Current Carrying Capacity for Cable

CURRENT CARRYING CAPACITY FOR CABLE

               Current carrying capacity is defined as the amperage a conductor can carry before melting either conductor or insulation. Heat, cause by an electrical current flowing through a conductor, will determine amount of current a cable can handle.  Cables may be seriously damaged, leading to early failure, or their service lives may be significantly reduced, if they are operated for any prolonged periods at temperature above those corresponding to the tabulated current-carrying capacities.

The tabulated current-carrying capacities are based upon an ambient air temperature 30^o C. When the surrounding temperature is higher than 30°C, the conductor’s operating temperature will also increase and the current carrying capacity of the conductor will be reduced. For other values of ambient air temperature it is necessary to apply a correction factor (multiplier) to obtain the corresponding effective current-carrying capacity.

The following formula applies.

a)      I_t =                      I_n                                                           

                              Ca x Cg x Ci x Ct                              

Iz            the current-carrying capacity of a cable for continuous service, under the particular installation condition concerned

It            the value of current tabulated in this appendix for the type of cable and installation method concerned, for a single circuit in an ambient temperature 30˚C.

Ib            the design current of the circuit, i.e. the current intended to be carried by the circuit in normal service.

In            the nominal current or current setting of the device protecting the circuit against overcurrent

I2            the operating current (i.e. the fusing current or tripping current for the conventional operating time) of the device protecting the circuit against overload

C             a correction factor to be applied where the installation conditions differ from those for which values of current-carrying capacity are tabulated in this appendix. The various correction factors are identified as follows:-

               Ca          for ambient temperature

               Cg          for grouping

               Ci            for thermal insulation

               Ct           for operating temperature of conductor

Determination of the size of cable to be used

Example for EMSB to ESSB-LP

I_n = 600 A                          

Ca= 0.97 (refer to Table 4C1, IEEE), assuming ambient temperature of 35°C for thermosetting cable)

Cg=0.8 (refer to Table 4B2, IEEE) I, total 3 core cable touching Horizontal/Vertical)

Ci=1.0 (cable not run in enclosed thermal insulation material)

Ct=1.0 (1.45/2=0.725 for BS3036 semi-enclosed fuse)

I_t  =                 600            

                0.97 x 0.8 x 1 x 1

    = 773.20 A

Referring to Table 4E1A, IEEE for XLPE/PVC cable,  4/1C x 400mm2 cable on tray has a current carrying capacity of 849 A.

Example for ESSB-LP to SUBMAIN NO.5

I_n = 200 A                          

Ca= 0.97 (refer to Table 4C1, IEEE), assuming ambient temperature of 35°C for thermosetting cable)

Cg=0.8 (refer to Table 4B2, IEEE), total 3 core cable touching Horizontal/Vertical)

Ci=1.0 (cable not run in enclosed thermal insulation material)

Ct=1.0 (1.45/2=0.725 for BS3036 semi-enclosed fuse)

I_t  =                 200            

                0.97 x 0.8 x 1 x 1

    = 257.73 A

Referring to Table 4E1A, IEEE for XLPE/PVC cable,  4/1C x 95mm2 cable on tray has a current carrying capacity of 341 A.