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Topic Title: Overload omision
Topic Summary: 433.3.1 ?
Created On: 07 November 2012 07:25 AM
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 07 November 2012 07:25 AM
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SherlockOhms

Posts: 325
Joined: 05 April 2011

Chaps,

Job as installed as follows -

TT. 32A RCBO. 4mm SWA to external box. Link to FCU then 2,5mm SWA (install method D) into garage. Small dist. bd with incomer then 1 x 16A MCB and 1 x 6A MCB.

My question, is the FCU required as it seems to limit the whole lot to 13A? Can the 2,5mm SWA overload? 433.3.1 ?

Regards,

S.
 07 November 2012 07:46 AM
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JZN

Posts: 562
Joined: 16 November 2006

No i would not use the 13A FCU. A quick look shows 28 amps for the 2.5mm SWA (method D) but thinking about you load (100watts of lighting and a couple of sockets chrging batteries etc) this would be unlikely for any period of time.

If in doubt remove FCU and swap RCBO for 20A.

John
 07 November 2012 11:44 AM
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SherlockOhms

Posts: 325
Joined: 05 April 2011

But am I correct in saying the 2,5 SWA can't overload and my interpretation of the reg. is correct?

S.
 07 November 2012 12:10 PM
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OMS

Posts: 19759
Joined: 23 March 2004

Well as it stands, it can't overload as you have a 13A CPD in place.

It's not easy to say that a socket outlet circuit won't overload but the 16A MCB takes account of that.

If you start with 16A and then go mad and assume FLC through the 6A MCB you have a total of 22A - which is less than the CCC of 2.5mm2 SWA - so you have an inherently safe system from an overload point of view.

I'd say 433.3.1 is satisfied

regards

OMS

-------------------------
Failure is always an option
 07 November 2012 12:19 PM
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SherlockOhms

Posts: 325
Joined: 05 April 2011

Thanks OMS.

S.
 07 November 2012 01:33 PM
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GB

Posts: 359
Joined: 04 September 2002

Why not change the 2.5mm swa or downsize Rcbo and comply with all the regs (probably)
 07 November 2012 01:44 PM
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SherlockOhms

Posts: 325
Joined: 05 April 2011

It's all installed as is. Nothing to do with me really, I'm simply queerying what's there.

The 4mm SWA feeds an external box which then feeds the hot tub.
The 2.5mm SWA tees off the box to the garage.

S.
 07 November 2012 02:50 PM
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AJJewsbury

Posts: 11581
Joined: 13 August 2003

What sort of lengths are involved?

(just thinking of fault protection - L-N loop impedance, as Iz < In)

- Andy.
 07 November 2012 03:52 PM
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SherlockOhms

Posts: 325
Joined: 05 April 2011

4mm approx 10m. 2.5 approx. 15m

S.
 07 November 2012 04:18 PM
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AJJewsbury

Posts: 11581
Joined: 13 August 2003

4mm approx 10m. 2.5 approx. 15m

So R1+Rn of about 0.11Ω for the 4mm² and 0.27Ω for the 2.5mm² - giving 0.38Ω overall, if we presume supply L-N loop impedance doesn't exceed 0.35Ω then we shouldn't be faced with a total of more than about 0.73Ω - so well within the requirements for a 32A B type.

Should be fine on fault protection too then.

- Andy.
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