Summary of Methods of Busbar Rating
Tables of Properties of HC Copper Conductors
Table 12. Current ratings, moments of inertia and section moduli - strips and bars
Table 13. a.c. current ratings of laminated bars
Table 14. Current ratings, moments of inertia and section moduli - tubes
Table 15. Current ratings, moments of inertia and section moduli - sections
Table 16. Moments of inertia, section moduli and current ratings - rods
Table 17. Comparison of flat bar d.c. current ratings for different ambient and working temperatures
Summary of Methods of Busbar Rating
The following examples summarise the rating methods detailed in section 3 and section 4 for typical cases. Unless otherwise stated, a temperature rise of 50°C above an ambient of 40°C and a frequency of 50 Hz have been assumed. The ratings may be increased by blackening the busbar surfaces. (see Radiation)
Case I d.c., single rectangular-section bar on edge in still air
Case II d.c., single circular-section bar (solid or hollow) in still air
Case III d.c., laminated bars in still air
Case IV a.c., single rectangular-section bar in still air
Case V a.c., single circular section bar, in still air
Case VI a.c., laminated bars, in still air
Case VIII Economical use of busbar configurations
Case I d.c., single rectangular-section bar on edge in still air
Apply formula 4 or read direct from Table 12, for standard sizes.
Example:
Copper bar l00 mm x 6.3 mm (A = 630 mm2, p= 212.6 mm)
I = 7.73 (630)0.5 (212.6)0.39 = 1570 A
(or read direct from Table 12).
Case II d.c., single circular-section bar (solid or hollow) in still air
Apply formula 6 or read direct from Table 16 for standard sizes.
Example:
50 mm diameter copper rod
I = 8.63 (1964)0.5 (157)0.36 = 2360 A
(or read direct from Table 16).
Case III d.c., laminated bars in still air
a) Apply formula 4, or read direct from Table 12 for one bar.
b) Multiply by appropriate factor from section 3
Example:
4 copper bars 100 mm x 6.3 mm with 6.3 mm spacing.
I = 1570 A per bar.
Multiplying factor for 4 bars = 3.20.
Hence I = 3.2 x 1570 = 5020 A
Case IV a.c., single rectangular-section bar in still air
Divide d.c. rating by appropriate value of
as obtained from Figure 7
Example:
Copper bar 100 mm x 6.3 mm (a/b = 100/6.3 = 16)
d.c. rating = 1579 A (Case I).
Rf/Ro = 1.12 from Figure 7
Ö1.12= 1.058
Hence I = 1570/1.058 = 1480 A
Case V a.c., single circular section bar, in still air
a) Divide d.c. rating by appropriate value of
as obtained from Figure 4 (solid rods or tubes).
Example:
50 mm diameter copper rod.
d.c. rating = 2360 A (Case II)
Hence Rf/Ro = 1.61, from Figure 4
Hence
Case VI a.c., laminated bars, in still air
a) Determine rating of one bar as for Case IV.
b) Multiply by appropriate factor, Table 8
Example:
4 copper bars 100 mm x 6.3 mm with 6.3 mm spacing.
d.c. rating per bar = 1570 A (as Case I)
a.c. rating per bar = 1480 A (as Case IV).
Multiplying factor for 4 bars = 2.3
Hence I = 2.3 x 1480 = 3404A
a) Multiply still air rating by appropriate constant (see Enclosed copper conductors) i.e.. by 0.6 to 0.65 for conductor configurations largely dependent on air circulation (e.g., modified hollow square arrangement, Figure 9c), or by 0.7 for tubular conductors or closely grouped flat laminations.
b) Multiply by further 0.85 if enclosure of thick magnetic material.
Example:
4 copper bars 100 mm x 6.3 mm arranged as in Figure 9c, to carry a.c.
d.c. rating, single bar = 1570 A (as in Case I).
a.c. rating, single bar = 1480 A (as in Case IV).
Multiplying factor for 4 laminations (Table 8) = 2.3
Multiplying factor for configuration of Figure 9c, (see Figure 11) = 1.28
Hence still air rating for this configuration = 1480 x 2.3 x 1.28 = 4360 A
Multiplying factor for non-magnetic enclosure (Enclosed copper conductors) = 0.60
Hence enclosed rating = 4360 x 0.6 = 2610 A
Multiplying factor for magnetic enclosure = 0.85
Hence rating in magnetic enclosure =2610 x0.85 = 2220 A
Case VIII Economical use of busbar configurations
Example:
Two channels, each 100 mm high x 45 mm flange width x 8.6 mm thick (A = 1430 mm2 per channel). a.c. 60 Hz, 30°C rise on 40°C ambient in still air. From Table 15, rating based on 50°C rise on 40°C ambient. = 5550 A
Use re-rating formula (equation 8) to obtain rating for 70°C working temperature and 40°C ambient.
Hence rating under conditions specified = 5550 x 0.756 = 4195 A
Equivalent 4-bar laminated configuration for same cross-sectional area = 118 mm x 6.3 mm per bar (A = 743 mm2, p = 249 mm).
Hence d.c., rating per bar for 50°C rise on 40°C ambient. = 1300 A (from equation 4, and application of appropriate conversion constant as above).
a/b = 118/6.3 = 18.7 (see Figure 7)
= 1.08 (from Figure 7 for 60 Hz).
Hence a.c. rating per bar = 1300/1.08 = 1190 A
Multiplying factor for 4 laminations = 2.3 (Table 8)
Hence a.c. rating for 4 laminations = 1190 x 2.3 = 2760 A
Thus the double channel arrangement is able to carry more current than laminated bars, in the ratio 1.52:1 for this cross-sectional area. This corresponds to the factor given in Figure 11. For larger cross-sectional areas this factor would be still greater, for smaller sections the increase would be rather less than this, the exact value depends on the ratio of web to flange lengths of the channel used, and on the thickness of web and channel; a rather wide spacing between "go" and "return" conductors is also assumed in Table 15, in order to approximate to the "equi-inductance line" condition (see Condition for minimum loss).
