Cable Sizing

The primary function of cable is to carry energy reliably between source and utilization equipment. In carrying this energy there are heat losses generated in the cable that must be dissipated.

The cable sizing procedure is based on the following considerations:

1.1.  Load Current Criteria

Full load current shall be calculated from following formula :

For AC systems, the formula of calculating full load current:

 I_fl  =  P / V . cos 0 . n

where:

P              : Active Power (W)

V              : Voltage (V)

I_fl              : Full load current (A)

η               : Efficiency

For DC systems, the formula of calculating full load current:

 I_fl = P/V

The minimum conductor size, before the application of any adjustment or correction factor, shall have an allowable ampacity 125% of full load current.

1.2.  Cable Derating Factor

  The given cable ampacity has to be derated base on the actual method and local environment. The derating factor are calculated with the following formula:
D_f  =k₁ x   k₂

a.    Temperature Derating Factor (k₁)

Temperature derating factor cable are considered as 1.00 base on NEC Table 310.15(B)(2)(a) for maximum conductor temperature 90⁰C and ambient temperature 26⁰C - 30⁰C

b.    Group Derating Factor (k₂)

Group derating factor cable are considered as 80% base on NEC Table EC Table 310.15 (B)(3)(a) for number of current carrying conductor 4-6

Over all derating factor based on above consideration to be used on the corrected cable ampacity (cable ampacity at 20⁰C) shall follow

 1.3.  Voltage Drop Criteria

For AC systems, the method of calculating voltage drops based on load power factor is commonly used. Full load currents are normally used, but if the load has high startup currents (e.g. motors), then voltage drops based on starting current (and power factor if applicable) shall also be calculated.

 

where

VD_3φ          : Three phase voltage drop in Volt

L                 : Length of cable in meter

I                  : Load current in ampere (RMS Value)

R                : Resistance of cable in ohms per kilometer

X                : Reactance of cable in ohms per kilometer

Cos θ         : Power factor of connected load

V                : Nominal system voltage (line to line)

n                 : Number of cable in parallel per phase

where

VD_1φ          : Single phase voltage drop in Volt

For DC system is calculated using the following formula: