Busway derating is the correction you apply when the conditions on site are not the conditions the rating was established under. A 1600 A busway rated at 35 °C ambient in free air does not carry 1600 A at 50 °C ambient, at 2000 m altitude, or inside a sealed shaft with poor ventilation. Applying derating correctly is the difference between a run that reaches its design life and one that runs hot, oxidises its joints, and fails early. This guide covers the three derating factors that matter most — ambient temperature, altitude, and enclosure — and how to combine them.
Why a Busway Rating Is Conditional
A busway current rating is not a fixed property of the product. It is the result of a type test performed under defined conditions and interpreted against temperature-rise limits. Under IEC 61439-1 and IEC 61439-6, an assembly is verified by temperature-rise testing, with limits set for the various parts of the assembly — busbars, connections, and accessible external surfaces — depending on the material, the surface treatment, and whether a part is accessible to an operator.
The rating therefore says: under these conditions, no part exceeds its limit. Change the conditions and the statement no longer holds. Derating is how you restore it.
The Physics in One Sentence
A busway reaches a steady temperature when the heat it generates (I²R losses) equals the heat it dissipates to its surroundings. Anything that reduces dissipation — hotter air, thinner air, restricted airflow — raises the operating temperature for the same current. Anything that increases dissipation — forced ventilation, better surface emissivity — lowers it.
Factor 1: Ambient Temperature
Busway ratings are commonly established at a reference ambient — frequently 35 °C for indoor assemblies under IEC practice, though other reference values appear depending on the standard and the product. When the actual ambient exceeds the reference, the available temperature rise is reduced, and so is the permissible current.
The relationship is not linear and is not universal — it depends on the design’s thermal resistance and the limiting component. The correct source is always the manufacturer’s derating table for that series.
Illustrative Derating Behaviour
| Ambient temperature | Illustrative available current | Note |
|---|---|---|
| At reference ambient | 100% of rated | Baseline from type test |
| Reference + 10 K | Roughly 90–95% | Reduce and verify against the derating table |
| Reference + 20 K | Roughly 80–90% | Materially affects rating selection |
| Reference + 30 K | Roughly 70–80% | Often forces a rating increase of one step |
| Below reference | May allow up-rating | Only where the manufacturer declares it |
Illustrative only. Actual factors vary substantially by design, bar material, plating, and housing ventilation. Use the manufacturer’s declared derating data.
Getting the Ambient Right
The ambient that matters is the air immediately surrounding the busway, not the comfort temperature of the room. Common errors:
- Using the room’s design temperature when the busway runs in a ceiling void that is 15 K hotter.
- Ignoring solar gain on a roof-level or external run.
- Ignoring heat from neighbours — a busway routed alongside a steam line, a large cable bundle, or a transformer is not in free air.
- Using the annual average instead of the worst-case sustained condition. The rating must hold on the worst day, not the average one.
Factor 2: Altitude
Air density falls with altitude, and with it the ability to remove heat by convection. Installations above the reference altitude — commonly 1000 m or 2000 m depending on the standard and product — require derating.
Altitude affects more than cooling. Reduced air density also lowers dielectric strength, which affects clearances and creepage distances, and therefore the insulation coordination of the assembly. The two effects are separate and both need addressing.
Illustrative Altitude Derating
| Altitude | Illustrative current factor | Additional consideration |
|---|---|---|
| Up to reference altitude | 1.00 | None |
| 1000–2000 m | Roughly 0.95–0.99 | Confirm reference altitude of the rating |
| 2000–3000 m | Roughly 0.90–0.96 | Clearance/creepage check becomes necessary |
| 3000–4000 m | Roughly 0.85–0.92 | Usually requires manufacturer confirmation |
Illustrative only. Altitude derating is product-specific; high-altitude projects should always be confirmed with the manufacturer, particularly where insulation coordination is affected.
Factor 3: Enclosure and Ventilation
This is the factor most often missed, and it is frequently the largest.
A busway in open air on a wall dissipates heat freely. The same busway inside a ventilated shaft, a covered trench, or a compartment with restricted air movement operates at a higher temperature. Where multiple runs are stacked, the upper runs sit in air preheated by the lower ones — a compounding effect that catches people out.
Conditions That Restrict Dissipation
- Enclosed shafts and ducts — restricted natural convection.
- Stacked parallel runs — upper runs see preheated air.
- Covers and shrouds — a decorative or protective cover over a run reduces dissipation.
- Fire barriers and penetration seals — locally restrict airflow; usually acceptable over a short length but worth noting.
- Insulated or fire-rated enclosures — where a run is boxed in for fire rating, dissipation drops materially and the arrangement needs specific verification.
- Proximity to other heat sources — treated as an ambient elevation, since the surrounding air is hotter regardless of the cause.
Other Derating Factors Worth Checking
- Mounting orientation — horizontal flat, horizontal edgewise, and vertical installations have different convective behaviour. Manufacturers publish ratings per orientation; using the wrong column is a common specification error.
- Solar radiation — for external runs, absorbed solar heat adds to the thermal budget.
- Harmonic content — non-linear load increases losses through skin and proximity effects, which raises heating at the same RMS current. Where harmonic content is significant, confirm the manufacturer’s guidance.
- Continuous vs intermittent duty — a genuinely intermittent load has thermal margin, but “intermittent” must be verified, not assumed.
Combining the Factors
Where more than one factor applies, they are generally combined multiplicatively — but confirm this against the manufacturer’s method, because some manufacturers publish combined tables and others specify a different approach.
Worked Example (Illustrative)
A 1600 A busway is installed vertically in an enclosed electrical shaft. Conditions:
- Ambient in the shaft: 20 K above the reference ambient
- Site altitude: 2500 m
- Installation: enclosed shaft with restricted ventilation, three runs stacked
Applying illustrative factors:
- Ambient factor: ≈ 0.85
- Altitude factor: ≈ 0.93
- Enclosure / stacking factor: ≈ 0.85
Combined: 0.85 × 0.93 × 0.85 ≈ 0.67
Available current: 1600 × 0.67 ≈ 1070 A
If the design load is 1400 A, a 1600 A busway is not adequate under these conditions — the next rating up, or a change to the installation arrangement, is required. Note that the enclosure and stacking factor is the largest single contributor here, and it is the one most likely to have been omitted from the original calculation.
Illustrative example only. Use the manufacturer’s declared derating data for the actual product and conditions.
Application Case: High-Altitude Industrial Plant
Scenario Constraints
A process plant at 2600 m altitude requires a 2000 A main busway run from the substation to the production hall, passing through an enclosed service tunnel for approximately 40 m of its 120 m length. Tunnel ambient reaches 45 °C in summer due to adjacent process equipment; the reference ambient for the product is 35 °C.
Selection Approach
Two separate sections with different conditions, so they are assessed separately rather than applying the worst case to the whole run:
- Tunnel section (40 m): ambient +10 K, altitude 2600 m, enclosed tunnel. Combined factor is materially below 1.0, so this section governs the rating. It may be appropriate to up-rate only this section, provided the manufacturer confirms that mixing ratings within one run is permitted and that the transition is detailed correctly.
- Open section (80 m): ambient +10 K and altitude, but free air — a more favourable factor.
Where mixing ratings is not permitted or is impractical, the run is rated on the worst section throughout. At 2600 m, the clearance and creepage implications are checked separately from the thermal derating — the two are independent and both must be satisfied.
Common Mistakes
- Applying only the ambient factor and ignoring the enclosure. In the example above, the enclosure factor is comparable in size to the ambient factor. Omitting it would understate the derating significantly.
- Rating on average conditions. The rating must hold at the worst sustained condition, not the mean.
- Using the wrong orientation column. Vertical and horizontal ratings differ; the vertical column is often lower for the same product.
- Ignoring stacked-run preheating. Three stacked runs are not three independent runs.
- Treating altitude as purely thermal. Dielectric strength reduction is a separate issue requiring its own check.
- Derating without documenting. An undocumented derating assumption is unresolvable at handover or during a later load increase.
Acceptance Checks
- Manufacturer’s derating tables obtained for the exact series, with the reference ambient confirmed.
- Site ambient measured or credibly estimated at the worst-case location along the route — not at the room thermostat.
- Altitude factor applied and, separately, confirmation that clearance and creepage requirements are met at that altitude.
- Enclosure and stacking assessment documented, including the number of parallel runs and their spacing.
- Combined calculation recorded, with each factor stated separately so it can be reviewed.
- Thermographic survey after commissioning at representative load, with joint temperature rise compared against bar temperature rise rather than against an absolute figure.
Frequently Asked Questions
At what ambient temperature is a busway rating based?
It depends on the standard and product — a commonly used reference for indoor assemblies under IEC practice is 35 °C, but other values appear. Always read the rating’s stated reference conditions rather than assuming.
Does higher altitude always require derating?
Above the product’s declared reference altitude, yes — both for reduced cooling and for reduced dielectric strength. Below the reference altitude, no derating is normally needed, but confirm the reference value first.
Can I compensate for derating with forced ventilation?
Physically, improved airflow increases dissipation. But forced ventilation is a maintenance-dependent measure — if the fan fails, the derating returns. Only rely on it where the manufacturer has declared a rating for that arrangement, and where the ventilation is monitored.
Do stacked busway runs need more derating than a single run?
Yes, generally. Upper runs operate in air preheated by lower runs, so each tier sees a higher effective ambient. The manufacturer’s data for multiple-run arrangements should be used rather than treating each run independently.
Does mounting orientation change the rating?
Yes. Horizontal flat, horizontal edgewise, and vertical mounting have different convective behaviour and therefore different ratings for the same product. Select from the correct column in the manufacturer’s table.
How does harmonic load affect a busway rating?
Harmonic currents increase losses through skin and proximity effects, producing more heat at the same RMS current. Where non-linear load is a significant proportion of the total, raise it with the manufacturer — some designs are more sensitive than others.
Should I up-rate the whole run or just the worst section?
Where the manufacturer permits mixed ratings and details the transition, up-rating only the affected section is economical. Where it does not, rate the run on its worst section. Confirm before specifying, because the answer is product-specific.
Get the Derating Right with BANGE Electric
BANGE Electric supplies busway with declared reference conditions and derating data for ambient temperature, altitude, and installation arrangement. Send us your route description, actual site ambient at the worst location, altitude, mounting orientation, number of parallel runs, and enclosure conditions — we will return a derated rating you can defend at review.
Review busbar sizing fundamentals or contact our technical team.
Requirements may vary by application, market, and applicable standard. Figures given here are illustrative examples; always confirm against manufacturer data and your project specification.