Estimate a busbar
Computes resistance, current density and losses — and includes the bolted joints. That is where critical heating almost always occurs, not in the bar itself: a single joint can produce as much loss as half a metre of copper.
- The permissible continuous rating of a busbar is established by type testing to IEC 61439, not by calculation. This is a plausibility estimate, not a design.
BUSBAR_TYPE_TEST_REQUIRED
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
Safety-relevant calculation. This is an estimate based on the stated assumptions. The final design must be checked by a qualified electrician against the rules that apply where you are.
Data as of: 2026-07-15 · The technical review of this safety-relevant calculator is still outstanding.
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Cross-section | w * t | 150 mm^2 | exact |
| Current density | I / A | 1.3333 A/mm^2 | exact |
| Bar resistance | rho(T) * L / A | 0.000067634 Ohm | exact |
| Total losses | I^2 * (R_bar + R_joints) | 6.7053 W | exact |
- Formula
R = ρ(T) · L / A · P_loss = I² · (R_bar + R_joints)- Valid for
- PLAUSIBILITY ESTIMATE, not a design — the binding rating comes from type testing per IEC 61439, and every output says so. Current density, bar loss (temperature-corrected resistance) and per-joint losses. Reference: 30 × 5 mm Cu, 0.5 m, 200 A → 1.33 A/mm², bar 2.7 W against 4 W in two 50 µΩ joints — the joints dominate, and exactly there the critical heating arises.
- Not covered
- Type testing and temperature-rise verification per IEC 61439 (the binding route), AC skin effect, short-circuit forces and withstand, external heating inside the assembly, joint ageing (maintenance: re-torquing, thermography).
- Data sources
- Official publications of the standards bodies and state authorities (NFPA, IEC, DKE/VDE, CEN) · retrieved 2026-07-15
Frequently asked questions
How do I estimate the resistance and losses of a copper busbar?
The cross-section is width times thickness, the resistance R = ρ(T) · L / A with temperature-corrected resistivity, and the loss P = I² · (R_bar + R_joints). Current density in A/mm² serves as a first plausibility measure, not as a limit. Operating temperature is deliberately an input rather than a result — a closed-form thermal formula would be false precision.
Why do the bolted joints matter more than the bar itself?
In busbar systems the critical heating almost never occurs in the bar but at the bolted joints: a single contact resistance of 50 microohms at 200 A produces the same loss as half a metre of bar. Joint resistance depends on torque, surface condition and ageing, so it is an input from a datasheet or measurement — the calculator does not assume it.
Can this calculator tell me the permissible continuous current of a busbar?
No — this is a plausibility estimate, not a design. The permissible continuous rating of a busbar is established by type testing to IEC 61439, not by calculation. Also not modelled are the thermal balance, skin and proximity effects, short-circuit withstand and the electrodynamic forces involved; rating tables are deliberately not stored.
Why do the joints matter more than the bar cross-section?
Because every contact point has its own resistance producing quadratic losses at full current: even 15 µΩ per joint equals centimetres to metres of extra bar length at common bar sizes — the tile computes it for your case. And unlike the copper, joints age: they loosen, corrode and become hotspots. The loss split shows the joint share regularly dominating.
What current density is acceptable for copper bars?
As rough plausibility: 1.5 to 2.5 A/mm² for continuous load on bare copper bars in indoor air — the same guide value the result card states. The reference case sits below it at 1.33 A/mm², i.e. conservative. In enclosed, densely packed assemblies the practical value drops; the binding answer always comes from the type test.
How do I keep contact resistances low permanently?
Contact faces bright and flat (with aluminium, add contact grease against oxide), defined torque with a torque wrench, serrated or disc springs against settling — and retorque once after the first month under load. In operation a thermal image is the fastest test: a warm joint under load is an early warning long before anything chars.
Why is this only a plausibility estimate?
Because a busbar's permissible continuous load is established in practice by type testing per IEC 61439 — on the tested assembly, not on a calculator. This page checks whether current density and losses sit in a plausible band, and says so in every output. A calculation posing as a design would be false precision.
Why do the bolted joints matter more than the bar?
In the reference case (30 × 5 mm, 0.5 m, 200 A) the bar itself loses 2.7 W — the two bolted joints at 50 µΩ each lose 4 W, more than the entire bar. Critical heating almost never arises in the copper but at the junction. Exactly why this page includes the joints, which table calculators do not.
Where do I get a joint's contact resistance?
From the joint system's datasheet or — better — from a micro-ohmmeter measurement on the finished joint. It depends on torque, surface treatment and ageing; 20 to 200 µΩ is the usual span. The 50 µΩ default is marked as an assumption; ageing is why switchboard maintenance includes re-torquing and thermography.
What does aluminium change versus copper?
About 60 % higher resistivity — the same bar loses correspondingly more, and the current-density rules of thumb drop. On top, the joint issue sharpens: aluminium creeps under pressure and oxidises; its bolted joints need special treatment. The material is selectable since 5 August 2026; the joint problem remains professional territory.
What current density is usual for copper bars?
As rough plausibility: 1.5 to 2.5 A/mm² for continuous load on bare copper bars in indoor air — the same guide value the result card states. The reference case sits below it at 1.33 A/mm², i.e. conservative. In enclosed, densely packed assemblies the practical value drops; the binding answer always comes from the type test.