Cable size calculations

Power connections and control wiring The loss within such components is measured by their resistance, which, in the case of cables, is a function of their size and length. The loss in the external power cables is calculated similarly, parts of which run inside the assembly to connect the various feeders, by measuring their average length inside the assembly. 

It could similarly be calculated for the individual outgoing circuits, or considered equivalent to half the cable size being used to feed the circuit. 

A house extension has a total load of 6 kW installed some 18m away from the mains consumer unit for lighting. A PVC insulated and sheathed twin and earth cable will provide a sub-main to this load and be clipped to the side of the ceiling joists over much of its length in a roof space which is anticipated to reach 35°C in the summer and where insulation is installed up to the top of the joists. Calculate the minimum cable size if the circuit is to be protected by a type B MCB to BS EN 60898. Assume a TN-S supply, that is, a supply having a separate neutral and protective conductor throughout. 

A cooker with a rating above 3 kW must be supplied on its own circuit but since it is unlikely that in normal use every heating element will be switched on at the same time, a diversity factor may be applied in calculating the cable size, as detailed in the On Site Guide. 

When there is no dedicated transformer and these circuits are connected on the system bus directly a large inductor will be essential at the incoming of the static circuits, sufficient to absorb the trapped charge within the transformer and the interconnecting cables up to the converter unit. The size of the inductor can be calculated depending on the size (kVA) of the distribution transformer, its fault level and the characteristics of its current limiting protective device. An inductor sufficient to absorb //, L of the transformer and the cables may be provided at the incoming of the sialic circuits. 

I,// 2 =//f = maximum resistance of the connecting leads from the CT terminals to the relay terminals. For calculating this, for an estimated length and size, refer to cable data in Table 13.15... 

For clarity, voltage drop in the next size of cable, i.e. 3 x 95 mm, is also calculated as... 

Calculate a suitable size for the load cable to satisfy the /-squared-f duty. 

When designing a lighting circuit it is customary practice to size the cables so that the farthest lamp from the supply receives no less than 95% of its nominal voltage. In addition it is assumed that all the hghting fittings are energised when this design calculation is made. 

A TN supply feeds a domestic immersion heater wired in 2.5 mm PVC insulated copper cable and incorporates a 1.5 mm CPC. The circuit is correctly protected with a 15 A semi-enclosed fuse to BS 3036. Establish by calculation that the CPC is of an adequate size to meet the requirements of lET Regulation 543.1.3. The characteristics of the protective device are given in lET Regulation Fig 3A2(a) of Appendix 3. 

To calculate the size of trunking required to enclose a number of cables ... 

When calculating the size of cable required to supply a single-storage radiator, it is good practice to assume a current demand equal to 3.4 kW at each point. This will allow the radiator to be changed at a future time with the minimum disturbance to the installation. Each radiator must have a 20 A DP means of isolation adjacent to the heater and the final connection should be via a flex outlet. See Fig. 3.57 for wiring arrangements. 

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