UL 508: Horsepower Rating

December 20, 2011
By Anonymous

I am sometimes asked if Panasonic’s precision components meet the various UL: Underwriters Laboratories industry standards. UL is a non profit independent product safety certification organization.  Basically, they use industry standards to develop tests and determine whether certain products are safe or hazardous in various applications.  For example, if an engineer is designing an elevator system for a new building, his design must first be submitted to UL for approval before his project can move forward. UL 508 is the standard for industrial control equipment.  In this standard, different load ratings are specified:

  • Resistive load
  • Tungsten load
  • Ballast load
  • Horsepower load rating
  • Pilot duty rating, etc.

When it comes to Panasonic’s relays, I have found that different customers will request that relays be UL certified for a specific load requirement in their application. For example, that the same customer designing the elevator system is in the process of selecting a relay to turn on the electric motor for a cooling fan.  Based on the horse power rating of that motor, UL specifies that specific testing conditions must be passed by the relay in order to be used in that motor. Example:  If the horsepower rating on the electric motor is 1/8, and the operating voltage is 120VAC, then from the table below, the full load motor running current (FLA) is 3.8A. Note: Locked rotor current (LRA) = 6 x FLA

Full load motor running current (FLA) 


110-120  Volts

Horse Power

Single Phase



























(Excerpt from Reference Table: UL508 – Industrial Control Equipment) 

UL specifies two types of testing conditions for Horsepower rating:

1)      Overload test: This tests the relay's ability to handle a higher than normal load for 50 cycles a) 50 cycles b) Locked rotor current (LRA) with power factor between 0.4 – 0.5 c) Specified voltage 2)      Endurance test: This tests the relay's ability to endure a heavy load for at least 6000 cycles a) Minimum 6,000 cycles b) Twice the Full Load Current (FLA) with power factor between 0.4 – 0.5 c) Specified voltage Let’s look back at the example from above and look at both tests: Overload Test The overload test specifies that the relay must be tested for 50 cycles.  It also calls out a locked rotor current (LRA) with power factor between 0.4 to 0.5.  The LRA can be calculated from the table above (LRA = 6 xFLA) to be 22.8A.  The rated voltage is given by the customer, which in this case is 120VAC. Endurance Test The endurance test specifies that a minimum of 6,000 cycles of operation must be handled by this relay with twice the FLA and a power factor between 0.4 – 0.5.  From the table we know that the FLA x2 is 7.6A. Both the overload and the endurance test must be passed in order for the relay to be UL certified. Over the years Panasonic has worked with different customers to meet UL standards.  Panasonic now has a wide variety of relays which have had different ratings added to their UL file. The JQ series of relays for example is widely used in applications such as washing machines, air conditioners and refrigerators.  Over time, different UL ratings have been approved for this relay as shown below: If you'd like to learn more  about Panasonic UL ratings please leave a comment below.