There are five facts and rules you should remember about Class 1 Circuits.  One, we can use smaller wire sizes than normal power circuits. This means we can use No. 16 and No. 18 AWG conductors.  They have to be insulated for 600 volts and they have to be protected at 10 amperes maximum for No. 16 and 7 amperes maximum for No. 18.  

 

     The second rule is if the job specification allows us to use these smaller conductors for Class 1 circuits and if we install these conductors in a conduit or raceway with other conductors they have to be functionally related.  Now what does this mean?  Well it means we can’t install Class 1 circuit conductors to start/stop stations in the same raceway as the motor power conductors if the start/stop station does not control the same motor unless  they are, somehow,  functionally related.  

 

     The third rule is that Class 1 circuit conductors have to be derated when they carry more than 10 per cent of their rated current continuously (for three hours or more) and where there are more than three current carrying conductors in the raceway.  

 

     The fourth rule is  generally Class 1 circuits shall be powered from a limited power supply no larger than 1000 volt-amperes and 30 volts maximum for power limited circuits.  But Class 1 signaling and remote control circuits are limited to 600 volts maximum and there is no limitation on the power supply.

 

     The fifth rule to remember is that Class 1 circuits are a potential shock hazard and source of ignition just like any other power circuit.  Therefore these circuits have to be wired like power circuits.  No open splices, no exposed terminals, and no open runs of single conductors.  The conductors have to be in a cable or raceway and spliced in boxes.  In other words, the wiring methods of Chapter 3 apply.

 

     So where do we see these circuits?  One of the most common applications is using 120 volts from a motor controller bucket control transformer to supply a control circuit outside the bucket that includes safety devices, thermostats, start/stop stations, signal lights, speed switches, limit switches, and more.  Many times the Class 1 control conductors of size No. 16 and 18 AWG can be run in the same raceway as the motor power conductors.  Since the smaller wire sizes can be used for Class 1 circuits most of the time we save time and expense, especially for rework.

 

 

     There is one important section in Article 725 that should be addressed for safety reasons, 725.11(A).  When a circuit controls equipment that can introduce a direct life safety hazard the circuit shall be Class 1.  Class 2 and Class 3 circuits are not allowed!  An example of this is the high limit thermostat switch for a boiler.  Another is the high level switch on a tank full of flammable vapors.   At a refinery located in Valdez, Alaska the entire process control wiring was done using 24 volt Class 2 and intrinsically safe wiring.  However, all the high limits and any other safety device that could possibly introduce a catastrophic failure were wired using Class 1 signaling circuits at 120 volts.