Design Criteria

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Certain criteria must be considered prior to inclusion of a bus bar in a system. The type of bus required will dictate whether or not an Atlee Standard can be used. If so, tooling charges are avoided provided, of course, specifications remain within the broad parameters noted.
 
Electrical Behavior of a laminar bus bar is determined by Capacitance(C), Inductance(L), Resistance(R), and Conductance(G).
 
Capacitance (C) is computed by using the following formula:
 
 
where K=dielectric constant of the insulation used, N=number of conductors, W=width, L-length, and D=thickness of dielectric.
 
Increased capacitance results in greater signal suppression and noise elimination. To obtain capacitance, the bus element or conductor must be made as wide as possible. In addition, the insulation or dielectric must have a high permittivity or K factor and be as thin as feasible.
 
Inductance (L) when maintained at low values, results in a characteristicly low impedence for better attenuation of noise. The thinner the dielectric and the narrower the conductor, the lower the inductance. Inductance also decreases as the frequency increases. The formulas follow:
 

External Inductance

 

Total Inductance at high frequency: for (T > > SD)

 
D=dielectric thickness, W=width of conductor, L=length of conductor, SD=Skin Depth,
 

DC Resistance (R) for a two-conductor bus at 20 C is calculated by:

 

DC Resistance for copper at 20 C

 
where L=length of conductor, N=Conductivity, W=width, and T=thickness.
 
Conductance (G) is also dependent upon the physical dimensions of the bus and the dielectric conductivity of the insulation
 
 
where U=dielectric conductivity at the operating frequency, W=conductor width, and D=dielectric thickness.
 
When designing a bus bar, environmental parameters must also be taken into consideration. For example, safe current-carrying capacity decreases as the temperature increases. Therefore, a bus that must operate in a heated environment should be designed heavier than one that operates in a cool atmosphere. Furthermore, a bus that will be used on a computer in a climate controlled room does not require the additional precautions, such as edge sealing or encapsulation, as one that operates in a radar on a fishing boat. Quite often a bus bar will serve a dual role: to carry current and ground and also to strengthen or stiffen a PC board.
 
Of course, of primary interest in determining current-carrying capacity are the cross-sectional dimensions put forth in the following chart:
 
 
 Wire Harness  Cross Sectional 
Area
 Typical Equivalent 
Bus Element
 Current 
 Carrying 
 Capacity 
 (amps) 
DC
 Resistance 
 of a 2 - 
 Conductor 
 Bus 
 (milliohm/ft) 
 Wire 
 Size 
 (Gauge) 
 Diameter 
 (inches) 
 Circ. 
 (mils) 
 Equiv. 
 Sq. 
 Inches 
 Thickness 
 (inches) 
 Width 
 (inches) 
22 .0253 640 .000503 .005 .101 1.52 32.44
20 .0320 1020 .000804 .005 .161 2.42 20.30
18 .0403 1620 .00128 .005 .256 3.85 12.75
16 .0508 2580 .00203 .005 .406 6.13 8.04
14 .0641 4110 .00323 .005 .646 9.75 5.05
12 .0808 6530 .00513 .010 .513 15.50 3.12
10 .1019 10380 .00815 .010 .815 24.62 2.00
8 .1285 16510 .01297 .015 .865 39.35 1.26
6 .1620 26204 .02061 .020 1.032 62.40 .792
4 .2043 41740 .03278 .030 1.092 99.20 .500
2 .2576 66400 .05215 .035 1.490 157.50 .320
0 .3249 106000 .08325 .045 1.850 252.00 .200
00 .3648 133000 .10446 .045 2.320 316.00 .160
 
Current-carrying capacity is based on use of 400 circular mils per amp. This can be converted to 314 square mils per amp in order to determine rectangular cross section.
 
EXAMPLE:
 
Requirement is for single bus element to carry five (5) amps:
5 X 314 sq. mils + 5% safety factor = 1648 sq. mils
A typical laminar bus element to meet these requirements would be .010"thick X .165"wide.
 
In certain instances when a maximum voltage drop must be specified, some adjustment to the dimensions of the conductors will probably be necessary.
 
Through the use of the stable and predetermined characteristics of a properly designed bus bar, circuit design can be simplified and performance improved.
 

 
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