Washington motor calculations practice questions

What motor calculations are testing

Motor questions check whether you know where the numbers come from. A motor has a nameplate, and the nameplate has an amp rating right on it, so the natural move is to use that number. The 2020 NEC does not let you. For sizing the branch-circuit conductors and the short-circuit and ground-fault device, you use the full-load current from Table 430.250, which is a standardized value keyed to horsepower and voltage. On the exam the question hands you a motor and asks for the conductor size or the minimum ampacity, and the nameplate amps are usually printed right there to pull you off the table.

The reason it shows up on the NEC & Theory section is that it forces you to apply the right rule to the right number. Two candidates can both multiply correctly and land on different answers because one used the table and the other used the nameplate. That is the trap the question is built around.

The rules, in plain terms

Before any math, get the source of each number right. The three that catch people on the exam:

• Full-load current comes from the table. For a three-phase motor you read Table 430.250 by horsepower and voltage. That table value, not the nameplate, drives conductor sizing and overcurrent protection.
• Conductors get 125 percent. A single continuous-duty motor is a continuous load, so 430.22 sizes the branch-circuit conductors at 125 percent of the table full-load current. That is the conductor calculation, start to finish.
• The overcurrent device is separate. The branch-circuit short-circuit and ground-fault device is sized under 430.52 with its own multipliers, which run higher than 125 percent. Do not size the breaker at 125 percent and do not size the wire at the breaker multiplier.

Worked example

Take a 10 HP, 230 volt, three-phase motor on a single continuous-duty branch circuit. Find the minimum conductor ampacity. Work the source of each number before you touch the calculator.

1. Read the full-load current. From NEC Table 430.250, a 10 HP, 230 volt, three-phase motor is 28 amps. Use the table value, not the nameplate.
2. Apply 125 percent. A single continuous-duty motor is a continuous load, so 430.22 sizes the conductors at 125 percent of the full-load current.
3. Run the math. 28 times 1.25 is 35 amps.
4. Read the result. The branch-circuit conductors must carry at least 35 amps.

Notice the work that mattered was the source of the 28 amps, not the multiplication. If you had used a nameplate amp value instead of the table, you would have run the same 125 percent and landed on a different ampacity, and that wrong answer is sitting right there in the choices waiting for you.

Why this topic costs points

Motor calculations cost candidates points for reasons that have nothing to do with whether they can multiply. The arithmetic is one step. The failure is in where you got the number, and the exam knows it.

• The table-versus-nameplate trap is built into the choices. Use the nameplate amps and you land on a number that is in the list, so you feel confident and move on. There is no flag telling you that you skipped the table.
• The conductor rule and the overcurrent rule get mixed up. Conductors are 125 percent under 430.22. The branch-circuit device is sized under 430.52 with its own multipliers. Apply the breaker multiplier to the wire, or 125 percent to the breaker, and both answers are wrong.
• Open book does not save you. Table 430.250 and 430.22 are in the book, but flipping to the right table mid-exam is slow, and that lookup eats time you needed for the other calculations in the NEC and Theory section.
• It is the 2020 NEC, not the field code. If you study from a 2023 NEC or a 2023-based course, the motor full-load current structure is close, but you are rehearsing on the wrong edition. The exam tests 2020.

How to drill motor calculations for the Washington exam

The fix is reps that force you to the table first, under time, not rereading the article. You want going to Table 430.250 for the full-load current and applying 430.22 at 125 percent to be automatic, so the only thinking left is reading the motor and the question. Practice problems that vary the horsepower and voltage, mix in a nameplate amp value as bait, and ask for the overcurrent device separately, so no single pattern lets you coast.

Where to practice it

Start with the free 15-minute Washington diagnostic. It scores you against the NEC & Theory section and the Washington Laws & Rules (RCW 19.28 + WAC 296-46B) section separately, so you can see whether calculations like motor sizing are actually what is costing you, or whether your points are leaking somewhere else. From there the practice platform drills the 2020 NEC calculation topics with worked steps, not just an answer key. We are not going to quote you a motor question count we have not verified. What we will do is route you to the right next thing to drill. For the rules behind the math, the motor sizing fundamentals hub walks the table lookup and the 125 percent rule end to end.