Calculate busbar rating I_z with full IEC 61439-1 ampacity derating, IEC 60865-1 short-circuit thermal and electrodynamic checks, voltage drop, and recommended cross-section for copper and aluminum busbar trunking systems.
Ampacity is derived from I_z = k · A^0.61 where A is the cross-section in mm² and k is a base constant depending on installation method (18.5 for horizontal open air, 13.5 for enclosed horizontal, 11.0 for vertical enclosed). The result is derated for ambient temperature above 30°C, grouping of multiple bars per phase, and enclosure type. This gives the maximum continuous current the busbar can carry under specified conditions.
IEC 61439-1 requires busbars to withstand the thermal effects of short-circuit currents without damage. The thermal equivalent current is calculated as I_th = I_k × √(t_k), where I_k is the RMS fault current and t_k is the fault duration. This must be less than the rated short-circuit current I_cw (typically 25–50 kA for standard assemblies). IEC 61641 defines the test requirements for busbar systems.
IEC 60865-1 §4 gives the peak electrodynamic force on parallel busbars during a short circuit as F_N = 0.2 × i_peak² / (d × h) newtons per meter, where i_peak = √2 × I_k is the peak fault current, d is the center-to-center spacing between phases, and h is the busbar thickness (height in the force plane). Results are expressed in kN/m and must be checked against the mechanical strength of busbar supports.
IEC 61439-1 limits the temperature rise of busbar components to 55°C above ambient at rated current (ΔT ≤ 55°C for bare copper or aluminum). The actual temperature rise is estimated as (I_r / I_z_base)^1.75 × 55°C. For enclosed or grouped arrangements, the base ampacity is reduced accordingly. Excessive temperature rise accelerates insulation degradation and can cause joint overheating.
Voltage drop is calculated as ΔU = √3 × I_r × L × (ρ × (1 + α(T_amb − 20)) / A) for a 3-phase system, where ρ is the resistivity at operating temperature (2.25×10⁻⁸ Ω·m for copper at 70°C), α is the temperature coefficient (0.00393/°C for copper), L is the run length, and A is the cross-sectional area. IEC 60439-2 recommends keeping voltage drop below 5% of the reference voltage for proper operation of downstream equipment.
Copper busbars offer higher conductivity (ρ = 1.72×10⁻⁸ Ω·m vs 2.82×10⁻⁸ Ω·m for aluminum), better corrosion resistance, and higher current density for the same cross-section. Aluminum busbars are 60–70% lighter and significantly cheaper per kilogram, but require larger cross-sections for the same ampacity, necessitating larger trunking enclosures. For rated currents above 2500 A, copper is typically preferred; below 1600 A, aluminum can offer cost savings on total installation weight and material cost.