Induction hob and double oven rating

[Kenn7]

Hello there,

My existing cooker installation is a New World double oven 5kW, an isolation switch on about 10m of 6mm (I think) cable, plus a gas hob.

I plan to buy and install a Neff Induction hob (7.2 kW max rating) and a Neff double oven, (6.8 kW max rating).
I'll be installing them in new locations (i.e. I'm moving the kitchen).
I need to know if the exiting 6mm cable is large enough to supply the new loads.
I'm not clear how the regulations on diversity work.

I've read an exiting post; "symphonysteve, 20-Aug-2006" which covers a similar instalation, but it's conclutions are far from definative.

My max total current draw would be around 60A, and the above post suggest two methods using diversity;

A) Taking 10A + 30% of the combined load would give a load current of 25A
i.e. Load current = 10A + 0.3(60A - 10A) = 25A, (30A if I opt for a socket on the isolation switch)

B) Taking 10A + 30% of each load and adding them would give a load current of 31.5A
i.e. Load current = (10A + 0.3(31.3A - 10A)) + (10A + 0.3(29.6A - 10A)) = 16.4A + 15.9A = 32.3A, (42.3A if I opted for sockets in each isolation switch)

Both seem to be well inside the 6mm cable rating of 47A, and the run is only about 4m from the MCB.
Because of the oven and hob locations I will need to install seperate isolation switches, but I was planing to spur the hob switch from the oven switch.
I can utalise the existing 6mm cable to run to the new oven isolation switch (only 4m), then take a spur to the hob isolation switch (a further 6m).
Thus both the hob and oven currents would only pass through the initial 4m of cable.

Are there more prescriptive regulations on the use of diversity?


Also, both positions are against block walls so the cables will need to be sunk into the walls above the switches.
I can't sink them 50mm, so will I need to run them through steel condutes, or will PVC be OK?
(The regs are a bit uncleare on this and seem to suggest only if a socket is included on the switch. I don't need sockets on either isolation switch).

Thanks

 

[EFLImpudence]

A) Taking 10A + 30% of the combined load would give a load current of 25A
i.e. Load current = 10A + 0.3(60A - 10A) = 25A, (30A if I opt for a socket on the isolation switch)

Correct. Definitively

B) NOT

Both seem to be well inside the 6mm cable rating of 47A, and the run is only about 4m from the MCB.

Yes, is fine.

Because of the oven and hob locations I will need to install seperate isolation switches, but I was planing to spur the hob switch from the oven switch.
I can utalise the existing 6mm cable to run to the new oven isolation switch (only 4m), then take a spur to the hob isolation switch (a further 6m).
Thus both the hob and oven currents would only pass through the initial 4m of cable.

OK.
Separate switches not necessary unless you want them.

Are there more prescriptive regulations on the use of diversity?

Not stricter.

I can't sink them 50mm,

Not allowed by Building Regulations, anyway.

so will I need to run them through steel condutes, or will PVC be OK?

PVC is fine but they should be protected by an RCD. Are there any in the CU already?

(The regs are a bit uncleare on this and seem to suggest only if a socket is included on the switch.

No, new concealed unprotected cable also requires RCD.

 

[Kenn7]

Thanks for the response. Yes the CU has not long been installed and does include 2 RCDs, it's a British General CUD16610 or IP2XC. The MCB the cooker is on is a B32, (I'm guessing this is a 32A breaker), is this OK for the new hob & oven? There is a spare B40.

 

[EFLImpudence]

Yes, 32A and that's fine.

 

[Kenn7]

Hi again. I read your comment on the 50mm sinking of the cables not being allowed by building regs. I originally read this, or possibly missread it, from (Section 4.4 "Electricians Guide to the Building Regulations 17 edition"), the note under figure 4.4 seems to require a depth of 50mm, however it also says... "or protected by 30mA RCD; something I missed.

Also, the reason I was putting in 2 isolation switches is because the hob and oven are about 3m apart and because of a window I cannot get both within 2m of a single switch. However on re-reading "para 4" it does say its only "recomended that the control switch is within two 2m of the appliences. One switch would be easier for me, but if I don't put in a second switch there would be nothing to warn future DIY'ers of a cable sunk into the wall; and a hob 6m from the switch might be pushing it a bit.

 

[EFLImpudence]

Hi again. I read your comment on the 50mm sinking of the cables not being allowed by building regs. I originally read this, or possibly missread it, from (Section 4.4 "Electricians Guide to the Building Regulations 17 edition"), the note under figure 4.4 seems to require a depth of 50mm, however it also says... "or protected by 30mA RCD; something I missed.

Yes, it is confusing.
RCD protection is required for cables less than 50mm. deep (out of reach for two inch screws) but
other BRs (elsewhere in EG to BR) you will see maximum depths allowed for chases.

Also, the reason I was putting in 2 isolation switches is because the hob and oven are about 3m apart and because of a window I cannot get both within 2m of a single switch. However on re-reading "para 4" it does say its only "recomended that the control switch is within two 2m of the appliences.

Yes, only recommended. No requirement to have one at all although it is a good idea - so up to you.

One switch would be easier for me, but if I don't put in a second switch there would be nothing to warn future DIY'ers of a cable sunk into the wall; and a hob 6m from the switch might be pushing it a bit.

Fair enough.