LV Busbar Sizing

Screen copper or aluminium rectangular busbar sizes from current or transformer kVA, with short-time conductor fault heating and clear IEC 61439 verification limits.

EstimatePreliminary result, not a design verification.

Before you calculate

LV busbar sizing: thermal area vs fault stress

Screen standard copper and aluminium rectangular bar arrangements for a known current or transformer kVA. A/mm² screening is not an IEC 61439 current rating; always verify enclosure temperature rise, joints and peak mechanical short-circuit withstand.

Learn more — illustrated guide →

Step 1 · LV Busbar application

Design inputs

Rectangular bare Cu/Al busbars in a low-voltage distribution assembly. The calculated size is a preliminary area screening — not a manufacturer-verified continuous current rating.

Design current (A)

Known load

Line to Line Voltage (L-L), VUsed to convert kVA to A; no power factor is needed.

Busbar material

Bars per phase

Bar thickness (mm)Focused on the two common LV panel bar thicknesses. Width and number of bars are selected separately.

Area screening current density (A/mm²)Editable planning assumption, NOT an IEC 61439 ampacity value.

Optional screening headroom (%)

Step 2 · Short-circuit heat

Prospective fault current

Symmetrical RMS fault current (kA)

Effective thermal fault duration (s)

Assumed initial conductor temperature (°C)

Assumed final short-time temperature (°C)

Screening assumptions: 25 kA / 1 s and 70 → 160°C are illustrative defaults, not site data or universal permissible busbar temperatures. Confirm the actual short-circuit duty, connections, insulation and support limits.

Step 3 · Preliminary result

Busbar selection

Design line current1,600 A

Minimum combined area requirement1,333.3 mm²

Minimum area-screened option3 × 100 × 5 mm

Per phase · 1,500 mm² total area · 1.067 A/mm² at design current

15% screening headroom option2 × 80 × 10 mm

Per phase · 1,600 mm² total area · 1 A/mm² at design current

Current-density area requirement1,333.3 mm²

Adiabatic fault area requirement217.4 mm²

Conductor k (selected temperatures)115 A√s/mm²

Engineering checks still required before approval

Continuous thermal rating: verify in the real enclosure under IEC 61439, including bar spacing, ambient, ventilation and joints. Mechanical short-circuit withstand: verify peak current (I_pk), phase spacing, support strength and span under IEC 60865 / verified assembly design. Additional: AC skin/proximity losses, earthing and N/PE bars, connection temperature, clearance and creepage.

1 · Convert kVA to current, if applicable

Three-phase apparent power does not require cos φ.

I=\frac{S}{\sqrt{3}\,U_{LL}}

I_{\mathrm{design}}=1,600\,\mathrm A

2 · Current-density area screening (not ampacity)

A_{J}=\frac{I}{J_{\mathrm{screen}}}

A_{J}=\frac{1,600}{1.2}=1,333.33\,\mathrm{mm}^2

3 · Bare-metal adiabatic fault heating

k=C\sqrt{\ln\!\left(\frac{B+\theta_f}{B+\theta_i}\right)}

A_{k}=\frac{I_k\sqrt{t}}{k}=\frac{25\times1000\times\sqrt{1}}{115.004}=217.38\,\mathrm{mm}^2

4 · Select standard geometry from both criteria

A_{\min}=\max(A_J,A_k)=1,333.33\,\mathrm{mm}^2

A_{\mathrm{headroom}}=\max\!\left(A_k,A_J(1+15/100)\right)=1,533.33\,\mathrm{mm}^2

Select a rectangular bar width, the chosen 5/10 mm thickness and number per phase meeting the required combined metal area. Equal current sharing is assumed for parallel bars, but must be demonstrated for a real assembly.