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Spec Sheet  Install Manual

Intermatic GMXST-O-120 24-Hour 120V Electromechanical Timer, 15 Minute Intervals, 21A, SPDT, Type 3R Outdoor Plastic Enclosure

The GMX series time switches are universal mechanical time switches. They are a low cost alternative for applications that do not require more than 21 A or 2 HP @ 240 VAC. The timer is mounted in a UL Type 1 indoor or Type 3R outdoor plastic enclosure. The GMX is a light-duty commercial time switch ideal for 24, 120 or 208/240 VAC single-phase general applications.


  • 21 A SPDT Switch
  • UL Type 1 indoor or Type 3R outdoor plastic enclosure
  • Manual 3-way override
  • Captive trippers with 15-minute intervals on 24-hour models and 2-hour intervals on 7-day models
  • True clock face, easy to set
  • Finger-safe terminals
  • Enclosure provides space for field-added contactor or other accessories
  • Synchronous drive or quartz drive ("Q" models) with self-charging battery backup


  • Aeration
  • Air Movement & Ventilation
  • Generator Exercising
  • Refrigeration Merchandiser
  • Septic Treatment


Input Voltage Range(s) 120 VAC, 60 Hz Mounting Options Wall
Resistive 21 A, 250 VAC, 60 Hz Switch Type SPDT
Enclosure Type Outdoor type 3R plastic Has Real-time Clock Face Yes
Warranty Period 1-Year limited Wiring Option Screw terminal accepts up to #12 AWG wire
Application Compatibilities CFL, Fluorescent, Incandescent, LED Backup Protection Time 7-day
Category Timer Controls Web Clock Drive Type Synchronous
Has Captive Trippers Yes Has Finger-Safe Terminals Yes
Has Override Switch Yes Maximum Cycle Time 24 hrs
Minimum Cycle Time 15 min Motor (Three-Phase) 1 HP, 120 VAC; 2 HP, 250 VAC
Operating Humidity 0 to 95%, Non-condensing Shortest switching time ON/OFF 15 minutes
Timing Interval 15 min Tungsten (NO) 1350 W, 120 VAC, 60 Hz
Wire Size Max #12 AWG Operating Temperature Max (°C) 85
Operating Temperature Max (°F) 185 Operating Temperature Min (°C) -40
Operating Temperature Min (°F) -40 Product Depth (in) 3 3/4
Product Depth (mm) 95 Product Height (in) 10
Product Height (mm) 254 Product Width (in) 6 1/2
Product Width (mm) 165 Unit Carton Height (in) 4.125
Unit Carton Length (in) 10.125 Unit Carton Weight (lb) 1.547
Unit Carton Width (in) 6.875

Switch Rating: 21A Resistive @ 120-250 VAC
1HP @120 VAC
2HP @ 240 VAC
1,350W Tungsten @ 240 VAC
Wiring Connections: Screw terminal accepts up to #12AWG
Operating Temperature: -40˚F to 185˚F (-40˚C to 85˚C)
-4˚F to 131˚F (-20˚C to 55˚C) with battery
Operating Humidity: 0 to 95% non-condensing
Enclosure Dimensions: 10"H x 6.50" W x 3.75" D
Battery Backup Option: 7-Day carryover
Supply Voltage: 24, 120, 208/240 VAC, 60 Hz models
Quartz: 120, 208/240 VAC 50/60 Hz
Shipping Weight: 2 lbs.

Product Depth (in) 3 3/4
Product Height (in) 10
Product Width (in) 6 1/2

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