Builder normally equals BODGER! they are some of the biggest cowboys going and should not be touching or even commenting on electrical matters, as 99% of them are not electrically competent and have not undergone formal electrical training or hold relevant qualifications. Where as most Part P registered electricians do.

A 4200 watt (4.2Kw) oven at 230volts (4200 divided by 230 ) = 18.26 amps. A 2.5mm cable is capable of supplying around 23-25 amps depending on the method of installation, so is fine, safe and compliant on a 20amp circuit breaker. Its very clear that the builder is not an electrician and probably knows little or nothing about circuit design and calculating circuit current. If he did then he could have done this quick calculation and realised it was perfectly compliant with BS7671:2008.

Hello,
A quick calculation for your problem:
A 4.2kw oven =4200w
4200/240=17.5amps
A 2.5mm short length of cable without diversity applied for insulation etc is capable of 24amps

Therefore your 2.5mm cable over the short distance IS SUITABLE for the capacity of the oven.
The 20amp fuse is the correct size fuse.

But in doubt always consult your ovens manual on there MINIMUM specification for electrical conections.

I hope this helps you.
Leighton Haynes

You have not given the method of installation, they are as listed below. Your builder is only quoting from limited knowledge that generally cooker supplies are run in 6.0mm T+E and does not know how to calculate the requirements. It sounds like your electrician has taken the cable to its design limit if using method B or C which is acceptable.
2.5mm T+E
Installation method A (enclosed in conduit in a thermally insulating wall, etc)
current carrying capacity 18.5A (max MCB 16A)
Installation method B (enclosed in conduit on a wall or in trunking, etc)
current carrying capacity 23.0A (max MCB 20.0A)
Installation method C (clipped direct)
current carrying capacity 27.0 (max MCB 20.0A)

Your electrician is right. The cooker will pull a max of 18amps and the cable is rated at around 24amps and protected by 20amp fuse so your good to go : )

hi
4200w divided by 230v = 18.26 amps
from memory 2.5 t and e in normal conditions can carry apprx 27 amps
so :
cable is rated at 27amps
fuse is rated at 20amps
oven is rated at 18amps

the numbers look fine to me , however there are other things to consider.

i.e if the cable run travels through more than 400mm (depth) of thermal insulation the current carrying capacity of the cable has to be halved so the cable will become the weakest link in the chain.

are there other issues the builder has noticed that you have not posted?
best regards