How big is the room space where the heaters are to be installed in cubic metres and to what degree is this space insulated?
For example a room that was 7mx4mx2.4m with a high degree of insulation would require about 3.5kW of heat.
Normally you would work out your design for the electrics, firstly taking in the consideration of what is required at the load side (garage)
Your basic requirements are lights, sockets and heating.
this could be divided in to two circuits if the heaters were plug in or three if they require a circuit of there own.
Also consideration would need to be taken for the use of the socket outlets, if you are using this as a workshop you may need a little more power to accommodate welders, lathe etc for machines with high current and inrush current demands.
So really depends what your requirements are?
If the cable is SWA three core, you will see that the cable is sheathed with black PVC and a protective metal braid/strips before a second inner PVC sheath then three conductor cores.
6.00mm can carry a max of 53A, so that could be protected at the cu by a 50A MCB, given you 6A for lighting 20A for sockets and 20A for heating circuit with two 2kW heaters on it.
As the derating of the cable is an issue for the power circuits, the cable could be in 2.5mm t&e and removed from any insulation routes or be of a higher CSA.
4.00mm t&e can carry max of 37A, derated by the 0.63 would allow 23A to be carried safely this can be protected by a 20A MCB.
Then the jobs a good one!
