Motor Calculations Practice Questions

A. Conductor ampacity, the current ratings of switches, and the current rating of branch-circuit short-circuit and ground-fault protection

Three items sit on that list and they are named in order in the subsection. The separate overload device is deliberately not among them, which is the entire point of the split. A question that gives you both currents is asking whether you know which list you are standing in.

NEC 2026 430.6(A)(1)

B. The rating of the separate motor overload protection

Overload protection is watching this particular motor, so it reads what this particular motor actually draws. The other three all protect or switch the circuit and come off the standardized table value. Protecting the motor means nameplate, protecting the wire means table.

NEC 2026 430.6(A)(2)

C. Motors built for low speeds under 1200 RPM, motors built for high torques, canned pumps and multispeed motors

The exclusion is written into the opening words of the subsection that establishes the table, so it is easy to read past on the way to the table itself. Canned pumps sit in that list alongside the low speed, high torque and multispeed cases. Any working habit that skips this check will confidently eliminate the right answer on those motors.

NEC 2026 430.6(A)(1)

D. Assume the horsepower that corresponds to the marked value in the tables, interpolating where necessary, then work from the table

This one reads like the nameplate rule and does the opposite. The marked amperes are used to find a horsepower, and the horsepower then sends you back to the table for the current you actually calculate with. Answering with the marked amperes is the error the option list is built around.

NEC 2026 430.6(A)(1)

A. 42.5 amperes

Conductors for a single continuous duty motor carry not less than 125 percent of the table full-load current, so 34 times 1.25 gives 42.5 amperes. The 39.1 figure is 115 percent and belongs to the disconnecting means, and 85 is 250 percent and belongs to an inverse time breaker. All three numbers come off the same starting current, which is why they all look plausible.

NEC 2026 430.22, Table 430.250

B. 8 AWG

The required ampacity is 42.5 amperes. In the 75 degree column, 10 AWG copper carries 35 amperes and 8 AWG carries 50, so 8 AWG is the first size that reaches the requirement. Reading the 90 degree column instead gives 10 AWG at 40 amperes, which is still short and is how the wrong answer gets built.

NEC 2026 430.22, Table 310.16

C. 37.44 amperes

The line side of the controller takes the usual 125 percent, but the winding conductors past the controller carry only part of the motor current, so the code sets them at 72 percent of the full-load current. That gives 52 times 0.72, or 37.44 amperes. The 32.5 figure is the part-winding percentage and is the distractor built from the neighbouring rule.

NEC 2026 430.22

D. 25 amperes

A part-winding connection splits the motor current between two winding circuits, so the conductors past the controller are sized at 62.5 percent of the full-load current. That is 40 times 0.625, or 25 amperes. The line side conductors still take 125 percent, which is the 50 ampere answer offered first.

NEC 2026 430.22

A. 18 amperes

The duty-cycle rule changes two things at once, the multiplier and the current it applies to. The table is indexed by duty classification down the side and by the time rating of the motor across the top, and intermittent duty at 30 minutes is 90 percent, so 20 times 0.90 gives 18 amperes. The 25 ampere answer is the continuous duty habit applied to a motor that is not on continuous duty.

NEC 2026 430.22(E), Table 430.22(E)

B. 24 amperes

The continuous rated column of the duty-cycle table sets varying duty at 200 percent, so 12 times 2 gives 24 amperes. That is larger than the ordinary 125 percent, which surprises people who expect a duty-cycle allowance to always reduce the number. Reading the wrong column produces 18 or 13.2, and both are on the list.

NEC 2026 430.22(E), Table 430.22(E)

C. Only where the nature of the apparatus the motor drives is such that the motor cannot run continuously with load under any condition of use

This is a classification of the driven machine and not a stopwatch reading, which is what separates it from the three hour continuous load rule in Article 210. A conveyor that could run all shift is continuous duty even if it does not. A time rating on the nameplate tells you which column of the duty-cycle table to read, not whether the table applies.

NEC 2026 430.33

D. 90 amperes

The table sets 250 percent for an inverse time breaker on this motor type, and 34 times 2.5 is 85 amperes. That is not a standard rating, and the permission that follows the table lets you take the next higher standard rating, which is 90. The 80 ampere answer comes from rounding down, which is what the feeder rule asks for and not the branch-circuit rule.

NEC 2026 Table 430.52(C)(1), 430.52(C)(1)(a), 240.6(A)

A. 60 amperes

A time-delay fuse rides through starting current by design, so its column carries a lower multiplier than the others: 175 percent, giving 34 times 1.75, or 59.5 amperes. The next higher standard rating is 60. Picking the row correctly and then reading across to the wrong column is the standard way to lose this item.

NEC 2026 Table 430.52(C)(1), 430.52(C)(1)(a), 240.6(A)

B. 110 amperes

A fuse with no intentional delay has to be sized high enough to survive inrush, so its column is 300 percent. That gives 34 times 3, or 102 amperes, and the next higher standard rating is 110. The 102 answer is on the list because a calculated value is not a rating you can buy.

NEC 2026 Table 430.52(C)(1), 430.52(C)(1)(a), 240.6(A)

C. The fuse columns and the inverse time breaker column all read 150 percent, well below the squirrel cage values

A wound-rotor motor starts with resistance in the secondary circuit, so it does not present the inrush a squirrel cage motor does and the device does not need the same headroom. Three of its four columns sit at 150 percent. A stem that names the motor type is handing you the row, and reading a squirrel cage percentage into a wound-rotor problem roughly doubles the answer.

NEC 2026 Table 430.52(C)(1)

D. 39.1 amperes

The general figure for a motor circuit disconnect is 115 percent of the full-load current, so 34 times 1.15 gives 39.1 amperes. That current is the table value, because a switch is one of the three items the table governs. Reaching for the 125 percent conductor habit produces 42.5 and is the miss the option list is built for.

NEC 2026 430.110(A)

A. A listed unfused motor-circuit switch with a horsepower rating not less than the horsepower of the motor

The exception trades an ampere rating for a horsepower rating, because a switch rated in horsepower has already been tested against the locked-rotor current the motor will throw at it. That is a different way of proving the same thing. Note that it is limited to a listed unfused switch, which rules out most of what is on the wall.

NEC 2026 430.110(A)

B. The 10 horsepower motor, because its table current of 50 amperes is larger than the 42 amperes of the three-phase motor

The code settles this in its own short section, and it settles it on rated full-load current selected from the tables rather than on horsepower. A single-phase motor and a three-phase motor of similar size draw very different currents, so the smaller horsepower can carry the larger current. Sort the group by table current before you apply any percentage.

NEC 2026 430.17, Table 430.248, Table 430.250

C. 83.5 amperes

Only the highest rated motor takes the 125 percent, and every other motor comes in at 100 percent, so the sum is 42.5 plus 27 plus 14, or 83.5 amperes. The 93.75 answer is what you get by applying 125 percent to all three, which looks careful and is wrong. The plain sum of 75 leaves the single-motor margin out entirely.

NEC 2026 430.24

D. 125 amperes

The feeder device sum starts from a device rating rather than a current: 90 plus the other two full-load currents of 27 and 14, which is 131 amperes. That section states a maximum, so where the sum is not a standard rating you take the next size down, which is 125. Carrying the next higher permission over from the branch-circuit rule produces 150 and puts you above the ceiling.

NEC 2026 430.62(A), 240.6(A)

A. At not less than the marked ampacity, without adding up the motors inside the cabinet

The manufacturer has already combined the motors and other loads that run at the same time and printed the result, so you select a conductor that carries at least that after your own corrections and the termination check. Applying another 125 percent doubles up a margin that is already in the number. You return to the several-motors sum only where the equipment is not factory wired and the individual nameplates are visible.

NEC 2026 430.25, 430.7(D)

B. The maximum operating current marked on the motor or the control nameplate, and where none is marked, 150 percent of the table values

This case has its own subsection alongside the general one, and it puts the marked maximum operating current in charge of everything. The fallback where nothing is marked is a multiplier on the table value rather than the table value itself. Answering from the general table method here misses the subsection entirely.

NEC 2026 430.6(C)

C. The locked-rotor current, and that nameplate value determines the conductor ampacity, the overload rating and the branch-circuit short-circuit and ground-fault protection

A torque motor is built to sit at stall and produce torque, so its normal operating condition is the condition every other motor only sees during starting. The code therefore treats locked-rotor current as the rated current and runs the whole calculation off it. One current serves all three steps here, which is unusual enough to be worth an item.

NEC 2026 430.6(B)

D. 14 AWG, unless one of the two small conductor allowances in the same subsection applies

The article puts a floor under motor conductors that has nothing to do with the calculated ampacity, then carves out narrow allowances for 18 AWG and 16 AWG copper under stated conditions of enclosure, cable or cord and a load limit. Read the conditions rather than the size when one of those answers looks attractive. A calculation that comes out under 15 amperes does not by itself buy you a smaller wire.

NEC 2026 430.22(G)

A. The temperature rating of the terminations at each end of the circuit

Termination temperature is a separate constraint that sits outside Article 430 and applies to every circuit, motor or not. Most equipment terminations are limited to the 60 or 75 degree column even where the conductor insulation is rated 90. That is why a motor answer read out of the 90 degree column is usually one size optimistic.

NEC 2026 110.14(C)