Short answer: a copper bar does not have an amp rating; it has a rating for a stated set of conditions. Ask “how much current does 20 × 5 mm carry” and the honest answer is “somewhere between about 250 A and 400 A” — the spread comes from whether the bar is in free air or inside a closed enclosure, whether it is bare, painted or sleeved, how many bars run in parallel, how close they are to each other and to steel, the ambient temperature and the permitted temperature rise. The number that actually goes on a drawing comes from a table plus the arrangement, or from a type test to IEC 61439-1.
We are an independent trading company: we do not manufacture busbars and we are not an appointed distributor of any brand. What we can do is make sure the bar that leaves here matches the rating the panel designer thought they were buying.
Three numbers that get confused
Every busbar decision is really three separate checks, and they fail in different ways.
| Check | Question | Governing value | Typical failure |
|---|---|---|---|
| Continuous current | Will the bar stay below its permitted temperature rise with the design load all day? | Ambient, temperature-rise limit, cross-section, cooling | Bar runs hot, insulation on connected cables degrades, terminals anneal |
| Short-circuit withstand | Will the bar survive the fault until the upstream device clears? | Icw or Ipk, cross-section, support spacing | Bar bends or fuses, supports shatter, fault spreads |
| Mechanical | Will it hold its shape and its clearances under electrodynamic force and its own weight? | Support pitch, bar thickness, insulator strength | Phase-to-phase contact after a fault, cracked supports |
A bar sized only on the first check is a bar that works until the day of the fault. A bar sized only on the second is expensive and often does not fit.
What actually changes the number
Cross-section is the first-order factor and the least interesting one. The rest explain why two suppliers can both be “right” and still be 60 % apart.
| Factor | Effect on continuous rating | Practical note |
|---|---|---|
| Free air vs enclosed | Enclosure can cost 20–40 % of the free-air rating | A closed IP54 box with no vent is the normal case, not the exception |
| Ambient | Roughly 1 % per degree above the reference | A 35 °C design in a 45 °C plant room is already 10 % short |
| Bare vs painted vs sleeved | Paint or matt black finish can improve heat rejection slightly; thick sleeving reduces it | Insulation is for clearance and touch safety, not for current |
| Bars per phase | Two bars are not twice one bar — expect roughly 1.6 to 1.8 times | Proximity effect and shared cooling surface |
| Bar spacing and steel nearby | Close spacing and steel panels reduce the rating | Ferrous enclosures heat from induced currents |
| Frequency | Minor at 50/60 Hz for bars under a few hundred mm² | Skin and proximity effect matter at higher frequencies and large sections |
| Orientation | Edgewise mounting cools better than flatwise | Also changes the depth the bar needs in the enclosure |
Indicative continuous ratings
The table below is a starting point for a single bare copper bar in still air at roughly 35 °C ambient with a temperature rise in the region of 65 K. It is a design aid, not a certificate: a bar inside a closed assembly has to be verified for that assembly.
| Bar size (mm) | Section (mm²) | Indicative current, single bar | Two bars per phase, indicative |
|---|---|---|---|
| 12 × 3 | 36 | 130–170 A | — |
| 15 × 3 | 45 | 160–210 A | — |
| 20 × 5 | 100 | 270–350 A | — |
| 25 × 5 | 125 | 320–400 A | — |
| 30 × 10 | 300 | 550–680 A | 900–1,100 A |
| 40 × 10 | 400 | 700–850 A | 1,150–1,400 A |
| 50 × 10 | 500 | 850–1,000 A | 1,400–1,700 A |
| 60 × 10 | 600 | 1,000–1,150 A | 1,600–1,950 A |
| 80 × 10 | 800 | 1,250–1,450 A | 2,000–2,400 A |
| 100 × 10 | 1,000 | 1,450–1,700 A | 2,300–2,800 A |
Anyone quoting a single figure for one of these sizes without stating the arrangement is quoting a laboratory condition. The two-bar column is included because it is the sizing question that most often gets answered by doubling the single-bar figure and then not fitting in the enclosure.
Short-circuit: the adiabatic check
For the fault case, the standard first approximation is the adiabatic equation: the minimum section is the fault current times the square root of the clearing time, divided by a material constant. For copper bars a k value around 143 is commonly used, which corresponds to starting from an initial temperature near 30 °C and allowing a final temperature in the region of 250–300 °C.
Rearranged for a one-second fault, the current a bar can take is roughly 143 × section in mm², expressed in amps. That gives the following, and it shows why short-circuit withstand and continuous rating pull in different directions.
| Bar size (mm) | Section (mm²) | Approximate 1 s withstand | Approximate 0.1 s withstand |
|---|---|---|---|
| 12 × 3 | 36 | 5.1 kA | 16 kA |
| 20 × 5 | 100 | 14 kA | 45 kA |
| 30 × 10 | 300 | 43 kA | 136 kA |
| 50 × 10 | 500 | 72 kA | 226 kA |
| 80 × 10 | 800 | 114 kA | 362 kA |
| 100 × 10 | 1,000 | 143 kA | 452 kA |
Two cautions. The adiabatic method assumes no heat leaves the bar during the fault, which is fair for short clearing times and increasingly optimistic as the time grows. And the Icw printed on a certificate is a tested value for a specific arrangement with specific supports — the calculation tells you whether the section is in the right range, not whether the assembly passes.
Inside an assembly, IEC 61439-1 is the reference
Once the bars are inside a switchboard, the continuous current is no longer a property of the bar. The assembly standard requires the temperature rise to be verified — by test, or by calculation, or by a combination of the two — and it sets limits for accessible parts and for the materials involved. That is why a busbar system sold as a kit comes with a rated current for that kit, at a stated ambient, inside that enclosure, and why the same bars used loose in a different box carry a different number.
For a buyer, the practical consequence is simple: ask for the rating at the ambient of the destination, not the rating at 20 °C, and ask whether it is a free-air figure or an in-assembly figure. Projects in West Africa, the Gulf and parts of South Asia routinely run panel interiors well above the European design ambient, and a bar chosen from a European catalogue page without derating is a bar chosen for a different climate.
Sizing sequence that avoids rework
Start from the design current and add the future margin the project expects. Pick the enclosure, because the enclosure decides the cooling. Choose the arrangement — one bar, two bars, edgewise or flatwise — and check the depth. Look up or calculate the continuous rating at the destination ambient, then apply the enclosure factor. Check the fault level against the adiabatic rating for the actual clearing time of the upstream device, not for one second by reflex. Check support spacing against the peak fault current. Only then choose the terminal and joint hardware, because the joint is usually the hottest point in the run.
What to put on the purchase order
Bar size and material (electrolytic copper, typically 99.9 %), whether tinned, bar length and the number of pieces, hole pattern or the drawing it must match, support insulator type and pitch, joint hardware and torque, the rated current at the destination ambient with the basis stated, Icw and Ipk if the assembly is type tested, and the destination country with the certificate that market requires. If the bars are going into an assembly that will itself be tested, say so — the bar data has to match the test report, not the other way round.
FAQ
Can I just double the current rating if I use two bars per phase?
No. Two bars in parallel share the current but not the cooling surface in proportion, and proximity effect adds losses. Expect roughly 1.6 to 1.8 times the single-bar rating rather than double, and verify against data for the actual arrangement.
Does tinning change the current rating?
Only marginally, and mainly through contact resistance at joints. Tinning is chosen for corrosion resistance and for joint quality, not as a way to increase ampacity.
Why does the same bar have different ratings in different catalogues?
Because the conditions differ: ambient, permitted rise, enclosure, spacing, number of bars and whether the figure was measured in free air or inside an assembly. Compare the stated conditions, not the headline number.
Is the adiabatic check enough for the fault case?
It is enough to choose a section, not enough to certify an assembly. The mechanical force between phases during a fault can bend bars and break supports even when the thermal check passes, so support spacing and insulator strength have to be checked separately, and Icw must come from a test for that arrangement.
Related: How to read an MCCB datasheet · Selectivity between MCB and MCCB · Breaker and cable coordination: Ib, In, Iz · Planning an enclosure in DIN modules · NF2 load break switch selection · MCCB range overview · Cross-reference an existing device · Request a quotation