Cables Motor feeder

The conductor size and length of the motor feeder cable need to be chosen carefully and the following points should be considered -... 

When calculating the volt-drop in low voltage motor feeder cables it is necessary to consider three factors in particular -... 

Table 9.33. Volt-drop in worked example the motor feeder cable for the ...

These are the parallel elements of the load in per-unit at the system base, f) Motor feeder cable. Convert to system base. 

Electrical. The plant electrical system is sometimes more important than the steam system. The electrical system consists of the utihty company s entry substation, any ia-plant generating equipment, primary distribution feeders, secondary substations and transformers, final distribution cables, and various items of switch-gear, protective relays, motor starters, motors, lighting control panels, and capacitors to adjust power factor. 

The power cables to the motor should preferably be buried or otherwise routed away from or protected against fire or other potential source of damages. The feeder circuit should be independent so that plant power can be shut-off without interrupting the pump operation. Conventional feeder overcurrent protection is not permitted by NFPA 20. 

A good rule-of-thumb guide is to assume that between 15% and 25% extra capacity will be required. Hence the chosen rating will be 115% to 125% of the best-known estimate at the early design stage. This requirement also applies to power transformers and their main cables or overhead power lines, and to outgoing feeder cables to auxiliary switchboards and motor control centres. It does not usually apply to individual motor consumers, see Chapter 1. 

Figure 8.4. Protection of an induction motor and its feeder cable.

Cables are used to supply power to individual loads such as motors, and to groups of loads as in the case of a feeder to a switchboard. Selecting the load current for the first case is reasonably simple. Most individual loads have a manufacturer s nameplate that gives details such as rated power, voltage, current and power factor. If the current is not given on the plate then it should be calculated from the output power, power factor and efficiency of the load. 

For high voltage cables, and low voltage feeder cables between switchboards, the disconnection time can be reduced from 5 seconds to not less than 0.2 seconds. The time of 0.2 seconds is determined from the total clearance time of a circuit breaker protected by a fast acting overcurrent relay. For high voltage motor circuits in which the short-circuit protection is provided... 

LV motors can occasionally be ignored when HV switchboard faults are being calculated but this will depend npon circumstances, e.g. the number of intermediate voltages exist in the system, whether there are many small motors or a few large motors, the average route length of motor and transformer feeder cables. On offshore platforms it is advisable to seriously consider the LV network. LV motor control centres will be inflnenced by their motor loads, and the effect of motor contribution will mainly be determined by the fnse, contractor and circuit breaker configurations. 

Note For small motors, e.g. 22 kW and below, the earth loop impedance inclnding the feeder cable armouring may be too high. When this is the situation a risk of electric shock exists dnring a short circnit at or near to the motor. To reduce the exposure to the risk it is necessary to nse a 51 N or a 50 N core balance current transformer and relay at the motor control centre. The choice of a 50 N is preferred subject to the contactor being properly coordinated with its upstream fuses. 

Now add the feeder cable impedance in series to obtain the total series impedance between the MCC and the motor. Call this total impedance Zmnic-... 

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