The busbar is the main artery of a switchgear cabinet, and its selection and jointing quality directly determine whether the cabinet can operate safely for a decade. But many engineers’ understanding of busbars stops at “copper is better than aluminium” and “thicker is better,” overlooking the details that really matter — ampacity derating, jointing torque, and copper‑aluminium transition. Here are 10 points, each learned from real‑world pitfalls. First, copper vs aluminium is not about “good vs bad” but about matching the scenario. Copper has a resistivity of about 0.0172 μΩ·m, aluminium about 0.0283 μΩ·m — at the same cross‑section, aluminium carries only 75%‑80% of copper’s current. But aluminium is cheaper and lighter, perfectly usable where ampacity requirements are moderate and space is ample. The key: copper and aluminium must never be directly jointed, or electrochemical corrosion will occur. Second, ampacity is not “cross‑section × a constant.” For the same cross‑section, vertical orientation carries 5%‑8% more current than horizontal because air convection is better. When multiple busbars are paralleled, you cannot simply multiply single‑bar ampacity by the number of bars — the more bars in parallel, the more each is derated (adjacent bars heat each other). Typically, 2 bars in parallel use a factor of 0.9, 3 bars 0.85, 4 bars 0.8. Third, ambient temperature and altitude require derating. Busbar ampacity is typically based on 35°C or 40°C ambient. If the cabinet internal temperature exceeds the base, derate by about 1.8% per 1°C rise. Above 2000m altitude, air density drops, heat dissipation worsens, and derating is required (typically 5%‑10% per 1000m). Fourth, quick reference for common copper bar sizes (TMY, 35°C ambient, vertical). 15×3 ≈ 200A; 20×3 ≈ 260A; 30×4 ≈ 400A; 40×5 ≈ 550A; 50×6 ≈ 700A; 60×8 ≈ 900A; 80×10 ≈ 1200A; 100×10 ≈ 1400A; 120×10 ≈ 1600A. Note: these are reference values — actual selection must be corrected per manufacturer data and derating conditions. Fifth, joint surface preparation: grinding, paste, torque — all three are essential. Joint surfaces must be sanded to remove oxide layers and expose metallic shine, then immediately coated with conductive paste (to prevent re‑oxidation), and finally tightened to standard torque. Grinding without paste leads to re‑oxidation within hours; paste without proper tightening allows the paste to be squeezed out and contact resistance remains high. Sixth, bolt torque is a life‑or‑death line. M8 bolts about 10‑12 N·m, M10 about 20‑25 N·m, M12 about 35‑40 N·m (follow manufacturer specifications). Insufficient torque causes high contact resistance, heating, and fire; excessive torque crushes the busbar or strips threads. Always use a torque wrench — relying on feel will eventually cause an accident. Seventh, copper‑aluminium transition must use transition bars or terminals. Directly connecting copper and aluminium in humid air forms a galvanic cell, corroding the aluminium and rapidly increasing contact resistance. The correct approach: use copper‑aluminium transition bars (flash welding or friction welding) or transition terminals. The copper end connects to copper, the aluminium end to aluminium, and the welded interface in between does not participate in electrochemical corrosion. Eighth, phase sequence and colour coding must not be scrambled. Facing the cabinet front, horizontal arrangement from left to right: L1 yellow, L2 green, L3 red, N light blue, PE green‑yellow. Vertical arrangement from top to bottom: L1, L2, L3, N, PE. Wrong colours create hidden hazards for future maintenance and mis‑wiring. Busbar heat‑shrink insulation colours must match the phase sequence. Ninth, temperature rise limits: busbar 70K, terminals 80K. IEC 61439 specifies that at 40°C ambient, busbar temperature rise must not exceed 70K (busbar temperature ≤110°C), and connection terminal temperature rise must not exceed 80K (terminal temperature ≤120°C). Beyond these values, insulation aging accelerates, contact resistance further increases, and a vicious cycle forms. An infrared thermometer is standard equipment for busbar inspection. Tenth, insulation support spacing and dynamic/thermal stability. Busbar insulator support spacing generally does not exceed 1m (shorter when short‑circuit currents are high). Excessive spacing allows electromagnetic forces during short circuits to deform busbars or break insulators. Dynamic stability is measured by peak withstand current (Ip), thermal stability by short‑time withstand current (Icw). When selecting busbars, both values must exceed the system short‑circuit current. Busbar selection and jointing leave no room for “close enough” — every parameter has its reason. Remember these 10 points, and your busbars won’t overheat, and your joints won’t catch fire