Bussco Bus Bar Electrical Design Characteristics

Delivering Voltage and Current

The starting point for bus bar design is identification of the voltage(s) and current(s) that the bus bar will be required to distribute. Then, a candidate cross sectional area can be selectred and an initial conductor layout can be designed. The elctrical properties of the conductor(s) must then be evaluated to determine if the voltage and current requirements will be met.

The most important electrical property is resistance, which applies to all types of voltages and currents. If the bus bar is carrying AC current or DC switching currents, two additional electrical properties need to be considered: capacitance and inductance. Each of these causes its own type of reactance (opposition to electrical change). Those reactances contribute to the impedance of the bus bar, which is resistance (to steady current) plus the total reactance.

Resistance

Conductor resistance is calculated from the resistivity of the conductor material and the cross-sectional area of the conductor:

Formula (2.1)
R = p / A ohms/foot

Where,
p = resistivity in ohms x sq mils per foot (from table 3)
A = cross sectional area in sq mils calculated from formula (2.7)

Current through the conductor will generate heat, and the resistance of the conductor will then increase proportaionlly to the heat. This sounds like an out-of-control spiral, but the system will eventually come to an equilibrium determined by the amount of heat dissipated by the surroundings of the bus bar. An allowable temperature rise will need to be determined (see ampacity table), then the resistance recalculated at that temperature to check the impact on bus bar performance.

Formula (2.2)
R2 = R1 [1 + α (T2-T1)] ohms/foot

Where,
R2 = resistance at new temperature in ohms/foot
R1 = resistance at 20° C in ohms/foot
T1 = 20° C
T2 = new operating temperature in ° C
α = temperature coefficient of resistivity of the material (from table 4)

Voltage Drop Calculation

The voltage drop can be calculated using Ohm's Law

Formula (2.3)
Δ V = R x l x l

Where,
Δ V = voltage drop in volts in the entire conductor length
R = resistance in ohms/foot as calculated from formula (2.1) or (2.2)
l = conductor length in feet
l = current in amperes given by the amperage requirements of the pplication

If the voltage drop does not meet the application requirements, consider increasing the cross sectional area to lower the conductor resistance.

Capacitance

The capacitance is directly proportional to the conductor area and the dielectric constant, and inversely proportional to the insulation thickenss, as shown by this formla:

Formula (2.4)
C = 0.224 (k) (w) (l) / d picofards

Where,
k = dielectric constant of the insulation used
w = conductor width in inches
l = conductor length in inches
d = thickness of dielectric in inches

Inductance

Low inductance is a critical element for controlled and efficient operation of the bus bar as it prevents excessive transient overshoots. The inductance of a two layer bus bar can be calculated by using this formula:

Formula 2.5)
L = 31.9 (l ) d / w nano Henrys

Where,
l = length of conductor in inches
d = dielectric thickness in inches
w = conductor width in inches

Characteristic Impedance

Low characteristic impedance improves the bus bar performance for AC loads, or during the transition when load currents are switching.

Formula (2.6)
Ζο = √(L/C) Ohms

Where,
L = inductance
C = capacitance
Assumption: Effective loss less conductors and dielectric