What busbar sizing has to satisfy
The busbar in a distribution board carries the whole current of the assembly to the outgoing ways. Sizing it means answering two separate questions at once. Can it carry the design current continuously without overheating its insulation or its enclosure, and can it survive the short-circuit current until the protective device clears the fault?
The two checks pull in different directions. Continuous current pushes you toward a larger cross-section; the fault check depends on the rated short-time withstand current of the assembly and on how quickly the protection operates. A bar that passes the first test can still fail the second on a board with a high prospective fault level.
Assemblies are covered by their own product standard, so the board you are filling will have a declared design current and short-circuit rating. The busbar you choose has to sit inside those limits, not outside them.
Step 1 — Establish the design current
Start from the current the board actually has to carry, not from the nominal rating of its incoming device.
- Total the expected load on the board, allowing for demand and diversity rather than the arithmetic sum of connected load.
- Convert power to current using the correct voltage and power factor for the system.
- Check the result against the rating of the incoming protective device and the declared design current of the assembly.
- Note any spare ways and future load the client has asked you to allow for.
The figure you take forward is the larger of the design current and the incoming device rating, since the bar must not be the weak link in the chain.
Step 2 — Choose a material and a section
Copper carries more current per square millimetre and resists corrosion better; aluminium is lighter and cheaper but needs a larger section for the same current. The choice is usually set by the assembly standard and by what the panel builder works with.
| Copper bar (single) | Section | Indicative free-air rating |
|---|---|---|
| 20 x 5 mm | 100 mm² | Around 600 A |
| 30 x 5 mm | 150 mm² | Around 800 A |
| 40 x 5 mm | 200 mm² | Around 1000 A |
| 50 x 10 mm | 500 mm² | Around 1500 A |
These figures are indicative for a single bar in free air and are given to show the order of magnitude. Enclosed bars run cooler than free-air bars because they shed heat less easily, so the real rating must come from the manufacturer data for the specific assembly.
Step 3 — Apply temperature and installation factors
A published rating assumes fixed conditions. Your board rarely matches them, so derating factors bring the figure back to reality.
- Ambient temperature above the reference value reduces the permissible current.
- Enclosure type and the number of bars run in parallel affect how much heat can escape.
- Painting, taping or sleeving a bar changes its ability to radiate heat.
- Grouping several current-carrying bars close together lowers each one's rating.
The safe habit is to take the manufacturer's base rating, apply the derating factors for your conditions, and then compare the result with the design current from step one.
Step 4 — Check the short-circuit withstand
A busbar must ride out the fault current for the time the protection takes to operate. This is expressed as a rated short-time withstand current over a stated duration, and it is compared against the prospective fault current at the board.
Two things matter. The peak current of the fault, which applies mechanical force to the bars and their supports, and the thermal energy let through during clearing. A high prospective fault current on a slow upstream device is the combination that most often catches designers out.
If the withstand is marginal, the practical answers are to reduce the clearing time, to increase the section, or to support the bars more closely so they resist the magnetic forces between them.
Step 5 — Verify mechanical and connection details
Electrical sizing is only half the job. The bar also has to be supported and terminated properly.
- Support spacing must resist the forces produced under fault conditions.
- Bends, offsets and joints reduce the effective section and need a larger allowance.
- Bolted joints need the correct torque, contact area and, for aluminium, an oxide-inhibiting treatment.
- Clearances between phases and to earth must satisfy the assembly standard.
A bar that is electrically correct but mechanically loose will fail at the joint long before it fails in the middle.
A worked outline
Suppose a board has a design current of 630 A and an incoming device rated at 630 A, with a prospective fault current of 25 kA cleared in 0.1 s.
- Continuous check: a 40 x 5 mm copper bar is indicative for around 1000 A in free air, comfortably above 630 A before derating.
- Derating: in a warm enclosure the same bar might be rated nearer 700 A, which still covers the load but with less margin.
- Fault check: confirm the assembly withstand rating covers 25 kA for 0.1 s, and select supports accordingly.
The section is then fixed by the tighter of the two checks, not by the one that happens to be easier to calculate.
How GorkyCAD supports busbar design
GorkyCAD keeps the distribution board in the same model as the rest of the installation. Board loads, cable sizes and protective devices all come from one source, so the design current you feed into a busbar check is the one the model already holds. The toolset includes a broad range of fault and thermal calculations and checks against IEC 60364, and the panel documentation flows into the PDF set.
Because the board is not a separate worksheet, a change to an outgoing circuit shows up in the totals immediately, which makes it harder to size a bar against stale numbers.
Common pitfalls
A few habits cause most busbar errors on real boards.
- Sizing from connected load instead of assessed demand, which oversizes the bar and the board.
- Using a free-air rating inside a closed enclosure with no derating.
- Checking the thermal fault energy but ignoring the peak mechanical force.
- Forgetting that a joint or a bend weakens the conducting path.
Writing the two checks down side by side, and noting the derating factors used, keeps the decision auditable.
Next steps
Busbar sizing rewards a disciplined sequence: design current first, then material and section, then the derating and fault checks, then the mechanical detail. Keep those steps visible and the choice will stand up to review.
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FAQ
How do I decide the busbar cross-section?
Establish the design current and the prospective fault current, then choose the smallest standard bar that passes both the continuous-current and the short-circuit withstand checks once derating is applied.
Is a bigger busbar always better?
Not necessarily. An oversized bar costs more, is harder to bend and may not fit the enclosure. Size for the actual duty and keep the fault check in view.
Why derate a busbar inside an enclosure?
A closed enclosure traps heat, so the bar reaches its permissible temperature at a lower current than in free air. Manufacturer data gives the factors for each board type.
What short-circuit check is needed?
Confirm the rated short-time withstand current covers the prospective fault current for the clearing time of the protective device, and check that the supports resist the peak force.