Page 971 - Industrial Power Engineering and Applications Handbook

Properties and ratings of current-carrying conductors 30/919

Table 30.2 Current ratings for single rectangular copper sections Table 30.3 Multiplying factors for copper sections
_______~
Conductor size Area of cross- Approximate rating Total area of cross-section Multiplying factors
(mm) section (mm2) Amp (a.c.)
(mm2) 2 bars 3 bars 4 bars
2.5 x 12.5 3 1.25 159
16.0 40.0 195 500 1.78 2.45 3.13
20.0 50.0 235 .72 2.36 3.00
25.0 62.5 287 1000
31.5 78.75 347 1500 .65 2.24 2.84
40.0 100.0 426
50.0 125.0 516 2000 .60 2.16 2.70
63.0 157.5 630
4 x 16.0 64 254 2500 .55 2. IO 2.60
20.0 80 305 3000 .52 2.02 2.52
25.0 100 367
31.5 125 445 3500 1.48 1.98 2.48
40.0 160 542
50.0 200 660 4000 1.44 1.96 2.45
63.0 252 802
80.0 320 900
100.0 400 1185 Source The Copper Development Association and the Electrical
Review.
6.3 x 25.0 157.50 473
31.5 198.45 569
40.0 252.0 693 Note
50.0 315.0 832 The space between the bars is considered to be equal to the thickness
63.0 396.9 1010 of the bars.
80.0 504.0 1220
100.0 630.0 1465
125.0 787.5 1755
160.0 1008.0 2145
10 x 50.0 500 1060
63.0 630 1260
80.0 800 1525 only 90% surface area compared to the smaller section,
100.0 1000 1800 and consequently less heat dissipation in the same ratio
125.0 1250 2150 and will require a higher derating.
160.0 1600 2620
200.0 2000 3140
250.0 2500 3710
16 x 100.0 1600 2220 Corollary
125.0 2000 2640
160.0 2560 3180 1 Thinner sections will have a relatively higher surface
200.0 3200 3760 area to dissipate heat compared to thicker sections.
250.0 4000 4500 The thinner the section, the better will be metal
315.0 5040 5370
utilization and vice versa.
Source The Copper Development Association and the Electrical 2 More bars will reduce the heat dissipation further and
Review. will require yet higher deratings.
3 Skin effect - the same theory is usually true for the
Notes skin effect. The thinner the surface, the smaller will
1 The ratings are based on a 50°C rise over 35°C ambient be the nucleus resulting in a higher concentration of
temperature in still but unconfined air.
2 AC ratings are based on spacings at which the proximity effect current at the surface and better utilization of metal.
is considered almost negligible (2 300 mm Section 28.8).
3 These are the basic maximum ratings, that a current-carrying
conductor can carry under ideal operating conditions. The rating
is influenced by the service conditions and other design We can derive the same inference from Tables 30.2,
considerations, as discussed in Section 28.5. Apply suitable 30.4 and 30.5, specifying current ratings for different
derating factors to arrive at the actual current ratings of these cross-sections. The current-carrying capacity varies with
conductors under actual operating conditions. the cross-section not in a linear but in an inconsistent
4 Ratings may be improved by approximately 20% if the busbars way depending upon the cross-section and the number
are painted black with a non-metallic matt finish paint. This is
because heat dissipation through a surface depends upon of conductors used in parallel. It is not possible to define
temperature, type of surface and colour. A rough surface will accurately the current rating of a conductor through a
dissipate heat more readily than a smooth surface and a black mathematical expression. This can be established only
body more quickly than a normal surface. Also refer to Section by laboratory tests.
3 1.4.4 and Table 3 1.1. Mechanical and electrical data for important rectangular,
5 The above ratings are for single bars. When multiple bars are circular and channel sections are also provided in Tables
used, apply the multiplying factors, as recommended in Table
30.3. These factors will account for the restricted heat dissipation 30.7, 30.8 and 30.9 respectively for reference. For more
and additional skin effect due to the larger number of bars. details contact the manufacturer.