Prior to about 1940 there was little concentrated effort made to standardize motors as to their physical dimensions. This meant that mountings for a motor made by one manufacturer would not fit another motor of the same type, size and speed made by another manufacturer, or, for that matter, by the same manufacturer. Realizing these conditions caused much confusion and inconvenience when it was necessary to replace a motor or even in their use in production line assembly, the NEMA ultimately evolved a standardized list of specifications for motor dimensions. The majority of the American motor manufacturers eventually conformed to these specifications.

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