ElectroHub

Busbar Size Calculator & Current Rating Chart

Two checks, and a bar has to pass both. Continuous current is a density rule — 1.2 to 1.6 A/mm² for naturally cooled copper. Fault current is the adiabatic equation, and on a switchboard it is usually the check that decides the bar, which is why switchboard bars so often look absurdly oversized for the load they feed.

Busbar size & current rating

Two checks, and both have to pass. Continuous current is a density rule — 1.2 to 1.6 A/mm² for naturally cooled copper. Fault current is the adiabatic equation A = I√t / k, and on a switchboard it is usually the one that decides the bar.

Section
Density
Capacity
Acceptable — 1.33 A/mm² is inside the 1.2–1.6 A/mm² band, but at the top of it the enclosure has to be verified.
Adiabatic minimum (k = 122)
Withstand

k = 122.1 for copper heating from 90 °C to 200 °C during the fault (IEC 60949). A shorter clearing time is worth more than more metal: the area needed falls with √t, so halving the time saves 29 % of the section.

  • Current density is a design rule of thumb, not a rating. The real limit is the temperature rise the enclosure allows — IEC 61439 type-tests the whole assembly, and a bar that is fine in open air can be well over temperature in a sealed panel.
Common trap: A bar that passes the density check can still fail the fault. At 36 kA for 1 second a copper bar needs 295 mm² whatever the load current is — which is why switchboard bars so often look absurdly oversized for the load they feed.
Current density is a design rule of thumb, not a rating. The real limit is the temperature rise the enclosure permits, and IEC 61439 type-tests the assembly as a whole — a bar that is fine in open air can be well over temperature in a sealed panel.

The formula

I = J × A ; A_min = I_sc √t / k ; k = 226 √(ln((234.5 + θ_f)/(234.5 + θ_i)))

J
current density, A/mm² — 1.2–1.6 copper, 0.8–1.0 aluminium
A
bar section, width × thickness × bars per phase, mm²
I_sc
prospective symmetrical fault current, amperes
t
time the protective device takes to clear, seconds
k
IEC 60949 adiabatic constant — 122 for copper from 90 °C to 200 °C

Worked example

A 50 × 6 mm copper bar carrying 400 A, on a board with 25 kA prospective fault cleared in 1 s.

  1. Section = 50 × 6 = 300 mm²
  2. Density = 400 / 300 = 1.33 A/mm², inside the 1.2–1.6 band
  3. k for copper, 90 → 200 °C = 122.1
  4. A_min = 25 000 × √1 / 122.1 = 204.7 mm²

300 mm² passes both checks — 1.33 A/mm² continuous, against a 204.7 mm² fault minimum.

Where you'll use it

Laying out a panel or an LT switchboard, and checking whether an existing board still has the withstand it needs after the transformer feeding it was uprated. Current density is a design rule of thumb, not a rating: the real limit is the temperature rise the enclosure allows, and IEC 61439 type-tests the assembly as a whole.

The laws behind it

Parts this applies to

Resistor (fixed)Varistor / MOVTVS / Transient SuppressorIGBTSCR (Thyristor)Linear Regulator (78xx / LDO)Switches (toggle/push/slide/rotary/DIP)Solid-State Relay (SSR)

Related calculators

Short Circuit Fault Current CalculatorI_sc = I_fl × 100 / %ZCable Size CalculatorI_z = I_table × k_ambient × k_groupingEarthing Conductor Size CalculatorS = √(I² t) / kResistivity Calculator (R = ρL/A)R = ρ × L / A