Kitchen redesign and electrical constraints

[mrmistoffelees]

Hi,

We're planning a redesign of our kitchen and have pretty much got it down to where we want it. At the moment, we have a large range cooker supplied by gas. The new kitchen will have two electric double ovens and an electric venting hob on the island. This is all fine.

Except, last night in bed, I had a horrible "Oh fk" moment when I realised that potentially I've got an issue with the peak load in that we're on a single phase 100A incomer.

The ovens we're intending to fit have a specified max load of 6300W (so ~26 Amps), and the hob is 7400W (so ~31 Amps) That gives me:

Ovens: 52 amps
Hob: 31 amps
EV charger: 32 amps

== 115 Amps excluding the rest of the house.

I think I need three phase, don't I?

[Bill]

How often are you likely to have all those things running flat out?

[mrmistoffelees]

Probably a handful of times a year - Christmas Day being the most likely candidate. We already use the three ovens we have in the range for that, and the full capacity of the hob we have at the moment so it's entirely plausible the whole kitchen will be being used then. EV charger? I'd guess maybe not...

[Bill]

But even then you won't turn both ovens on to warm up while you're using the full hob flat out.

[PhilboSE]

Look up “diversity” for electrical loadings. You’re allowed to have lower supply than theoretical max load because it won’t all be on.

If you ever did have an issue and things tripped, you could just turn off the EV charger while the Christmas bird was in the oven.

[LooneyTunes]

PhilboSE said:

Look up “diversity” for electrical loadings. You’re allowed to have lower supply than theoretical max load because it won’t all be on.

If you ever did have an issue and things tripped, you could just turn off the EV charger while the Christmas bird was in the oven.

True, but his concern is probably the risk of popping the DNO fuse, which would be a PITA (even though it probably won’t go at dead on 100A anyway).

Seems to be easily and completely avoidable by not charging the EV if he is making full use of the kitchen.

It’s also usually possible to set a lower max current draw for induction hobs, possibly ovens too, in case the cabling isn’t up to the job of handling full power.

[Mr Pointy]

Diversity may be your saviour: you don't have to allow for the maximum rating of each appliance in the kitchen, there's a calculation that takes care of the fact not everything is on flat chat at the same time. Calculate the cooking load, take off 10A & then calculate 30% of the remainder, add that to the 10A & then another 5A if there's a socket outlet in the cooker switch.

6300+7400 = 13,700 W
13700/240 = 57A
10 + (47x0.3) = 10+14 = 24A

Presumably your EV charger is a 7.4kW jobbie so about 31A maximum giving s total of 55A, which would indicate that an 80A might be adequate as long as you don't have too many electric showers on at the same time. Many EV chargers have the ability to monitor the total house current via a current transformer in the meter box & can back off the charger load when the rest of the house is using a lot of current.

[mrmistoffelees]

Mr Pointy said:

Diversity may be your saviour: you don't have to allow for the maximum rating of each appliance in the kitchen, there's a calculation that takes care of the fact not everything is on flat chat at the same time. Calculate the cooking load, take off 10A & then calculate 30% of the remainder, add that to the 10A & then another 5A if there's a socket outlet in the cooker switch.

6300+7400 = 13,700 W
13700/240 = 57A
10 + (47x0.3) = 10+14 = 24A

Presumably your EV charger is a 7.4kW jobbie so about 31A maximum giving s total of 55A, which would indicate that an 80A might be adequate as long as you don't have too many electric showers on at the same time. Many EV chargers have the ability to monitor the total house current via a current transformer in the meter box & can back off the charger load when the rest of the house is using a lot of current.

Ah - that's what I was looking for - perfect. And, yes, the EV charger will back off. Phew. I was envisaging another few grand from the money printer on top of an already painfully expensive project. Thanks!

[Mr Pointy]

mrmistoffelees said:

Mr Pointy said:

Diversity may be your saviour: you don't have to allow for the maximum rating of each appliance in the kitchen, there's a calculation that takes care of the fact not everything is on flat chat at the same time. Calculate the cooking load, take off 10A & then calculate 30% of the remainder, add that to the 10A & then another 5A if there's a socket outlet in the cooker switch.

6300+7400 = 13,700 W
13700/240 = 57A
10 + (47x0.3) = 10+14 = 24A

Presumably your EV charger is a 7.4kW jobbie so about 31A maximum giving s total of 55A, which would indicate that an 80A might be adequate as long as you don't have too many electric showers on at the same time. Many EV chargers have the ability to monitor the total house current via a current transformer in the meter box & can back off the charger load when the rest of the house is using a lot of current.

Ah - that's what I was looking for - perfect. And, yes, the EV charger will back off. Phew. I was envisaging another few grand from the money printer on top of an already painfully expensive project. Thanks!

Out of interest what size MCB is your sparky installing for the kitchen cooker circuit/s?

[mrmistoffelees]

Mr Pointy said:

Out of interest what size MCB is your sparky installing for the kitchen cooker circuit/s?

We're at the point between kitchen has been designed and got the contractor on board that's dealing with the install/everything (plastering, plumbing, etc). The sparks work is being subbed out, but I'm already aware that the CU I've got is woefully inadequate going forwards in that it has no spare capacity at all, primarily. The electrician hasn't yet visited/been engaged so I've not yet got anyone to bounce this off.

I've said that I want a circuit per device (ie one per cooker) and also a separate kitchen ring and a separate lighting circuit. I'd imagine it'd be a 40A per circuit for the cookers and hob, ring would probably be a 32 and the lighting a 6. The CU is within a few metres of the kitchen and the ceilings are coming down anyway so makes sense to do it once and do it properly. Other option would be a dedicated sub board for the kitchen coming off Henley blocks but that feels a bit messy.

[essayer]

How does diversity work if you turn on a couple of hob rings and both ovens at the same time? (While you’re charging the EV)

I think the only answer is make sure the EV charge has current measurement built in, so it can throttle back if there’s a risk of exceeding the main fuse rating

[Sheepshanks]

essayer said:

How does diversity work if you turn on a couple of hob rings and both ovens at the same time? (While you’re charging the EV)

That would be fine. He’d only have a problem if he turned all 4 ovens on and all 4 hob rings.

Anyway he’s already said the charger will back itself off.

[Mr Pointy]

mrmistoffelees said:

Mr Pointy said:

Out of interest what size MCB is your sparky installing for the kitchen cooker circuit/s?

We're at the point between kitchen has been designed and got the contractor on board that's dealing with the install/everything (plastering, plumbing, etc). The sparks work is being subbed out, but I'm already aware that the CU I've got is woefully inadequate going forwards in that it has no spare capacity at all, primarily. The electrician hasn't yet visited/been engaged so I've not yet got anyone to bounce this off.

I've said that I want a circuit per device (ie one per cooker) and also a separate kitchen ring and a separate lighting circuit. I'd imagine it'd be a 40A per circuit for the cookers and hob, ring would probably be a 32 and the lighting a 6. The CU is within a few metres of the kitchen and the ceilings are coming down anyway so makes sense to do it once and do it properly. Other option would be a dedicated sub board for the kitchen coming off Henley blocks but that feels a bit messy.

This is where you are possibly going to make it hard for yourself by speccing individual radials to tthe cooker & the ovens as now the overall diversity calculation doesn't apply when sizing the MCBs for them - you'll have three calculations.
7400/240 = 31A then 10+(21*0.3) = 16.3A
6300/240 = 26A then 10+(16*0.3) = 14.8A

If he fits 3x 20A MCBs then you could pull 60A via the kitchen cookers (pretty unlikley though, except on Christmas Day morning). Time to see what your electrician says.

[speedyman]

Ovens have thermostats to control the heat, so once up to temperature the power is cut to the element until the temperature drops a little and the process cycles. So the ovens load will be on and off during use.

[Trustmeimadoctor]

Also the hob likely will not allow all rings to be on the full draw

[98elise]

speedyman said:

Ovens have thermostats to control the heat, so once up to temperature the power is cut to the element until the temperature drops a little and the process cycles. So the ovens load will be on and off during use.

Same with the hob. That's why you consider diversity. You're unlikely to have everything on at once, and even then you wouldn't be pulling the maximum current 100% of the time.

[gotoPzero]

Just don't have the car charging on those rare days you will have all the other stuff on?


Also, not sure on your oven specs but mine are neff and if you look at the different modes there is only one mode that draws full beans.
I forget which one it is but it runs all 3 elements at once plus the fan. In "normal" non super death kill mode (think its called eco therm or something) its just the rear element with fan.

HTH.

[CharlesElliott]

You won't pull that current. A hob or oven pulls larger currents for short periods of time and diversity calcs also support that.

Secondly, most car chargers can monitor incoming load and reduce output if needed.

I looked at getting three phase when we did refurbishment but the cost was prohibitive (£20K+). If you add up all the RCBOs in my consumer unit it gets to 400A+, but the largest actual draw I've ever seen is in the region of 75A, and that was for a brief period.

[LooneyTunes]

Comments on this thread make little sense to me.

mrmistoffelees said:

The ovens we're intending to fit have a specified max load of 6300W (so ~26 Amps)

The manufacturer says the max load of the single appliance is 6300W

and:

mrmistoffelees said:

I've said that I want a circuit per device (ie one per cooker)

Mr P says that you can't apply diversity to their own radials (which logically makes sense) but then seemingly suggests having 20A MCBs fitted?

Mr Pointy said:

This is where you are possibly going to make it hard for yourself by speccing individual radials to tthe cooker & the ovens as now the overall diversity calculation doesn't apply when sizing the MCBs for them - you'll have three calculations.
7400/240 = 31A then 10+(21*0.3) = 16.3A
6300/240 = 26A then 10+(16*0.3) = 14.8A

If he fits 3x 20A MCBs then you could pull 60A via the kitchen cookers (pretty unlikley though, except on Christmas Day morning). Time to see what your electrician says.

We then have these comments:

98elise said:

speedyman said:

Ovens have thermostats to control the heat, so once up to temperature the power is cut to the element until the temperature drops a little and the process cycles. So the ovens load will be on and off during use.

Same with the hob. That's why you consider diversity. You're unlikely to have everything on at once, and even then you wouldn't be pulling the maximum current 100% of the time.

It makes no sense whatsoever to me why you would apply any diversity/undersize a MCB to a single appliance on its own radial that could well run all of its elements simultaneously and at full power (either for rapid warmup, self-cleaning, or on certain cooking programmes).

I get it with a hob where you might not, and can choose not to, run all rings at the same time (albeit I would still want the circuit sized to take full power or limit the load drawn in the hob's settings) but I can't get my head round why people are suggesting seemingly undersizing the MCB on a radial to a single appliance. You'd end up with it constantly tripping?

What am I missing?

Surely the manufacturer isn't overstating max load just to take into account people deciding that it won't actually draw that?

Edited by LooneyTunes on Monday 26th August 14:43

[CharlesElliott]

You can calculate what you think the max load will be (although then you need to know what the 'actual' draw of a cooker or appliance will be, rather than the rated draw), or you can use diversity rules, or some combination of your choosing. But as a starting point, the diversity rating for a domestic cooker is 10A + 30% of the remaining rated value......which is approx 17A or 50% of a 32A rated appliance.

Edited by CharlesElliott on Monday 26th August 14:49