Wiring to hob and oven

[Bob Martin]

I'm wiring a new hob and oven. I have a 30A breaker in the consumer unit, 55A cable to a 45A cooker switch with 3-pin socket. I've wired the oven to the switched output via a 13A fuse box but am a bit stumped about the hob. The instructions say to use

2.5 mm cable (the terminal block won't take bigger) and to protect it with a 20A fuse. How do I do this? If I wire the hob directly to the switched output then the cable should be able to carry the 30A if I understand the regs correctly but this doesn't seem possible if the hob and oven are to be connected to the same consumer unit point.

[Sparks]

They basically want you to run a separate circuit from the CU with a 20A breaker on it.

What is the rating of the hob on the data plate? What is the rating of the oven on the data plate?

[David Hansen]

On Fri, 11 Aug 2006 15:14:32 GMT someone who may be Bob Martin wrote this:-

Both electric, presumably.

What do the instructions for the oven say about the rating of the protective device for that?

Is there anywhere suitable to place a small two way consumer unit?

[Andy Wade]

By "fuse box" I presume you mean a fused connection unit.

Assuming clipped direct installation conditions apply, the rating of 2.5 mm^2 cable is 27 A. If the max. load of the hob doesn't exceed this figure (= 6.2 kW) the cable cannot be overloaded. If that is the case

*and* the total max load of hob plus oven is under 60 A (13.8 kW) *and* this is in a domestic rather than a commercial kitchen, then it will be OK to feed the hob from the switched output of the cooker control unit in parallel with the oven.

Reasoning:

1. The 2.5 cable is a short run and will be adequately fault protected by the 32 A breaker (c.f. an unfused spur on a ring circuit), and overload protection for that cable is not necessary, as established above.

1. If the total cooker load is 60 A, applying the standard domestic cooker diversity rule requires the circuit design current to be the first 10 A, plus 30% of the remaining 50 A (= 15 A), plus 5 A for the socket-outlet, i.e. 30 A total, which is satisfied.

[WiseOldSage]

Bob

1 Hob and Oven should be on separate supplies 2 If there is a socket on the cooker plate you should consider an RCD 3 Under the wonderful new Prescott tax (part P of the building regs) you probably shouldn't be doing this in a kitchen anyway!:)

[Frank Erskine]

Does the domestic cooker diversity calculation (10A+30%) apply equally to "separates" and a "traditional" combined cooker? I'm interested in this since I'm at the initial thinking stage of remodelling the kitchen, and pondering a separate double oven and hob. Almost certainly the existing feed will be inadequate - it's a 32A MCB feeding a (I think!) 4mm cable to a CCU which has a 13A socket (only currently (sorry) used for a fridge/freezer). I'll certainly do away with the CCU as I want a separate non-RCD feed for the f/f.

[Frank Erskine]

Where else would you (normally) wire a hob and oven?

What's a "Prescott" anyway?

[David Hansen]

On Fri, 11 Aug 2006 21:47:16 +0100 someone who may be WiseOldSage wrote this:-

Why?

[Andy Wade]

I don't see why not, nor have I ever seen any official or quasi-official guidance to the contrary. Of course if you have reason to suspect that the loading will be higher in a particular situation then you must design appropriately.

Eyebrows will be raised if you don't upgrade the cable to 6 or 10 mm^2. 4 mm^2 is actually OK for 37 A (clipped direct, no grouping, 30 deg. C ambient and no BS 3036 rewireable fuse) but the voltage drop and Zs calculations might lead to quite a short max. circuit length. It's something of a hint that the combination of a 30 A OPD and 4/1.5 mm^2 cable does not appear in the table of conventional circuits in the OSG (Table 7.1).

I agree that CCUs with sockets should be consigned to the bin - and indeed to history - and doing that does give you another 5 amps to play with on the diversity calc., allowing another 16 A or so of MD.

[Andy Wade]

I's a processor chip surely?

appropriately noted for generating a lot of heat.