The mounting form determines how a motor is fitted to your installation, feet, flange, or a combination. Choose wrong and mechanical fitment fails or you pay for features you don't need. This guide explains the main IEC mounting forms, typical applications, and the price differences.
The four main mounting forms
Most industrial motors use one of four IEC IM-code mounting forms:
- B3B3 (IM 1001): horizontal foot mounting. Feet integral with or bolted to the frame. Most commonly used, about 70% of all industrial motors. Lowest cost
- B5 (IM 3001): large flange (FF series) at the drive end, no feet. Used for direct-coupled pumps, blowers, gearboxes. Mounting via through-holes in the flange.
- B14 (IM 3601): small flange (FT series) with tapped holes (threaded). Typically for smaller motors up to frame 132. Used where space is tight.
- B35B35 (IM 2001): combination, feet plus B5 flange. Universal fitment, higher cost. Useful when the exact mounting is uncertain
Frame size: IEC 56 to 450
The frame size (or "shaft height" / "H") is the dimension from the center of the motor shaft down to the mounting surface. IEC frame sizes run from 56 mm (very small) to 450 mm+ (large industrial). Common sizes: 63, 71, 80, 90, 100, 112, 132, 160, 180, 200, 225, 250, 280, 315, 355, 400, 450.
Frame size correlates with power range, frame 63-100 typically covers 0.09-3 kW, frame 132-180 covers 5.5-22 kW, frame 200-280 covers 30-132 kW, frame 315+ for 160 kW and above. Always match frame size to both power AND mechanical fitment constraints.
Price differences: B3 vs B5 vs B35
As a rule of thumb, B3 (foot) is the base price. B5 (flange) typically adds a premium because of the heavier flange casting. For our Dormot Core range we add 5% for B5 (Asian manufacturing), for Dormot Pro we add 2% for B5 (European manufacturing, tighter tolerances). B35 (foot + flange) costs roughly the same as B5 but offers more flexibility.
For OEM applications where large series of identical motors are ordered, the form is defined from day one and price is fixed. For replacement markets, B5 or B35 is often preferred because existing installations may have been built with either foot or flange mounting.
Choosing the right form
Start with mechanical constraints: is there a flange or foot mounting point? Is space tight (B14 preferred) or generous (B3 fine)? Is the motor directly coupled (B5 better) or via belt (B3 usually preferred)? For replacement, match the existing motor's IM code exactly, don't substitute B3 for B5 without adapter plates.
For uncertain cases or where multiple mounting options may be needed over the motor life, B35 is a safe choice. For our standard stock we hold B3 and B5 versions of most motors; B14 and B35 variants typically have a few weeks lead time.
Frequently asked questions
Can I convert a B3 motor to B5 later?
Not easily. B3 and B5 differ in the front end-shield casting. Some manufacturers offer convertible designs but most require the correct form from new. Using an adapter plate adds cost, weight and tolerance stack-up.
What is the difference between B14 and B5 flanges?
B5 uses a large flange with through-holes for bolts from the equipment side. B14 uses a smaller flange with tapped (threaded) holes for bolts from the motor side. B5 handles higher radial loads; B14 is compact for smaller installations.
Why does B5 cost more than B3?
The heavier flange casting requires more material and machining. Our pricing: Core +5% for B5 over B3, Pro +2% for B5 over B3, reflecting respective manufacturing origin and tolerances.
Need advice on motor selection?
Our sales engineers are happy to help you find the right motor for your application.