Spur off MCB?

[loply]

Hi folks,

I have a 2.5mm^2 ring circuit in my garage, wired off it's own consumer unit. The consumer unit has two MCBs - one for the 2.5mm ring, and one for the lighting circuit.

I wired it all up myself, the unit is fed from a 10mm^2 cable coming from in the house.

I'm fitting a bigger air compressor in the garage and it will potentially pull 30-odd amps on startup, so I don't really want to put it on the 2.5mm ring.

I'm wondering if I should just wire up a single run of 6mm^2 cable off the same breaker as the 2.5mm ring, and stick a 32a commando socket on the end? the 6mm^2 cable has a higher current rating than the 2.5mm ring and seems like it should be enough.

But is it it badly frowned upon to have what is effectively both a ring and a spur coming off one MCB?

Ideally I'd like to avoid changing the consumer unit to one which can take more MCBs, as it would be a pain.... I know I should have fitted a bigger one in the first place!

Thanks,
Rich

 

[EFLImpudence]

A ring and spur from the MCB is not frowned upon; it is specifically allowed.

However, two 2.5mm²s and one 6mm² fitted in the MCB will not be ideal and it will need ensuring that they are adequately secured and connected.

4mm² would probably be adequate.

 

[loply]

Thanks EFLImpudence, why do you say it would not be ideal to mix? Just because of the mechanics of trying to fit it all in?

I'll give some thought to just using 4mm instead. I wish I had wired the ring with 4mm in the first place!

Final question I didn't think of previously - I can't seem to find anywhere that sells flex cable sufficient to go between the compressor and it's 32A plug. Should I just use solid core here? If I do that I might as well just get rid of the plug and wire it straight in to an isolator switch...?

 

[Risteard]

Such an arrangement would be likely to allow small overloads of long duration and leave the circuit breaker running very hot.

It is not a spur as with a ring final circuit.

This should be two circuits - not one.

 

[securespark]

What does it say on the ratings plate of the compressor?

 

[EFLImpudence]

why do you say it would not be ideal to mix? Just because of the mechanics of trying to fit it all in?

Yes, I just meant the mechanics.

Not lack of room; there will be too much.
There will be seven cores of 4 or 6mm² and two of 2.5mm² gripped by two (more or less flat) pieces of the MCB clamp.
Make sure all are tightly held.

I don't see there is much wrong with doing it this way.
It may not be classed as a spur in the normal ring circuit sense - but then look at it the other way; you will have a few sockets on your compressor circuit. They didn't need to be wired in a ring, anyway.

 

[JohnW2]

It is not a spur as with a ring final circuit.

That's an interesting one. As I understand it, the existing circuit is a ring final, and one is certainly allowed to have spurs from such a circuit originating from the MCB (Appendix 15 even says this). However, if it were regarded as 'a spur from a ring final', the guidance in App 15 certainly would not 'allow' it to supply a 30A load. On the other hand, having a 30A load supplied via 6mm² (probably also 4mm²) cable from a (I presume) 32A MCB is not, in itself, a problem.

This should be two circuits - not one.

That would certainly be the ideal. More pragmatically, a lot depends upon how much current the compressor really draws when running (not starting), and for long at a time (it is presumably intermittent) - and what loads are likely to be simultaneously on the (existing) ring.

Kind Regards, John

 

[loply]

Thanks for the help everyone.

I'm trying to find out what the normal operating current of the motor is, it isn't in the manual and the thing hasn't arrived yet, but I presume it will be around 23A when it's at full load (4hp motor, 240v).

The maximum inrush current on the motor is 'up to' 40 amps, presumably only for a few fractions of a second.

The MCB which this will run off is currently a 32A type B, I was thinking I would need to change it to a type C which might withstand the inrush current? Or would it just trip still?

I guess if keeps tripping a C32 MCB then I will need a different circuit / new CU, because I guess I can't up the size of the MCB otherwise my 2.5mm ring could overheat before the MCB trips?

Cheers,
Rich

 

[EFLImpudence]

4hp is 3kW - 12.5A.

You cannot up the rating of the MCB with the circuit as it is.

You can only have a C type if the Zs is low enough.

 

[loply]

I would imagine the Zs will be low as it's just one cable from the MCB straight to the single commando socket? There are no breaks/connections.

As it happens I think I've decided just for peace of mind to just put in a new consumer unit, with 3 MCB slots... then I'll just put the compressor on it's own MCB... for the sake of half an hour to rewire the CU. I guess then I have to choose between a C32 or a B40 on the compressor circuit...? Neither seems ideal though (40a is a bit high for 6mm, but a C32 might trip??)

 

[JohnW2]

I would imagine the Zs will be low as it's just one cable from the MCB straight to the single commando socket? There are no breaks/connections.

It not quite as simple or certain as that. The 'loop' whose loop impedance (Zs) one is concerned about includes all the wiring back to, and through, your house, and then all the way back to the substation. In theory, the impedance from your compressor to the local CU could be 'almost zero', yet the Zs (including the rest of the 'loop', as described above) could be too high for a C32.

As it happens I think I've decided just for peace of mind to just put in a new consumer unit, with 3 MCB slots... then I'll just put the compressor on it's own MCB... for the sake of half an hour to rewire the CU.

I presume that you realise that replacing that CU (and probably installing the one you already have) constitutes 'notifiable' work? Whatever you choose to do about that, if you took this course, I'd personally go for something more than 3-way - since you might, for some reason, find yourself wanting/needing to upgrade your 3-way one to a 4-way or 5-way in a few months' time!

However, this 'upgrading' begs the question of the adequacy of the supply. What, in the house, is feeding this garage CU?

I guess then I have to choose between a C32 or a B40 on the compressor circuit...? Neither seems ideal though (40a is a bit high for 6mm, but a C32 might trip??)

If the Zs is low enough (it needs to be measured), the C32 (or even a D32, which would require an even lower Zs) would seem the obvious option.

Kind Regards, John

 

[sparkwright]

I presume your garage board is only 2 way, and no available spares?

Why not just replace the board for a 3 or 4 way one?

You've already stated you wired it originally, so presumably you could replace this.

 

[loply]

Hi John,

Ah sorry, I didn't realise the impedance was measured out to the substation. How on earth do you measure that?!

I am aware that replacing the CU is notifiable...

The CU in the garage is fed by a 10mm armored cable from a B40 MCB on the house CU (which has a 40a RCD on it too).

I think I will just put another B40 in the garage to protect this new circuit, I think previously I thought 6mm cable was only rated for 30-something amps, just realised that clipped direct it's 46a so a B40 would do the job fine and I don't need to worry about Zs etc?

I guess the use of the B40 on the new circuit would mean that the B40 upstream in the house could trip first in the event of an overload but I suppose that's fine?

Thanks for all your help.

 

[JohnW2]

Why not just replace the board for a 3 or 4 way one?

You're not paying attention - that is precisely what the OP has now said that he proposes to do!

Kind Regards, John

 

[sparkwright]

Where has he said about replacing the board?