LV Busbar Sizing Explained

What does this busbar calculator actually size?

Our first version covers bare rectangular copper and aluminium bars used in low-voltage AC distribution assemblies. You can start with a known design current or three-phase transformer kVA, then choose the number of bars per phase.

Its job is to determine a reasonable starting cross-section by comparing two independent areas: an engineer-selected current-density screening assumption and an approximate short-time adiabatic heating requirement. The larger of these sets the minimum area for the preliminary selection.

Width, thickness and parallel bars

A rectangular bar is described by width × thickness. For a 100 × 10 mm bar, one bar contains 1,000 mm² of metal. With two identical bars per phase, the combined geometric area is 2,000 mm² per phase.

The number of bars does not guarantee perfectly equal current sharing. AC skin and proximity effects, bar spacing and joint design can change each bar's share, while mechanical supports must accommodate the entire arrangement.

The calculator focuses on the commonly encountered 5 mm and 10 mm rectangular bar thicknesses, with widths of 20–120 mm and one to four bars per phase. You can restrict it to 5 mm or 10 mm, or let it evaluate both. It chooses the smallest combined metal area satisfying the two preliminary tests; this is not a manufacturer's assembled-busbar recommendation.

From LV transformer kVA to busbar current

For three-phase transformer nameplate apparent power, the full-load current at the secondary rated voltage is:

A 1,600 kVA transformer at 400 V gives about 2,309 A on the LV side. No power factor is required when the rating is already in kVA. Use the actual demand and operating arrangement if the busbar is not intended to carry the full transformer output.

Alternatively, select Current (A) and enter the design current directly. The load selector converts between A and kVA without introducing cos φ.

Why A/mm² cannot prove continuous ampacity

The simplified initial area check is:

Here J_screen is an editable planning value such as the initial 1.2 A/mm² for Cu or 0.8 A/mm² for Al. These are illustrative sizing heuristics, not IEC current capacities, and should not be interpreted as a general safe rating for enclosed switchboards.

Actual temperature rise depends on I²R heat, conductor temperature, exposed surface area, emissivity, air circulation, neighbouring phases, joints, support insulation and the enclosure. The European Copper Institute design guide explains these mechanisms and shows why shape matters. See the Copper for Busbars design guide.

LV assemblies are covered by the IEC 61439 series; temperature-rise verification must use the applicable verified design or permitted verification method. A simple area density factor cannot replace it.

Will the bar survive the fault heat?

During a short circuit, current can rise to tens of kiloamperes. In an approximate adiabatic metal-heating model, the required conductor area is:

The coefficient k changes with material and the selected initial/final conductor temperatures. For bare metals the calculator uses:

The 25 kA / 1 s default is only an example. Actual prospective fault current, effective heating duration and thermal limits must be determined for the installation. The final temperature assumption is not automatically permitted for bolted joints, plated connections, busbar supports or insulation.

What the fault-current kA input does not prove

Short-circuit current creates electromagnetic forces between the phases. These depend strongly on the peak current I_pk, not only the RMS fault current. Phase-to-phase spacing, support span, bar orientation and support strength determine whether a bar bends or breaks its mountings.

IEC 60865-1 provides calculation methods for the thermal and mechanical effects of short circuits. The present screening checks conductor heat only and does not calculate dynamic stress, peak withstand current or support forces.

For IEC 61439 switchboards, use a properly verified assembly configuration and its manufacturer documentation. Never label a busbar assembly “25 kA withstand” merely because this calculator says the conductor metal meets the adiabatic area screen.

1,600 kVA transformer, 400 V, copper busbars

Assume 1,600 kVA at 400 V, an illustrative Cu screening density of 1.2 A/mm², fault level 25 kA for 1 s, 70°C initial temperature and 160°C final temperature.

Here the area-based current-density screen is the governing condition, not the fault-heating screen.

Neither of these dimensions should be ordered or installed without verifying the assembled-panel temperature rise, mechanical withstand and the manufacturer's permitted bar arrangement.

What must be checked before an installation is approved

Mistakes to avoid

Standards and engineering references

Published engineering references guide the model but do not replace a current licensed standard, manufacturer design verification or responsible engineering review.