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Join Date: Oct 2008
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Busbar Selection and Ampacity

11/11/2009 7:59 AM

I would like to know how to select busbars ( Cu or Al ) based on load current.If number of runs increased how to apply derating factor.What is vertical and horizontal busbar ? Any web site is available for manual calculation?

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Re: Bus bar selection based on ampacity

11/11/2009 8:11 AM
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Join Date: Mar 2008
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Re: Bus bar selection based on ampacity

11/11/2009 9:46 AM


To select busbars, please consider the following factors;


Given a set of electrical, mechanical and environmental bus bar requirements, the focus of the design process is to find the optimum combination of materials, configurations and manufacturing processes that will yield a finished product that meets the requirements of the application.

A) Electrical

1) 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
selected and an initial conductor layout can be designed. The electrical 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

2) Resistance

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

R = ρ / A ohms/foot


ρ = resistivity in ohms x sq mils per foot
A = cross sectional area in sq mils calculated

Current through the conductor will generate heat, and the resistance of the conductor will then increase proportionally 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, then the resistance recalculated at that temperature to check the impact on bus bar performance.

R2 = R1 [1+ α (T2-T1)] ohms/foot


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

3) Voltage Drop Calculation

The voltage drop can be calculated using Ohm's Law.

∆V = R x ℓ x I (2.3)


∆V = voltage drop in volts in the entire conductor length
R = resistance in ohms /foot as calculated from formula

ℓ =conductor length in feet

I = current in amperes given by the amperage requirements of the application

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

4) Capacitance

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

C = 0.224 (k)(w)(ℓ) / dpicofards


k = dielectric Constant of the insulation used w = conductor width

ℓ = conductor length
d = thickness of dielectric
(ℓ, w and d are in inches)

5) 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:

L = 31.9 (ℓ ) d/w nano Henrys


ℓ = length of conductor
d = dielectric thickness
w = conductor width
(ℓ,d & w are in inches )

6) Characteristic Impedance

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


L= inductance
C= capacitance

Assumption: Effective loss less conductors and dielectric.

B) Physical

1) Cross Sectional Area Considerations and Determination

The required cross sectional area of a copper conductor for a given amperage requirement
and a temperature rise of 30°C max from ambient can be determined by the following

A= 300 x I x [1 + .075(N-1)] Sq. Mills


I = current in amperes
N = number of conductors

For multiple layer bus bars, the cross sectional area calculated for each conductor should be increased by approximately 7½% to account for the decrease in heat dissipation between conductors. This is already accounted for in formula above

2) Conductor thickness and Width Calculations

The width calculation for a given cross sectional area can be determined by selecting an appropriate standard thickness and using the following formula:

w = A/(1x10 6) / tconductor width in inches Where,

A = cross sectional area as calculated from formula (2.7)

t = conductor thickness in inches selected from the list below

Available Standard Alloy 110 Thickness copper conductor:

0.020", 0.032", 0.40", 0.062", 0.093", 0.125", 0.187", 0.250", 0.375", 0.500", and 0.750" For a given cross sectional area and taking into consideration the space and structural application requirements, the combination of a very thin and wide conductor, or having a maximum w / t ratio, has the following benefits:

It will give the width, per formula for a given current, a given conductor thickness and a temperature rise of 30°C max from ambient A series of widths is calculated for the same current using all the available standard widths. The corresponding resistivity and inductance / length are calculated for each thickness and calculated width.

C) Mechanical

In addition to the electrical design characteristics of the bus bar, the mechanical design characteristics should also be addressed. The following are some aspects to consider.

1) Shapes

Bus bar shape will be affected by termination locations, enclosure constraints, operating
environment, and mounted components. When determining overall dimensions for a
laminated bus bar, allow for sufficient insulation extension beyond the conductor. The exact
amount of extension depends on insulating material used, overall thickness of the bus bar,
operating environment, operating voltage and method of sealing. Ultimately the final shape
of the bus bar is a trade off between application requirements, manufacturing capability and

Basic shapes:

Planar conductors - square, rectangle, circular, or zigzag

On-edge conductors - straight, "L", "U", "S", "T", and zigzag

Formed conductors - flat or on-edge

2) External Stresses

Temperature and vibration are a couple stresses to consider that could affect the performance and reliability of the bus bar. Also note that for some applications, the bus bar can act as a reinforcement member or stiffening component for the assembled system.

3) Termination Methods

In some applications, termination options are fixed, such as for IGBTs. For other methods, consider the type of environment the connection will need to withstand. Ease of assembly and field service accessibility are issues to consider as well.

4) Mounting Methods

The mounting method used can depend on a number of factors that include the weight, size,
ease of assembly, termination locations, enclosure constraint and accessibility to other
system components. In most cases the bus bar is secured using fasteners through the bus
bar body or by the terminations. Since holes through a conductor reduce the local cross
sectional area, the designer must compensate for any reduction in current carrying

5) Tolerances

Manufacturing capability and cost must be considered when specifying tolerances.

D) Environmental

The environment of the end application in which the bus bar operates will play a role in determining the type of materials and construction methods used for product fabrication. The main environmental factors are temperature extremes, moisture/humidity, and vibration, but many applications will impose additional concerns.

Another consideration, which is sometimes overlooked, is the type of environment the bus bar and assembled system will be subjected to during transportation. During the design phase the environmental suitability of the following should be considered: conductor material, out gassing, plating material and thickness, inner and outer insulation material, termination method, edge-sealing method, back filling of bushings and conformal coating if used.
Choosing the right materials and construction methods will help to assure that the bus bar performs well under the specified environmental conditions.


Electrical Bus Bar Requirements:

Current Carrying : 300 Amps operating current @30°C max temp rise.

Application Dependent Parameters: Minimum Voltage drop Max. Capacitance, and Minimum Inductance.
Mechanical and Physical Requirements:

Product Configuration: Two Layer, Rigid Epoxy Glass Board, Edge Potting;

Shape: Planar; Dimensions: 24" long by 1.5" wide max; Materials: Copper alloy 110, Mylar Tedlar Inner Insulation,; Termination Method: Threaded Fastener; Mounting Method: Insulated thru holes Humidity: High humidity environment Vibration: Minimum.

Design: Formulas and Tables Used

A= 300 x I x [1 + .075(N-1) ) (2.7)

I= 300 Amps

N =2 layers

A= 300 x 300 x [1 + .075(2-1) ) =96,750 sq mils or 0.097 sq in

w= A / t (2.8)

t= Selected thickness values from the available Std thickness to get the maximum w / t ratio and practical to the application

A=0.097 sq in

Thickness (t) 0 .125" 0.093" 0.062"

Width (w) 0.776" 1.043" 1.564"

w / t Ratio 6.20 11.21 25.23

The width requirement is 1.5" max therefore 11.21 (.093"/1.043") is the max w / t ratio practical to the application

(Optional method)

Use Ampacity Table A in the appendix and select the combination of w & t practical to the application and which will yield the lowest inductance (max w/t ratio)

R = ρ / A ohms/foot (2.1)

ρ = 8.1(Ω • sqmil/ft) at Ambient Temp. 20 °C, Table 3

A=96,750 sqmil

R = 8.1 / 96,750 Ω/foot =. 084 Milli Ω / foot @ 20 °C

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

R1= 0.084 Milli Ohms, as calculated above,

α = .393 from Table 3,

(T1-T2) =30 °C,

R2= 0.084 [(1+0.393(30)]= 1.074 Milli Ω / foot @ 50 °C
∆V = R x ℓ x I (2.3)

ℓ (Conductor Length) = 2 ft

R = 0.084Milli Ohm / foot at ambient temperature

I = 300 Amps

∆V = 0.084 * 2 *300= 50.4 Milli Volts at ambient temperature

R2=1.074 Milli Ohm / foot at the 50 °C ( The max allowed


∆V =1.074* 2*300 = 644.4 Milli Volts or 0 .644 Volts at 50 °C (If this voltage drop is too large , increase cross sectional area ) C = 0.224 (k)(w)( ℓ ) / d picofards (2.4)

K (Dielectric constant Mylar tedlar) =8.5 Table 4

w (width)=1.040",

ℓ (length) =24"

d (dielectric thickness) = 0.005"

C=(0.224)(8.5)(1.040)(24)/.005= 9504 picofards or

0 .0095 microfarads

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

ℓ =24"



L=31.9 (24) (.005/1.040)= 3.68 nano Henrys

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Anonymous Poster
In reply to #2

Re: Bus bar selection based on ampacity

08/05/2010 5:52 AM

Dear sir ,

How calculate the resistance in case the busbar as following ;

1.) 4 pcs of busbar size 200x10mm per phase

2.) 4 pcs of busbar size 120x10mm per phase

3.) 2 pcs of busbar size 200x10mm per phase

4.) 2 pcs of busbar size 80x10mm per phase

I have a formula resistance at temperature Φ °C , RΘ=R20 [ 1+∞20 (Θ-20 ) ]

use for 1pc of busbar per phase.

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