following was posted in another forum:

I can answer why Dom installers are hated so much in one question get it right and ill be very shocked. 3k heater wired in 2.5/1.5 over a 18 meter run bunched in trunking with a ze of 0.18 ohms 230v . Is this cpc ok? And give me the minimum cpc size you could use using calculations.
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im still learning and tried to do the above calc, but dont i need to know how many cables/circuits are bunched in the trunking to get the correct answer (Cg). please help and advise or correct me thanks

It's only asking about the adequacy of the c.p.c. size, not the live conductors size, so you don't really need Cg to answer that bit.

You should be able to work out R1 and R2 from the cable length & c.s.a.s.

Even so, I'm not sure how it can be answered without knowing the characteristics of the protective device...

I presume the OP means a 3kW heater, rather than a 3 kelvin one, and there aren't any submains.

- Andy.

thanks, i was just trying to test myself to see if i fully understood all the calcs involved, i thought something was missing from question