Definite Purpose Contactors

Definite Purpose Contactors

A definite purpose contactor is meant for use with HVACR (heating, ventilation, air conditioning, and refrigeration) applications such as hot tubs, dry cleaning equipment, and ovens.

Why use a definite purpose contactor vs. a general purpose one?

A definite purpose contactor is cheaper and rated for lighter duty use compared to general purpose contactors. In other words, you can use a general purpose contactor to switch power in HVACR systems, but definite purpose contactors provide a cheaper option.

General purpose contactors are designed for a broad range of applications and must be able to switch higher currents. It will have larger contact heads (to lower resistance) and wider gaps between contacts (to limit damage from arcing) than a definite purpose contactor. These features, plus other design characteristics of general purpose contactors, give them a longer operating lifetime than cheaper definite purpose contactor options. Aside from the higher cost, there is no drawback in using general purpose contactors for HVACR applications. In fact, general purpose contactors will most likely outlast any HVACR equipment they are installed in.

Air Conditioning

For air conditioning applications, definite purpose contactors fall into two main groups depending on whether the air conditioning unit is being used in residential buildings (e.g. houses and apartments) or commercial/industrial locations (e.g. factories and hospitals).

Residential

Compact frame single pole (SP) or double pole (DP) definite purpose contactors are the most popular units for homes. These are generally rated for 20A and 40A.

Commercial/Industrial

Standard frame (i.e. larger than compact frames) 2 and 3 pole contactors are generally rated from 15A to 360A for commercial and industrial buildings.

Standards and Ratings

NEMA vs. IEC

The National Electrical Manufacturers' Association (NEMA), which is based in the United States, approaches standard ratings with an emphasis on selection simplicity and broad applicability. In contrast, the International Electrotechnical Commission (IEC) approach standard ratings with the emphasis on being application specific. Though it takes slightly more effort to select a contactor with proper IEC ratings, the contactor will often be cheaper. On the other hand, NEMA contactors can be used for a wider range of applications, so there is less chance of equipment damage due to selection error.

Only NEMA rated contactors are marketed as “definite purpose” because IEC ratings allow for the smaller and cheaper contactors preferred by HVACR manufacturers.

Standard Industrial Classification (SIC) and North American Industry Classification System (NAICS)

First established in the U.S. and later used by other countries, the 4-digit SIC codes refer to various commercial industries. The first digit refers to which of the 10 divisions (A to J) an industry is classified under.

Division A: 0
Division B-C: 1
Division D: 2-3
Division E: 4
Division F-G: 5
Division H: 6
Division I: 7-8
Division J: 9

For example, division A refers to Agriculture, Forestry, and Fishing industries while division D refers to Manufacturing (everything from food to electrical equipment).

Since 1997 the more specific NAICS codes have been gradually replacing SIC codes. The following chart lists industries that often use definite purpose or equivalent IEC rated contactors.

4 Digit SIC Numbers

6 Digit SIC Numbers