Cable size for hobs and oven

[teaks]

I have an 8.4kw hob, a 3.4kw main oven and a 3.4kw second combi oven.

What size cable do I need?

It is a long way (25m) to the nearest distribution board (consumer unit?) which is full anyway. It is about 40m to the supply inlet where there is a distribution board with spare slots. This distribution board says 80A, 30mA on it.

I am about to get an electrician in but want to know a bit more first.
From what I understand I will need one or possibly two 10mm cables?

Any comments would be much appreciated.

 

[PrenticeBoyofDerry]

without applying diversity
you have current load of 66.1A, using reference method c (cable clipped direct through out route) this means the cable will not run through insulation or be bunched with other cables or enclosed in conduit.
If using twin and earth your cable CSA would be 16mm.
Now we could apply diversity to the calculation, which would mean assuming that all the elements of all the appliances would not be used at the same time, for long periods.
This will get the CSA of cable down.

 

[Taylortwocities]

This distribution board says 80A, 30mA on it.

.

That is the rating of an RCD and nothing to do with how much power you have available.

It is impossible to judge even if the supply is enough for the new loads without a site survey. Also the length of the feeds from the supply means that larger cable than normal may be needed (because of volt drop).

Get a registered electrician on site - best to get one recommended, and then get three quotes.

 

[teaks]

Thanks for the speedy replies. I should have been more specific with my question. I have an electrician coming to quote but want to know more before he comes. I am fitting the kitchen. I have run two 10mm cables under tongue and groove flooring and can not change them. Sounds like one 16mm cable or separate live/neutral/earth may have been better but should two 10mm be sufficient? For most of their length they are clipped to the joists and have air circulating round them but over 40m there are quite a few corners!

 

[ajrobb]

Edit: It is not clear if this is domestic or not. The following only applies to domestic cooking appliances:

The IEE On-site Guide states that a 32A supply is adequate for 15 kW of domestic cooking appliances plus a kettle. You have 15.2 kW so you need more. You could use a 32A supply without a kettle socket for up to 19 kW. All these figures use diversity.

You should look at the appliance manufacturer's instructions as they may require extra protection. For instance, both your ovens are rated at 3.4 kW and could be manufactured using 1.5 mm² internal wiring. The MI might state maximum 16A protection.

Perversely, a single 16A supply could be used for both ovens as it can supply up to 6.9 kW of cooking appliances using diversity. You hob's full load current is 37A and would need a minimum supply of 18A. These are bare minimums and not recommendations.

The diversity calculation for domestic cooking appliances is 10A + (full-load current - 10A) * 0.3 and can be applied to circuits of more than 15A.

A single 32A 6 mm² cable might be adequate up to 49m (depending on RCD or building supply type). Unfortunately, an electrician might not be able to connect your circuits as he/she cannot inspect them. Even then, I think it would be illegal. I think only a Building Inspector can sign off other people's electrical installations.

 

[Steve]

Get a gas hob. That will halve your power demand, and your cooking energy bill too.

Is this in a commercial premises?

Its not about what size cable you need. You need to dig more into circuit design. For the specs you have quoted, I'd run a 16mm cable to somewhere local and install a submain. Fit two 16A MCBs and a 40A MCB. You need three isolators local to the unit (if the submain isnt within 2 metres of the appliances).

No RCD protection is nessecary unless the circuit supplies an accessible socket, or the cables are buried in plaster.

 

[Taylortwocities]

For most of their length they are clipped to the joists and have air circulating round them but over 40m there are quite a few corners!

Don't worry about the corners, they don't slow down the passage of the electricity.

 

[teaks]

Great I feel as if I am making progress here but feel I need to stay focused. (So no gas suggestions please - I am not on a gas main - my wife hates gas - I have already bought the gas hob.) My electrician is coming this afternoon (I hope) and I am confident he really knows his stuff but I like to investigate things myself as well.

I already have in place two 10mm cables.
Question 1. Is there any possiblity that this will not be sufficient for the hob and the two ovens?
Question 2. Is it possible to use these cables for the kitchen ring main as well.

In the kitchen - I assume the two 10mm cables need to be wired to a box (two separate boxes) on the wall, probably with a fuse and switch in it - and from there secondary wires to the hob/oven etc.
Question3. Is this correct and do these boxes on the wall need a switch and a fuse?

At the supply.
I have various distribution boards (consumer units?) dotted around the house. It might be easier to add yet another distrution board for these two cables.
Question4. If they are wired into an existing distribution board then what slots/capacity will be needed or if another distribution board is added what should it be?

PS. It is not a commercial premises just a large house.
PPS. The two cables are clipped separately but touch one another at a few corners.

 

[ajrobb]

As far as I can make out, a single 6mm² 32A supply could have been enough for your 15.2 kW of domestic cooking appliances. So one of your 10mm² cables should probably be OK for 32A, even de-rated by grouping (you're supposed to keep them 2 major diameters apart). Check your appliance MIs for maximum fuse values.

The other 10mm² cable could be used as a 32A supply for sockets but you'll probably need a junction box as it won't fit into a 13A socket.

The appliances should be connected through accessible double-pole isolators within 2m of each appliance and not directly above it. The two ovens could share an isolator if it's within 2m of both.

 

[teaks]

Thanks ajrobb, sounds like good advice. It is good to know 10mm will be enough.
For the worktop sockets is it ok to move the existing ring main sockets? I would prefer to dedicate the two 10mm cables for the hob and ovens.
I will spend a bit of time separating the cables at the corners - a bit tedious but it makes sense (not sure what the "major diameter" is - and I am a mathematician! (the max width of the cable I assume)).
My sparky is coming on Monday to fit it all.

 

[ajrobb]

I think that socket circuits can supply different areas (20A up to 50m² and 32A up to 100m²). I don't think it essential to have kitchen sockets on a separate supply. Within that area, the number of sockets is unlimited.

Recommendations from OSG Appendix 8.7 Number of socket-outlets, Table 8B

• Small kitchen (up to 12m²) 6 sockets (at least 3 above worktops)
  Medium kitchen (to 25m²) 8 sockets
  Large kitchen (over 25m²) 10 sockets.

"It is recommended that wall mounted socket outlets above a work surface are spaced at not more than 1 m intervals along the surface."

Your sparks should KNOW all this. I mention these things so you can expect them. Good luck.

PS Have you worked out the fuse requirements for the ovens yet? If they are OK on 32A, that would make things simple. A 32A supply to an isolator then split to the ovens (if not too far apart).