When the term horsepower (HP) comes up, it’s easy to picture Tim “the Toolman” Taylor cranking up a belt sander on Tool Time and watching things spiral out of control. While the comedy is exaggerated, it captures the idea that “more power” isn’t always straightforward, especially when it comes to electrical ratings.
In electrical devices like relays, HP ratings provide critical information about how a component will perform when controlling motors. Let’s explore why an HP rating is important when installing a relay that will control a motor.
Functional Devices builds solid, dependable relays with a variety of HP ratings for different applications. Additionally, we test each relay before it leaves our production floor, giving you peace of mind that you’re purchasing a device that’s up to our high standards. Contact our team for more information on our relays with HP ratings.
Why an Amp Rating Alone Isn’t Always Enough
Besides HP ratings, relays also have amperage (amp) ratings, which rate the device for the maximum amount of electricity it can handle. In some applications, an amp rating might be enough to make a selection; however, if the relay is managing a motor, you need to dig further into the details.
Unlike simple resistive loads, motors demand a lot of energy to get started, as much as six to 10 times the amount it needs to run normally (the steady-state operating current). This energy is called an inrush current and can cause stress on relay contacts, leading to pitting, arcing, or even welded contacts if the device isn’t properly rated. Your relay could fail quickly or worse, pose a serious safety risk.
An amp rating is a starting place, but you need to check the HP rating to know if the relay can manage high inrush currents. HP ratings take motor characteristics, not just steady-state current, into account. That’s why our manufacturer datasheets list not only amp but also HP ratings, giving installers a quick reference to ensure safe and reliable motor control.
Now you know why you need to check HP ratings, but let’s back up and ask some fundamental questions: what exactly is horsepower and what do watts have to do with anything? How do you interpret an HP rating?
What Is Horsepower?
Horsepower (HP) is a unit of power, measuring the rate at which work is done. Specifically, it's the power needed to lift 33,000 pounds one foot in one minute.
Another unit used to describe the power of a motor is watts. One watt is directly related to HP, with one HP defined as 746 watts of mechanical power. In real-world applications, however, efficiency losses in motors mean the actual electrical input is higher than the mechanical power output.
With greater power comes greater responsibility — specifically, the responsibility to select a relay with the proper HP rating! Here’s a chart illustrating the electrical current typically drawn at each HP level, serving as a helpful guide for matching loads to relay capacity.
Horsepower to Watts and Current Conversion
|
Horsepower (HP) |
Power (Watts) |
Approx. Current @ 120V |
Approx. Current @ 240V |
|
1/4 HP |
~186 W |
~2 A |
~1 A |
|
1/2 HP |
~373 W |
~4 A |
~2 A |
|
1 HP |
~746 W |
~8 A |
~4 A |
|
2 HP |
~1,492 W |
~16 A |
~8 A |
|
3 HP |
~2,238 W |
~24 A |
~12 A |
|
5 HP |
~3,730 W |
~40 A |
~20 A |
These values are approximate and based on ideal conversions (1 HP = 746 W). Actual motor current will be higher due to inefficiencies and power factor. Locked-rotor or startup current can be six to 10 times higher than the listed running current, so proper relay selection must take these spikes into account.
HP Ratings for RIB® Relays Explained
Now let’s look at Functional Devices’ RIB relays and their HP ratings specifically.
The HP ratings listed on the 10A, 20A, and 30A rated relays (seen below, respectively) are proven by the UL testing that is performed on the RIB relay devices. With those listed ratings, it is important to notice the specific voltages that are stated and not exceed them.
Could our relays handle a 1 HP motor at 240Vac? Perhaps it could, but we cannot guarantee how long the relay will last. With the listed specifications, the RIB relay devices are guaranteed to last a long life of switching the motor load on and off.
Another important note is that these HP ratings are for conventional alternating current (AC) motors. Electronically Commutated Motor (ECM) units do not behave, electrically speaking, like a conventional AC-powered motor. UL has not yet made standard specifications for ECM units because manufacturers have different ways of making a motor into an ECM unit. For a more reliable contact rating for ECM units, look at the "electronic ballast" rating.
Practical Implications of Improper Ratings in the Field
If you ignore HP ratings or don’t understand them, it can spell trouble. While oversizing your relay is often safer in motor control applications, choosing a relay that’s either under or overrated can pose some risks.
If your relay is underrated (undersized), you could see:
- Welded contacts
- Premature failure
- Fire hazard
- Motor damage
- High impedance (motor doesn’t start reliably)
If overrated (oversized), you could experience:
- Increased cost
- Poor motor protection
- Poor switching performance
- Inefficient use of space
To ensure you’re selecting the correct HP rating, always check the motor nameplate data for the Full Load Amperage (FLA) and Service Factor (SF). Then use this data to calculate the needed overload protection based on the National Electrical Code (NEC) guidelines. This will help you determine the HP and amp ratings needed for your relay.
HP-Rated Relays for Your Application
Horsepower ratings may seem like just another line on a datasheet, but they play a critical role in selecting the right relay for your application. Choosing a relay with the proper HP rating ensures that your equipment can handle both the steady-state current and the inrush demands of the motor it’s controlling.
An underrated relay may fail prematurely or create unnecessary downtime, while an overrated relay may add unnecessary cost. By understanding the relationship between HP, watts, and current draw, you can make smarter design choices that keep your systems efficient, reliable, and safe.
That’s why our RIB relay datasheets clearly list HP rating, giving you a reliable benchmark when planning your next controls project. If you’re ever unsure, don’t leave it to guesswork. Our engineering team is available to provide guidance and help you double-check your selection, which can save you costly mistakes down the road. Reach out for a quick conversation; we’re always here to back you up with expert advice.