Clarity on wires and fuses for ovens

[BecciG1978]

I’m hoping someone can help. We recently had an extension built to house a working kitchen for my new business. The original sparky wasn’t available to fit the oven when we arrived, so we hired another, who said he couldn’t install it as the wrong wire and fuse were fitted.
When the wiring & fuse was installed, I’d planned to use a 4.2kw double built in oven, but as it was not available, ordered a different one of 4.7kw.
Original sparky installed a 2.5 cable with a 40 breaker, the second sparky says we need a 6ml cable with a 32 breaker, and the manufacturer says it needs a 20 breaker. Which one is it?!!! Original sparky’s arguing with our builder saying what he installed is fine, second sparky says it’d be dangerous, could melt, and catch the house alight, which is obviously the last thing we want, especially with two young kids in the house. Help!!!!!

 

[SJD]

You cannot put 2.5mm2 cable on a 40A circuit breaker. The breaker is there to protect the cable, and the cable could melt before the breaker tripped.

However, the oven only takes approx. 20A. So you could have the circuit changed to a 20A circuit breaker, and it should be fine, depending on how the 2.5mm2 cable has been installed (how much insulation etc). The second sparky should be able to confirm if this is OK.

Is assuming with this that there is nothing else connected to this circuit (e.g. a hob) since you didn't mention anything else!

 

[Baddegg]

I’d always go with manufacturers spec...just calcs for cable depending on its run/installation method....

 

[BecciG1978]

You cannot put 2.5mm2 cable on a 40A circuit breaker. The breaker is there to protect the cable, and the cable could melt before the breaker tripped.

However, the oven only takes approx. 20A. So you could have the circuit changed to a 20A circuit breaker, and it should be fine, depending on how the 2.5mm2 cable has been installed (how much insulation etc). The second sparky should be able to confirm if this is OK.

Is assuming with this that there is nothing else connected to this circuit (e.g. a hob) since you didn't mention anything else!

No, no hob, just the oven. The cable runs in a cavity between either cinder block or brick (depending on the wall), and the plasterboard, and the cavity between the ceiling plasterboard and the flat roof. No insulation in that cavity.
So as long as we replace the fuse with an appropriate one, it’ll be safe?

 

[SJD]

So as long as we replace the fuse with an appropriate one, it’ll be safe?

Your second sparky can confirm if it will be safe.

 

[Wilko]

I’d planned to use a 4.2kw double built in oven, but as it was not available, ordered a different one of 4.7kw.

Hi - what make and model oven is it, if I may ask?

 

[BecciG1978]

It’s an Indesit IDD6340WH Aria.

 

[Wilko]

Thanks - having a quick read of the installation manual it does say-

5.1kW
20A
4mm cable

My guess is the manufacturer recommends a 4mm cable to err on the side of safety as the maximum current is likely 22A or less and 4mm will carry that in even the most adverse installations.