25A MCB on 2.5mm kitchen radial

[Joeyscurlock]

First post here,

Wired 2 new 2.5mm t&e radial circuits for a kitchen. Single oven being installed so haven't used a dedicated circuit.

Stupid of me not to check but oven is 3.5kw, large for a single oven id say!

I've tried to work out max load on the circuit and I've worked out if I was to put in a 25A MCB in place of the 20A I originally planned I should be okay. Looking at around 5500-5700W when everything is in use, roughly 23-24A.

Cables are clipped direct then under the plinths, run in free air. Ref method C, which some say 25-27A

Bit worried on the loading on the cable. Can anyone clarify if I'm okay to be doing this?

Thanks

 

[Taylortwocities]

Hmm. You're not up on circuit design are you?

First of all, we do not necessarily use the maximum operating current of each appliance when calculating loads and current demand.

The Oven may be rated at 3.5KW but we use a technique called DIVERSITY to work out what the need for the oven will actually be. Even at max full, sack the oven only wants about 15amp (and it wont get to that).

But really one of those radials should be a dedicated circuit for the oven (you'll need an isolating switch for it).

You say teh total is 5.5-5.7KW so that means you can EASILY have one 20A radial for the oven and another 20Aradial for the other 2KW ish.

 

[davelx]

beaten to it

 

[Joeyscurlock]

Thanks lads, I know bit of a cock up.

 

[JohnW2]

You say teh total is 5.5-5.7KW so that means you can EASILY have one 20A radial for the oven and another 20Aradial for the other 2KW ish.

From the fact that he was contemplating the need for a 25A MCB, I assumed that the OP was taking about a possible 5.7kW load on just one of the radials.

If that interpretation was correct, then .... if (as is likely) the oven is 3.5kW at 240V, hence about 14A at 230V, that would amount to about 11.2A at 230V after diversity. If diversity was not applicable for the other 2.2kW, that would add about 9.6A, taking the total to 20.8A - no problem at all for the cable, but theoretically (if one is being 'correct'/pedantic) very slightly naughty for a 20A MCB. Even if the oven were 3.5kW at 230V, the total would only rise to about 21.6A. Of course, if diversity could be applied to any of the 'extra' 2.2kW, then all would be well, even for the pedantic.

Kind Regards, John

 

[OwainDIYer]

It's a single oven. Its element is either on or off. I don't see how diversity applies. It's not a cooker which will have various bits on at different times.

Also if it's > 13A it can't be wired through a FCU and so shouldn't be be on a standard circuit shared with BS1363 socket outlets.

Dedicated radial for oven required with 16A or 20A protection. Mfr's instructions may require protection at 16A or 20A for the appliance/flex.

 

[JohnW2]

It's a single oven. Its element is either on or off. I don't see how diversity applies. It's not a cooker which will have various bits on at different times.

Everything you say is true, except that I very much doubt that 3.5kW is a single element. Whatever, I think it's a case of "go tell that to the OSG" (the regs themselves say nothing specific about diversity!) - which says that the 10A+30% diversity calculation applies to any 'domestic cooking appliance'! To be fair, they probably are (may be) thinking in terms of 'diversity over time' - i.e. the average current demand over a significant period of time.

Also if it's > 13A it can't be wired through a FCU and so shouldn't be be on a standard circuit shared with BS1363 socket outlets.

I can't argue with that either - but, again, the manufacturer may say that it can be protected by a 13A fuse if they are thinking in terms of an after-diversity current demand (which I would have thought a bit naughty).

Joeyscurlock: Do the oven's instructions say that it is OK with a 13A fuse in a plug or FCU, or do they talk of 'hard wiring' and some higher current protective device?

Kind Regards, John

 

[PrenticeBoyofDerry]

Sounds like hash up to me!
Why was a dedicated circuit for the oven not installed?

 

[EFLImpudence]

Mfr's instructions may require protection at 16A or 20A for the appliance/flex.

I would not say 'appliance/flex' - they would demand one or the other.

That is - if 16A protection demanded, it would be because of flex supplied, or -
if 16A supply demanded it would be because of appliance requirement and no flex supplied.

 

[Joeyscurlock]

There will be 5.5-5.7KW on one of the radials alone. Plus maybe 3.5kw on the other radial.

Yeah bit of a cock up, I didn't know the cooker size before the first fix thought it was less than the 3.5kw.

Diversity applied, a clamp meter to check with all the appliances on and see from there. Thanks for the advice.

 

[JohnW2]

There will be 5.5-5.7KW on one of the radials alone. Plus maybe 3.5kw on the other radial.

Thanks for confirming. That's what I thought. What is the nature of the loads which make up the other 2.0-2.2kW on the cooker circuit?

Yeah bit of a cock up, I didn't know the cooker size before the first fix thought it was less than the 3.5kw. ... Diversity applied, a clamp meter to check with all the appliances on and see from there. Thanks for the advice.

That's all very well but, as I asked, what does the manufacturer of the oven say about the required supply and protective device? Does it say that you can use a 13A plug or a 13A FCU, or does it say something else?

As I wrote before, manufacturers usually quote power at 240V (to make their products sound more powerful) but circuit design is done at 230V. Ig the 3.5kW is at 240V, that would correspond to around 14A (about 3.2kW) at 230V - but even that is still (without diversity) over 13A.

Kind Regards, John

 

[Joeyscurlock]

There will be 5.5-5.7KW on one of the radials alone. Plus maybe 3.5kw on the other radial.

Thanks for confirming. That's what I thought. What is the nature of the loads which make up the other 2.0-2.2kW on the cooker circuit?

Yeah bit of a cock up, I didn't know the cooker size before the first fix thought it was less than the 3.5kw. ... Diversity applied, a clamp meter to check with all the appliances on and see from there. Thanks for the advice.

That's all very well but, as I asked, what does the manufacturer of the oven say about the required supply and protective device? Does it say that you can use a 13A plug or a 13A FCU, or does it say something else?

As I wrote before, manufacturers usually quote power at 240V (to make their products sound more powerful) but circuit design is done at 230V. Ig the 3.5kW is at 240V, that would correspond to around 14A (about 3.2kW) at 230V - but even that is still (without diversity) over 13A.

Kind Regards, John

13A Plug top

 

[knowall]

First post here,

Wired 2 new 2.5mm t&e radial circuits for a kitchen. Single oven being installed so haven't used a dedicated circuit.

Stupid of me not to check but oven is 3.5kw, large for a single oven id say!

I've tried to work out max load on the circuit and I've worked out if I was to put in a 25A MCB in place of the 20A I originally planned I should be okay. Looking at around 5500-5700W when everything is in use, roughly 23-24A.

Cables are clipped direct then under the plinths, run in free air. Ref method C, which some say 25-27a


Bit worried on the loading on the cable. Can anyone clarify if I'm okay to be doing this?


Thanks

I would advise you to add a separate circuit for the oven.
Then you will be able to sleep !

KA

 

[SimonH2]

Also if it's > 13A it can't be wired through a FCU and so shouldn't be be on a standard circuit shared with BS1363 socket outlets.

Does that apply to radials ?

I know it's a specific rule about RFCs which allow for a cable with a CCC less than the protective device rating, but I can't see a valid reason to impose it on a radial (as long as the device is suitable for the size of breaker or you fit local protection).

 

[JohnW2]

Also if it's > 13A it can't be wired through a FCU and so shouldn't be be on a standard circuit shared with BS1363 socket outlets.

Does that apply to radials ? I know it's a specific rule about RFCs which allow for a cable with a CCC less than the protective device rating, but I can't see a valid reason to impose it on a radial (as long as the device is suitable for the size of breaker or you fit local protection).

I don't think it's anything to do with the type of final circuit. I think that OwainDIYer's point was that, if the oven drew more than 13A, it should not be on a circuit protected by a 13A fuse (design current >In of OPD is a general no-no) - either in a plug or an FCU. However, since the manufacturer apparently says that a plug (hence 13A fuse) is OK, I can but presume that they are thinking in terms of the after-diversity current demand - which surprises me a little.

Kind Regards, John