybe he has read the manual and the cooker only needs 2.5 te although I cannot imagine this to be the case unless it is a gas cooker and the ignitions are the only power it requires.

Also 20A is the maximum breaker you can have on a 2.5 te radial circuit.

Martin

PS what did he buy the cable or cooker. the cooker right?

 

[JohnnyG]

13.7kw - 60a - 25a after diversity - 1.8% VD
I haven't seen the manual as yet.
The existing cooker is gas, the new one is wholly electric.
Yes he bought the cooker.

He's apparently been told that the cable is ok...

 

[Taylortwocities]

He's apparently been told that the cable is ok...

By a qualified electrician? Or by the cooker salesperson who had never been to the site

 

[martinxxxxxx]

Sorry I am half asleep, I just noticed the heading to your question.

The manual should reveal all.

But to answer your question -no I would not fit it.

Let him sign it off if he wants to, unless the manual say otherwise then it is down to the manufacturer.
##Martin

 

[ColJack]

It is against the regulations to have a circuit that is slightly overloaded. Someone is going to ask which reg and I will have to go to the van and get the red book.

er..

433.1 Co-ordination between conductor and overload protective device

Every circuit shall be designed so that a small overload of long duration is unlikely to occur.

that one?

Also 20A is the maximum breaker you can have on a 2.5 te radial circuit.

not true.. as long as it satisfies the reg below then it's a correctly designed circuit..

433.1.1 The operating characteristics of a device protecting a conductor against overload shall satisfy the following conditions:

(i) The rated current or current setting of the protective device (In) is not less than the design current (Ib) of the circuit, and

(ii) the rated current or current setting of the protective device (In) does not exceed the lowest of the current carrying capacities (Iz) of any of the conductors of the circuit, and

(iii) the current (I2) causing effective operation of the protective device does not exceed 1.45 times the lowest of the current carrying capacities (Iz) of any of the conductors of the circuit.

 

[martinxxxxxx]

What is I2 for a 25A breaker?

Would it comply with item (iii)

Martin

 

[ColJack]

to be perfectly honest I don't know.. or remember where in the book to look..
would that be the magnetic trip point or the thermal trip point?

B type siemens says it's 3-5 x the rating for magnetic,
1.13 - 1.45 for thermal.. actually it says that's the thermal for all curves..

after a few vodka and cokes I can only reason that magnetic is surge ( ie shorts etc ) and thermal is overload.. ( ie sustained. )..

in that case, worst case ( 25 x 1.45 ) is still less than ( 27 x 1.45 )..

You will see from the curves that the inverse time / current
characteristic which provides overload protection is the same on all
three. This is because the British Standard requires the breaker to
carry 1.13 times the rated current without tripping for at least one
hour and when the test current is increased to 1.45 times the rated
current, it must trip within one hour, and again from cold if the
current is increased to 2.55 times the rated current the breaker must
trip between 1 and 120 seconds. The inverse time delay
characteristic of all MCBs claiming compliance with BS EN 60898
must operate within these limits.

so going by that, any breaker to BS EN 60898 will comply as long as the cable is rated higher than it.. ?

If I've gone off in the wrong direction then please excuse me and correct me.. as I said I'm into my 3rd or 4th large drink so my thinking is a bit fuzzy.

 

[ColJack]

for future reference and after some googling I found a list on te IET forum..

BS EN 60898: I2 = 1.45 x In
BS 88 : I2 = 1.6 x In
BS 1361 : I2 = 1.5 x In

 

[ColJack]

oh now I feel silly...

433.1.2 Where the protective device is a general purpose type (gG) fuse to BS 88-2-2, a fuse to BS 88-6, a fuse to BS 1361, a circuit-breaker to BS EN 60898, a circuit-breaker to BS EN 60947-2 or a residual current circuit-breaker with integral overcurrent protection (RCBO) to BS EN 61009-1, compliance with (i) and (ii) also results in compliance with condition (iii) of Regulation 433.1.1.

now I remember why I don't know where to find I2, you don't really need to unless it's a none standard fuse / breaker..
BS 3036's are delt with in the next reg 433.1.3 where it says to comply with (iii) the fuse rating can't be more than 0.725 x cable rating.