Most industrial gearbox designs are rated for continuous duty at 20°C ambient temperature. 
The design manuals of geared motor manufacturers give consideration to operating conditions above and below 20°C ambient, as well as the duty cycle of the unit, and modify the thermal capacity ratings as required.
A good rule of thumb that is as old as the hills is if you can touch the unit and keep you hand on it for a few seconds without losing skin, the temperature is not a concern.
Below is a table from the Bonfiglioli A Series catalogue for bevel-helical gearboxes, identifying that 20°C ambient is the continuous duty rating, with a thermal de-rate for hotter ambient conditions and a capacity increase for lessened duties.
| Temp (°C) | Continuous Duty | Intermittent Duty | |||
| 80% | 60% | 40% | 20% | ||
| 40 | 0.80 | 1.10 | 1.30 | 1.50 | 1.60 |
| 30 | 0.85 | 1.30 | 1.50 | 1.60 | 1.80 |
| 20 | 1.00 | 1.50 | 1.60 | 1.80 | 2.00 |
| 10 | 1.15 | 1.60 | 1.80 | 2.00 | 2.30 |
This table can only be read in conjunction with the individual kW ratings of each gearbox in the range, which have a thermal rating for both 4 pole and 2 pole input speeds. The de-rate is higher for the faster 2 pole electric motor.
For probably the best, most succinct explanation of heat in industrial gearboxes, follow this link to Dr. Gerard Antony’s page which outlines how heat is caused and some of the best ways to limit its build up.
