Square D

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Square D is an American manufacturer of electrical equipment headquartered in Andover, Massachusetts. Square D is a flagship brand of Schneider Electric, which acquired Square D in 1991.
Electricity is and will continue to be, a critical driver of world economic growth. As the global specialist in energy management and automation, Schneider Electric will play a key role in enabling that growth through innovative, connected Square D power solutions.

Wildcat Electric carries a broad range of Square D products. Click here to request a quote or ask about Square D Solutions or call 713-676-0600. 

Featured Products: 
  square D Limit Switch
Heavy Duty Industrial Limit Switches are Used in Material Handling, Mechanical Conveying, Automotive, Machine Tool, Packaging Applications

$158.67 / ea

Square D Circuit Breaker

The Square D Extreme Duty QOU Circuit Breaker was Developed Specifically with the Unique Challenges of Harsh Environments in Mind

$29.75 / ea

Square D Contactor

Definite Purpose Contactors Are Ideal for HVAC, Heating, Air Conditioning, Refrigeration, Data Processing, Food Service Equipment

$60.12 / ea


Introduced in 1955, the Square D "QO" line of miniature 3/4-inch circuit breakers may be their best-known product line, used in the electrical distribution boards of many residences in North America. The "QO" designation stands for "Qwik-Open"; Square D claims these are the fastest-opening breakers in the industry, responding within 1/60th of a second, or just one full power cycle on a 60 Hz AC line. QO breakers feature a window with a visual trip indication flag, making it easy to identify a tripped breaker in a panel. A "QOB" version is also offered, which stands for "Qwik-Open Bolted", featuring a screw that secures the beaker in the panel. This is useful in industrial installations where vibration might cause a QO snap-in breaker to loosen and prevents them from accidentally being popped out while an adjacent breaker is being changed. A QO breaker could easily be clipped into a QOB panel. The QO and QOB series breakers are by far their most popular commercial products.

A second miniature circuit breaker line is sold under the brand name Homeline, marked "HOM". Designed for value, they lack the fast-trip feature and the visual trip flag of the "QO" series, cost one-third to half less, and are physically not interchangeable with "QO" breakers.

In the 1960s Square D introduced the Safety Line distribution centers for large industrial electrical power loads. This design had solid or tubular busing with each large fused switch clamping onto the busing. The company also sold the B line 277-480 volt panels, generally used for lighting in the 1970s and 1980s.

Another well-known Square D product line is the Powerlink circuit breaker, created for lighting control in commercial buildings. B Line and D Line were the breaker panels in which Powerline breakers were used. Eventually, the company branded Powerline and Powerlink panels and breakers. Powerlink was a heavy duty 277-volt lighting breaker, used in large buildings with 277-volt lighting loads. These were also favored for their robust design, and functional reliability. This means less panel space and wire are needed, and why commercial buildings frequently utilize higher voltage for general task lighting.

The I-Line series of distribution panelboards is favored by many electricians for its ease and safety of adding new breakers. The panel uses a stacked bus system that protects the energized bus from accidental contact and the breaker can be easily installed using a flat blade screwdriver contacting only de-energized parts of the backplane of the panel. The design of the breakers and panel require that extra attention be paid to phase rotation, as the right (even-numbered) side of the panel will have the phases in a C-B-A configuration whereas the left (odd-numbered) side of the panel will have a more standard A-B-C arrangement.

Square D also made disconnect switches, both fused, and unfused, as well as Heating Air Conditioning and Refrigeration (HACR) rated switch boxes that held an HACR breaker for use as a disconnect. The QO type breaker fits this disconnect box. These were popular with food service, grocers, and other cold storage users.

Other companies used terms like "Quick Lag" and similar brand name letter coding on their circuit breakers, e.g., QL, QLRB, etc. No one knows who started the brand coding for sure. QO is an industry recognized standard, like "Kleenex" is used to refer to a tissue regardless of brand. These brand-specific letter codes had nothing to do with National Electrical Manufacturers Association (NEMA) specifications or coding, although they appear to have been intended to emulate NEMA codes.


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