Key takeaways
- Design the panel as a map of runs: incoming, main bus, droppers, neutral and earth. Each has its own current and fault duty.
- Repeat the same bend and hole pattern across ratings so the assembly team can build the same way at every size.
- Support spacing, clearance and joint access decide more installations than the bar section does.
- A build that can be checked on the bench is faster and safer than one that is only checked in the panel.
What panel builders need from a busbar
A panel builder buys a busbar system to do four things: carry the current, fit the enclosure, assemble quickly and pass the routine test. The bar that is cheapest per kilogram can fail all four if it needs a new bend, a new support or a new joint position on every project.
That is why the useful unit of design is not the bar, it is the run. A run is a current path between two points: the incoming terminals to the main bus, the main bus to a breaker, a breaker to an outgoing cable, the neutral bar to the outgoing terminals. Each run has its own current, its own fault exposure and its own access requirement. Designing the runs first makes the bar shapes follow from the layout rather than the other way round.
Map the runs before the shapes
A typical low-voltage panel has five run types. The incoming run carries the full supply current from the cable or the transformer to the main breaker. The main bus distributes that current across the section. The droppers take current down to the breakers. The neutral run carries the return current, which may include harmonics. The earth run carries fault current and must bond the enclosure, the doors and the instrument earth.
The current in each run is not the same, and the fault duty is not the same either. The incoming run and the main bus see the full prospective fault current. A dropper sees the fault current of its own circuit, which is lower but may last longer if the protection is slower. The earth bar must take the fault current for the clearing time without overheating or damaging its joints.
Drawing the map first shows where a section change is needed and where a joint can be avoided. Every joint is a potential hot spot and a cost, so the shortest route with the fewest joints is usually the best route.
| Run | Current it carries | Main design focus | What to check |
|---|---|---|---|
| Incoming | Full supply current | Termination space and fault withstand | Joint torque and cable landing |
| Main bus | Full current, split across sections | Section, support spacing and clearance | Temperature rise and bracing |
| Dropper | One circuit or a group | Bend radius and breaker alignment | Hole pattern and bend angle |
| Neutral | Return current, including harmonics | Section for harmonic content | Temperature and joint access |
| Earth | Fault current for the clearing time | Continuity and bond resistance | Bond resistance and labels |
The run map is a design aid, not a standard. The current and fault values come from the schematic and the protection study.
Shapes, bends and hole patterns
Most panel busbars are flat bars with a few bends and a set of holes. The shape looks simple, but the bend sequence decides whether the part fits. A bend that is made after a hole can distort the hole. A bend that is too close to a hole pulls the hole out of position. A bend radius that is too tight for the bar thickness cracks the outside face or thins the section.
The hole pattern is the part that carries from project to project. If the same breaker family is used, the same hole pattern can be repeated across ratings, and the assembly team learns one build. A new pattern for every rating adds setup, inspection and the risk of a wrong bar in the wrong panel.
Allow for the tolerance stack. The bar sits between a breaker terminal and a support, and both have their own tolerance. A slot instead of a round hole can absorb a small misalignment without preloading the joint. A slot needs its own edge distance and washer coverage, so it is a design decision, not a shop floor fix.
Supports, clearance and fault bracing
Supports do two jobs: hold the bar in place and keep the clearance. Under fault current the bar is pushed sideways, and the support has to take that force without letting the bar touch a live part or the enclosure. Support spacing is therefore a fault design variable, not a convenience. Closer supports reduce the bending moment in the bar and the reaction at each support.
The support material has to suit the environment. A moulded insulator with the right creepage and tracking group is the usual choice. A metal support must not create a path or a loop, and it must be bonded if it is part of the enclosure. A plastic support that creeps under the weight of a long bar can reduce the clearance over time, so the support should be rated for the load and the temperature.
Clearance is checked at the tightest point in the installed position. That includes the bar corner, the bolt head, the washer, the cable lug and the door. A busbar that passes a bench check can fail when the door is fitted, because the door swings closer than the drawing suggested.
Plating and joint practice
Tin plating is the common choice for copper panel bars. It protects the joint face, it is soft enough to conform under pressure, and it is cost effective. Silver is used for high-current or high-temperature joints, and nickel for wear or corrosion. The plating should be specified on the joint faces and on the whole bar if the environment needs it, because a bare cut edge is where corrosion starts.
Joint practice is where panels differ most. A bolted joint needs flat faces, the right washer, the right torque and a mark so a later inspection can see movement. A joint that is assembled with an impact driver and no torque control is a future hot spot. A joint that is tightened before the bars are aligned puts a bending load into the bar and the breaker terminal.
Keep the joint access in the layout. A joint that is buried behind a component cannot be re-torqued, and a joint that is close to the door cannot be reached with a wrench. Draw the wrench envelope, not just the bar.
Build it on the bench where you can
A busbar subassembly that is drilled, plated, bolted and checked before it goes into the enclosure is easier to inspect and faster to fit than one assembled in place.
An assembly sequence that repeats
The sequence below is the one that keeps a panel build consistent. It also gives the inspector a place to check before the next step covers the work.
- 01
Lay out the runs
Mark the incoming, main bus, droppers, neutral and earth on the layout, with the current and fault duty for each.
- 02
Fix the interfaces
Lock the breaker terminal positions, cable landings and support points before the bar shapes are drawn.
- 03
Draw the bar
Use a flat pattern, then add bends and holes. Check the bend radius, the hole edge distance and the wrench access.
- 04
Check the supports
Space the supports for the fault force and the bar weight, and confirm the creepage and clearance at each one.
- 05
Plate and insulate
Plate the joint faces and the bar, mask the contact areas, and add sleeving or barriers where the layout needs them.
- 06
Assemble and mark
Bolt in the specified order, torque to the value on the drawing, mark the fasteners and record the readings.
Documentation for the first build
The first build is where the drawing meets the panel. Send the panel layout, the schematic, the breaker datasheets, the current and fault values, the enclosure size and the incoming cable details.
Ask for a dimensional report on the first article that includes the hole positions, the bend angles, the overall length and the distance between the terminal faces. Add the plating thickness at the joint faces and the millivolt drop on a sample joint. If the bar is insulated, add the coating thickness and a visual check of the masked areas.
Frequently asked questions
What current rating should a panel busbar have?
It should be sized for the continuous current of the run, the ambient inside the enclosure, the allowable temperature rise and the fault withstand. The incoming and main bus usually need a larger section than a dropper. Use the schematic and the protection study, not a single rating for the whole panel.
Should panel busbars be tinned or bare copper?
Tin plating is usual for copper panel bars because it protects the joint faces and conforms under pressure. Bare copper can work in a dry, clean enclosure but oxidises quickly. Silver is used for high-current or high-temperature joints. The environment and the joint temperature decide.
How often should busbar joints be re-torqued?
There is no universal interval. It depends on the joint temperature, the washer type, the vibration and the duty cycle. A joint that runs hot or cycles often needs a shorter interval. The maintenance plan should include a torque check and a millivolt drop check, not torque alone.
Can I use the same busbar drawing for a higher rated panel?
Only after checking the section, the support spacing, the clearance and the fault withstand for the higher rating. A drawing that is valid at one rating may be unsafe at another even if the hole pattern still fits. The shape can be reused; the rating has to be recalculated.
Sources and further reading
- IEC 61439-1, Low-voltage switchgear and controlgear assemblies
- IEC 60947-1, Low-voltage switchgear and controlgear, general rules
- Qumek panel busbar layout and build reviews
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Building a panel around a new bar?
Send the layout, schematic, breaker datasheets and fault level. The engineering desk will review the run map, bends and joint access before quoting.
Send the panel layout