What does “RES” mean on a contactor or relay?

Look closely at a contactor on your truck, and you may find some interesting ratings you never noticed—things like terminal ambient temperature ratings and torque specs. One reading you may overlook is the RES AMPACITY of the contactor or relay. The RES rating is the RESISTIVE LOAD AMPACITY (amperage capacity) or rating.

Remember, a contactor and relay is both a switch (contacts) and a load (coil) that controls the switch (contacts). The FLA, RES, and LRA are the ratings of the switch/contacts/points portion of the relay/contactor, not the coil portion. The coil is just rated for voltage and cycle rate (Hz).

You will notice that the RES (resistive load) rating is higher than the FLA (full load amperage) rating and much lower than the LRA (locked rotor amperage) rating. So, the contactor above would be sufficient to control/switch a 40 amp full load INDUCTIVE load and a 50 amp RESISTIVE load.

So, what is the difference between an inductive and a resistive load?

An inductive load is a magnetic load that converts electrical energy into kinetic energy (motion) through electromagnetic force (magnetism). That would usually be motors and solenoids in HVAC/R, or really anything where magnetism and motion are involved (transformers and inductive ranges being examples of induction WITHOUT motion for the purist).

An inductive load will experience a spike in current when voltage (potential) is first applied; this is called locked rotor amps (LRA) because it is the current a motor will draw when it starts up from a locked or stalled position.

A resistive load is a load that converts electrical energy directly to light or heat as the electrons flow through a resistive conductive path. These would be things like heat strips, crankcase heaters, and incandescent light bulbs. Resistive loads do not have any internal variation in load, and the voltage and amperage are completely in phase. This means that when a contactor, switch, or relay controls a resistive load, there will be less load variation.

The conclusion is that often contactors and relays can handle a higher amperage across the contacts when the load is resistive (light/heat) than when it is inductive (motor/motion).

—Bryan

Related Tech Tips

It Has a Flat For a Reason
When tightening down a blower wheel or a fan blade on a motor shaft, ONLY tighten it on the flat of the shaft. If you have more than one screw but only one flat surface on the shaft, only tighten the one set screw. Also… Refrain from over-tightening set screws; they need to bite into […]
Read more
Don't Get Depressed (Unless You're a Core)
This website is for professional HVAC techs and those actively learning to become professional HVAC techs, not DIY homeowners. I feel like that needed to be said before I state these seemingly obvious facts, but these very obvious facts are so commonly mistaken that it needs to be covered. Take a look at the two […]
Read more
Milivolt Systems w/ Modern Thermostats
Jesse Grandbois is a tech who reads the tech tips, and he wrote a few tips that he wanted to share on some gas furnace control basics. This tip is about how to use a fancy digital stat on a millivolt system. For those of you who don't know, a millivolt system uses a thermocouple/thermopile […]
Read more

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from