Cable Rating to Supply a Hob and an Oven / Microwave

[WhatNext]

Hi

My oven has a rated maximum load of 3.7kw and the microwave grill has a rated maximum load of 3.6kW. Total maximum load of 7.3kW.

Both are supplied from a single 6 sq.mm cable protected with a 40A MK MCB. The cable length is approximately 12 - 15m long. It has been run clipped to timber joists and is covered with rockwool insulation. The electrician originally had a 32A MK MCB, but increased it to 40A when I requested it.

My hob has a maximum load of 10.8kW ( 5 rings ).

It is supplied with a dedicated single 6 sq.mm cable protected with a 40A MK MCB. The cable length is approximately 15m - 20m long. It has been run clipped to timber joists and is covered with rockwool insulation. The electrician originally had a 32A MK MCB, but increased it to 40A when I requested it.

I understand the principle of diversity and the effect that insulation has on cable ratings.

Do you think that 10 sq.mm cable should have been run or will the 6 sq.mm cables provide sufficient power safely. I think that I may have been wrong to ask for 40A MK MCBs and plan to have the electrician change them back to 32A - the appliances have yet to be wired up.

Any comments ?

 

[BS3036]

Your max load is just under 32A. Your cable run, as you define it, is rated at 35A if I read the table right. So your MCB should be 32A as you have deduced. For your existing setup that will be fine, (and it will be many years before you will need a new oven).

Ooops.. missed the hob!

 

[pompeygit]

The cable size will need to be derated because of the insulation by a factor of 0.55 what you need is a 32amp breaker with 10mm2 cable for the oven/microwave. As for the oven at 10.8 kw you will need a 50amp breaker and 25mm2 cable(if the cable is buried in insulation). Best put the 32amp breaker back temporarily to give the cable a bit more protection. Are you sure the cable is buried most sparkies would not allow it to be buried wherever possible, are you sure it isnt clipped clear of the insulation. Insulation covering cables has a really bad effect on cable sizing. Ive had a f*)king crap day so I amy be wrong........can anyone else do the calcs and confirm what I worked out

 

[BS3036]

For the ovens, doesn't TABLE 6F in the OSG apply? 6mm, column 3, 35A. No diversity needed.

For the hob, with diversity, 47A would be 21A. With 5 rings, diversity makes sense, so again 6mm in col 3 gives 35A. 10mm would have given 47A. Either way an MCB less than the wire capacity is required.

 

[securespark]

I think, because of the insulation issues, you cannot stick with the 6mm², especially if you want to design the circuit to run at flc.

 

[davelx]

for both cases the OP states:

"It has been run clipped to timber joists and is covered with rockwool insulation"

I'm assuming that this is above a ceiling where the cable has been clipped to the side of the joist and then insulations installed between the joists.

I think this is one area where the IMs in the regs are not adequate - none of the methods really fits - it's not really "buried" in insulation as one side is in contact with a thermally conducting surface. On the other hand, it's not really method 4 or 15 either, but this could probably apply if the coefficient of heat transfer from the joist is > 10W/m^2K

I wonder if there is anything better in the 17th?

 

[WhatNext]

Hi

That's correct.

The manjority of the cable runs through the ceiling where it was clipped to the side of the timber joists. The rockwool insulation was then installed between the joists.

From what I have read the MCBs should be changed from 40A to 32A which is what the electrican had fitted before I asked for the change.

As this particular situation seems to fall between Table 4D5A Method 15 and Reference Method 1 ( Clipped Direct ) would this possibly be the reason why he was happy to fit the 40A MCBs ?

The electrician is superb and I was just worried that I may have asked him to do something that he was not completely happy with ?