Busbar Derating Calculation: How to Correct Current Carrying Capacity
Busbar current carrying capacity: The factors that limit busbar current capacity, the derating coefficients involved and a step by step worked example. For the table values of standard sizes, see our copper busbar current carrying capacity table.
ContentsBusbar current carrying capacity is the current a given cross-section can carry continuously without exceeding its temperature limits. The figure printed in a catalogue for a 60×10 mm copper bar and the current the same bar can actually carry inside a sealed enclosure, at 50 °C ambient, as one of four bars stacked side by side, are not the same number. Derating is the calculation that closes that gap.
Why the catalogue value cannot be used directly
Catalogue and standard tables are produced for a defined reference condition: typically 35 °C ambient, free air, vertical mounting, a single bar and a stated surface finish. A real switchboard meets none of those conditions exactly. The bar sits inside a closed compartment, neighbouring bars heat each other, warm air rising from the cable zone collects in the busbar chamber, and the IP rating of the enclosure limits natural ventilation. The practical result is that real capacity is often 20 to 35 per cent below the catalogue figure.
The factors that drive derating
| Factor | Effect | Typical coefficient range |
|---|---|---|
| Ambient temperature | The dominant factor; every degree above 35 °C reduces capacity | 0.95 – 0.60 |
| Mounting orientation | A bar mounted on edge cools better than a flat mounted one | 0.90 – 1.00 |
| Proximity and number of parallel bars | Bars stacked side by side block each other heat rejection | 0.80 – 0.95 |
| IP rating | From IP54 upwards natural ventilation is practically absent | 0.75 – 0.95 |
| Surface finish | Painted or darkened surfaces radiate heat better | 1.00 – 1.10 |
| Altitude | Above 1000 m air density falls and convection weakens | 0.92 – 1.00 |
| Harmonic content | Skin and proximity effects increase effective resistance | 0.85 – 1.00 |
Step by step derating calculation
The logic is simple: take the capacity at reference conditions and multiply it by every applicable coefficient to obtain capacity under real conditions.
- Establish the rated current of the incoming device and the real load profile
- Read the reference capacity of the chosen cross-section from the applicable table
- Apply the ambient temperature factor, using the worst case ambient from the specification
- Apply the factors for mounting orientation, proximity and number of parallel bars
- Apply the enclosure factor for the IP rating and ventilation arrangement
- Apply altitude and harmonic factors where relevant
- Compare the result with the rating of the incoming device and increase the section or the number of bars if it falls short
Example: a section rated 1600 A at reference conditions, assessed with 0.78 for a 50 °C ambient, 0.90 for three bars side by side and 0.88 for an IP54 enclosure, drops to roughly 1600 × 0.78 × 0.90 × 0.88 = 1098 A. That section is not adequate for a 1600 A incomer, so the section has to grow, a second bar has to be added, or forced ventilation has to be provided. For the influence of the material choice on capacity, see our copper versus aluminium busbar comparison.
Temperature rise limits and IEC 61439
IEC 61439-1 defines temperature rise limits for the different parts of an assembly. For busbars and connections the limit follows from the insulation material and the connecting hardware in use; a common design target for copper busbars is an absolute temperature around 105 °C, which corresponds to a 70 K rise over a 35 °C reference. Limits for manual operating handles, doors and accessible external surfaces are considerably lower.
Compliance with those limits is demonstrated either by a full temperature rise test or by verification rules derived from a design that has already passed type testing. The key point is this: a derating calculation is a design tool, not a verification certificate. The rated current of the assembly is the value that stays inside the verified design limits.
Seven recommendations that work in practice
- Take ambient temperature from the worst case in the specification, not from an annual average
- Separate the busbar chamber from the cable zone so warm air does not accumulate
- Mount high current bars on edge rather than flat
- When using parallel bars leave a gap at least equal to the bar thickness
- Place the load centre close to the incomer to shorten the busbar run
- In sealed enclosures account for forced ventilation with filter fans or a heat exchanger
- Record a thermal camera survey at commissioning as the baseline for later inspections
Key takeaways: busbar current carrying capacity
- Why the catalogue value cannot be used directly — Catalogue and standard tables are produced for a defined reference condition: typically 35 °C ambient, free air, vertical mounting, a single bar and a stated surface finish.
- Step by step derating calculation — The logic is simple: take the capacity at reference conditions and multiply it by every applicable coefficient to obtain capacity under real conditions.
- Temperature rise limits and IEC 61439 — IEC 61439-1 defines temperature rise limits for the different parts of an assembly.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Which standard provides the derating factors?
There is no single universal table. IEC 61439-1 sets the verification framework, while the current tables and correction factors normally come from the verified data of the busbar manufacturer or from the type test results of the assembly manufacturer. That is why two suppliers can quote different values for the same cross-section.
Do two parallel bars double the capacity?
No. Once two bars sit next to each other, proximity effect and mutual heating usually limit the combined capacity to about 1.7 to 1.9 times that of a single bar. Leaving a gap between them improves that ratio.
What should be done with harmonic rich loads?
High harmonic content raises effective resistance and can push neutral current above expectations. In installations dominated by third harmonic the neutral bar should be sized the same as the phase bars, and in some cases larger.
Related articles
- Cable Sizing and Voltage Drop Calculation
- Busbar Trunking or Cable? Choosing the Right Distribution Method
- Residual Current Device Types: The Difference Between AC, A, F and B
- Switchboard Service Conditions: Altitude, Temperature and Humidity Derating
- Busbar Support and Insulator Selection: Short-Circuit Forces
- All technical articles
At Devpan we calculate busbar capacity against the ambient temperature, IP rating, mounting arrangement and harmonic profile of the actual project, and we size it inside verified design limits. Share your specification and we can present cross-section options together with a temperature rise estimate.
