flex size connecting multiple ovens

But that means that the three 2.5mm flexes are only protected by a B40, which to my simple mind sounds wrong. Which is why I'm hoping someone wiser than me will explain about overload,  short circuit, diversity, the non-diabetic equation, and lovingly quote some 5XX.X.X at me, such that I feel comfortable with 2.5mm.

If you use 2.5 mm² flex (may be smaller) are the appliances capable of overloading that? Presumably not.


Which leaves fault protection. The extra impedance of the flex will be negligible.


Look at it another way: would the internal cables within a freestanding cooker be 6 mm² or smaller?


ETA: if you have three 2.5 mm² cables, the total is greater than the 6 mm² fixed wiring.

I guess the thing I'm not getting is, if there was a L-E short at the cooker outlet then the (Sk)² of the 6mm² T&E's E conductor will be sufficient to withstand the I²t of the short circuit current, while if the the short was 2m further away at the far end of the flex 2.5mm² flex, Zs is not significantly higher, so I²t remains the same, while (Sk)² is much reduced and not sufficient. I feel I'm missing something very obvious here.

Are you sure it  is not enough ? Usually it is, the range of Zs for a far end fault for which the 2.5mmsq cable does not get too hot in the breaker trip time is quite wide.


For a period of 5 seconds or less, the relationship I²t = k²S² characterizes the time in seconds during which a conductor of c.s.a. S (in mm ²) can be allowed to carry a current I, before its temperature reaches a level which would damage the surrounding insulation.  so for 2,5mmsq, say 0.08  x 10^6, l say 80 000

allowable thermal stress for cables I²t (expressed in ampere² x second x 10⁶) so for a typical 40A breaker



so any fault less than about 4kA, and high enough to take out the breaker fast, say 5* 40A plus 10% for luck, say any fault current in the range from  250A to 4000A  will be just  fine for  full fault protection for a 2.5mm cable on a B40.

Something similar a while back.

Kitchen refit.

Old cooker out double oven and hob as two seperate unit at opposite ends of kitchen (small domestic).

Manufacturer stated each appliance 32A each circuit.

Total load each appliance about 5.5KW.

I reckon treating as one appliance not exceeding 13KW would probably have been more appropriate therefore one breaker shared between the two after diversity

I guess the thing I'm not getting is, if there was a L-E short at the cooker outlet then the (Sk)² of the 6mm² T&E's E conductor will be sufficient to withstand the I²t of the short circuit current, while if the the short was 2m further away at the far end of the flex 2.5mm² flex, Zs is not significantly higher, so I²t remains the same, while (Sk)² is much reduced and not sufficient. I feel I'm missing something very obvious here.

What's the size of the c.p.c in a 6mm² T&E? (answer: 2.5mm²), what's the size of all the conductors (including the c.p.c) in a 2.5mm² flex? (answer of course 2.5mm²) - does that reassure you?


   - Andy.

What is the difference in resistance between the 6mm and 2.5mm for your tail to the cooker? If you really are that close to the mcb  not tripping, I suggest you have far more wrong with the circuit. 


Regards,


Alan.

A Double accessory box behind the ovens will work in most cases, unless the manufacturer of the oven stipulates the ovens need protecting by a 16 or 20 amp device.


It allows you to use either flex outlets, SFCU or FCU in any combination. Use the 35 mm deep box for easier wiring.


 Andy Betteridge

I think the penny has dropped now, thanks guys.


Yes, I completely forgot to take into account that the flex's cpc will be full-sized. So ... in general:


a B40 won't protect 2.5mm flex from overload, but we assume the ovens can't be overloaded (and we choose a flex large enough for the load).


Whether a B40 can protect against short-circuit depends on factors like PFC and the I2t of the breaker. In this specific case, since the cpcs are the same size (2.5/2.5mm flex, 6/2.5mm T&E) we can assume that what's good for the goose is good for the gander. If I was to (hypothetically) use 1.5mm flex, then this wouldn't apply and I'd have to consider the adiabatic equation to decide whether its ok in these circumstances.