Best way sub board supply

[mattyevz]

Hi peeps,
I usually stick to domestics but have been asked to do abit of commercial so bear with me.Just woundering what would be the best way to do this.
Its a shop im working on .upstairs is going to be a new consumer unit 17th rcd 2x20 radials skts and 1x 6 lighting circuit all trunking.i need to get a supply to the sub board. Downstairs is a 3 phase board no rcds but has spare ways. Could i put in a 50 or 40amp breaker with 10mm twin & e a seperate 6mm earth if diversity is ok.the run is about 10meters .All cable is surface mount so no need for an rcd correct? or is there a better way? or would it be better to split one of the tails and put into a small 50 amp DP in an inclosure? the main supply is located in the same place as the 3 phase board. the customer wants cheapest possible.
thanks

 

[HRL]

What is the sun main for? Domestic?

 

[HRL]

sorry sub

 

[mattyevz]

no its commercial for a shop. Downstairs is a hairdressers( 3phase board). upstairs (where the new con unit is going) is a photography studio with 2x 20 radials and 1xlight circuit.

 

[johnboy6083]

i would do as you suggested, mate, and run the cable in MT4 or something similar. like you said, as its on the surface, then no RCD required.
i would only split the tails if you need to completely separate the installations,

 

[mattyevz]

thanks mate.do i need to run a seperate 6mm earth aswell?

 

[johnboy6083]

i dont think so, unless you are connecting any bonding to that board. im sure others will correct if im wrong.