The 40 °C nobody reads: motor sizing in calculator #009

A standard IEC motor is rated for 40 °C ambient at up to 1000 m, and almost nobody reads the second half of that sentence properly. IEC 60034-1 applies no correction to the temperature-rise limits between 1000 m and 4000 m — not because altitude does not matter, but because the standard assumes the ambient falls as you climb, by about 1 % of the limiting rise per 100 m. Table 11 states what it therefore assumes: at 2000 m with class 155 (F) insulation, at most 30 °C. Put that motor at 2000 m in a 50 °C plant room and both halves of the assumption have failed at once. This guide runs the motor sizing tool through a real selection and through that trap. The worked example is a real run — every figure below was taken from the calculator, not typed in by hand.

What the tool is for

Single-speed three-phase induction motors on a sinusoidal supply, per the scope of IEC 60034-30-1:2014 clause 1: 0,12 kW to 1000 kW, rated voltage above 50 V up to 1 kV, 2, 4, 6 or 8 poles.

It sizes the motor, fills the efficiency from the standard rather than from a guess, produces the rated and starting currents, and screens four things that decide whether the selection actually works on your site: the voltage dip at start, the torque margin against the load, the duty cycle, and the ambient-and-altitude rating.

P_required = P_load × design margin
I          = P_out × 1000 / (√3 × U × η × cos φ)
I_start    = I × starting multiple          (DOL, star-delta, soft starter)
           = I × converter overload %       (VFD)

A worked selection: a 72 kW load

The efficiency is worth pausing on, because it is the input most often invented. IEC 60034-30-1:2014 tabulates nominal efficiency limits by class, frequency, rating and pole count — Table 7 is 50 Hz IE3 — and for 90 kW at 4 poles it reads 95,2 %. The tool fills the field from that table when you pick a class and leaves it editable, so the starting point is the standard's own minimum rather than a round number. Change the pole count and it changes with it: the same 90 kW IE3 motor is 95,0 % at 2 poles, 94,9 % at 6 and 93,4 % at 8, and the rated current moves accordingly — 159,00 A instead of 158,67 A on the 2-pole machine.

Note also what the 90 kW is. IEC 60034-1 rates a motor by its mechanical shaft output, so 90 kW out means 94,54 kW drawn from the supply at rated load, and the feeder is sized for the input, not the nameplate.

Starting: the number that sizes everything upstream

The same motor, started two ways on a 25 kA source:

dip = S_start / (S_sc + S_start) × 100

That is a screening estimate and the tool says so on the result: it ignores the source impedance angle and the feeder impedance, both of which make the real dip worse. What it is good for is the decision it informs — a 3,67 % dip against a typical 10 % limit is comfortable, and the tool plots the dip against source strength so you can see how weak the supply would have to be before it is not. On a 5 kA source the same DOL start is a different conversation entirely.

The torque check runs beside it, because a soft starter or a converter that fixes the dip can just as easily leave the motor unable to accelerate the load. The tool puts the motor's available starting torque against the load's demand as a percentage of full-load torque — 150 % against 100 % here — and reports the margin. Both charts matter: reduce the starting current and you reduce the starting torque with it, and a fan starts fine on 40 % torque while a loaded conveyor does not start at all.

The altitude and ambient trap, in the standard's own words

IEC 60034-1:2017, Table 10, item 3a — altitude, general rule, for 1000 m < H ≤ 4000 m with the maximum ambient air temperature not specified:

No adjustment. It shall be assumed that the reduced cooling resulting from altitude is compensated by a reduction of maximum ambient temperature below 40 °C and that the total temperature will therefore not exceed 40 °C plus the Table 8 and Table 9 temperature rises.

with the footnote that this assumes the ambient decrease is 1 % of the limiting rises for every 100 m above 1000 m, which is where Table 11 — Assumed maximum ambient temperature comes from:

Above 4000 m it is by agreement with the manufacturer.

Read that table as the condition on the no-adjustment rule, not as a curiosity. A class 155 (F) motor at 3000 m carries its nameplate output only if the ambient there stays at or below 19 °C. It very often does — that is why the rule works — and it very often does not, and the two cases are indistinguishable on a datasheet.

So the tool asks for the site altitude and the site ambient separately, prints which Table 11 figure applies, and when the entered ambient exceeds it, fails the case with the reason. Enter 2000 m and 50 °C on the 90 kW selection above and it returns:

Site ambient 50 °C exceeds the 30 °C that Table 11 assumes at 2000 m for thermal class 155 (F). A derating factor is required and IEC 60034-1 does not tabulate one — take it from the manufacturer and enter it below.

That last clause is the honest part. There is no IEC table of altitude-and-temperature derating factors for motors; there are manufacturer curves, and they differ between manufacturers. So the derating factor is a field you fill from the vendor's document, and the tool applies it to the available output — 90 kW × 0,92 = 82,80 kW — rather than inventing a factor of its own. A fabricated derating that looks authoritative is worse than a blank that names the document you need.

For the Gulf the mirror case is the common one: sea level, where the assumed ambient is a flat 40 °C, and a plant room at 50 °C. Altitude is irrelevant and the rating is broken just the same, which is why the tool checks the two conditions independently and quotes clause 6.3 — the ambient shall not exceed 40 °C — alongside 6.1, which requires special consideration outside the rated conditions.

How the implementation is checked

Thirteen cases run on every load, and each is a published value:

• IEC 60034-30-1:2014 efficiency tables — Table 7 (IE3, 50 Hz): 90 kW, 4 poles = 95,2 % · Table 5 (IE2): 200 kW and above, 8 poles = 93,5 % · Table 9 (IE4): 315 kW and above, 6 poles = 96,6 %
• IEC 60034-1:2017 Table 11 at four points — 1000 m = 40 °C · class 155 (F) at 2000 m = 30 °C and at 3000 m = 19 °C · class 200 (N) at 4000 m = 0 °C
• the sizing and current chain — 72 × 1,10 = 79,20 kW selecting the 90 kW preferred output of IEC 60072-1 · shaft output 90 kW at η 0,95 giving 94,74 kW input · 90 000 / (√3 × 400 × 0,95 × 0,86) = 159,00 A · DOL 159,00 × 6,0 = 954,01 A · VFD 159,00 × 150 % = 238,50 A · derating 90 × 0,92 = 82,80 kW

All thirteen pass.

What it does not do

This is a preliminary sizing and feeder-screening tool. Final selection needs the manufacturer's IEC 60034 nameplate data, and specifically:

• duty type — the tool screens an RMS load from a duty cycle percentage; a real S1 to S10 duty is a thermal calculation, and IEC 60034-1 Table 10 itself adjusts the permissible rise for S2, S9 and S10 duties (item 5 raises it by 10 K for short-time duty below 5000 kW)
• temperature rise, insulation class, enclosure, mounting — the thermal class here selects a Table 11 row, which is not the same thing as verifying a temperature-rise test
• the load torque curve — one number for demanded starting torque is a screening input; a breakaway torque, a pull-up torque and an acceleration time against inertia are not
• the allowed voltage dip and the source impedance angle, which need the real supply impedance rather than a short-circuit level
• converter data — the VFD overload percentage, ramp and torque limits are drive inputs, and the result says to verify them against the selected converter's datasheet
• harmonics, bearing currents, cable length limits and dV/dt on a converter-fed motor, all of which belong to IEC 60034-25 and the drive manual

The short version

Three numbers come out of a motor selection and all three are downstream of the standard's tables rather than of arithmetic. The rated current follows from the efficiency, which IEC 60034-30-1 tabulates by class, rating and pole count. The starting current follows from the starting method, and it sizes the feeder, the protection and the acceptable weakness of the supply. And the output you can actually use follows from the ambient and the altitude, through an assumption IEC 60034-1 makes silently and Table 11 makes explicit.

The first two are usually calculated. The third is usually assumed, and it is the one that shows up as a motor running hot two summers later.

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