Page 238 - Handbook of Electrical Engineering

222 HANDBOOK OF ELECTRICAL ENGINEERING

9.4.3.4 Cable-sizing tables

It is common practice to prepare cable-sizing tables for low voltage cables that are to be used for a
particular project. These tables are usually prepared for,

• Three-phase motors.
• Three-phase static loads.
• Single-phase static loads.
• DC static loads.

Each table should state the operating conditions that apply e.g.,

• Ambient temperature.
• Cable type and construction.
• Cable conductor maximum operating temperature.
• Derating factor for ambient air temperature.
• Derated cable current for each size of cable.
• Motor or static load kW rating.
• Motor or static load running power factor.
• Motor or static load running current.
• Type of protection e.g., fuses, moulded case circuit breakers.

Table 9.25 is a typical example for induction motors and EPR insulated cables.

9.4.3.5 Heat dissipation during short circuits
When a short circuit occurs in a cable the surface temperature of the conductor will rise rapidly. If the
short circuit is allowed to persist the temperature will increase to values that will permanently damage
the cable insulation. Protective devices such as fuses or circuit breakers will normally operate well
before damage can occur. However, the cable manufacturers design cables to withstand a certain level
of current for a specified length of time. The level of current will depend mainly on the insulating
material used. Table 9.5 shows the maximum temperature that can be allowed to exist for a period
of 5 seconds for different insulating materials, see IEC60502, IEC60364, Chapter 9 of Reference 4.
The heat Q developed in the conductor due to its resistance R when current I passes for a
time of t seconds is,
2
Q = I Rt joules
This amount of heat is absorbed into the insulation material. If the initial temperature θ 1 of
◦
◦
the conductor is its maximum continuous value e.g., 70 C for PVC, 90 C for XLPE and EPR when
rated current I fl flows, and the temperature limit θ 2 is the maximum allowed for the insulation e.g.,
◦
◦
160 C for PVC, 250 C for XLPE and EPR when a short-circuit current I sc flows, then an equation
relating current and time can be given as (see Reference 4 Chapter 9, or Appendix A of IEC60364),
2 2
A k θ 2 + 234.5
2
(I sc − I fl ) = log e (9.6)
t θ 1 + 234.5