Hager mcb type MTN and NCN same bus bar.

[aladinsane339]

Hello one and all, new to the forum.
Need a bit of info, and some help please.
Got a six year old house [2010 CU] 17th ed electrics,Hager consumer unit ,all the mcbs along the bus bar are MTN B ratings, but I want to put in a 16A dedicated socket on a commando blue plug and wall socket [ type used for caravans] with an mcb type NCN C or D rating at 32A to cope with the electric motor start surge, it is for a compressor, it will work in isolation, nothing else will be on that circuit. Problem is having bought the NCN C rated mcb which is almost identical to the MTN it will not fit the bar. I am assured it is the right one,but the electrical sales person says they should not be mixed anyway. eh! dont understand, all help appreciated.

 

[securespark]

Is the earth fault loop impedance of your designed circuit low enough for a C type?

 

[ban-all-sheds]

I want to put in a 16A dedicated socket on a commando blue plug and wall socket

If you live in England or Wales do you intend to comply with the Building Regulations?

C or D rating at 32A to cope with the electric motor start surge, it is for a compressor,

You might well need permission from your DNO to have such a load

Problem is having bought the NCN C rated mcb which is almost identical to the MTN it will not fit the bar. I am assured it is the right one,but the electrical sales person says they should not be mixed anyway. eh! dont understand,

There is one reason not to mix.

The other is that consumer units are type-tested assemblies. Their whole standards compliance hinges on them being certified as a complete unit by the maker, and they do not do that for a mixed selection of MCB types.

 

[flameport]

Problem is having bought the NCN C rated mcb which is almost identical to the MTN it will not fit the bar.

NCN can be fitted into single phase domestic consumer units just as MTNs can be.
If you can't work out how, you really shouldn't be doing this.

It's unlikely that any circuit in a domestic setting could permit use of a type D 32A MCB.
https://www.electricalsafetyfirst.org.uk/mediafile/100106752/master_EARTHFAULTLOOPtable.pdf

 

[JohnW2]

https://www.electricalsafetyfirst.org.uk/mediafile/100106752/master_EARTHFAULTLOOPtable.pdf

I'm not quite sure why they have ('arbitrarily') quoted minimum Zs figures which are "80% of those in BS7671", but as a 'warning' to anyone who actually wants to work to such "80%" figures, that table seems to be based upon pre-Amendment 3 BS7671 figures (i.e. does take into account "Cmin") and therefore are actually about 84% of current BS7671 figures.

If anyone wants "80% of (current) BS7671" figures, they therefore should multiple the maximum Zs figures in that tabulation by about 0.95.

Kind Regards, John

 

[yorkspark]

I want to put in a 16A dedicated socket on a commando blue plug and wall socket [ type used for caravans] with an mcb type NCN C or D rating at 32A

why would you think it acceptable to install a 16amp bs 60309 skt then fit a 32a mcb? what size cable are you proposing to install and whats your measured zs for the install?
Has your db spare ways on the rcd protected side, if not, your going to need an rcbo installed instead of an mcb as it will need rcd protection as well

 

[EFLImpudence]

why would you think it acceptable to install a 16amp bs 60309 skt then fit a 32a mcb?

Isn't it acceptable? You wouldn't want it the other way round.

 

[JohnW2]

Isn't it acceptable?

As a generalisation, a 16A socket on a 32A circuit is, per se, no less acceptable than a 13A socket on a 6A circuit (which, as you have recently written, in not necessarily unacceptable).

However, I suspect that yorkspark was considering the fact that we are talking about a 16A 60309 socket, hence without a fused plug, so that, unless the load were such that it was 'unlikley to create an overload', the flex connected to the corresponding plug would have to be rated for 32A (i.e. pretty hefty, at least 4mm²).

Kind Regards, John

 

[EFLImpudence]

As a generalisation, a 16A socket on a 32A circuit is, per se, no less acceptable than a 13A socket on a 6A circuit

Haven't you also got that the wrong way round?
It surely is comparable to a 13A socket on a 32A circuit - which is quite common, I believe.

However, I suspect that yorkspark was considering the fact that we are talking about a 16A 60309 socket, hence without a fused plug, so that, unless the load were such that it was 'unlikley to create an overload', the flex connected to the corresponding plug would have to be rated for 32A (i.e. pretty hefty, at least 4mm²).

Yes, as Yorkspark went on to query. It will need the correctly rated cable.
Is 4mm² pretty hefty?

 

[JohnW2]

Haven't you also got that the wrong way round? It surely is comparable to a 13A socket on a 32A circuit - which is quite common, I believe.

Yes, my mind and/or typing fingers that all upsdie down and back to front - probably because that other thread was in my mind . As you say, a 16A socket on a 32A circuit is, in some senses, no worse than a 13A socket on a 32A circuit - but I wonder what you would say about a 5A or 15A socket (with unfused corresponding plug) on a 32A circuit? (it would be daft to have to connect cable rated for 32A to a 5A plug, even if it would fit).

Yes, as Yorkspark went on to query. It will need the correctly rated cable.

It did not seem to be the (current) 'rating' of the cable he was querying but, rather, whether the Zs (presumably of the 'fixed wiring') would be low enough for the 32A breaker. On the other hand, I was talking about the current rating of the flex downstream of the (unfused) plug.

Is 4mm² pretty hefty?

For flex (and that's what I was talking about), I would say that it's pretty hefty, yes - I'm not at all sure that I have any in my house (that may be because I haven't got any electric cookers, but I would suspect that 2.5mm² would probably be enough even for most of them, given diversity).

Kind Regards, John

 

[EFLImpudence]

As you say, a 16A socket on a 32A circuit is, in some senses, no worse than a 13A socket on a 32A circuit - but I wonder what you would say about a 5A or 15A socket (with unfused corresponding plug) on a 32A circuit?

As we have recently discussed/decided, in most cases the fuse in the plug is probably not necessary, so as long as it is properly designed, all will be well.

It did not seem to be the (current) 'rating' of the cable he was querying but, rather, whether the Zs (presumably of the 'fixed wiring') would be low enough for the 32A breaker.

Same difference.

On the other hand, I was talking about the current rating of the flex downstream of the (unfused) plug.

Yes, I know.

 

[JohnW2]

As we have recently discussed/decided, in most cases the fuse in the plug is probably not necessary, so as long as it is properly designed, all will be well.

In some situations, yes. However, it's largely down to the nature of the load. No matter how well designed the circuit is, the cables (both fixed wiring and post-socket flex) need to be protected by some device with a rating ≤CCC of the cable unless the load is such that "an overload is unlikel".

Same difference.

Not necessarily. Zs is dependent upon cable length, whereas CCC is not. It's therefore quite possible for a long cable to have adequate CCC but for the Zs of the circuit to be 'too high'. Conversely, a very short run of a small CSA cable might have a low enough Zs for ADS, but too small a CCC for the circuit/load.

Kind Regards, John

 

[EFLImpudence]

In some situations, yes.

Do you mean the properly designed ones?

However, it's largely down to the nature of the load. No matter how well designed the circuit is, the cables (both fixed wiring and post-socket flex) need to be protected by some device with a rating ≤CCC of the cable unless the load is such that "an overload is unlikel".

Yes, I know.

Not necessarily. Zs is dependent upon cable length, whereas CCC is not.

Yet both are dependent on c.s.a. Who was talking about length?

It's therefore quite possible for a long cable to have adequate CCC but for the Zs of the circuit to be 'too high'.

Yes, I know but both are dependent on c.s.a. Who was talking about length?

Conversely, a very short run of a small CSA cable might have a low enough Zs for ADS, but too small a CCC for the circuit/load.

Yes, I know but both are dependent on c.s.a. Who was talking about length?.

A solution to either high Zs or low CCC has very rarely been solved by altering the length of a cable so, that leaves c.s.a.
I know. Let's use 4mm².

 

[JohnW2]

Do you mean the properly designed ones?

I was responding to ...

... in most cases the fuse in the plug is probably not necessary...

... and, as I said, no matter how well designed is a circuit, whether or not the fuse in the plug is needed (not that we usually have any choice in the UK) will depend upon the nature of the load that is plugged in, something which the designer cannot foresee (and has no control over).

Yet both are dependent on c.s.a. Who was talking about length? ... Yes, I know but both are dependent on c.s.a. Who was talking about length? .... Yes, I know but both are dependent on c.s.a. Who was talking about length?.

I was responding to your "Same difference" comment and was merely illustrating that adequately low Zs does not necessarily mean adequately high CCC, or vice versa.

Kind Regards, John

 

[aladinsane339]

Well thank you all for the comments, seems to have started a great debate, argument even, however can someone please answer the query, keeping to the point to help me please. I would much appreciate it. I propose to wire it like so and from the CU direct not taking in other loads, it will be on no party line with other 13A loads, kettles or anything like that, it is in isolation so from the CU 32 amp C mcb, why 32 amp, because it is not unknown for major non brushless motors not to surge and trigger the breaker. Some motors [non brushless ] can surge 45 A for milliseconds until the motor gets away so 32 A to cope with these surges that a 3 HP motor will create, and lets hope none of these surges reach 45A even if for milliseconds otherwise nuisance tripping will prevail. So using 2.5 mm cable from breaker of a C rating to a switched spur, and straight from the CU a length of cable only 3 metre long to the 16A commando socket mentioned, this is how I hope to wire it, that is if I can work out why this ncn breaker will not fit.