Oven circuit

[Edddd]

Sorry, this might be a bit of a tired subject...

I have just bought a replacement double oven and I'm trying to work out what I need to do about the supply.

The existing circuit is 2.5mm T+E radial on a 20A breaker and the new oven is rated at 5.9kW. That is a 25.7A load so I'll need a 32A MCB. I'm assuming that diversity is not applicable here because it's entirely likely that both oven elements will be on continuously and simultaneously when they are turned on from cold.

The cable route to the cooker switch is approx. 2m through galvanised trunking, 7m through a floor space/clipped to wall and 200mm buried behind tiles (tile singular really). 4mm T+E with method B for the trunking can supply 30A - correct?

Can I install a 32A MCB on a cable rated at 30A for a fixed-load circuit that has no requirement for overload protection anyway? Or do I need a bigger cable?

The trunking also contains two ring finals and an underfloor heating circuit. However, if I apply a 0.65x grouping factor for 4 circuits I'm faced with a 10mm cable for the oven. Do I need to consider grouping? I can't guarantee that the other circuits won't be fully and continuously loaded at the same time as the cooker circuit, but it is rather unlikely.

Thanks in advance!

 

[ericmark]

It is very hard to work out what power is required by an oven over the time taken to over heat a cable. It is dependent on the mass of the oven and quantity of air and food that needs heating. With a industrial installation it is common to have cables fully loaded, but with domestic it is rare. The auto trade taught me a lot about cables grouped together, get an overload of extreme proportions and the wire is reduced to an uninsulated cable out of the harness, but inside the harness which is always stripped just in case, the overloaded cable is A1. The other cables have absorbed the heat better than the cable being in free air.

So likely you are worrying about something which will never happen, if the cable is protected with a thermal overload it takes less time for the thermal overload to heat up than the cable, and you would be surprised to see what overload you can get away with. My son ran a 2.7kW washing machine from a 6A supply, the heater was not on long enough to trip the overload.

I am not saying you should overload any cable, but the standard 6mm² cable used for most cooker supplies should be ample. Using a range cooker than because it has such a large mass to heat up, yes there could be a problem, but most ovens reach temperature within 20 minutes and from that point they switch on and off, and the space is larger than the mark in the mark/space ratio.

Not a good idea, but likely a 20A MCB with the existing 2.5mm² would hold in, so swapping to 4 or 6mm² you really have no problems.

 

[Taylortwocities]

Diversity DOES apply to the oven. The existing circuit with a 20A braeker is adequate. Don't worry about it, I have a bigger double oven than that on a 20A breaker and I have had several years (including Christmas Days when its all going full blast) wnd nothing has tripped, or caught fire yet.

PS, if you are really worried (you should not be), you could put a 25A MCB on the 2.5mm² cable.

It looks like you are assuming that your supply voltage is 230v, I'll bet its more than that. Get your Ohms Law out and see what the current is at the good old English 240v

 

[EFLImpudence]

t looks like you are assuming that your supply voltage is 230v, I'll bet its more than that. Get your Ohms Law out and see what the current is at the good old English 240v

...and THEN what it will be at 230V.

 

[Edddd]

Hmm good point. The oven is rated for 220-240V. They must assume 240V for the quoted power consumption so that saves me an amp. But then my (uncalibrated) meter says 244V which, if true, would push it back up to 25A.

 

[Taylortwocities]

would push it back up to 25A

Mathematically, yes. But as I said, diversity is applied to domestic ovens, you will never pull anything like 25A.
Read https://www.tlc-direct.co.uk/Book/6.5.2.htm

The design demand for your oven is
10 + (15/3) = 15AMP

The existing circuit and MCB is just fine.

 

[Edddd]

Ok, thanks for the reassurance. Can always upgrade at a later date if it does start tripping. Wouldn't be the worst way to spend a Boxing Day.

 

[Taylortwocities]