Changes to connect a 16A (hardwired) oven to 13A socket

[JohnW2]

Playing devils advocate here... Now before you run off and all point at 433.1.5 it refers to accessories... could you call a MCB in an enclosure an accessory?

I presume you are talking about 433.1.103 (per BGB). If so, what it refers to is accessories to BS1363 - and I don't think you'll find 'an MCB in an enclosure' which qualifies as that!

Kind Regards, John
Edit: mind/fingers obsession with BS7671 corrected to what it should have said!

 

[ban-all-sheds]

Or even BS 1363...

 

[JohnW2]

Or even BS 1363...

Whoops .... corrected! Thanks.

Kind Regards, John

 

[studentspark]

THis is a problem I come about all the time, and according to Appendix 15 , it can't be done.



Adding a cooker to a ring final circuit circuit is a bad design as the loads of all the items could add up to a large current draw.

A cooker on its own circuit is the way to go, and in the scheme of things could be there for 20 years, so a bit of effort now will save headache later on.
There is always a way to get power somewhere

Saying that though I've always wondered about some of the maths and reasoning, to why one thing is acceptable and not another.

A fix load of max 16 amps will not in itself overload the circuit.
However we can add a double socket with no fixed load, that could use 26 amps plus the fusing factor of the 13amp fuses

You could argue that fitting a cooker connection plate in place of a socket that has been supplying s 13amp cooker for years would not cause much issue and could actually reduce the ability to pull more current and an oven is not constantly drawing its full power.


433.1.103
The load current in any part of the circuit should be unlikely to exceed for long periods the current carrying capacity of the cable


Now is that the rating of 27amps (in ideal conditions) for the 2.5 T&E

What is a long period?

or the current carrying capacity of a ring final circuit (What is the CCC of a ring final?)

So the current in any part of the ring, must be an individual point, and not a combined amount - so say 16 amps - so that is not exceeding the CCC of the cable

 

[EFLImpudence]

Assuming roughly mid point on the ring, if you spurred a 2.5 twin and earth off a point or a joint box and took it to an enclosure containing a 16A B type breaker and onto your 3.6kw load.

Now before you run off and all point at 433.1.5 it refers to accessories... could you call a MCB in an enclosure an accessory?

It says accessories to BS1363

If I was to fit two single socket outlets on the ring supplying 1.8kw loads each which were run together. Would that be a problem?

No, but you could still do that with an oven at mid point plus everything else plugged in.

What about a 15A switch-fuse off a busbar chamber in 2.5mm singles for a fire alarm?

BS1363

We could prove that its sound engineering practice, but perhaps falls fowl of the exact wording in BS7671. Or do you disagree, what would your comment be upon finding such an arrangement on a EICR

You can't prove it is sound on a ring.
That's why the regulation is specific, although, having said that, the same would apply to a radial socket circuit.

Now... I've recently been looking at some luxury appartments where the electrical contractor has taken the ring into the supply terminals of a 20A double pole switch and taken a cable off the load side to supply a socket for a freezer / washer/ etc. What do you think the answer of the manufacturer was when I asked them if the supply terminals were rated to be part of a 32A ring final circuit? .

Did he know what he was talking about and how things work?
What current is going through the terminals?

20A switches are not BS1363 but we've discussed that before and, I think, concluded that even so it is fine.



In general.

How many sockets added to a cooker circuit would anyone deem satisfactory?

 

[JohnW2]

Adding a cooker to a ring final circuit circuit is a bad design as the loads of all the items could add up to a large current draw.

I think that most of us would probably agree that it's not too good an idea. However, as being discussed, cooking appliances which can be supplied via a 13A plug or FCU are commonly fed from ring finalsand, in some senses, are not much worse than any other '13A'load.

A fix load of max 16 amps will not in itself overload the circuit. However we can add a double socket with no fixed load, that could use 26 amps plus the fusing factor of the 13amp fuses ...

Sure, but when you consider that a ring final may well have 6, 8 or more double sockets, you are really only talking about the tip of a potential iceberg.

433.1.103 The load current in any part of the circuit should be unlikely to exceed for long periods the current carrying capacity of the cable ... Now is that the rating of 27amps (in ideal conditions) for the 2.5 T&E ... What is a long period?

How long is a piece of string? I have heard people citing 30 mins - but given that the many of our calculations implicitly involve the I2 of a Type B MCB, maybe 1 hour would be more reasonable?

....or the current carrying capacity of a ring final circuit (What is the CCC of a ring final?) .... So the current in any part of the ring, must be an individual point, and not a combined amount - so say 16 amps - so that is not exceeding the CCC of the cable

As you say, I'm sure they are talking about the current in any part of the cable of the ring. As you also say, a 16A load alone cannot possibly overload cable that is required to have a minimum CCC of 20A - but, in deciding whether the CCC 'is likely to be exceeded for long periods', the designer has to make assessments/judgements about other loads which might simultaneously be connected to the circuit.

Kind Regards, John

 

[mfarrow]

The rest of 433.1, for a start.

Yes well, poorly worded question.

Now I have the book in front of me, I thought 433.3.1 was closer to 433.1.103 than it actually is. The key difference being how it regulates overload current, with .103 allowing overload current for "not long" periods. However I can't see anything else which 433.1.103 adds in terms of derogation which isn't covered elsewhere.

So, we have a conundrum. Do we stick rigidly to 433.1.103 or look towards allowing those "not long" overloads in a circuit otherwise satisfied by 433.3.1? We've all been in the discussion about the slightly overloaded 43A shower on the 40A MCB. Would those electricians see this as any different?

 

[JohnW2]

Now I have the book in front of me, I thought 433.3.1 was closer to 433.1.103 than it actually is. The key difference being how it regulates overload current, with .103 allowing overload current for "not long" periods.

That surely makes reasonable sense, particularly if, as I suggested before, one regards 'a long period' as being about 1 hour. With any circuit other than a ring, there is the requirement that In ≤ Iz, which means that the cable cannot be subjected to more than 1.45In for more than an hour - and it seems that the 1.45 times the tabulated CCC (Iz) figures we use is deemed to be acceptable/safe for up to an hour.

With a ring, since Iz may appreciably exceed Iz that 'automatically OK' situation afforded by the MCB is no longer present - so it is down to the designer to make an assessment/judgement about whether an appreciable 'overload' of the cable is likely to arise 'for a long time' (say, more than about 1 hour).

So, we have a conundrum. Do we stick rigidly to 433.1.103 or look towards allowing those "not long" overloads in a circuit otherwise satisfied by 433.3.1? We've all been in the discussion about the slightly overloaded 43A shower on the 40A MCB. Would those electricians see this as any different?

As above, I think the designer input/judgement (particularly that involving the judgement abou 'not likely to be exceeded for a long time) arises specifically in relation to the ring final (because In > Iz, so that one does not have the 'deemed to always be safe', regardless of load, situation), and does not really apply to any other type of circuit. That's how I see it, anyway!

Kind Regards, John

 

[ban-all-sheds]

Now I have the book in front of me, I thought 433.3.1 was closer to 433.1.103 than it actually is.

433.3.1 is about omitting OPD devices - has nothing to do with 433.1.103.

The key difference being how it regulates overload current, with .103 allowing overload current for "not long" periods. However I can't see anything else which 433.1.103 adds in terms of derogation which isn't covered elsewhere.

What 433.1.103 adds is the ability to have a ring final socket circuit where Iz < In. Without it such a circuit falls at 433.1.1 (ii).

So, we have a conundrum.

No we don't.

What we have are people seeking to contravene one of the requirements of 433.1.103 and yet still use the exemption in 433.1.103 (which of course ceases to be valid once some of the conditions for compliance with 433.1.103 are no longer met) from 433.1.1.

What we have are people who want to design or modify a circuit so that it contravenes both 433.1.1 and 433.1.103 because it is inconvenient to comply with them.

Do we stick rigidly to 433.1.103 or look towards allowing those "not long" overloads in a circuit otherwise satisfied by 433.3.1?

"Otherwise satisfied"? A 32A/2.5mm² ring final does not comply with 433.3.1, and does not become compliant if you also take it out of compliance with 433.1.103.