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The difference between a light switch and an industrial disconnect switch

In a previous post I listed the NEC requirements for “permanently installed devices” such as an air compressor in a dry pipe sprinkler system. #5 on that list states that the air compressor on a dry pipe sprinkler system must “Be connected to a listed motor-circuit switch rated for horsepower”(NEC 430.109) which is also referred to as an industrial duty disconnect switch.

I have spoken with many installers through our training programs who have said that they have never seen an industrial duty disconnect switch before. They have told me that their only experience with a power switch for an air compressor in a dry pipe sprinkler system has been a standard light switch. I won’t tell you the exact phrasing that was used when our engineering department got word of this but I will say that “surprised” is an extreme understatement.

The reason for their shock is concern for the safety of our air compressors. There is one primary reason that an industrial duty disconnect must be used, as opposed to a standard light switch: AMP draw.

Industrial Duty Disconnect Switch
Example of an Industrial Duty Disconnect Switch (or Motor Circuit Switch Rated for Horsepower)

When an air compressor, or any device that uses a motor, starts up it draws up to 7 times it’s running AMPs. This is referred to as the start up AMP draw. While it only lasts for an instant it is a huge draw and any components that are not built to suit that kind of draw will, over time, fail. A standard light switch that sees this kind of AMP draw every time the air compressor pressurizes the dry pipe sprinkler system can burn up causing a wide variety of concerns. When you use a correctly sized industrial duty disconnect switch you are addressing start up AMP draw problems before they occur (and you are following NEC as well.)

Another practical advantage to the use of an industrial duty disconnect switch is that it is far less likely to be shut off by accident. Many times where a light switch is used as the air compressor disconnect we have seen people shut them off by accident when they are searching for the switch that turns the lights off.

The code referenced here is in section 430 of the NEC. If you would like clarification on our interpretation of that code please feel free to contact us at 800-345-8207 to speak with one of our engineers or send us an email or comment below.