IEC Motor Frame Sizes Explained (160 to 355+)

A frame size is a standardised set of mounting dimensions for an electric motor, defined internationally under IEC 60072. Its purpose is interchangeability: a motor of a given frame from one maker should bolt into the same footprint as the equivalent frame from another.

The headline number in an IEC frame — 160, 200, 280, 355 and so on — is the shaft height in millimetres: the distance from the centre of the shaft to the bottom of the mounting feet. A 355 frame therefore has its shaft 355 mm above the mounting plane.

The frame number fixes the key mounting geometry — shaft height, and, with its letter suffixes, the mounting-hole pattern and shaft details — so the motor is mechanically compatible with its base and coupling.

It does not, by itself, define power. A given frame can house a range of power ratings depending on speed, cooling and design. Larger frames generally accommodate more power, but the exact rating comes from the datasheet, not the frame number alone.

As a rough guide, bigger frames hold bigger motors: more active material means more torque and power capacity, and more surface area to reject heat. Speed matters too — at a given frame, a higher-speed motor can deliver more power than a lower-speed one.

This is why product ranges are described both by a power span and a frame span. A range covering roughly 20–800 kW, for example, typically spans frames from the low hundreds up to 355 and beyond.

Frames in the 160–250 band cover small-to-medium industrial motors used in many pumps, fans and general machinery. Frames 280–355 cover larger industrial drives — bigger pumps and compressors, heavy conveyors and process equipment.

Above 355, motors move into large-machine territory. The '+' notation simply indicates that a range extends into these larger frames for higher-power applications.

Standard frames make replacement and second-sourcing straightforward: an efficient modern motor in the correct frame can drop into an existing installation without redesigning the mechanical mounting.

For an example of a product range described by both power and frame span, see the ferrite motor page, or get in touch to check fit for a specific application.