Pair of ovens

[EFLImpudence]

You have 18kW which at 240V (which it will be) is 75A.
(at 230V this is 16.5kW and 71.9A.)
If you have a cooker switch with integral socket, you must add 5A.

Diversity calculation -
65 x 0.3 = 19.5 + 10 = 29.5A
(61.9 x 0,3 = 18.57 + 10 = 28.57)

So, in fact, with 32A MCB (if the cable is clipped direct or buried in masonry - this is called Method C) 4mm² cable would suffice.
This is why I keep stating that the standard 32A and 6mm² should be fitted.
However, if Method C, you could have 40A or 45A MCB with the 6mm² cable.

If you insist on 10mm² cable, Method C (which, as has been said, is used as the main supply for many whole premises), then you could have a 63A MCB, but there is no point.

If the cable is not installed to Method C but runs in conduit, then you should use the next size up cable (although with 32A MCB, 6mm² is already the next size up).

 

[Dave Thornley]

The 10mm2 cable is already in place. All the kitchen is fed from the red phase . 100A main switch switch, 32A RCBO for the sockets, 6A MCB for the lights and a 32 A MCB feeding the 10mm2 to an old type cooker unit, cooker switch with 13A socket, which is to be dispensed with, and hence onto where an electric hob would be fitted and terminates in a cooker outlet. Also coming out of the cooker unit was a feed to supply the orignal double oven in 2.5mm2 T&E.
It is a Hager CU. So keeping the 100A switch I can fit a 45A RCBO to do the hob and the ovens. Other manufacturers do bigger RCBOs ie 50A and 63A so may go for one of those. Also may fit a box with some din rail near to the ovens and feed each of those via a
16A MCB each with a dedicated switch.

 

[flameport]

You have two choices:

1. Use the existing 10mm² cable with a 32A RCBO. The cable is oversized, but as it's already there it may as well be used.

2. Install all the other totally unnecessary things you keep mentioning and use grossly oversized cables and excessively large ratings of protective devices for everything. No idea why you would want to do that - but ultimately it's your money and time you will be wasting, so if that is the goal, then crack on.

 

[Dave Thornley]

I am just worried 32A is not big enough. If only one oven is on and a couple of the hob zones that is as near as damn it 7KW

 

[securespark]

But the standard cooker circuit of 32A/ 6mm cable allows diversity. This means it allows for the fact that not everything will be on at full power simultaneously.

 

[AndyPRK]

well IF it were to be come a problem you could change the MCB to 40 or 45 A

 

[flameport]

I am just worried 32A is not big enough.

A 32A circuit breaker does not pop off the instant the current reaches 32.0001A.
At 40A, it will never trip.
At 45A, you have got many hours before it trips.
At 50A, it will still take 10s of minutes
Even at 60A, it's still a couple of minutes.

That is why it's perfectly acceptable for the total load of a cooker, ovens/hobs or other combinations of things to be greater than 32A.
A short duration overload will not damage anything, it won't trip circuit breakers, and the overload won't last long because all hobs, ovens cookers and similar have multiple heating elements all with thermostatic control. The elements(s) will be switched off once it reaches the set temperature and then just cycle on/off to maintain the temperature.

Cables do not instantly burst into flames if the current is slightly high either.
Their operating range is all carefully coordinated with the way the circuit breakers and other devices react to overloads.
Even if some overload situation occurred for a long time which did cause the circuit breaker to trip, the cable still won't be damaged.

 

[Dave Thornley]

Okay persuaded I will fit a 32A RCBO. If I do experience any problems I can always put a larger on in at a later date.

 

[Risteard]

A 32A circuit breaker does not pop off the instant the current reaches 32.0001A.
At 40A, it will never trip.
At 45A, you have got many hours before it trips.
At 50A, it will still take 10s of minutes
Even at 60A, it's still a couple of minutes.

That is why it's perfectly acceptable for the total load of a cooker, ovens/hobs or other combinations of things to be greater than 32A.
A short duration overload will not damage anything, it won't trip circuit breakers, and the overload won't last long because all hobs, ovens cookers and similar have multiple heating elements all with thermostatic control. The elements(s) will be switched off once it reaches the set temperature and then just cycle on/off to maintain the temperature.

Cables do not instantly burst into flames if the current is slightly high either.
Their operating range is all carefully coordinated with the way the circuit breakers and other devices react to overloads.
Even if some overload situation occurred for a long time which did cause the circuit breaker to trip, the cable still won't be damaged.

It must not operate at 36.16A (230 * 1.13), but it must operate within the conventional time of one hour at 46.4A (230 * 1.45). But that doesn't alter the reality that 15kW of cooking loads can be fed from a 32A circuit taking diversity into account.

 

[scousespark]

It must not operate at 36.16A (230 * 1.13), but it must operate within the conventional time of one hour at 46.4A (230 * 1.45). But that doesn't alter the reality that 15kW of cooking loads can be fed from a 32A circuit taking diversity into account.

I think you mean 32 as opposed to 230 in your calcs. Also at 46.4A the breaker should not trip within an hour.

 

[Risteard]

I think you mean 32 as opposed to 230 in your calcs. Also at 46.4A the breaker should not trip within an hour.

Yes indeed - 32A. At 1.45 times the rating of the protective device it is required to operate in the conventional time (one hour). It must not ever operate below 1.13 times the rating.