Oven Flex Rating - one for the electricians

[thepook]

Morning all,

I’m hoping one of the electricians around here may be able to help me with this one…

I’ve got a built-in Siemens eye-level double oven to connect to an existing circuit. The oven is rated 5.5kW @ 230V, i.e. 23.9A. They do not specify a fuse size or a cable size.

My existing circuit is run from a 32A RCBO, via 10mm t+e to a 45A Cooker Control Unit with socket, and on to a 45A Cooker Connection Unit.

As I understand it, the typical diversity rules for a ‘cooker’ consider a typical appliance with 4 hotplates, and oven and a grill. Am I correct that this cannot really be applied to a double oven, as the characteristics of that are more like a single fixed load?

If I am correct not to apply diversity, then the existing 32A breaker would appear to be the correct choice, and the cable/controls more than adequate.

The problem I have is with the flex to connect the oven to the connection unit. The manufacturer’s instructions insist on H05VV-F flexible cable or a higher rating, so that rules out ordinary t+e for me. I have got some of this flex in 4mm form (H05VV-F 3G4), and as far as I can tell, this is rated at 32A in Table 4F3A, but I would imagine that the ambient temperature at the back of an oven could be expected to be over 30 degrees, therefore derating the flex.

I am concerned that while 4mm would still be adequate for the designed load of the oven even if derated for 40 degrees (26A), it may not be adequately protected from overload by a 32A breaker. This brings me to another point – can I consider the oven to be a fixed load that does not require overload protection at the origin, and therefore use the 4mm flex provided that it is adequately protected from fault currents?

An alternative I have is to use H07RN-F flex in 6mm, would that be a better option? The problem with this is that it will be a very tight squeeze in the Cooker Connection Unit due to the overall diameter of it as a round cable.

In either case, I will of course be fitting bootlace ferrules to the flex before I fit it to the cooker connection unit, and the terminals in the oven itself appear to be suited to flex.

Thanks in advance for your help with this.

Dave

 

[flameport]

4mm is fine.

Overload is irrelevant, there is no way this cable could be overloaded as it's connected to one fixed appliance.

The 32A breaker is only really there for short circuit protection, and 4mm is far larger than the minimum required size.

Diversity applies to almost everything. It certainly will in this case, as the element(s) in the oven will not be on 100% of the time.

 

[thepook]

Hi,

Thanks for the reply.

Using diversity, would you then suggest that I get an electrician in to change the breaker from 32A to 20A? I feel confident to do that myself, but i know the rules.....

I feel that 20A is a bit close for comfort, but the diversity calc gives 10A + (0.3x13.9A) + 5A for the socket, so that is around 19.2A.

Dave

 

[Taylortwocities]

NO

The breaker in the consumer unit is there to protect the cable(s) from damage if there is high fault current.

6mm and 4mm cables are both rated at more than than 32amp, so you are fine to leave it like it is.

 

[thepook]

4mm is fine.

Overload is irrelevant, there is no way this cable could be overloaded as it's connected to one fixed appliance.

The 32A breaker is only really there for short circuit protection, and 4mm is far larger than the minimum required size.

The breaker in the consumer unit is there to protect the cable(s) from damage if there is high fault current.

I guess both of these responses are what I was trying to suggest when I asked whether I could consider the oven to be a fixed load that does not require overload protection at the origin, but should still consider fault/short circuit current protection.

Thanks both for the confirmation of this.

Dave

 

[Taylortwocities]

I could consider the oven to be a fixed load that does not require overload protection at the origin, but should still consider fault/short circuit current protection.

As I said, the fuse at the origin is to protect the fixed wiring. The oven won't be a fixed load, it'll be switching on and off. If its a cooker, with integrated hob, then the rings will be switching on and off, not all at the same time.

You don't have to worry about that UNLESS the manufacturer specify a protective fuse/device for the APPLIANCE. Then a different approach may need to be taken.