MI motor calculations practice questions

What motor questions actually ask

A motor calculation question on the Michigan journeyman exam hands you a motor by horsepower, voltage, and phase, then asks for the branch-circuit conductor ampacity, or sometimes the overcurrent device, or the disconnect size. Each of those answers comes from a different rule in Article 430. The conductor question is the one you will see most, and it is the one with the cleanest process, so it is the right place to start.

The work is always the same shape. You look up the motor full-load current in NEC Table 430.250 for a three-phase motor, or Table 430.248 for single-phase, then you apply the percentage the rule calls for. There is no judgment call once you know which table and which percentage. The answer is a number, and it is either right or it is not.

The two rules that carry most motor questions

Two pieces of Article 430 sit under most of the motor calculation questions you will face. Get these straight and the rest is arithmetic.

• Use Table 430.250 for full-load current. For a three-phase motor you read the full-load current straight off NEC Table 430.250 by horsepower and voltage. You use this table value, not the motor nameplate amps, for branch-circuit conductor sizing.
• Size conductors at 125 percent in 430.22. A single continuous-duty motor needs branch-circuit conductors rated for at least 125 percent of that full-load current. So you multiply the table value by 1.25 to get the minimum ampacity.
• The protective device is a separate rule. The breaker or fuse is sized under 430.52 with its own percentages, not the 125 percent in 430.22. Conductor and overcurrent device are two different numbers, so do not mix them.

We will not reprint the NEC table here, but the method is fixed. Table 430.250 gives you the starting current, and 430.22 sets the percentage for the conductors. Those two together answer the most common motor question on the Michigan exam.

Worked example

Here is a motor you will see a version of on the exam. A 10 HP, 230 volt, three-phase motor. Find the minimum ampacity for the branch-circuit conductors feeding a single continuous-duty motor. Work it in two stages: read the table value first, then apply the percentage.

Step 1: read the full-load current from the table

1. Go to NEC Table 430.250, find the 10 HP row at 230 volts, three-phase. The full-load current is 28 amps.
2. Use this table value, not the nameplate amps. Branch-circuit conductor sizing always starts from the Table 430.250 full-load current.

Step 2: apply the 125 percent rule

NEC 430.22 requires the conductors for a single continuous-duty motor to carry at least 125 percent of that full-load current. Multiply the table value by 1.25:

So the conductors have to be rated for at least 35 amps. On the exam, the trap answers come from starting with the wrong number. People grab a nameplate value the question planted, or they skip the 125 percent and answer 28, or they reach for the 430.52 overcurrent percentages instead of the 430.22 conductor rule. The method is fixed. Read full-load current from Table 430.250, multiply by 1.25, and the conductor ampacity falls out.

Why this topic costs points

Motor math looks heavy, so candidates either over-study it or avoid it, and both groups lose points the same way. The arithmetic is not the problem. The problem is which number you start from. Pull the current off the nameplate the question dangles in front of you instead of Table 430.250, and your answer is wrong before you ever multiply. The table-versus-nameplate choice is where the points actually go.

The second loss is mixing rules. Branch-circuit conductors come from 430.22 at 125 percent. The branch-circuit protective device comes from 430.52 with different percentages. A candidate who answers the conductor question with an overcurrent percentage gets a clean, confident, wrong answer. Knowing that 430.22 and 430.52 are separate calculations is half the battle on motor questions.

Then there is the clock. The Michigan exam is 80 questions in 2 hours 30 minutes, which is under two minutes a question, and you need 75% to pass. A motor question you have drilled is a table lookup and one multiplication, under a minute. One you are working cold, flipping through Article 430 to find which percentage goes where, can burn four minutes and still come out wrong. It is open book, but page-flipping for a rule you should know cold is how the time runs out before question 60.

How to practice motor calculations for the Michigan exam

Drill the two-step method until it is automatic, then practice it under a timer in your own tabbed 2023 NEC, the same book you will carry into the PSI test center. We will not claim a fixed number of motor questions in the Michigan bank, because we have not counted them per topic and we do not invent numbers. What we will tell you is where it sits in your prep.

• Start with the free 15-minute diagnostic. It shows whether motor calculations and the rest of Article 430 are actually costing you points, or whether your time is better spent on grounding or box fill.
• Drill motor calculations inside the Michigan bank, which trains the LARA journeyman exam on the 2023 NEC, not the master or contractor exam, and cites the NEC article behind every answer.
• Rehearse the lookup. Tab Article 430 and Table 430.250 in your own book so a motor question is a one-minute table read and multiply, not a four-minute page hunt.