Spare Feeder

Similarly, 100 A and 25 A spares are also not considered, assuming that the utility of such feeders would be rare and for short durations (such as for maintenance). 

Spare motor feeders are also not considered for similar reasons. 

Service entrance equipment must have a current rating equal to or greater than the present and future service entrance conductors ampacity. Moreover, this equipment must have an lAC current rating that is higher than the available short circuit current at the utility company s transformer. Spare service entrance conduits should be installed for future service entrance conductors. The service entrance equipment should include spaces for future feeder circuit breakers to supply future loads. All incoming service entrance conductors must be 3 phase, 4 wire and should be installed in underground metal conduit. 

The application of the M — 1 philosophy is less complicated with incoming feeders e.g. underground cables, overhead lines. N is usually chosen as 2 because it is not usually economical to use three or more feeders for one switchboard. Both feeders are usually in service and so the spare does not usually exist. However, each feeder is rated to carry the full demand of the switchboard. Therefore with both in service each one carries half of the demand, and can rapidly take the full demand if one is switched out of service. This approach also enables a feeder to be taken out service for periodic maintenance, without distmbing the consumers. 

Moulded case circuit breakers are also available for incoming and busbar section purposes, with ratings up to 6000 A and service voltages between 220 V and 660 V. (At 415 V a 4000 A circuit breaker would satisfy the duty of a 2500 kVA feeder transformer with about 15% spare capacity.) These are also available as 4-pole units. Circuit breakers having ratings of 800 A and above are often provided with several adjustments that widely modify the shape of the complete protection curve, as described in Chapter 12. This enables the curve to coordinate with almost any other protective device or equipment that is immediately upstream or downstream of the circuit breaker. Some circuit breakers with the higher rated currents are also provided with integral earth fault protection facilities. 

The supply capacity normally consists of two parts. One part to match the known or initial consumption and a second part to account for keeping a spare generator or feeder ready for service. 

Diesel generator sets and fuel tanks, batteries, and charging system Plant lighting—alternate lights to have supplies from separate feeders A functional maintenance repair facility with adequate stocks of essential spares Storage tanks for all products, fuels, and starting materials—calibrated and tested for leaks... 

Wire and cables are usually sized exactly for the loads they serve. The sizes of feeders running to panelboards or switchboards usually allow for all spare circuits built into these devices, computed as if they were half-loaded. 

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