The busbar is the main artery of a switchgear cabinet, and its ampacity calculation and jointing quality directly determine whether the cabinet can operate safely for a decade. Many engineers select busbars based only on cross‑sectional area and judge joints only by “how tight it feels,” overlooking derating corrections, torque control, and copper‑aluminium transition — the details that really matter.

Ampacity is not determined by a single table — correction factors are the key. Busbar ampacity tables are typically based on 25°C ambient, altitude below 1000m, vertical orientation, and bare‑bar installation. In practice, multiple correction factors must be applied. The temperature correction factor is Kt=(70−T)/(70−25)Kt​=(70−T)/(70−25)​, where 70°C is the maximum allowable copper bar temperature and T is actual ambient temperature. At 30°C, Kt≈0.94Kt​≈0.94; at 40°C, Kt≈0.82Kt​≈0.82. Above 1000m altitude, air density drops and convection cooling degrades; the altitude correction factor is Kh=1−0.005×(H/1000)Kh​=1−0.005×(H/1000) (H ≤ 4000m). The correction factor for enclosed cabinet installation is about 0.8‑0.85, and vertical orientation carries 10%‑15% more current than horizontal.

Consider a cabinet rated 630A, 30°C ambient, enclosed cabinet, vertical orientation. A 50×5 copper bar rated 630A at 25°C vertical: multiply by 0.94 (temperature) = 592A; multiply by 0.85 (enclosed) = 503A — far below 630A. A larger size is required, such as 60×6 (820A at 25°C vertical), corrected to 820 × 0.94 × 0.85 ≈ 655A — just sufficient. This is why “look up the table and pick a busbar” often fails — three correction factors multiplied together can reduce ampacity to 60%‑70% of the table value.

Parallel busbar derating is not simply multiplying by the number of bars. When multiple busbars are paralleled, adjacent bars heat each other, and each bar’s actual ampacity must be derated. Two bars in parallel use a factor of 0.9, three bars 0.85, four bars 0.8. Moreover, the more bars in parallel, the more uneven the current distribution — due to skin effect and proximity effect, outer bars carry more current while inner bars carry less. Selecting based on “total current ÷ number of bars” means inner bars run underloaded and outer bars overloaded, accelerating aging.

Jointing: grinding, paste, torque — all three are essential. Joint surfaces must be sanded to Ra ≤ 3.2μm, exposing metallic shine, then immediately coated with conductive paste (e.g., Dow Corning 340), 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.

Bolt torque is a life‑or‑death line. M8 bolts about 20‑25 N·m, M10 about 35‑40 N·m (copper bar connections require 30% extra to prevent thermal expansion loosening), M12 about 45‑50 N·m. Aluminium alloy busbars require 10%‑15% lower torque (lower yield strength), and vibration environments require spring washers and 5% higher torque. Always use a calibrated torque wrench — relying on feel will eventually cause an accident. During acceptance, use a 0.05mm feeler gauge to check gaps; insertion depth must not exceed 5mm. Re‑tighten bolts annually during inspections; if torque decays by more than 15%, replacement is required.

Copper‑aluminium transition: without treatment, it’s a time bomb. Directly jointing copper and aluminium forms a galvanic cell — aluminium is more active than copper, acts as the anode, and is continuously corroded, rapidly increasing contact resistance and joint temperature, forming a vicious cycle of “electrochemical corrosion → increased contact resistance → joint overheating → accelerated corrosion.” The thermal expansion coefficients of copper and aluminium differ greatly; during thermal cycling, microscopic displacement occurs at the joint, and aluminium’s softness leads to creep, causing bolt loosening. Aluminium’s oxide layer (aluminium oxide) is an insulator with extremely high resistivity, further degrading connection quality.

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 metallurgically bonded interface in between does not participate in electrochemical corrosion. In dry indoor conditions, copper bars may be tinned before connecting to aluminium, but outdoor or near‑100% humidity indoor locations require copper‑aluminium transition joints.

Phase sequence and colour coding. Facing the cabinet front, horizontal arrangement from left to right: L1 yellow, L2 green, L3 red, N light blue, PE green‑yellow striped. Vertical arrangement from top to bottom: L1, L2, L3, N, PE. No painting within 10mm on either side of connections or support edges. Wrong colours create hidden hazards for future maintenance.

Busbar selection and jointing leave no room for “close enough.” Every correction factor, every torque value, every copper‑aluminium transition determines whether joints overheat and cabinets catch fire.