Can I mix "B type" and "C type" MCBs in

[Reynardine]

Electrician about to install a new Wylex Part 17 CU in my old flat.

Problem is previous wiring situation in that the whole flat ( apart from the lighting ) is currently hanging off one 32amp MCB.

Cabling in flat is 2.5mm T&E throughout. So the amount of protection I'm currently gettiung from my 32amp MCB is next to nothing.

For safety, electrician wants to fit a much lower rated MCB so that going forwards, the cabling is protected as it should be to 20 Amps. He has warned me it could "trip a lot" once done.

My question : can I compromise by asking him to install a type C 20amp MCB instead of a type B ? This way, hopefully it won't trip so often and I'll still have protection? In general is it allowed to mix type B and type C MCB's on the same consumer unit ?

Regards,

 

[JohnW2]

Cabling in flat is 2.5mm T&E throughout. So the amount of protection I'm currently gettiung from my 32amp MCB is next to nothing.

If the circuit were a 'ring', then then a 32A MCB would be normal, and would provide adequate protection.

For safety, electrician wants to fit a much lower rated MCB so that going forwards, the cabling is protected as it should be to 20 Amps. He has warned me it could "trip a lot" once done.

If the circuyit is not a ring then the electrician is correct. The maximum permissible ('safe') MCB rating would then usually be 20A (under some circumstances 25A, if you could find one).

My question : can I compromise by asking him to install a type C 20amp MCB instead of a type B ? This way, hopefully it won't trip so often and I'll still have protection? In general is it allowed to mix type B and type C MCB's on the same consumer unit ?

Mixing Type B and C MCBs (on different circuits) is not in itself a problem, but use of a Type C MCB is not a satisfactory (or likely to be effective) way of addressing your problem. If I were you, I would discuss with the electrician the possibility of splitting your sockets circuit into two or more 20A circuits, each with their own B20 MCB.

Kind Regards, John

 

[PrenticeBoyofDerry]

Electrician about to install a new Wylex Part 17 CU in my old flat.

you mean 17th edition?

Problem is previous wiring situation in that the whole flat ( apart from the lighting ) is currently hanging off one 32amp MCB.

and how big would this flat be?

Cabling in flat is 2.5mm T&E throughout. So the amount of protection I'm currently gettiung from my 32amp MCB is next to nothing.

Not sure I understand what you mean by the amount of protection you are getting? A 2.5mm2 (T&E) ring final will deal with a 32A MCB quite happily.

For safety, electrician wants to fit a much lower rated MCB so that going forwards, the cabling is protected as it should be to 20 Amps.

What safety issues?

He has warned me it could "trip a lot" once done.

well he should not be installing it until he can remedy the issues that would cause any trips.

My question : can I compromise by asking him to install a type C 20amp MCB instead of a type B ? This way, hopefully it won't trip so often and I'll still have protection?

And our reasoning behind that is?

In general is it allowed to mix type B and type C MCB's on the same consumer unit ?

It is not not allowed, but you/your alleged electrician would need to confirm the Zs values are suitable, but I cannot see how this would improve your situation.
Did the electrician perform an EICR, and if he did what codes if any where remarked upon?

 

[EFLImpudence]

the whole flat ( apart from the lighting ) is currently hanging off one 32amp MCB.

Do you not have a cooker circuit and an immersion heater circuit?

 

[JohnW2]

For safety, electrician wants to fit a much lower rated MCB so that going forwards, the cabling is protected as it should be to 20 Amps.

What safety issues?

Reading between the lines, I suspect that the electrician has detected that the entire flat is served by a single 2.5mm² radial circuit protected by a 32A MCB.

He has warned me it could "trip a lot" once done.

well he should not be installing it until he can remedy the issues that would cause any trips.

I assume that 'the issue which would cause (overload) trips' would be that all the sockets (and maybe more!) would be (if he downgraded the MCB) supplied by a single 20A circuit.

Kind Regards, John

 

[securespark]

As John I feel is getting close to the root of the problem, you need to have a good chinwag with your spark (who obviously hasn't made it crystal clear to you what the problem or its solution(s) is/are).

If you have a broken or incomplete or incorrectly installed ring final, with two legs of 2.5 at the unit, it would be ideal to split them up into two 20A circuits, with one on each RCD, but that goes without saying.

If there is only one leg of cable, you will indeed only be able to have one 20A radial circuit for the socket outlets. However, having had to do this remedial work before, I can say that I have rarely been recalled for nuisance tripping, even on a 20A breaker. If you have only one cable, try and get a 25A breaker. Some manufacturers make them, some don't.

Hager make them and they are a decent trustworthy brand.

But your first port of call is that chat with your spark.

 

[ericmark]

For a B20 MCB to trip on the magnetic part requires 100 amp to flow so in real terms neither the C20 or B20 MCB should trip on the magnetic part of the trip and the thermal bit is the same for both trips so really there will be no difference using type B or type C as far as tripping goes.

So even if you are allowed to use type C there is really no point.

As to if allowed it’s all down to the impedance of the circuit.

 

[JohnW2]

If there is only one leg of cable, you will indeed only be able to have one 20A radial circuit for the socket outlets. However, having had to do this remedial work before, I can say that I have rarely been recalled for nuisance tripping, even on a 20A breaker.

That doesn't surprise me. A B20 (or a C20, come to that) will allow >30A to flow for >15 minutes - plenty of time to boil a kettle, toast some bread, dry ones hair or for the water heating part of a WM or DW cycle to complete - so only two or more persistently high loads (heaters, dryers etc.) would probably trip a B20.

Kind Regards, John