Cable size for 32a breaker?

[Milleniumaire]

I've just had a conversation with an electrician who will be quoting me for a job, which includes installing two 600W towel rail elements.

Each element will be connected to a double pole switched fuse spur as required by the installation instructions.

The instructions also state the circuit must be connected to a 30mA MCB, which isn't an issue for our 4 year old consumer unit.

As there are free slots in our CU he has suggested running a new 2.5mm RADIAL circuit to both towel rails, which will be protected by a 32A MCB. Each of the switched fused spurs will have 13A fuses.

With my limited knowledge, I was surprised at the recommended use of a 32A MCB on this RADIAL circuit. I understand that 32A would be acceptable for a FINAL RING circuit as the load is halved, so shouldn't the new towel rail RADIAL circuit be protected by a 20A MCB, rather than a 32A MCB?

I guess because there are fused spurs, each with 13A fuses, this would actually give the protection required, so does it then really matter about the size of the MCB and it is the speed (30mA) that is important?

I'm sure this is bread and butter to you guys, but I would just like to understand the options a little better.

 

[EFLImpudence]

As there are free slots in our CU he has suggested running a new 2.5mm RADIAL circuit to both towel rails, which will be protected by a 32A RCD. Each of the switched fused spurs will have 13A fuses.

MCB not RCD.
A bit pointless, surely.

With my limited knowledge, I was surprised at the recommended use of a 32A RCD on this RADIAL circuit. I understand that 32A would be acceptable for a FINAL RING circuit as the load is halved, so shouldn't the new towel rail RADIAL circuit be protected by a 20A RCD, rather than a 32A RCD?

32A MCB - Miniature Circuit Breaker.
Well, the cable will be protected by the fuses so it doesn't really matter but you are correct.
I wouldn't call it a recommendation, surely. Is that what is already there in the CU?

I guess because there are fused spurs, each with 13A fuses, this would actually give the protection required, so does it then really matter about the size of the RCD and it is the speed (30mA) that is important?

It's nothing to do with the RCD. It is an 32A MCB.

RCDs - Residual Current Devices must be 30mA (or less).

 

[AndyPRK]

Yes a 20A mcb would be much better

 

[Milleniumaire]

MCB not RCD.
It's nothing to do with the RCD. It is an 32A MCB.

RCDs - Residual Current Devices must be 30mA (or less).

32A MCB - Miniature Circuit Breaker.

Yes, I'm sorry, showing my ignorance as I always get RCD and MCB mixed up. Good job I never became an electrician

I've edited the original question, replacing RCD with MCB.

Looking at the consumer unit, there are two "banks" of MCB's, each protected by an RCD. There are 4 spare slots.

Well, the cable will be protected by the fuses so it doesn't really matter but you are correct.
I wouldn't call it a recommendation, surely. Is that what is already there in the CU?

A bit pointless, surely.

It's possibly just as easy to run a new cable from the CU as it is to connect upto the existing upstairs circuit as there will be lots of floorboards up to run some new piping to the new bathroom UFH manifold. Also, we had a loft extension about 15 years ago and the existing upstairs final circuit was extended into this loft room. I mentioned this to him, but not sure if that affected is decision to introduce a new towel rail circuit. Our en-suite, where one of the heating elements will be installed is directly above the garage and the consumer unit.

 

[winston1]

600w is only 2.5 amps at 240v. So 2 600w loads is only 5 amp. Why on earth does he/she want a 32 amp MCB. A 10 amp MCB with 1.5mm cable would be ideal. The requirement for FCUs would go away then as well. (Don’t worry about manufacturers instructions, they are guidance only and FCUs are not available nor compliant in the rest of Europe where they probably also sell).

Are you sure they are 600w, that seems an awful lot for a towel rail?

 

[Milleniumaire]

600w is only 2.5 amps at 240v. So 2 600w loads is only 5 amp. Why on earth does he/she want a 32 amp MCB. A 10 amp MCB with 1.5mm cable would be ideal. The requirement for FCUs would go away then as well. (Don’t worry about manufacturers instructions, they are guidance only and FCUs are not available nor compliant in the rest of Europe where they probably also sell).

Are you sure they are 600w, that seems an awful lot for a towel rail?

Thanks Winston.

Yes they are definitely 600W each. The towel rail is rated at over 700W with a 65 degrees CH feed. During the Summer, it would only be used for short bursts - an hour at a time, just to dry the towels after a shower.

It sounds like he has over-spec'd the MCB, but with the two SFCU's I guess it doesn't really matter.

 

[sparkwright]

Perhaps he just made a mistake.

Tell him you want 20amp.

 

[Milleniumaire]

....(Don’t worry about manufacturers instructions, they are guidance only and FCUs are not available nor compliant in the rest of Europe where they probably also sell).

It's interesting that the manufacturers fitting instructions don't make any reference to regulations, but are VERY specific about using a fused spur:

"....With the permanent installation (cable connected without plug) it is also mandatory to have an omnipolar cut-off for disconnecting the device on all poles, by points of contact with the clearance of 3mm.".

As this was going to be done anyway and to ensure any warranty conditions are upheld I don't think there's any harm in using SFCU's.

Perhaps he just made a mistake.

Tell him you want 20amp.

I'll have another chat with him when I get his quote, assuming it is reasonable

 

[ericmark]

Manufacturers instructions are a problem, you can by a battery operated device and it still says things like ensure power is isolated before opening, it is so clearly a preamble used for everything they sell be it mains or battery, and now the

134.1.1 Good workmanship by competent persons or persons under their supervision and proper materials shall be used in the erection of the electrical installation. Electrical equipment shall be installed in accordance with the instructions provided by the manufacturer of the equipment.

has been changed, not got new version, but the instructions were so often clearly wrong, some common sense has to be used.

The bathroom fan would often say a 3 amp fuse should be used, if powered from the lights, the 6 amp MCB would always trip before a 3 amp fuse ruptured, and the windings of the fan used wire well under 3 amp rating, likely well under 1 amp rating so a 3 amp fuse when powered from lights is useless.

The regulations tell us with TN we don't have to switch the neutral, however with TT systems often we do, but to write down instructions that are different for TN and TT would cause confusion, so they write the instructions for worse case scenario.

The problem is of course because they have gone OTT so often, people start to ignore manufacturers instructions as a result, so when some thing which is important is written down, that is also ignored. There have be discussions on this forum about the type of RCD used, and some manufacturers have stipulated type A and not all consumer units can take type A, and look at an advert for a RCD FCU and it often does not even say what type it is. And even as electricians we argue as to how important.

However in your case it is a new circuit, as a result there will be an insulation certificate, and a compliance certificate issued, for the electrician to do work which does not comply, he risks being thrown off the scheme, so they will not break the rules lightly. As said the MCB and RCD protect both cable and appliance, but with a towel rail there is very little which can go wrong because the protective device is wrong size, if massive over size an earth wire could burn out before the MCB trips, but a 10 amp MCB would trip with any short circuit, and if the towel rail element goes, then it goes, no fuse or MCB will change that. Supplied from a RCBO (MCB and RCD combined) there is no real need for twin pole switching, but where the RCD feeds many circuits, twin pole switching is a good idea so if there is a fault the circuit can be isolated and other items fed from the same RCD can continue to be used. So even with a low amp MCB a FCU is a good idea not for the fuse, but because it has a double pole switch, so it can be taken fully out of circuit, the FCU has a fuse carrier with provision for locking off, not necessary a lock a simple cable tie would do, but it means can't be accidentally switched on if a fault is found, so stops the RCD tripping, but does need the switch more than fuse, as fuse single pole, so a earth - neutral fault could still cause the RCD to trip, so any double pole switch able to be locked off, well yes regulations say lock off, but in real terms a bit of insulation tape on the switch shows anyone there is a fault, and if they did ignore it the RCD would trip so any double pole switch 6 amp or more.

It is a case of how far do you go, if looking at worst case scenario then a faulty boiler could cause the RCD in the consumer unit not to trip with an earth fault on a towel rail, so could say fit a RCD FCU, however that is really going OTT, the chances are so slim. Like saying pubs and bars should have a reinforced roof in case a helicopter has a problem and crashes on it, it has happened, but no one would say fit reinforced roofs as a result.

So I would fit a MCB of between 10 and 25 amp with 2.5 mm² cable and use a switched FCU as they have double pole switching and a cord outlet, but there are many other ways which would still be OK.

 

[SUNRAY]

I've just re-read the OP and if you have correctly relayed the information from the electrician I think it's looking a lot like he is incompetent, we have all been been concentrating on the 32A MCB but now I realise I misunderstood his FCU fuse rating, it shouldn't be 13A as the heater elements usually only have 0.5mm² flex. The fuse could usefully be 3A or 5A max.
Following on from Erics excellent post I'd possibly suggest the new dedicated radial would be well designed if using a 6A MCB and double pole switches without fuses. However FCU's tend to be cheaper and argueably more convenient.

 

[Milleniumaire]

Thanks for the post Eric. It's good to know others think manufacturers installation instructions and warranty conditions are OTT.

I recently purchased a mixer tap for my bathroom refurbishment project and noticed the warranty said something like ".... must be installed by qualified person...." and then mentioned specific plumbing qualifications, which I obviously don't have as I'm not a plumber. I was so surprised by this I contacted the manufacturer and they confirmed that the warranty would be void if not installed by a plumber with those qualifications.

I had no intention of hiring a plumber to install a tap so I sent it back and bought a different one!

Although it was a lot of hassle, and it would have only been an issue if I had to claim on their warranty, I decided to do this as a point of principle. Their loss and the sellers as I got the return postage reimbursed on the grounds that their website didn't specifically state anything about the OTT warranty conditions. I did feel bad for the seller!

There are obviously some things that qualified persons must install, but there are laws and regulations that dictate this requirement and I don't think it should be the manufacturers. I can imagine the old Monty Python sketch "what have the Romans ever given us..." ensuing now as people point out scenarios that I haven't considered.

 

[SUNRAY]

There are obviously some things that qualified persons must install, but there are laws and regulations that dictate this requirement and I don't think it should be the manufacturers.

Are you really saying the manufacturers should not provide any installation instructions?

 

[SUNRAY]

Thanks for the post Eric. It's good to know others think manufacturers installation instructions and warranty conditions are OTT.

I recently purchased a mixer tap for my bathroom refurbishment project and noticed the warranty said something like ".... must be installed by qualified person...." and then mentioned specific plumbing qualifications, which I obviously don't have as I'm not a plumber. I was so surprised by this I contacted the manufacturer and they confirmed that the warranty would be void if not installed by a plumber with those qualifications.

I had no intention of hiring a plumber to install a tap so I sent it back and bought a different one!

Although it was a lot of hassle, and it would have only been an issue if I had to claim on their warranty, I decided to do this as a point of principle. Their loss and the sellers as I got the return postage reimbursed on the grounds that their website didn't specifically state anything about the OTT warranty conditions. I did feel bad for the seller!

There are obviously some things that qualified persons must install, but there are laws and regulations that dictate this requirement and I don't think it should be the manufacturers. I can imagine the old Monty Python sketch "what have the Romans ever given us..." ensuing now as people point out scenarios that I haven't considered.

When I quote your post this bit comes up...

 

[EFLImpudence]

Are you really saying the manufacturers should not provide any installation instructions?

Any instructions they provide should be limited to the product fitting and usage requirements; not what they think are electrical regulations.

Most of them say the product should be fitted by a qualified electrician. No one seems to take any notice of that instruction.
Why then do they include wiring diagrams? Qualified electricians do not need one.

Some instructions are clearly wrong so how do we know which are correct without knowledge?

In another thread you quote several shower manufacturer's instructions; one of which states:

It doesn't say 'minimum' or 'at least' 45A so would a 50A switch be following the instructions?

 

[Milleniumaire]

Are you really saying the manufacturers should not provide any installation instructions?

Certainly not, most are very useful.

I understand that electrical fittings come with warnings in their instructions stating they must be fitted by a qualified electrician. If it was a new light fitting and was being used to replace an existing light fitting, I wouldn't think twice about installing it myself. However, if the warranty explicitly stated that it would be null and void if NOT installed by a qualified electrician, I would send it back as I believe that is a manufacturer being unreasonable. I can understand why they might do this as they want to be sure if the light fitting fails it is due to the light fitting itself, not because of the way it was installed.