Busbar sizing and selection for low-voltage panels: current rating, copper vs aluminium, ampacity, insulation and how to specify it for switchboards.
A busbar is the solid conductor that distributes current from the main incomer to every breaker, contactor and fuse in a low-voltage panel. It is usually a flat copper or aluminium bar, or a pre-formed DIN-rail comb, that carries the whole board's load. Get the size wrong and the bar overheats, loosens its own joints and fails under load; get it right and the assembly runs cool for decades. This guide covers what a buyer or panel builder should actually specify.
Size a busbar the same way you size any conductor: the design current Ib must be carried continuously, the selected bar rating In must satisfy Ib ≤ In, and the installed arrangement must not become the weak link. The most common mistake is picking a bar by its physical width alone. A 30 × 5 mm copper bar rated 430 A in free air may only carry about 310 A once it is stacked between insulators inside a sealed enclosure, because the surrounding heat has nowhere to go.
| Copper cross-section | Free-air ampacity (approx.) | Enclosed panel (approx.) |
|---|---|---|
| 10 × 2 mm | 130 A | 95 A |
| 20 × 3 mm | 240 A | 175 A |
| 30 × 5 mm | 430 A | 310 A |
| 40 × 5 mm | 560 A | 400 A |
| 50 × 6 mm | 730 A | 520 A |
These are planning values for copper at a 65 °C rise; always confirm against the manufacturer's derating curve for your enclosure, ambient temperature and altitude.
Copper has roughly 60% higher conductivity by weight and a far more stable contact surface, so it stays tight under thermal cycling and resists crenellation at the joint. Aluminium costs about a third as much and is far lighter, which matters for very large incomers, but it forms an oxide layer and creeps under load, so every aluminium joint needs anti-oxide compound, Belleville washers and a periodic re-torque. For most control panels and distribution boards copper is the safe default; reserve aluminium for the heavy busway runs where weight and cost dominate.
Ampacity is only half the story. During a fault the bar must survive the magnetic forces and I²t heating for the few tenths of a second before the breaker trips. You need a bar whose rated short-time withstand current (Icw, typically 10 to 50 kA for 1 s) meets the prospective fault level at its point in the board. A bar that is more than big enough for the load can still be physically torn apart by a fault it was never rated to withstand, so match Icw to the upstream protective device's breaking capacity.
Bare bars must keep their phase-to-phase and phase-to-earth clearances (IEC 61439 gives the minimums by rated insulation voltage). In tight boards use insulated (PVC-sleeved or epoxy-coated) busbar, or move to a pre-insulated DIN-rail comb that clips straight onto the rail. Support spacing matters too: bars deflect and resonate under fault forces, so fix them at the interval the supplier specifies, typically every 200 to 300 mm for a 5 mm-thick bar.
Match the form to your volume: a comb saves hours on repetitive distribution boards, while flat bar wins for a unique, high-current assembly.
ELEHPD stocks SP038 and DIN-rail busbar systems, fork and U-model connectors and matching insulators rated for typical low-voltage panel duties, supplied to IEC practice and ready to assemble alongside MCB, RCCB and contactor ranges.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.