Derating is reducing the usable output of a machine below its nameplate rating because the site it will run on is hotter, higher or electrically weaker than the conditions the nameplate was measured under. Almost every African installation we supply needs it, and the reason is simple. A motor nameplate is not a promise about your site. It is a promise about a test condition. WEG rates its W60 line for continuous S1 duty per IEC 60034-1 at an ambient of minus 30 to plus 40 degrees Celsius and an altitude not exceeding 1,000 metres above sea level. Change either and the number on the plate stops being true.
TL;DR
Equipment rated at the standard 40 degrees Celsius and 1,000 metres needs output derating as ambient rises above 40 and as altitude rises above 1,000 metres, and most African industrial sites break at least one of those. Weaker supply quality and wider voltage fluctuation argue for greater insulation and starting-current margin than the equivalent Indian specification carries. The single most valuable line in a tender is not the kW rating. It is an insulation class and temperature rise class, stated separately.
Why does a nameplate not apply to your site?
A nameplate rating is measured under defined reference conditions. IEC 60034-1 sets those at 40 degrees Celsius ambient and 1,000 metres altitude, with a balanced sinusoidal supply at rated voltage and frequency. Every derating factor is a departure from one of those four.
All four penalties act through one mechanism: winding temperature. The machine does not care why it is hot. It cares how far above its insulation limit it sits, and for how long. That is why derating is not a safety margin you can negotiate away in a price discussion. It is arithmetic.
New India Electricals Ltd supplies AC motors from principals including WEG, ABB, Siemens and Crompton Greaves, from closed cage LT ranges upward, and our export specification is typically IP55, Class F insulation, altitude-rated. We do not build the motors. What we do is specify and supply them derated for the site the customer actually has, which is a different job from reading a catalogue.
How much output do ambient temperature and altitude actually cost you?
WEG publishes a combined derating factor for ambient and altitude together in its W60 technical catalogue, applied as Pmax = Pnom × derating factor. Selected values:
| Ambient | 1,000 m | 1,500 m | 2,000 m | 2,500 m | 3,000 m |
|---|---|---|---|---|---|
| 40 °C | 1.00 | 0.97 | 0.94 | 0.90 | 0.86 |
| 45 °C | 0.95 | 0.92 | 0.90 | 0.88 | 0.85 |
| 50 °C | 0.92 | 0.90 | 0.87 | 0.85 | 0.82 |
| 55 °C | 0.88 | 0.85 | 0.83 | 0.81 | 0.78 |
| 60 °C | 0.83 | 0.82 | 0.80 | 0.77 | 0.75 |
Read one row and the scale of the problem is obvious. A machine on the Zambian Copperbelt, at roughly 1,300 metres, driving a centrifugal pump in a sheet-roofed pump house at 50 degrees, is delivering somewhere near 90% of its plate. On the Kenyan highlands or the Gauteng plateau, both near 1,750 to 1,800 metres, the altitude alone takes 5 to 6%.
Two cautions on that table. It is WEG’s data for the W60 line, not a universal curve, and factors differ between manufacturers and between lines from the same manufacturer. Take the factor from the datasheet for the machine you are actually buying. And because that table is already combined, do not then multiply a separate altitude factor on top of it. Double-counting produces an oversized machine, an oversized starter and a wasted budget.
The commonest error is not in the table at all. It is using the wrong ambient. Engineers take the outdoor shade temperature for the town when what matters is the air entering the machine, in the worst month, with any ventilation fan assumed to have failed. A sealed MCC room, a pump house under a steel roof, or a position next to a rolling mill line is nowhere near the meteorological number.
Why is high altitude not automatically a derating?
Because IEC 60034-1 permits altitude and ambient to trade against each other. A standard machine rated at 40 degrees and 1,000 metres can often be used higher without derating if the ambient falls as the altitude rises. That is why a genuinely cool site at 2,000 metres can need no altitude derating at all, while a hot site at 1,300 metres needs a substantial one. Altitude on its own is not the question. Altitude combined with the real ambient is.
Two related traps. An underground mine is not a high-altitude site. Air density increases below the collar, so the penalty at a shaft-bottom pump is ambient and humidity, not elevation. Applying a surface elevation factor there misses the actual problem. And a machine ordered site-rated for, say, 55 degrees at 2,000 metres already has the derating designed into it. Its nameplate kW is the site kW. Derate the standard product, not the one that was built for your conditions.
What does weak supply do that a temperature calculation will not show?
Voltage unbalance is the factor that does the most damage per unit of size, because it is invisible on a single-phase voltmeter reading and it does not appear anywhere in an ambient or altitude calculation. An unbalanced supply produces a negative-sequence current that rotates against the rotor, does no useful work, and dumps its energy into the rotor as heat.
The reference document is IEC 60034-26, Effects of unbalanced voltages on the performance of three-phase cage induction motors. Its second edition was published in January 2026 and cancels and replaces the 2006 first edition, and it carries a specific clause on derating the motor to prevent overheating. If your specification still cites the 2006 edition, it is citing a withdrawn document.
When we rank what actually kills a rerolling mill duty motor, two of the five causes are electrical rather than mechanical. Over-fluxing from voltage and frequency excursions on weak site supply is one, and the answer is voltage monitoring plus sizing margin. Supply imbalance across phases is the other, particularly on sites with large drive loads on the same bus, and the answer is phase-balance monitoring. Neither is fixed by buying a bigger frame.
There is a formal limit here that gets ignored. IEC 60034-1 classifies combined voltage and frequency variation as Zone A or Zone B. A machine must perform its main function continuously in Zone A, but WEG is explicit that temperature rises can be higher than at rated voltage and frequency. In Zone B, deviations are larger, temperature rises are probably higher still, and extended operation there is not recommended. A feeder that spends its life in Zone B is not a supply problem to be tolerated. It is a specification input.
Class F insulation with Class B rise: the line most tenders leave out
This is where you buy margin back, and it is the most commonly mis-specified line in a motor tender. Class F insulation with Class B temperature rise means the machine is built with a Class F insulation system but designed to rise only 80 K at rated load. WEG supplies the W60 stator windings that way as standard, and offers Class H insulation with the same Class B rise as an option.
The gap between what the insulation can take and what the design actually reaches is thermal reserve, and you get to choose what to spend it on. Spend it on life, and a critical, hard-to-access drive lasts considerably longer between rewinds. Spend it on site conditions, and the machine tolerates a hotter or higher location without exceeding its insulation limit.
What you cannot do is spend it twice. A machine already running at 55 degrees ambient on that reserve has nothing left for voltage unbalance. That is the error that turns a correctly calculated selection into a failed installation.
The practical instruction is short. Never write “Class F insulation” alone in a specification. A Class F insulated machine designed to Class F rise has zero reserve and sits at its limit on day one, at reference conditions, and it is perfectly compliant with a tender that says only “Class F”. Write insulation class and temperature rise class as two separate lines, and the cheapest compliant bid is still a machine you can use.
What else changes in an African order that is not about power?
Dust, humidity and corrosive air, and they are specification lines in their own right. When we rank motor failures on a rerolling line, scale and dust ingress through seals and bearing housings comes first, ahead of every electrical cause. The fix is the correct IP rating plus a real seal inspection schedule, not simply a higher IP number on a datasheet.
Humidity has a defined threshold worth knowing. WEG states that machines in environments at around 95% relative humidity need no protection beyond a space heater to prevent internal condensation, but above 95% an epoxy coating on internal parts, commonly called tropicalised painting, is recommended. Coastal West African and Indian Ocean sites cross that line routinely, and so does any cooling tower motor sitting in the drift.
Corrosion has a measurable specification too, rather than a promise. The W60 painting plan is stated to give a minimum of 1,000 hours of salt spray resistance to ASTM B117-03 and corrosion category C5 (I) to ISO 12944-2. That is the kind of line to put in your enquiry, because “suitable for coastal conditions” is not a specification and cannot be verified on delivery.
| Specification line | Typical Indian inland order | What we change for a hot, high or coastal African site |
|---|---|---|
| Ambient basis | 40 °C reference | Measured worst-month air at the machine intake, fan assumed failed |
| Altitude | Usually below 1,000 m, no factor | Factor from the manufacturer’s combined table, or a site-rated machine |
| Insulation and rise | Class F insulation stated alone | Class F insulation with Class B rise, stated as two separate lines |
| Enclosure | IP55 general purpose | IP55 as the floor, higher IP or weather-rated variants where dust or rain demand it |
| Corrosion and humidity | Standard paint | Named salt-spray hours and corrosion category, tropicalised internals above 95% RH |
| Supply | Assume near-balanced | Phase-balance and voltage monitoring, wider starting-current margin, conservative protection settings |
What does derating do to everything downstream?
It moves the whole circuit, and this is the step that turns a correct calculation into a failed installation. If the derating pushes you from one frame to the next, you are now buying a larger machine with a higher full load current. The cable, the contactor on AC-3 duty, the overload relay setting, the feeder and the busbar are all sized on the machine you actually installed, not the load you were trying to drive.
That is where our two halves meet. We supply the motor derated for the site, and we build the low voltage panel around the machine that was actually selected, in a 30,000 sq ft ABB licensed unit rated at 50 panels per month with more than 70 trained engineers and technicians. Getting the motor right and leaving the panel schedule unchanged is a common and expensive way to be half right.
One more thing worth saying plainly. Not every field failure is a derating failure. A customer in Africa once wired a motor incorrectly and attributed the fault to the equipment. We flew an engineer out, the issue was resolved in a few minutes, and the site team was trained on correct installation practice. We keep field service teams in Zambia, Malawi, Mumbai and Raipur for exactly this reason, alongside work such as a containerised secondary substation project currently running in Africa, and we hold more than 4,000 motors in ready stock so a genuine failure does not become a three-month outage.
What should you send us before we quote?
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The absorbed power of the driven load at its duty point, not the nameplate of the machine being replaced. Oversized replacements propagate for decades because nobody ever went back to the load.
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The ambient where the machine breathes, in the worst month, with any ventilation fan assumed failed. Not the town’s shade temperature.
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Site altitude, and whether the installation is above or below ground. These are different problems with different answers.
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Measured voltage unbalance at the board, plus how much of that board is drive load. Do not assume unbalance is zero because the average voltage looks correct.
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The environment is full. Dust, humidity, salt air, chemical exposure, indoor or outdoor, and whether the area is classified, which moves the selection to flame proof motors.
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For a replacement, photographs of the nameplate, the existing datasheets and any old drawings. This is the fastest route to a correct quotation.
Contact our team with those six items and you will get a rating you can hold us to, rather than a catalogue number.
Frequently asked questions
- What does derating a motor mean?
- Derating means reducing the usable output of a machine below its nameplate rating because site conditions are harsher than the conditions the rating was established under. IEC 60034-1 sets those reference conditions at 40 degrees Celsius ambient and 1,000 metres altitude with a balanced sinusoidal supply. Above either figure, or on an unbalanced or distorted supply, the machine cannot dissipate its losses as effectively and will exceed its insulation limit if run at full plate output.
- At what altitude do I need to derate a motor?
- The standard reference altitude is 1,000 metres above sea level, so derating is generally considered above that. But altitude alone does not settle it, because IEC 60034-1 permits altitude and ambient to trade against each other, and a cool site at 2,000 metres may need no altitude derating while a hot site at 1,300 metres needs a significant one. Use the manufacturer’s combined ambient and altitude table for the specific line you are buying.
- How much does high ambient temperature reduce motor output?
- It depends on the machine, which is why you must use the manufacturer’s own figures. In WEG’s published table for its W60 line, a machine at 1,000 metres carries a factor of 0.95 at 45 degrees Celsius, 0.92 at 50 degrees and 0.88 at 55 degrees. Those numbers are for that line and should not be applied to another manufacturer or another range. The commonest error is not the factor but the input: using outdoor shade temperature instead of the air actually entering the machine.
- Why does voltage unbalance matter so much?
- Because an unbalanced supply produces a negative-sequence current that rotates against the rotor, does no useful work and turns into rotor heat, and none of that shows up in a temperature or altitude calculation. IEC 60034-26, whose second edition was published in January 2026 and replaced the 2006 edition, describes these effects and carries a clause on derating to prevent overheating. Measure unbalance at the board rather than assuming it is zero.
- What is the difference between Class F insulation and Class B temperature rise?
- Insulation class describes what the insulation system can withstand. Temperature rise class describes how hot the machine is designed to actually get at rated load. Class F insulation with Class B rise means a Class F system designed to rise only 80 K, leaving thermal reserve. A Class F insulated machine designed to Class F rise has no reserve at all. Both are legitimately “Class F insulated”, which is why a specification must state insulation class and rise class separately.
- Does an underground installation need altitude derating?
- No. Air density increases below the collar, so an underground pump or fan does not need the surface elevation factor. Applying it there wastes money and, more importantly, distracts from the real penalties underground, which are ambient temperature and humidity. Treat the shaft bottom as its own environment and measure it.
- What changes in an export specification versus an Indian one?
- Equipment rated for the standard 40 degrees Celsius ambient needs output derating as ambient rises above that, and altitude above roughly 1,000 metres reduces cooling air density enough to require further derating even at the same ambient. Weaker supply quality and wider voltage fluctuation argue for greater insulation and starting-current margin, and more conservative protection settings, than the equivalent Indian specification would carry. The final specification always depends on the customer’s requirements and actual site operating conditions.
- Does derating change anything other than the motor?
- Yes, and this is where correct calculations still produce failed installations. If derating moves you up a frame, the full load current rises, and the cable, contactor on AC-3 duty, overload relay setting, feeder and busbar must all be sized on the machine actually installed rather than on the load being driven. Re-size the whole circuit, not just the machine.
