Busbar Sizing Calculator

Amps per mm² is not a constant

Ask around a panel shop and you will be told copper carries 1.6 A/mm². Ask about a 100 × 10 bar and the same rule gives 1600 A, which is optimistic. The rule of thumb is only true near the size the person quoting it usually works with.

The reason is geometry. A bar generates heat in proportion to its cross-section and sheds it from its surface. Double the area of a bar and its surface does not double, so the amps per mm² it can sustain has to fall. Fitting a power law to published manufacturer tables gives:

I ≈ C × A^0.765     A in mm²
C = 7.3 copper, 4.6 aluminium
(bare bar, still air, 30 K rise over 40 °C)

That tracks real tables within a few per cent from 12 × 3 up to 120 × 10. It is a good number for a quotation and for choosing what to buy. It is not a substitute for a type test — an assembly to IS/IEC 61439 is rated by measured temperature rise on the actual panel, and the enclosure, the joints and the cable entries all change the answer.

Two bars are not twice one bar

Paralleled bars sit close enough to heat each other, and at 50 Hz the proximity effect pushes current towards the outer faces. Practical multipliers over a single bar:

The returns fall off sharply. Past three or four bars per phase you are paying for copper that carries very little, and a sandwich or type-tested busbar trunking system becomes the better buy.

Derate for the panel, not the room

The ambient that matters is the air temperature inside the enclosure, which on a loaded panel in an Indian plant room runs 10–15 °C above the room. A sealed IP54 or IP55 enclosure with no ventilation costs another 20 %. Between a 50 °C internal ambient and a sealed box you can lose a third of the bar rating before anything has gone wrong.

The fault is a mechanical problem

Two parallel conductors carrying current in opposite directions push each other apart, with a force that goes as the square of the current:

F = 2 × 10⁻⁷ × I²ₚₑₐₖ / d    newtons per metre

Use the peak asymmetrical current, roughly 2.5 × the symmetrical rms value for LV systems. At 25 kA rms on bars 75 mm apart, that is a peak near 62 kA and about 10 kN per metre trying to tear the bars off their supports — comparable to hanging a tonne off every metre of bar, for a few milliseconds.

This is why busbar supports are spaced the way they are, and why the spacing tightens as the fault level rises. Halve the phase spacing and you double the force; double the fault current and you quadruple it. The support spacing shown here is an indicative maximum for standard SMC supports — an assembly declaring a short-time withstand rating must prove it by test.

Joints fail before bars do

In practice, panels do not fail because the bar was a size too small. They fail at the joints. A bolted lap joint carries current through the small fraction of the mating surface that is genuinely in contact, and if that contact degrades the joint heats, oxidises and heats further.

• Overlap by at least the bar width, and use a minimum of two bolts.
• Tighten to the supplier's torque — a torque wrench, not a spanner and judgement. Under-tightened joints run hot; over-tightened ones creep and relax.
• Use Belleville washers on aluminium. Aluminium creeps under a constant load and a flat-washer joint loses its clamping force within a year.
• Clean aluminium joint faces and apply joint compound immediately — a fresh oxide layer forms in minutes and it is an insulator.
• Thermal-image the panel at full load after commissioning. Every joint should be within a few degrees of its bar. The one that is not is the one that will fail.

Skin effect, and why thickness stops helping

At 50 Hz, alternating current does not distribute itself evenly through a conductor. It concentrates toward the surface, and the depth at which current density has fallen to about 37 per cent of the surface value — the skin depth — is roughly 9.3 mm in copper and 12 mm in aluminium at power frequency and normal operating temperature.

For a 6 mm bar that is academic; the whole section is inside the skin depth and the AC resistance is barely above the DC value. For a 20 mm bar it is not. The core carries proportionally less current than the faces, the effective resistance rises, and the extra copper you paid for is doing less work per square millimetre than the copper at the surface.

This is the real reason large ratings are built from several thin bars rather than one thick one, and why the table on this page rewards width more than thickness. It is also why a laminated set of bars must be spaced, not stacked tight — bars in contact behave electrically like one thick bar and thermally like a solid block with no cooling surface between them. A gap of about one bar thickness restores most of what stacking takes away.

Bare, plated or painted

A bare copper bar oxidises. The film that forms is thin and reasonably conductive, so on the bar itself it costs little, but on a bolted joint it grows into the contact interface and raises resistance over years — which is the slow failure mode behind most busbar joint problems.

Tin plating is the usual answer in Indian panels: cheap, solderable, and it keeps the joint interface stable. Silver plating performs better and is specified where the joint carries very high current or runs hot, and nickel is used where the atmosphere is corrosive. What matters is that plating is for the joint faces. Plating the whole bar is common and does very little for the run itself.

Paint does something different and is often misunderstood. Bare bright copper is a poor radiator — its emissivity is around 0.05 — so almost all its heat leaves by convection. A matt painted surface has an emissivity near 0.9, and radiation becomes a real second path. On a bar running warm in still air inside an enclosure, painting can lower the temperature rise by 10 to 15 per cent, which is why heavily loaded busbars in switchboards are frequently black rather than bright. Never paint the joint faces.