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Topic Title: Combining circuits on CCU to free up breaker
Topic Summary: This isn't right but which reg
Created On: 09 July 2011 04:16 PM
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 09 July 2011 04:16 PM
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johndpar

Posts: 27
Joined: 28 October 2003

Just visited a site where the immersion heater and garage radial have been combined onto one MCB in order to free up an MCB for a new circuit.

The MCB is 16A, and both circuits are 2.5/1.5.

Now the immersion is your standard 3kW so that's 13A at 230V and the garage has a small CCU with the usual breakers for the sockets and lights.

In the garage there is a chest freezer, electric door opener, battery chargers etc.

Clearly the 16A breaker is insufficient for the expected load even with a bit of diversity, but which reg best covers this?

533.2.1 looks possible.

Any thoughts?

Many thanks

John
 09 July 2011 04:39 PM
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johndpar

Posts: 27
Joined: 28 October 2003

OSG 8.5 (17th) covers this but does not list a reg.
 09 July 2011 04:43 PM
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ant1uk

Posts: 2376
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I agree 533.2.1 seems fit for this situation.

If you wanted a cheap fix for this I can't see a problem adding the garage to a ring circuit, just make sure there are MCB's at the garage end to prevent overloading the ring.
 09 July 2011 04:59 PM
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johndpar

Posts: 27
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Hi ant1uk

Not sure about the cheap fix. The 2.5/1.5 cable to the garage would not be sufficiently protected by the 32A mcb feeding the ring. There would need to be a protective device of some sort between the ring and the garage feed. At least in part this feed is 2.5/1.5 (although I hope that changes to SWA somewhere as the garage is detached) so for installation ref method A the max rating is 20A.

(I'll have to check but I suspect the garage CCU has a 16A mcb and 6A mcb so there is another problem anyway as there is no discrimination between the mcb in the main ccu and the mcb in the garage ccu.

Regards

John
 09 July 2011 05:35 PM
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johndpar

Posts: 27
Joined: 28 October 2003

Yup, that would protect the cable but then the mcb in the garage ccu does nothing.

The garage should probably be on a 20A mcb in the main CCU and (I'll have to check the discrimination) then the 16A fuse in the garage CCU might be OK.

Getting back to my original question, does anyone have any other offers of regs that back up OSG App 8.5?

Thanks
John
 09 July 2011 06:20 PM
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ant1uk

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Usually a garage is on a separate 20A mcb. Is there no way you can rearrange any other circuit in the CU to make it a better arrangement. Is it a split 2 rcd board?
 09 July 2011 06:37 PM
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johndpar

Posts: 27
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Yep - easy to re-arrange circuits actually - I'm only reporting on what's wrong at the moment.

John
 09 July 2011 06:46 PM
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johndpar

Posts: 27
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No Probs.

Hope the bad news is not too serious.

John
 09 July 2011 06:55 PM
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davezawadi

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Joined: 26 June 2002

Well a 16A fuse and a 20A breaker will almost always trip the breaker, but this doesn't matter really. Where does this 20A rating for 2.5/1.5 come from, 16 + 6 = 22A which is fine in my book except in exceptional circumstances (domestically speaking). Just have the garage as a spur if you wish, protection is perfectly adequate. The 16A breaker circuit cited in the OP is also probably adequate unless the owner gets a large appliance in the garage. A 16 A breaker will withstand 24A for a long time (Fig 3A4 in regs), so all is well anyway until the owner buys a large welder. MCBs are not necessary Ant, if the ring is overloaded its 32A breaker will protect it just fine. 533.2.1 does not appear to be violated, does it? Has the owner ever tripped the installed 16A breaker?

-------------------------
David
CEng etc, don't ask, its a result not a question!
 09 July 2011 07:44 PM
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alancapon

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You could even make the garage a spur at the CU if you wanted. Some don't like it, but there is no reg against it, and it may save another junction box! (head down, tin hat on! )

Regards,

Alan.
 10 July 2011 09:41 AM
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johndpar

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Re the 20A rating, I missed some detail, and also said ref method A when it should have been C. Max rating for 2.5/1.5 ref method C from table 4D5 is 27A but this CCU only has 20A and 32A rated mcbs available and without checking deeper I'm not sure other types will fit, hence saying 20A.

Making the garage a spur at the CCU would mean the cable would be unprotected in the event of a fault - so no, I don't think so.

The main point here is that the circuits should not have been combined by the spark who was making the recent changes.

John
John
 10 July 2011 10:38 AM
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alancapon

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Originally posted by: johndpar
. . . Making the garage a spur at the CCU would mean the cable would be unprotected in the event of a fault - so no, I don't think so. . .

No it wouldn't. A spur at the CU has exactly the same amount of fault protection as a spur anywhere else. The overcurrent protection you have in the garage then covers the spur cable for overload. Perhaps you would like to quote a regulation that prevents a spur from the CU?

Regards,

Alan.
 10 July 2011 10:46 AM
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johndpar

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Hi Alan

Where are you suggesting the spur is connected inside the CCU?

John
 10 July 2011 10:56 AM
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alancapon

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A spur to a ring main taken from inside the Consumer Unit is a third connection to the mcb supplying the ring. The reference for doing this, is Appendix 15 of BS7671:2008.

Regards,

Alan.
 10 July 2011 11:00 AM
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johndpar

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Hi Alan

So then the cable to the garage is protected by the 32A mcb for the ring which is too high for the cable that is installed.

Think what happens if the garage CCU is turned off. The cable is only protected by the mcb at the source end (in any installation).

The mcbs in the garage CCU are only there to protect the cables in the garage.

Regards

John
 10 July 2011 11:31 AM
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bajb

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Originally posted by: johndpar

So then the cable to the garage is protected by the 32A mcb for the ring which is too high for the cable that is installed.



Are you sure it is not protected? Remember the cable only has to be protected against fault not overload, the latter function being provided by the garage CU.

Regards
Bruce
 10 July 2011 11:46 AM
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johndpar

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Hi Bruce

Yes of course - not had any coffee yet, sorry Alan, note to self - engage coffee then brain!

I can see the spur from a mcb feeding a ring would be OK.

Thanks

John
 10 July 2011 12:06 PM
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ant1uk

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Thats what I suggested yesterday If you restrict the load to 20A at the garage end then the cable won't overload.
 10 July 2011 12:41 PM
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davezawadi

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Well 22A Ant, 16 + 6 = 22. Please try and be accurate, otherwise it leads to miles of post and confusion!

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David
CEng etc, don't ask, its a result not a question!
 10 July 2011 12:50 PM
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johndpar

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I think 20A is OK, allowing for diversity at the garage CCU.

Am I wrong?

Otherwise the main cut-out would need to be way higher for everyones installation.

John

PS. This is probably irrelevant anyway. The garage circuit will probably end up back on its own MCB
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