What size electrical cable for oven and hob?

[Great_scot_selfbuild]

We're laying first fix wiring with our electrician but the only size cable we have on site (at the moment at least) is 1.5TE (light), 2.5TE (power) and 3CE (blinds). I've not yet seen any radial circuits for the key appliances that I would expect to have on a dedicated circuit. Having done some searching and ChatGPT (but never fully trusting it), responses come back referring to oven/hobs on a 6 or 10mm² cable and appliances could be on a fuzed spur from a ring, but radial is preferred.

 

I'd much rather have more robustness in our design for the sake of a small cost of some cable whilst all the walls are open. Looking for a second opinion from those who know about this stuff.

 

Detail:

Induction hob, cable run = 25m

double oven & warming drawer, cable run = 30m

 

 

TIA

 

[markc]

What oven and hob are you having? Nothing worse than a big hob on a limited supply. Generally good practice to have a 6mm radial for oven and hob but a few years ago I fitted a powerful 6 section hob and double oven so I ran separate 6mm radials to them.

[Great_scot_selfbuild]

3 minutes ago, markc said:

What oven and hob are you having? Nothing worse than a big hob on a limited supply. Generally good practice to have a 6mm radial for oven and hob but a few years ago I fitted a powerful 6 section hob and double oven so I ran separate 6mm radials to them.

Part of the challenge - we haven't decided yet, but equally this could change in the future anyway, so I'm just looking to install the wiring to be the most robust / good design we can. If it future-proofed the design I'd even go for 10mm, but if this is HUGE overkill and only really needed for industrial systems, then I don't want to waste the money.

 

Based on me searches, I'm inclined to run separate 6mm radials. I assume a double oven is one (not two) cable, and a warming drawer is another(?)

[markc]

@Great_scot_selfbuild yes a twin oven is a single supply, if you run 6mm cables you will be future proof. Warming drawer is better on a switched spur off ring main or off cooker supply as they are quite low power.

[Russell griffiths]

I’m no electrician 

but we have two ovens and a hob, all 3 items are on their own dedicated circuit 

hob with a 10mm cable and the two ovens on 6mm each. 
don’t be using fused spurs, not in a new house. 
and look into your isolation switches, get the location right, no neon switches on show, all hidden nicely. 

[garrymartin]

If you've got an electrician, why aren't they doing the necessary calculations? You have to consider voltage drop over the distances you mention, as well as the route the cable will take and whether any of that route is insulated. Some induction hobs are in the 11kW range. If your oven uses pyrolytic cleaning, and it's a double oven, even 6mm2 may not be enough.

 

At the extreme, an 11kW induction hob at the end of a 25m run of cable enclosed in insulation could need a 25mm2 cable and a 50A RCBO or equivalent.

 

Can't stress strongly enough that your electrician should be looking at these sorts of considerations and advising you on the appropriate cable sizes.

 

Like @Russell griffiths we have 10mm2 to an induction hob, 6mm2 to a pyrolytic single oven, and 6mm2 to a combination pyrolytic oven/microwave which also serves a warming drawer, all on radial circuits with their own RCBOs.

 

 

[FarmerN]

Our 110cm Range cooker is rated at 17 kW .

Interesting as our transformer is rated at 15 Kw !

 

The other wiring we put in which my wife likes is a 5amp socket circuit for side lights switched at door separate to main lights.

Make sure you make provision for a car charger, even if you have no intention of getting one yet.

[Great_scot_selfbuild]

15 minutes ago, garrymartin said:

Can't stress strongly enough that your electrician should be looking at these sorts of considerations and advising you on the appropriate cable sizes.

Couldn’t agree more. I’ve yet to find an element of this project where doing some due diligence and checking of my own hasn’t thrown up very valuable questions or even identified glaring errors.

 

I equate it to doing a QA ‘dip check’ and then finding I quickly need to ramp up to a 100% check 😭

[Great_scot_selfbuild]

14 minutes ago, FarmerN said:

Make sure you make provision for a car charger, even if you have no intention of getting one yet.

Mandated in our planning conditions - seems to be a default on all planning approvals from our council (not sure if wider as they all seem to vary so much).

[Great_scot_selfbuild]

22 minutes ago, garrymartin said:

At the extreme, an 11kW induction hob at the end of a 25m run of cable enclosed in insulation could need a 25mm2 cable and a 50A RCBO or equivalent.

@garrymartin Out of interest, how is this calculated?

 

In terms of ‘enclosed in insulation’, I’ve just been thinking this over, how enclosed is enclosed (if this makes sense). Our situation:

There will be 75mm batt insulation in the 89mm stud walls (3m high), then cables running above the insulation in the ceiling space (425mm deep posi joists, with 100mm acoustic insulation), and then out into the plant room. 

[Mattg4321]

So many people don’t take diversity into account when calculating cable sizes for cooking appliances. 
 

Add the first 10 amps to 30% of the remainder. It’s never failed me in 25 years of using this. 6mm2 is almost certainly enough unless there’s huge amounts of insulation and/or a very long run.
 

My induction 110cm range at home is on 6mm2 and 32A RCBO when total load is over 16kW (around 70A). Don’t think I’ve ever known it to go above about 25A. 
 

Running 25mm2 is just madness. 
 

On the flip side, the amount of arguments I get into when talking about cable sizes for electric showers. People run 6mm2 when it’s very often not big enough as there is no diversity allowed. 
 

Sometimes the same guys who run in 10mm2 for a 5 ring hob, will run 6mm2 to an electric shower. Absolute muppets. 

[garrymartin]

35 minutes ago, Great_scot_selfbuild said:

Out of interest, how is this calculated?

 

There's a whole series of tables and calculations related to cable and protective device sizing in BS 7671. Start with this video to understand how complex it can get and why your electrician should be doing the relevant calculations.

 

By the way, 100% agree with the comment about personal research so that you are informed and can pose sensible questions and query statements. But you really want the electrician to be professionally responsible for the calculations and installation.

 

 

[garrymartin]

5 minutes ago, Mattg4321 said:

So many people don’t take diversity into account when calculating cable sizes for cooking appliances.

 

Diversity only applies where there are multiple loads on a single cable.

 

Whilst it might be appropriate for my combination oven and warming drawer example, it's not appropriate for the induction hob, as this is the only load on the radial circuit. Same reasons as for an electric shower - diversity isn't applied because it's the only load on the circuit.

[-rick-]

10 minutes ago, garrymartin said:

Diversity only applies where there are multiple loads on a single cable.

 

Whilst it might be appropriate for my combination oven and warming drawer example, it's not appropriate for the induction hob, as this is the only load on the radial circuit. Same reasons as for an electric shower - diversity isn't applied because it's the only load on the circuit.

 

Don't think that's right. There are multiple elements in cookers/hobs. Not all are on at the same time for long time periods.

 

Video I posted in another thread showed regs saying a 32A (7kw continuous) feed was sufficient for a cooker rated up to 18kw due to diversity calcs. (I'm not an electrician so don't rely on what I say)

[Temp]

2 hours ago, -rick- said:

There are multiple elements in cookers/hobs. Not all are on at the same time for long time periods.

 

I dare you tell my wife at Christmas she can only have 4 out of 6 rings on 😟

[-rick-]

9 minutes ago, Temp said:

I dare you tell my wife at Christmas she can only have 4 out of 6 rings on 😟

 

That's the point. Even if every ring is 'on' most of them won't be drawing much power. Most will be drawing a fraction of their rating to maintain the temperature.

 

The only time a ring will draw maximum power for more than a handful of seconds is after you first switched it on and say you have a pan of water you want to bring to the boil. Then that ring will be on for like 2-3mins continuously. It is exceedingly unlikely that your wife (or anyone) goes into the kitchen, gets out six pans and puts them all on the hob at the same time and sets them all to max. Even in that case, the rating on a circuit is designed for cable protection, ie, a 32A circuit can sustain a 32A draw continuously over many hours on the hottest day of the year without melting/starting a fire/etc. The cable (and the breaker) can sustain much higher currents for short periods of time and that's what these diversity calculations take into account. They allow for a short period of very high usage (much higher than rating) so long as the average over time is below the rating by enough.

Edited 8 hours ago by -rick-

[Temp]

Do the thermostats reduce power or just switch on and off (PWM)? I thought it was the latter but if it's the former I agree.

[-rick-]

1 minute ago, Temp said:

Do the thermostats reduce power or just switch on and off (PWM)? I thought it was the latter but if it's the former I agree.

 

Depends on your device. Expensive ones can do some form of variable power, though I would call PWM a smart form. The dumb control is bang-bang control (https://en.wikipedia.org/wiki/Bang–bang_control)

 

But again this is irrelevant to the discussion. Even the bang bang control leads to the average current draw for the hob being much lower than the max draw. Again, if there are short periods where everything aligns and every hob is on simultaneously for some seconds that's accounted for.

[garrymartin]

4 hours ago, -rick- said:

Video I posted in another thread showed regs saying a 32A (7kw continuous) feed was sufficient for a cooker rated up to 18kw due to diversity calcs. (I'm not an electrician so don't rely on what I say)

I think the calculations you are referring to relate to the design current for the protective device, not the size of the cable you should put in. But caveat, I'm not an electrician either... 😉

[Mattg4321]

8 hours ago, garrymartin said:

 

Diversity only applies where there are multiple loads on a single cable.

 

Whilst it might be appropriate for my combination oven and warming drawer example, it's not appropriate for the induction hob, as this is the only load on the radial circuit. Same reasons as for an electric shower - diversity isn't applied because it's the only load on the circuit.

 

Not true. See below in Table A2 of the On Site Guide. Yes, this is only a 'guide', but as above, I've been doing this for 25 years and have probably fitted thousands of cookers and following this formula has never yet caused me a problem and often saves my customers lots of unnecessary cost and disruption in an existing property.

 

8 hours ago, -rick- said:

 

Don't think that's right. There are multiple elements in cookers/hobs. Not all are on at the same time for long time periods.

 

Video I posted in another thread showed regs saying a 32A (7kw continuous) feed was sufficient for a cooker rated up to 18kw due to diversity calcs. (I'm not an electrician so don't rely on what I say)

 

Correct

 

3 hours ago, garrymartin said:

I think the calculations you are referring to relate to the design current for the protective device, not the size of the cable you should put in. But caveat, I'm not an electrician either... 😉

 

Yes, which has a large influence on cable size. The others being installation methods, correction factors and volt drop usually. It's fairly unusual for 6mm2 not to be big enough in a domestic property. I am an electrician 😉

Diversity - On Site Guide - 18th Ed BS7671.pdf

[garrymartin]

I think we're going to have to agree to disagree on this one @Mattg4321 - even the OSG states "The current demand of a final circuit is determined by adding the current demands of all points of utilisation and equipment in the circuit and, where appropriate, making an allowance for diversity." I'd argue that it isn't appropriate when you have a final circuit (radial) feeding a single appliance such as an induction hob or an oven as there aren't multiple points of utilisation.

 

AFAIK, the Cooking Appliances calculation has been in the guides with the same information since at least 1966, so I'd argue it's long overdue for an update to take into account modern cooking appliances and installation approaches! 🙂

 

[-rick-]

2 minutes ago, garrymartin said:

I'd argue that it isn't appropriate when you have a final circuit (radial) feeding a single appliance such as an induction hob or an oven as there aren't multiple points of utilisation.

 

Do you not have separate controls for each ring on a hob? If you only have a single point of utilisation, all the rings turn on when you turn one on?

 

Modern induction hobs usually have ratings for the individual rings and even break out groups of rings to different wiring terminals (to allow you to split the load over multiple phases).

[Mattg4321]

14 minutes ago, garrymartin said:

I think we're going to have to agree to disagree on this one @Mattg4321 - even the OSG states "The current demand of a final circuit is determined by adding the current demands of all points of utilisation and equipment in the circuit and, where appropriate, making an allowance for diversity." I'd argue that it isn't appropriate when you have a final circuit (radial) feeding a single appliance such as an induction hob or an oven as there aren't multiple points of utilisation.

 

AFAIK, the Cooking Appliances calculation has been in the guides with the same information since at least 1966, so I'd argue it's long overdue for an update to take into account modern cooking appliances and installation approaches! 🙂

 

 

You'd say an induction hob with multiple rings doesn't have multiple points of utilisation? It's a moot point really though. I have 25 years experience in this exact area, doing this sort of thing every day of the week. If someone wants to ignore my opinion, and that of the IET On Site Guide, then I don't think I'll even attempt to change their mind.

[Nickfromwales]

1 hour ago, -rick- said:

 

Do you not have separate controls for each ring on a hob? If you only have a single point of utilisation, all the rings turn on when you turn one on?

 

Modern induction hobs usually have ratings for the individual rings and even break out groups of rings to different wiring terminals (to allow you to split the load over multiple phases).

Plus these individual hobs will modulate with thermostatic control, so will be all switching on / off sporadically and independently.

 

32a is a boatload of juice to use up in fairness; I say this whilst considering the amount of homes I've serviced over the last 30+ years that have only had a 60a fuse in the supply head, feeding the entire (sometimes sizeable) residence.

[mjc55]

13 hours ago, Russell griffiths said:

 
don’t be using fused spurs, not in a new house. 
 

That's interesting - why do you say that?