Derating factors

To restrict the end temperature to less than the permissible limits, it is essential that the motor output be reduced, or for a required output, a higher-capacity motor be chosen. Table 1.7 gives the approximate derating factors for different ambient temperatures. Figure l.l I is based on these figures from which the derating factor even for intermediate temperatures can be quickly determined. 

Appendix 1 Selection of Power cables 16/531 A 16.1 Introduction 16/531 A16.2 Technical details 16/544 A16.3 Service conditions 16/544 A 16.4 Recommended derating factors 16/544 A 16.5 Voltage drop 16/544... 

The selection process of power cables is almost the same as that of a bus system discussed in Section 28.3. For simplicity we consider only the basic data for selection which would suffice the majority of applications. For accurate calculations a similar approach will be essential as for the bus systems (Chapter 28). For site conditions and laying arrangements which may influence the basic rating of a cable, corresponding derating factors have also been provided. The information covered here will be useful to users to meet their cable requirements, although the data may vary marginally for different manufacturers. For more data on cables, not covered here, reference may be made to the cable manufacturers. 

For higher ambient temperatures, current capacity should be suitably reduced to maintain the same end temperature during continuous operation. Refer to Tables 28.3(a) and (b), recommending the derating factors for a higher ambient temperature or a lower temperature rise for the same end temperature of 85° or 90°C respectively. For intermediate ambient temperatures, see Figure 28.10. 

Figure 28.7 Space factor for rectangular conductors (Courtesy The Copper Development Association) Table 28.3 Derating factors on account of higher ambient temperature or restricted temperature rise...

Ambient temperature C Permissible bar temperature rise Derating factor Ambient temperature C Permissible bar lemperalitre rise °C Derating factor... 

Figure 28.10 Derating factors for different ambient and maximum operating temperatures...

For MS enclosures, which will have both hysteresis loss B and eddy current loss a higher derating factor must be... 

These are the basic maximum ratings that a current-carrying conductor can carry under ideal operating conditions. They are influenced by the service conditions and other design considerations, a.s discussed in Section 28.5. Apply suitable derating factors to arrive at the actual current ratings of these conductors under actual operating conditions. 

Table 14-3 indicates the derating fectors for the effect of altitude on standard Class B motors. Motors are designed to operate within Class B temperature rise limits when operated at rated horsepower at altitudes up to 3,300 ft. For operation of this class of motor at altitudes greater than 3,300 ft at less than the rated horsepower, the derating factors shown in Table 14-3 should be used. 

For operation of a standard Class B motor at altitudes greater than 3,300 ft at less than rated horsepower, use the derating factor. 

The maximum permitted value for the combined biaxial stress is kST, where S is the specified minimum yield strength per para. PL-3.7.1(a), T is the temperature derating factor per Table PL-3.7.1(g), and k is defined in (b) and (c) below. 

T = temperature derating factor obtained from Table PL-3.7.1(b)(8) t = nominal wall thickness, in. 

GENERAL NOTE For intermediate temperatures, interpolate for derating factor. 

The application of derating factors is fraught with inconsistent practices and confusion. Caution is required. The following need to be carefully specified ... 

The flooding mechanism to which the derating factor applies (entrainment, downcomer backup, downcomer choke, or all these) must be specified. 

Derating factors vary from source to source, and may depend on the correlation used as well as the system. For instance, some caustic wash applications have a track record of foaming more severely than other caustic wash applications (see note in Table 14-9). The derating factors in Table 14-9 are a useful guide, but are far from absolute. 

Example A transformer with a full load current rating of 1000 A is subjected to a load with the following nonlinear characteristics. The transformer has a rated winding eddy current loss density of 10.0% (0.10 pu). Find the transformer derating factor. 

The transformer derating factor is 0.928 that is, the maximum nonlinear current of the specified harmonic makeup that the transformer can handle is 928 A. 

Derating factors are often vaguely related to the foaming tendency... 

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