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Old February 12th 19, 02:18 AM posted to uk.d-i-y
John Rumm John Rumm is offline
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Default Question about sub-main sizing

On 11/02/2019 11:25, wrote:
On 10/02/2019 21:26, John Rumm wrote:
On 07/02/2019 19:17,
wrote:
On 07/02/2019 18:50, Cynic wrote:
I've not got tables to hand but check your earth loop impedance
against disconnection times for the 40A mcb

The sub-main earth loop impedance (20m of 10mm2 and 20m of 16mm2)
should be about 60mOhm (37mOhm + 23mOhm) so about 0.4 ohms total
(TN-C-S) - about 600A fault current. A type C MCB needs 10x rated
current, so 500A for a 50A MCB. Unless I've boobed it should be OK.



I think I must be missing something here... where does the 20m of
10mm^2 come into this?

Sadly, it was me that was missing something - I'd forgotten about the
extg CPC being in parallel with the new single. I must be getting old.


Ah, ok, sorry I though I had missed and extra cable somewhere rather
than being the extra CPC/Bonding conduction for the submain


--
Cheers,

John.

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