High-Altitude Derating for Magnetic Components: Cooling and Clearance

Altitude Changes Two Things, Not One

Equipment installed above roughly 2,000 m is derated for two independent reasons: the air is thinner, so convective cooling is weaker, and the dielectric strength of air falls with pressure, so the clearance distances that were adequate at sea level may not hold. In a magnetic component, both effects land at once — the part runs hotter and its external clearances are less effective.

Catalogs often cite an altitude derating curve for current without mentioning clearances, and standards such as IEC 62368-1 and IEC 61558 treat altitude explicitly. Buyers specifying for high-altitude sites (mines, plateaus, mountain telecom, Andean or Himalayan installations) need both halves in the RFQ.

The Two Effects, Quantified

Altitude effects and typical engineering responses
Altitude	Air pressure (approx)	Cooling effectiveness	Clearance rule of thumb
0–2,000 m	101–80 kPa	Reference	Nominal distances
2,000–3,000 m	80–70 kPa	−10 to −15 %	Increase clearance roughly 10 % or use the standard’s factor
3,000–4,000 m	70–61 kPa	−20 % or more	Increase clearance roughly 20 % or apply the tabulated multiplier
4,000–5,000 m	61–54 kPa	−30 %	Consider potting to remove air from the barrier

These are planning figures. The normative multipliers are in the applicable standard’s altitude table and should be read from there; the point is that both cooling and clearance must be handled, and that they are not the same correction.

Effect on the Magnetic Design

Thermal derating reduces the current the part can carry at the same hotspot limit. In a convection-cooled design, the reduction can reach 20–30 % at 4,000 m, which either lowers the rating or forces a larger core. Two practical responses:

Change the cooling path. A conduction-cooled or potted part with a thermal path to the enclosure is far less sensitive to air density than an open-frame convection-cooled one. For high-altitude installations, this is usually the right first move.
Reduce flux and current density at design time. Designing to 80 % of the sea-level thermal capability at nominal altitude leaves headroom for the installation site.

Effect on Clearances and Insulation

Thinner air means a lower breakdown voltage across the same gap, and also easier partial discharge inception at voids. For a component with an internal air gap in the barrier, the altitude effect is worse than for a solid insulation system, which is why high-altitude parts are often potted or VPI’d. The external creepage path along surfaces is governed by pollution degree rather than pressure, so the creepage requirement usually does not change — but the clearance does.

What to Put in the RFQ

Installation altitude and the standard’s altitude category (for example “operating up to 5,000 m, category 2000 m per IEC 62368-1”).
Ambient temperature at that altitude, including the daily range.
Cooling method available at the site (forced air, natural convection, conduction to a plate).
Required clearance and creepage values, or the standard clause to apply.
Whether potting or VPI is required, and the insulation class at the derated hotspot.
The derated current rating that must be guaranteed at the site condition — not the sea-level rating.