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Busbar & Busduct Sizing

IEC 61439 / IEC 60865

Calculation Mode
Electrical Parameters
Material & Arrangement
Busbar Dimensions

The smallest passing busbar size will be selected automatically.

Environment
Physical Parameters
Short Circuit

Enter busbar parameters to begin

Results will appear here after calculation

How Busbar & Busduct Sizing Works

The busbar sizing calculator determines the required busbar dimensions based on the continuous current rating, short circuit withstand, and thermal limits for switchgear assemblies.

The current rating is calculated from the conductor cross-sectional area, material (copper or aluminium), and maximum temperature rise per IEC 61439-1 (typically 70K above 35 degrees C ambient for bare copper). Short circuit withstand is verified using the adiabatic equation, ensuring the busbar temperature does not exceed the material limit during fault conditions. Electromagnetic forces between parallel busbars during short circuits are calculated as F = (mu_0 / (2 x pi)) x (I^2 x L / d), where L is the busbar length and d is the spacing.

NEC Article 408 covers switchboard and panelboard busbar requirements. IEEE C37.20 defines metal-enclosed switchgear standards. Results include busbar dimensions, current rating, temperature rise, short circuit withstand time, mechanical force, and support insulator spacing.

Copper Busbar Current Rating (single bar, painted)

Size (mm)Rating (A)Temp Rise (K)Reference
20 × 520050IEC 61439-1
25 × 525050IEC 61439-1
30 × 530050IEC 61439-1
40 × 540050IEC 61439-1
50 × 550050IEC 61439-1
60 × 1080050IEC 61439-1
80 × 10100050IEC 61439-1

Source: IEC 61439-1 Clause 10.10

Frequently Asked Questions

How do I size a busbar for continuous current rating?
Busbar sizing for continuous current starts with selecting a material (copper: 1,700 micro-ohm-cm, or aluminium: 2,800 micro-ohm-cm resistivity) and determining the current density. For copper busbars, IEC 61439-1 and common engineering practice recommend 1.5-2.5 A/mm2 for enclosed busbars depending on cooling conditions. The cross-sectional area is A = I / J, where I is the rated current and J is the current density. For a 2000A copper busbar at 2.0 A/mm2: A = 2000/2.0 = 1000 mm2. This could be achieved with 2 bars of 80mm x 10mm per phase (1600 mm2, allowing margin for heating). Temperature rise must be verified per IEC 61439-1 Clause 10.10 (not exceeding 65K or 105 degrees C for bare busbars).
How do I check busbar short circuit withstand?
Busbar short circuit withstand has two components: thermal and electrodynamic. Thermal withstand ensures the busbar temperature does not exceed the short-time limit (250 degrees C for copper per IEC 61439-1) during a fault: A >= I x sqrt(t) / k, where k = 143 for copper (or use 13 for Aluminium per IEC 60865-1). Electrodynamic withstand checks that the peak electromagnetic force between parallel busbars does not exceed the busbar and support insulator mechanical strength. Per IEC 60865-1, the force per unit length is F = 0.2 x ip^2 / d (N/m), where ip is the peak short circuit current and d is the centre-to-centre spacing between phases in metres. Support spacing must limit busbar deflection and stress below yield limits.
What is the effect of skin effect and proximity effect on busbar rating?
At power frequency (50/60 Hz), skin effect causes current to concentrate near the conductor surface, increasing the effective resistance. For busbars thicker than approximately 10mm for copper at 50 Hz, the skin effect becomes significant. IEC 60287-1-1 and IEEE C37.23 provide skin effect factors. Proximity effect from adjacent conductors further distorts current distribution. For rectangular busbars, the skin effect factor increases with the ratio of bar thickness to skin depth (about 9.3mm for copper at 50 Hz). Using two thinner bars instead of one thick bar (e.g., 2x5mm instead of 1x10mm) can reduce AC resistance by 15-20% and improve current rating.
What busbar material should I choose — copper or aluminium?
Copper busbars have 60% higher conductivity than aluminium but cost more and weigh more (8,900 vs 2,700 kg/m3). For the same current rating, an aluminium busbar needs approximately 60% more cross-sectional area but weighs only about 55% as much. Copper is preferred for compact switchgear (IEC 61439), corrosive environments, and high fault-level applications. Aluminium is common in open-type busbars, outdoor substations, and cost-sensitive applications. Per IEEE C37.20, aluminium bus connections require special attention to prevent galvanic corrosion and creep-related joint loosening, using Belleville washers and anti-oxidant compound at all bolted connections.
How do I calculate voltage drop across a busbar?
Busbar voltage drop is calculated using Vd = I x Z x L, where I is the current, Z is the impedance per unit length (R + jX), and L is the busbar length. For a rectangular copper busbar, DC resistance per metre is R = rho / (width x thickness) in micro-ohms/m. AC resistance includes the skin effect factor: Rac = Rdc x ks. The reactance for a single-bar-per-phase system is approximately X = 0.2 x ln(2d/b) micro-ohms/m, where d is the phase spacing and b is the bar width. Per IEC 61439-1 Clause 10.11, the voltage drop across the busbar system must be included in the overall installation voltage drop calculation per IEC 60364-5-52 Clause 525.

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Key Terms

busbar ratingshort circuitcable sizing

Related FAQ

standards FAQ

Standards Reference

  • IEC 61439-1 — Switchgear assemblies
  • NEC Article 408 — Switchboards and panelboards
  • IEEE C37.20 — Switchgear