What the Electrical Busbar Size Calculator does
This calculator estimates the current a copper or aluminium busbar can carry, from its cross-section and a current density, with named derating for the number of bars per phase, the enclosure and the ambient temperature. It also gives the bar's resistance at its operating temperature, the I²R loss over the run and the volt drop along it.
It is an estimate and it says so on the page. A busbar's real rating is not a calculation at all: it comes from a temperature-rise type test to IEC 61439 with the actual bar arrangement, spacing, plating, supports, enclosure and ventilation. What this page is good for is a first sizing, a sanity check on someone else's figure, and the loss and volt-drop arithmetic, which are straightforward physics.
How to use it
- Enter the bar width and thickness, and how many bars make up each phase.
- Choose copper or aluminium, and say whether the bars sit in open air, a ventilated enclosure or a sealed one.
- Set the ambient temperature and the temperature rise you are designing to. Capacity falls roughly with the square root of the headroom that is left, so a hot enclosure costs real amperes.
- If the bar maker publishes a current density for this arrangement, enter it as an override - their figure beats the generic one every time.
- Enter the run length and the operating current to see the losses and the volt drop, or leave the current blank to use the estimated capacity.
- Compare the size sweep table to see what the next bar width would buy you.
Reading the results
The current density is the whole basis of the estimate. 1.6 A/mm² for copper in still air and about 1.0 for aluminium are the common first-estimate figures for a bar with roughly a 50 K rise; forced ventilation and a good surface finish raise them, a sealed enclosure lowers them.
The bars-per-phase factor is below one per bar on purpose: two bars do not carry twice the current, because they shield each other thermally and share the current unevenly. The usual design allowances are about 1.8 for two bars, 2.5 for three and 3.1 for four.
Loss is continuous and it is paid for twice - once on the bill and once in the cooling the enclosure has to provide. A 2 kW busbar loss in a sealed panel is a genuine thermal design problem.
Volt drop along a busbar run is small in absolute terms but it is worth checking on long risers, where 20 or 30 metres of bar can lose a meaningful fraction of a volt per metre at full load.
Worked example: a 50 x 10 mm copper bar in a panel
The cross-section is 500 mm². At the still-air copper density of 1.6 A/mm² that is 800 A before any derating.
At 35 °C ambient against a 50 K permitted rise there is no ambient penalty, and with one bar per phase the proximity factor is 1.0, so the estimate stands at 800 A in open air. In a ventilated enclosure at 0.9 it becomes 720 A, and in a sealed one at 0.8, 640 A.
The bar's operating temperature is 35 + 50 = 85 °C, where copper resistivity is 1.724 x 10⁻⁸ x (1 + 0.00393 x 65) = 2.164 x 10⁻⁸ ohm.m. Over 3 m of 500 mm² that is 1.299 x 10⁻⁴ ohm per phase.
At 800 A the loss per phase is 800² x 1.299 x 10⁻⁴ = 83.1 W, or 249 W for all three phases over the 3 m run. The volt drop is 800 x 1.299 x 10⁻⁴ = 0.104 V per phase, which is negligible in a panel and would matter on a 30 m riser.
Doubling to two bars of 50 x 10 gives 1000 mm² but only about 1,440 A rather than 1,600 A, because of the 1.8 proximity allowance. That 10% penalty is why panel builders often prefer one thicker or wider bar to two thinner ones where the physical space allows it.
Formulas and scoring rules
- Cross-section
A = width x thickness x bars per phaseIn square millimetres.- Capacity estimate
I = A x J x (product of the derating factors)J is the current density in A/mm². Every factor is named in the result.- Proximity allowance
1 bar 1.00, 2 bars 1.80, 3 bars 2.50, 4 bars 3.10Applied as a factor of n bars, so two bars give 0.9 of twice the single-bar capacity.- Ambient headroom
factor = sqrt((rise - excess ambient) / rise)Capacity falls roughly with the square root of the temperature headroom that remains.- Resistance at temperature
R = rho20 x (1 + alpha x (T - 20)) x L / AT is the ambient plus the permitted rise. Copper rho20 = 1.724e-8, alpha = 0.00393.- Loss
P = 3 x I^2 x RAll three phases, over the run length entered. DC resistance only - the AC resistance is higher.- Volt drop
V = I x R per phaseResistive drop only; the bar's own inductance is not included.
Why a busbar rating cannot really be calculated
A conductor's current rating is set by how fast it can get rid of heat, and for a busbar that depends on things no formula captures: the spacing between bars and between phases, whether the bars are mounted flat or on edge, the surface finish (a painted or plated bar radiates far better than bright copper), the enclosure volume, the vent placement, the proximity of other heat sources and the supports that conduct heat away.
IEC 61439 settles it the only honest way: a type test, in which a real assembly carries its rated current until the temperature stabilises and the rise at every specified point is measured against a limit. That is why manufacturers' figures for apparently identical bars differ by 20% or more, and why the number here is called an estimate.
Skin and proximity effects
At 50 or 60 Hz, alternating current does not distribute itself evenly through a thick conductor - it crowds towards the surfaces. The skin depth in copper at 50 Hz is about 9.3 mm, so a 10 mm bar is only mildly affected, while a 20 mm one carries much of its current in the outer few millimetres and has a noticeably higher AC resistance than its DC resistance.
That is the real reason busbars are wide and thin rather than square, and why several thin bars spaced apart beat one thick bar of the same area. The proximity effect compounds it: current in one bar pushes current in its neighbour towards the far face. The loss figures on this page use DC resistance, so they are optimistic for thick bars - by a few per cent at 10 mm and substantially more at 20 mm.
Short-circuit forces
Thermal capacity is the everyday question; short-circuit withstand is the one that destroys switchboards. Parallel conductors carrying fault current repel or attract each other with a force proportional to the square of the current and inversely proportional to their spacing. At 50 kA on bars 100 mm apart the force is thousands of newtons per metre, applied as a half-cycle impulse.
That is a mechanical design problem: support spacing, support strength, bar stiffness and the peak current the assembly must survive, all covered by IEC 61439-1 and verified by test. Nothing on this page addresses it.
Limitations: what the result does not prove
- It is a current-density estimate, not a rating. A real busbar rating comes from a temperature-rise type test to IEC 61439 on the actual assembly.
- Skin and proximity effects are not modelled, so the AC resistance and the losses are understated for thick bars.
- Short-circuit withstand - the electromagnetic forces on the bars and their supports - is not considered at all, and it is what sets support spacing.
- Surface finish, bar spacing, mounting orientation, joint resistance and enclosure ventilation all change the real answer substantially, and none of them is an input here.
- Nothing here is a compliance statement. A qualified engineer must verify the design against IEC 61439 or the applicable standard and the local regulations.
Privacy: where your data goes
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Standards and sources
- IEC 61439-1 - Low-voltage switchgear and controlgear assemblies - checked 19 Sep 2026
- IEC 61439-6 - Busbar trunking systems
- IEC 60865-1 - Short-circuit currents: calculation of effects
Frequently asked questions
What current can a 50 x 10 mm copper busbar carry?
Around 800 A as a first estimate in open air at a 50 K rise, falling to roughly 640 to 720 A inside an enclosure. Published manufacturer figures for the same bar range from about 700 A to over 1,000 A depending on the arrangement and the rise allowed, which is exactly why the type test rather than the calculation is the authority.
What current density should I use for busbars?
For a first estimate, 1.2 to 1.6 A/mm² for copper and 0.8 to 1.0 for aluminium in still air, rising to 2.0 and 1.3 with good ventilation. These are conventions rather than standard values: the figure that applies to your assembly is the one the bar maker publishes for that arrangement.
Do two bars carry twice the current of one?
No, about 1.8 times. The bars shield each other thermally, so the inner faces cannot lose heat, and the proximity effect makes the current share unevenly between them. Three bars manage about 2.5 times and four about 3.1. Spacing the bars a bar-thickness apart recovers some of the loss.
Copper or aluminium busbar?
Aluminium has about 61% of copper's conductivity, so it needs roughly 1.6 times the cross-section for the same current - but it is around a third of the weight and much cheaper, which is why large distribution boards and rising mains often use it. The real design issue is jointing: aluminium oxidises instantly, so joints need proper preparation, plating or bimetallic washers.
How much do busbar losses cost?
More than people expect. Three phases of 250 W over a 3 m panel run is 750 W continuously, which is about 6,570 kWh a year - and all of it ends up as heat inside the panel, which then has to be removed. Oversizing the bar by one step is often paid back by the loss saving alone.
Does the tool cover short-circuit withstand?
No. The forces between bars during a fault are proportional to the square of the peak current and can reach several kilonewtons per metre at high fault levels. Support spacing, support strength and bar stiffness are a mechanical design verified by test under IEC 61439-1, and they are a separate exercise from the thermal question here.
Why does the operating temperature matter for losses?
Copper resistance rises about 0.393% per degree, so a bar at 85 °C has about 25% more resistance than the same bar at 20 °C - and therefore 25% more loss at the same current. Using a cold resistance to estimate the heat output of a hot busbar understates it by a quarter.
Can I use this for a busbar trunking system?
Only as a sanity check. Proprietary busbar trunking is a tested assembly with its own published current rating, impedance, volt drop and short-circuit withstand figures, all measured rather than calculated. Use the manufacturer's data - it is more accurate than any estimate, including this one.
Last reviewed by the A2Z.Tools team against the sources listed above.
