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.

definite purpose contactor

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).


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.


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


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.

definite purpose contactor definite purpose contactor Enclosed definite purpose contactor NEMA 4X

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

2449 Hot tubs, manufacturing
3088 Hot tubs, plastic/fiberglass manufacturing
3563 Air gas compressors manufacturing
3564 Fans/blowers/air purification manufacturing
3567 Industrial processing furnace/oven manufacturing
3581 Automatic vending machine manufacturing
3582 Dry cleaning and pressing machine manufacturing
3585 Air conditioning and heating equipment manufacturing
3629 Battery chargers rectifying/non-rotating manufacturing
3842 Whirlpool baths/equipment manufacturing
3548 Welding and cutting apparatus, gas or electric manufacturing
3634 Humidifiers, electric, portable manufacturing
3582 Washing machines, commercial manufacturing
3639 Water heaters, electric, manufacturing
3556 Overns, bakery manufacturing
3821 Ovens, laboratory manufacturing
3556 Meat grinders
3589 Garbage disposers, commercial manufacturing

6 Digit SIC Numbers

308802 Hot tubs and spas manufacturers
356701 Industrial oven manufacturers
358101 Vending machine manufacturers
358298 Commercial laundry dry clean Mmachine manufacturers
369402 Battery charging equipment manufacturers
363908 Water heaters - manufacturers
399934 Tanning salons equipment manufacturers
358501 Air conditioning equipment manufacturers
358598 Air conditioning/heating/refrigeration equipment manufacturers
356301 Air compressors manufacturers
356398 Air and gas compressors manufacturers
354801 Welding equipment supplies manufacturers
356498 Industrial and commercial fans and blowers
356401 Industrial and commercial fans manufacturers
356402 Air cleaning and purifying equipment
355605 Ovens (bakers) manufacturers
356413 Ventilating equipment manufacturers
356789 Industrial process (furnaces/ovens)
358903 Industrial/commercial waste compactors manufacturers
358512 Marine refrigeration and air conditioning manufacturers
358513 Air conditioners room units manufacturers
356303 Spraying equipment manufacturers
358203 Pressing machine manufacturers
363410 Water jet cutting