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[email protected] nothanks@aolbin.com is offline
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Default Question about sub-main sizing

On 10/02/2019 20:30, ARW 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.


You might have boobed itÂ* - twice, if I am not mistaken!


1. You are using 240V not 230V for your fault current. (Max Ze for a 50A
C type is 0.37ohm and remember that is when the cables are at 40 deg)


2. Your resistance calculations should be 16mm earth and 16mm live
(unless you wish to chop out the 6mm cpc of the 16mm T&E and not bother
using it)

Either way you are over the 0.37 ohms allowed.

It might be better to swap the head end with some thing like this

https://www.tlc-direct.co.uk/Products/CGMSF100.html


Yes, it's always better when boobs appear as a pair ;-)

I'd done the quick check at 230 but old habits (my fingers initially
typed old hobbits) die hard so 240 reappeared when I did the proper check.
When I decided to add the 10mm2 cable the brain forgot about the
existing CPC. I can't think of an excuse for that :-(

As you spotted, there was a third boob in your calcs ... brings back
memories of a character from HHG2TTG ;-)