Copper Busbar Current Carrying Capacity Table

Copper current-carrying capacity is the maximum current a copper busbar or conductor can carry continuously without exceeding the permitted temperature rise. In low voltage switchboards this value depends on the busbar cross-section, surface condition, number of bars per phase, ambient temperature and mounting orientation. The reference condition is usually 35 °C ambient temperature and 65 °C busbar temperature.

Quick access: Current rating table · Correction factors · Busbar selection

Definition

What Is a Copper Busbar and Why Is It Used?

A copper busbar is a rectangular conductor that distributes energy inside a switchboard from the main incoming point to the outgoing feeders. Busbars are used instead of cables because at high currents they provide more compact, lower-impedance and mechanically more stable distribution. Copper’s high conductivity, good mechanical strength against short-circuit forces and low contact resistance at connection points make it the standard solution in low and medium voltage switchboards.
Resistivity (20 °C)0.0178 Ω·mm²/m
Density8.89 g/cm³
Melting point1083 °C
Material gradeE-Cu / Cu-ETP
Factors

Factors Affecting Current-Carrying Capacity

The current a busbar can carry does not depend on its cross-section alone. What matters is how quickly the busbar can dissipate the heat it generates to its surroundings.
01

Surface Condition

A dark, matt painted surface increases heat dissipation by radiation and raises capacity by about 15%.
02

Number and Spacing of Bars

Bars placed side by side heat each other; because of proximity and skin effects, the gain is not linear.
03

Ambient Temperature

As the temperature inside the switchboard rises, the permitted temperature difference narrows and the current that can be carried falls.
04

Mounting Orientation and Ventilation

Vertical mounting enhances convection; capacity is reduced in enclosed spaces without ventilation.
Table 1

Copper Busbar Current-Carrying Capacity Table

The values below are approximate reference values for one bar per phase, vertical mounting, 50 Hz alternating current, 35 °C ambient temperature and 65 °C busbar temperature. Resistance values are DC resistance at 20 °C.
Scroll the table sideways to see all columns.
Cross-section, weight, resistance and current-carrying capacity by copper busbar size
Busbar (mm) Cross-section (mm²) Weight (kg/m) Resistance (µΩ/m) Bare (A) Painted (A)
12×2 24 0.21 742 123 140
15×2 30 0.27 593 145 165
15×3 45 0.40 396 180 205
20×2 40 0.36 445 182 210
20×3 60 0.53 297 226 260
20×5 100 0.89 178 313 360
25×3 75 0.67 237 274 315
25×5 125 1.11 142 371 425
30×3 90 0.80 198 321 370
30×5 150 1.33 119 428 490
40×3 120 1.07 148 414 475
40×5 200 1.78 89.0 541 620
40×10 400 3.56 44.5 848 975
50×5 250 2.22 71.2 651 750
50×10 500 4.45 35.6 1000 1150
60×5 300 2.67 59.3 761 875
60×10 600 5.33 29.7 1150 1325
80×5 400 3.56 44.5 979 1125
80×10 800 7.11 22.3 1450 1665
100×5 500 4.45 35.6 1190 1370
100×10 1000 8.89 17.8 1740 2000
120×10 1200 10.67 14.8 2020 2325
Table 2

Correction Factors

The Table 1 value is multiplied by the factors below according to the actual project conditions. The factors are multiplied together.
Copper busbar current-carrying capacity correction factors
Factor Condition Coefficient
Surface condition Bare (bright) copper 1.00
Surface condition Dark painted copper 1.15
Bars per phase 1 bar 1.00
Bars per phase 2 bars 1.80
Bars per phase 3 bars 2.45
Bars per phase 4 bars 3.00
Ambient temperature 25 °C 1.10
Ambient temperature 30 °C 1.05
Ambient temperature 35 °C (reference) 1.00
Ambient temperature 40 °C 0.94
Ambient temperature 45 °C 0.88
Ambient temperature 50 °C 0.81
Mounting orientation Vertical (busbar face vertical) 1.00
Mounting orientation Horizontal 0.95
Iperm = Itable × ksurface × kbars × ktemp × kmountExample: 40×10 mm busbar, painted, 2 bars per phase, 40 °C ambient, vertical mounting → 848 × 1.15 × 1.80 × 0.94 × 1.00 ≈ 1650 A

Table values are for reference

Busbar selection is not based on continuous current alone. Short-circuit thermal and dynamic withstand, busbar spacing, insulator spacing, connection torques and temperature rise inside the switchboard must also be verified under IEC 61439. Devpan performs busbar calculations and design verification for your switchboards on a project basis.Contact us for busbar calculations
Application

Steps for Busbar Selection

  1. Determine the continuous operating current. Take into account the rated current of the main incoming breaker and the diversity factor.
  2. Define the ambient conditions. Clarify the temperature inside the switchboard, mounting orientation, ventilation and surface treatment.
  3. Select a candidate cross-section from the table. Correct the Table 1 value with the Table 2 factors and compare it with the operating current.
  4. Verify short-circuit withstand. Calculate the thermal and dynamic stresses for the expected short-circuit current and duration.
  5. Calculate support and insulator spacing. The spacing of busbar supports is determined according to short-circuit forces.
  6. Plan the connection points. Hole layout, contact surface and tightening torque are as important as the cross-section.
Calculator

Copper Busbar Size Calculator

Enter the required current and installation conditions; the calculator applies the table values and correction factors above to suggest a suitable busbar size.

Results are for preliminary sizing, with 35 °C ambient and 65 °C busbar temperature as reference. Short-circuit withstand, temperature-rise verification and IEC 61439 design verification must be carried out for each project.

Request a sizing check for your project
Caution

Common Mistakes

  • Using the table value directly without applying the ambient temperature correction.
  • Assuming that two bars per phase double the capacity.
  • Ignoring the loss of effective cross-section caused by connection holes.
  • Sizing the neutral busbar smaller than the phase busbar under harmonic loads.
  • Not applying connection torques or not checking them periodically.
  • Setting busbar support spacing by eye rather than according to short-circuit forces.
Summary
  • Material: E-Cu electrolytic copper, at least 99.9% purity
  • Resistivity: 0.0178 Ω·mm²/m (20 °C)
  • Reference condition: 35 °C ambient, 65 °C busbar temperature, vertical mounting, bare surface
  • Painted busbar: dark matt paint increases capacity by about 15%
  • Multiple bars: 2 bars ≈ 1.80×, 3 bars ≈ 2.45× (not double)
  • Example: a 40×10 mm bare copper busbar carries about 848 A under reference conditions.
Frequently Asked Questions

Frequently Asked Questions About Copper Current-Carrying Capacity

What is copper current-carrying capacity?

Copper current-carrying capacity is the maximum current a copper busbar or conductor can carry continuously without exceeding the permitted temperature rise. In switchboard applications the reference condition is usually 35 °C ambient temperature and 65 °C busbar temperature.

How many amps can a 30×5 copper busbar carry?

Under the reference conditions of Table 1 (35 °C ambient, one bar per phase), a 30×5 mm copper busbar carries 428 A bare and 490 A when painted dark. For other ambient temperatures or several bars per phase, apply the correction factors in Table 2.

How is a copper busbar cross-section selected?

First, the facility’s continuous operating current is determined; then the table value is corrected with the factors for surface condition, bars per phase, ambient temperature and mounting orientation. The resulting value must be equal to or greater than the operating current. Short-circuit withstand is verified separately.

Why does a painted copper busbar carry more current?

Dark matt paint increases the surface emissivity of the busbar and allows heat to be dissipated to the surroundings more quickly. As a result, about 15 percent more current can be carried with the same cross-section.

Does using two bars side by side double the capacity?

No. Bars placed side by side heat each other, and current distribution is distorted by skin effect. Capacity increases by about 1.8 times for two bars and about 2.45 times for three bars.

What happens when the ambient temperature rises?

As the temperature inside the switchboard rises, the busbar’s ability to dissipate heat decreases and the current it can carry falls. At 50 °C ambient, capacity drops to about 81 percent of the 35 °C reference value.

Should copper or aluminum busbars be preferred?

Copper has a higher conductivity than aluminum; for the same current, a cross-section about 35–40 percent smaller is sufficient. Aluminum is lighter and generally more economical. Space constraints and connection reliability favor copper; weight and cost favor aluminum.

Does drilling holes in a busbar reduce its capacity?

Yes. Connection holes reduce the busbar’s effective cross-section and cause current concentration around the hole. Holes should be kept as small as possible and should not be drilled in rows perpendicular to the direction of current.

What should be considered in busbar connections?

Contact surfaces must be clean and flat, tightened to the correct torque, and a suitable interface must be used between dissimilar metals. Loose or oxidized connections are the most common cause of local overheating; periodic checks with a thermal camera are recommended.

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