Maryland motor calculations practice questions

The rule, in plain terms

Motor calculations scare people because Article 430 is long. The branch-circuit conductor part is not. The code hands you the current in a table and tells you to add a margin. Two articles do the work: Table 430.250 gives the full-load current for a three-phase motor, and 430.22 says a single continuous-duty motor needs conductors rated at 125 percent of that current.

• Full-load current: read it from NEC Table 430.250 by horsepower and voltage. Use the table value, not the motor nameplate amps.
• The 125 percent step: a single continuous-duty motor needs branch-circuit conductors rated at 125 percent of that full-load current, per 430.22.
• Overcurrent is separate: the breaker or fuse size comes from 430.52, a different rule with different percentages. Do not blend it into the conductor math.

That is the whole shape of it. The trap is reaching for the nameplate amps instead of the table, or doing the conductor and the overcurrent math as one step. Keep them separate and motor sizing becomes free points.

Worked example

Here is the exact kind of setup the PSI exam asks. Work it the same way every time so the lookup and the multiply are muscle memory by test day.

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

From NEC Table 430.250, a 10 HP, 230 volt, three-phase motor has a full-load current of 28 amps. Use this table value, not the nameplate amps. That is the number the conductor calculation is built on.

Step 2: apply the 125 percent rule

A single continuous-duty motor needs branch-circuit conductors rated at 125 percent of the full-load current, per 430.22. So 28 × 1.25 = 35 amps.

Step 3: read the answer

The conductors must carry at least 35 amps. That 35 amp minimum ampacity is what you take into the conductor table to pick the wire. Notice the question stopped at conductor sizing, so you never touched the overcurrent rule here.

Notice what the open-book format is really testing. You did not have to memorize 28 amps. You had to know the full-load current lives in Table 430.250, flip to it fast, ignore the nameplate, and apply 125 percent from 430.22 without second-guessing it. That is the skill the Maryland exam pays for.

Why this topic costs points

Motor sizing looks like one calculation, which is exactly why it bleeds points on the real exam. Three things go wrong under the clock.

• The nameplate trap: people size conductors off the nameplate amps instead of Table 430.250. The branch-circuit rule wants the table value every time, and the wrong starting number sinks the whole answer.
• Skipping the 125 percent: the full-load current is not the conductor rating. A single continuous-duty motor needs 125 percent under 430.22, and people who stop at the raw table value pick a conductor that is too small.
• Blending in the overcurrent rule: conductor sizing is 430.22, but the breaker or fuse comes from 430.52, with its own percentages. Treat them as one calculation and both answers come out wrong.

None of that is hard once you have run a dozen of these. It is hard the first time you see it under pressure with the clock running, and it is worse on the 90-question master exam, where calculations are 30 of the 90 questions. That is the whole argument for drilling it now.

Practice motor calculations inside the Maryland bank

The fastest way to make this automatic is not reading about it. It is working original Article 430 questions on the 2020 NEC until the table lookup and the 125 percent step are reflex. Start with the free diagnostic to see whether motor and calculation articles are one of the families costing you points, then put your reps there.

Want the rule across every state and every motor type before you drill? Read the motor sizing fundamentals hub.

Ready to see where motor calculations rank against your other weak spots? Take the free Maryland diagnostic.