Capacity on ring main

[Sparra]

Hi,
I'm a DIY'er, new to this forum.

My situation is, I'm laying a new kitchen ring circuit for my sister. The ring will supply the obvious kitchen appliances including 3kW oven, fan, washing machine, dishwasher, f/f, microwave, toaster, kettle, plus an elec fire at the far end. Potentially an iron. It's unlikely that all those will be on and at full load at the same time, but technically possible. Boiler and hob will be on separate supplies. The ring will have MCB and RCD protection.

In terms of cable and protection, I was going to use the logic of the previous wiring and also the wiring of my own home (which is part p certified and been inspected/tested when I moved in). That is, 32A MCB and 2.5mm cable.

However, I'm confused about how this can be sufficient. Can you explain?

• All those appliances add up to something like 14kW. But if we've got a 32A breaker then doesn't that give a max capacity of 7360W? (I'm confused because surely most kitchens have more than this wattage in them)?
• And the 2.5mm cable, which radially carries 27A or 6.2kW, and could carry 12.4 kW in a ring. But again, surely most kitchens have appliances of more than this without meltdown

I've presumably got my thinking fundamentally wrong here. In which case, apologies for dumb question. However, I would like to know if it's ok to lay this ring in 2.5mm t&e to a 32A MCB. Or not. And I'd also really appreciate an explanation so I understand.

Thanks!
H

 

[Pauls0212]

I would give the oven its own supply and the rest on the ring, but im sure other people will tell you washing machine and dryer also on its own ring. Having the oven on with the ring is asking for overload trips.
the reason for it not tripping is diversity.

 

[pc1966]

One basic assumption in the Ring Final Circuit is the loads are suitable distributed so both legs of the ring are carrying broadly similar currents, really the assumption is 2.5mm can carry at least 20A so as long as you have no worse than a 1.5:1 split in current then a 32A breaker is fine to protect that.

Second assumption is diversity: that all loads are not likely to be on simultaneously for any significant length of time. This is one reason what any fixed loads above 3kW should not be put on a RFC, they should have their own dedicated radial.

If you look at the guidance for domestic cookers the IET guidance is 10A plus 30% of any current above that. So a typical 50A large cooker would be taken as 10 + 0.3*(50-10) = 22A for circuit rating. Again, this is domestic, if you are designing for a professional kitchen they have significantly higher usage.

 

[Pete999]

Hi,
I'm a DIY'er, new to this forum.

My situation is, I'm laying a new kitchen ring circuit for my sister. The ring will supply the obvious kitchen appliances including 3kW oven, fan, washing machine, dishwasher, f/f, microwave, toaster, kettle, plus an elec fire at the far end. Potentially an iron. It's unlikely that all those will be on and at full load at the same time, but technically possible. Boiler and hob will be on separate supplies. The ring will have MCB and RCD protection.

In terms of cable and protection, I was going to use the logic of the previous wiring and also the wiring of my own home (which is part p certified and been inspected/tested when I moved in). That is, 32A MCB and 2.5mm cable.

However, I'm confused about how this can be sufficient. Can you explain?

• All those appliances add up to something like 14kW. But if we've got a 32A breaker then doesn't that give a max capacity of 7360W? (I'm confused because surely most kitchens have more than this wattage in them)?
• And the 2.5mm cable, which radially carries 27A or 6.2kW, and could carry 12.4 kW in a ring. But again, surely most kitchens have appliances of more than this without meltdown

I've presumably got my thinking fundamentally wrong here. In which case, apologies for dumb question. However, I would like to know if it's ok to lay this ring in 2.5mm t&e to a 32A MCB. Or not. And I'd also really appreciate an explanation so I understand.

Thanks!
H

It is advisable to supply loads of 2KW and above with their own dedicated circuit

 

[Sparra]

One basic assumption in the Ring Final Circuit is the loads are suitable distributed so both legs of the ring are carrying broadly similar currents, really the assumption is 2.5mm can carry at least 20A so as long as you have no worse than a 1.5:1 split in current then a 32A breaker is fine to protect that.

OK, but it seems that even with even distribution of current, those appliances would see more than 20A on each leg?

Second assumption is diversity: that all loads are not likely to be on simultaneously for any significant length of time. This is one reason what any fixed loads above 3kW should not be put on a RFC, they should have their own dedicated radial.

None of the appliances will be over 3kW. The oven is 3kW. And potentially a kettle could be 3kW too. So I was planning to put these on the ring in order to avoid another MCB which there isn't space for in the CU. That ok?

If you look at the guidance for domestic cookers the IET guidance is 10A plus 30% of any current above that. So a typical 50A large cooker would be taken as 10 + 0.3*(50-10) = 22A for circuit rating. Again, this is domestic, if you are designing for a professional kitchen they have significantly higher usage.

So using that logic, the total amps of all those appliances is, say 60A (1400kW/230). so circuit rating needs to be at least 10 + 0.3*(60-10) = 25A. If that was on a radial then 2.5mm would be insufficent, but on a ring it's fine. Right?
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It is advisable to supply loads of 2KW and above with their own dedicated circuit

Oh.
But surely lots of people have a 3kW kettle that they don't have a dedicated circuit for?
It's so tricky not knowing how these rules work

 

[Pete999]

OK, but it seems that even with even distribution of current, those appliances would see more than 20A on each leg?



None of the appliances will be over 3kW. The oven is 3kW. And potentially a kettle could be 3kW too. So I was planning to put these on the ring in order to avoid another MCB which there isn't space for in the CU. That ok?



So using that logic, the total amps of all those appliances is, say 60A (1400kW/230). so circuit rating needs to be at least 10 + 0.3*(60-10) = 25A. If that was on a radial then 2.5mm would be insufficent, but on a ring it's fine. Right?
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Oh.
But surely lots of people have a 3kW kettle that they don't have a dedicated circuit for?
It's so tricky not knowing how these rules work

not a fixed load though is it?

 

[telectrix]

oven..... put it on it's own dedicated circuit.
kettle..... although can be 3kW, it's only on for a couple of minutes. fine on the ring.
washer and dryer.... fine on the ring. each is only about 10A when heating . you could give them a dedicated circuit, but not necessary.

 

[Sparra]

oven..... put it on it's own dedicated circuit.
kettle..... although can be 3kW, it's only on for a couple of minutes. fine on the ring.
washer and dryer.... fine on the ring. each is only about 10A when heating . you could give them a dedicated circuit, but not necessary.

OK, makes sense. Thanks.
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not a fixed load though is it?

No you're right it isn't.

 

[pc1966]

Cable and the related protection (fuse, MCB) are able to deal with modest overloads for short times. That is why you can run a lot of stuff that occasionally goes above the rating to 25% or so without incident, and the protective devices are designed to roughly match the cable characteristics so they are a good match for protection.

Provided big loads like cookers or showers (that can be one for many minutes or even an hour or so) are not on the ring and taking the majority of its capacity.

 

[davesparks]

None of the appliances will be over 3kW. The oven is 3kW. And potentially a kettle could be 3kW too. So I was planning to put these on the ring in order to avoid another MCB which there isn't space for in the CU. That ok?

But surely lots of people have a 3kW kettle that they don't have a dedicated circuit for?
It's so tricky not knowing how these rules work

The oven should be on a dedicated circuit, not on a general purpose socket circuit as per the wiring regulations.
A lack of space in the CU is not a valid reason to make a departure from the wiring regulations.
You mention adding MCB's, how are you providing residual current protection for the new circuits?

If you don't know the wiring regulations and how to apply them to the work you are doing then you should not be doing the work.

Do you know how to carry out the necessary inspection and testing of your new work to issue a certificate?

 

[G1ZQC_David]

Don't be mean, he is asking all the right questions.
But yes the oven really needs its own radial (and isolator switch nearby), 6mm cable and at least a 40 amp MCB, better still one of the newer RCBOs, which protect both the cable, AND the user from earth faults.
Any ring circuit is 32A MCB or RCBO protection.

 

[shaun1]

As you have identified, the kitchen is where most high current loads are usually located. This means there is a much higher chance of causing overheating and potentially a fire, if you dont get it right. It is even more inportant to make sure all connections are secure and well made, and that everything is installed correctlly.

As already mentioned, the circuit will need to have a full set of tests (incliding Insulation resstance, polarity, earth loop impedance, end to end resistance and others) once finished. End to end testing is particulaly important here to make sure the ring circuit is complete, as it will function normally with a break in it but is much more likely to become overloaded.