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Paul_K

Hi guys,


I have brain freeze and need some help!


I'm currently looking to run in a radial socket outlet circuit to a bedroom from a spare way in the consumer unit. The total circuit length will be approx 10 meters of 2.5 t&e. The supply is a TNC-S arrangement with a Ze of 0.13ohm (by direct measurement). The circuit protection is by way of a 20amp Type B BSEN60898 mini circuit breaker. NO RCD!!


Normally I would just follow the typical values in the OSG. This time though, I decided to just calculate it... Have to keep doing it once in a while or I'll forget!! :-/
The cable itself is fine (obviously) until I check the cpc size using the adiabatic equation. Can someone tell me if I've got this right? :-/


Ze = 0.13


R1+ R2 = 0.23 (adjusted for temperature @ 10 x 19.51 x 1.2)


Zs = Ze + R1 + R2
= 0.36


230/0.36
= 639A


K factor = 115
Assuming instantaneous time of 0.1sec


S = sroot(639*x0.1)/K
= 1.76??


Thanks in advance! :)
 
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looks like youre gonna need a cable with at least a 1.75mm cpc , which will be 6mm t&e ;-)


or maybe look at time current graphs for a 20a mcb ............


which will probably end up giving you a min cpc size of around half that.
 
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Lets end this now, a 2.5mm2 cable to a socket as a single radial to a bedroom 10 mtrs away from the dist board?? yes okay get on with it, 16 amp mcb and you will be fine, fluck the calculations on this occassion.
 
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