Understanding Short-Circuit Protection for Motors: A Quick Guide

The branch circuit short-circuit protection device for a 10 horsepower 230 volt single-phase wound rotor motor shall not exceed what rating?

• A. 110 amperes
• B. 125 amperes
• C. 115 amperes
• D. 120 amperes

Answer: (B)

In determining the appropriate branch circuit short-circuit protection device rating for a 10 horsepower 230 volt single-phase wound rotor motor, the key is to understand the National Electrical Code (NEC) guidelines regarding motor circuits. For continuous duty motors, the NEC specifies that the short-circuit and ground-fault protective device can typically be sized based on the motor's full-load current (FLC). The FLC can be calculated using the formula: FLC = (Horsepower × 746) / (Voltage × √3 × Efficiency × Power Factor) However, for fractional and small industrial motors, a simplified approach is often used based on standard sizing charts provided in NEC and manufacturer's guidelines. For a single-phase 10 horsepower motor at 230 volts, the FLC under typical conditions is approximately 50 amps. When applying NEC guidelines, specifically in Article 430 regarding motors, the circuit protective device can be set at a maximum of 125% of the motor’s FLC. Calculating 125% of 50 amps results in 62.5 amps. However, for larger motors like a 10 hp, the maximum branch circuit protection must also be considered alongside standard ratings. While one might initially think smaller ratings apply, in this

When preparing for the HOLT Electricians Exam, one of the topics you'll likely encounter is determining short-circuit protection for motors, particularly regarding branch circuits. Understanding the nuances of short-circuit protection can feel tricky, but once you get the hang of it, it’s a piece of cake. So, let’s break this down, shall we?

You might be wondering: “What’s the big deal with protection ratings?” Well, it’s all about keeping our electrical systems safe. A 10 horsepower 230 volt single-phase wound rotor motor is a common piece of equipment found in many places—factories, workshops, even some home setups. With this motor, the short-circuit protection device rating does not exceed 125 amperes. Sounds straightforward, but why?

To comprehend this fully, we need to unpack a couple of key concepts, particularly the National Electrical Code (NEC) guidelines. The NEC is your go-to document that lays the groundwork for electrical safety. Knowing how to leverage these guidelines while taking your exam will give you an edge. Here’s the thing: for continuous duty motors, the NEC provides insight into sizing short-circuit and ground-fault protective devices based on the motor's full-load current (FLC).

Now, calculating the FLC isn't as daunting as it sounds. You can do it using the equation:

FLC = (Horsepower × 746) / (Voltage × √3 × Efficiency × Power Factor).

Don’t worry if you’re not an engineer—we can keep this simple. A single-phase 10-horsepower motor typically pushes around 50 amps under standard conditions. But you don’t just stop there—because according to NEC Article 430, the circuit protective device can be sized at a maximum of 125% of this FLC.

So, here comes the math. If you take 125% of 50 amps, you get 62.5 amps. However, for larger motors like this beauty, we also need to consider maximum branch circuit protection alongside standard ratings. You might think that lower ratings apply, but that’s not the case for 10 hp motors.

Feeling overwhelmed? Don’t sweat it; let’s smooth it out. The reason we keep the rating at 125 amps is that it aligns perfectly with safety measures required for motors of this size under the NEC standards. But remember, knowing the calculations is one thing; applying this knowledge to real-world practices is where the magic happens.

Something else to consider is how the efficiency and power factor can fluctuate, which might affect your calculations slightly. For example, as motors work harder, they might draw more current. So tip your hat to variability in real-world applications. You might be amazed at how often you’ll refer back to these guidelines once you’re on the job.

In your studies, try to visualize scenarios where you might need this knowledge. Picture yourself on-site, repairing or installing equipment, pulling up that NEC guideline in your mind. How reassuring would it be to know you have the proper calculations down, ensuring not just compliance, but safety too?

You may find hints in your practice questions that sneak in variations of this theme—testing your knowledge on branch circuits, motor calculations, or NEC applications. So, keep pushing your understanding of short-circuit protection devices. Confidence is key, and understanding these calculations can set you up for success not just on the exam, but throughout your electrical career.

So, remember: the branch circuit short-circuit protection device for your 10 horsepower 230 volt single-phase motor shall not exceed 125 amperes. Keep this in your back pocket as you study, and you just might surprise yourself on that exam day!