Current Carrying Capacity of Cables

Current Carrying Capacity of Cables Points : Current Carrying Capacity of Cables, Determination of Total Load, Determination of Number of Sub-Circuits, Determination of Rating Of Main Switch And Distribution Board, Determination of Size of Conductor, The current carrying capacity of any conductor depends directly on its cross-sectional area and inversely on its resistivity. Ambient temperature and shape of together become of reduced capacity. The type of insulation also limits the current strength of a cable. So, selection of a cable for particular job is not simple. Tables of cables help in selecting cables size.

The current carrying capacity of a cable depends on its conducting material’. A25 mm dia bare copper and a 25mm dia hare aluminum have capacities of 940 A and 710 A respectively.

The current rating depends on the shape of the conductor also. A flat bar (25mm x 4mm) has larger surface area than a circular bar 11.3 mm dia) have the same length and cross-section, and can hence dissipate heat more readily and carry a larger working current. Determination of Total Load For determination of load of an installation the following ratings may be assumed unless the values are known or specified.
(i) Fluorescent lamps — 40 watts
(ii) Incandescent lamps, fans and socket outlets —60 watts
(iii) Power socket — outlets 1,000 watts
(iv) Exhaust fans — as per capacity of exhaust fans. Determination of Number of Sub-Circuits The number of sub-circuits are decided as per number of points to be wired and total load to be connected to the supply systems. In one light and fan sub-circuit the maximum load that can be connected in 800 watts and the maximum number of points, which can be wired is 10. In one power sub-circuit the maximum load that can be normally connected is 3,000 watts and the number of socket-outlets, which can be provided is 2. Determination Of Rating Of Main Switch And Distribution Board The current rating of the main switch is decided as per total current of the circuit to be controlled by it. The number of ways and current rating of the distribution board is decided as per number of sub-circuits to be connected to it and current of the sub-circuit having highest current rating. Determination of Size of Conductor There are three points, which must be taken into account, while determining the size of conductor for internal wiring for given circuit.
(i) Minimum size mainly for mechanical reasons.
(ii) Current carrying capacity.
(iii) Voltage drop. (i) Minimum Size of The Cable The conductor used in domestic wiring (except flexible and fitting wires), according to the regulation in our country, must riot be of size less than 1/1.12 mm in copper or 1/1.40 mm (1.5mm2) in aluminum wire. For flexible cords and fitting wires, a smaller lowest size is permissible viz. 14/0.193 mm may be used. (ii) Current Carrying Capacity The wire or cable should be of size sufficient to carry the maximum circuit current continuously without overheating. From table X the size of wire of cable can be determined corresponding to maximum current of the circuit. The following points are worth noting:

(a) The maximum possible currents are not in exact proportion to the sectional areas of the conductors. The amount of heating developed due to flow of current is proportional to the resistance of the conductor and consequently in inverse proportion to the sectional area, but the temperature rise does not only depend on the rate of dissipation of heat from the surface of conductor. The size of the conductor of the cable increases, the permissible current density decrease.

(b) Paper insulated or lead covered cables are allowed to carry larger currents than VIR cables of the same size (except in the smaller size in which the maximum permissible currents are the same). This is due to the fact that the impregnated paper insulation can withstand higher temperature than VIR cables without injury.

(c) When there are several cables bunched together in the same conduit or more than one conductor in lead covered cable the current carrying capacity for the same x-sectional area of conductor is less than that of a pair of cables in conduit.

(iii) Voltage Drop: Maximum voltage drop from supply terminals to any point on the installation is not to exceed the prescribed limit viz 2% of the supply voltage plus one volt for light load wiring and 5 percent of declared supply voltage for power load wiring. Tablex

1. The table X gives the current rating for single, twin or multi-core cable with VIR, PVC, Polythene insulation including TRS, PVC lead or aluminum sheathed. run singly or in conduit, troughing, casing or in free air. The ratings are based on an ambient temperature of 3 0°C. To Calculate Volt Drop in a Cable (a) Calculate maximum allowabic volt drop as above.
(b) Find or calculate, current to be taken by load.
(c) From the appropriate Tables I to 20, look up size of cable which has the nearest rated current above load current.
(d) Also from the Table note the length of circuit run for this cable, for 1 volt drop at rated current.
(e) First of all calculate the volt drop of this cable over the length of circuit run as if it were carrying its rated current. This is found from:
Length of circuit run
Length of run given in tables for I volt drop at rated current.
(f) Having got this value, multiply t by the proportion
Load current to be taken by cable/Rated current of cable given in tables
In order to obtain the actual volt drop in the cable when carrying this load current.
(g) If this calculated value of volt drop is greater than the maximum permissible as calculated in (a) above, then recalculate volt drop with the next largest size cable.