Motor full-load current tables

Delgado Fabrication's service upgrade was finished, and Hector Delgado was already planning the next step: a new air compressor, a second CNC mill and a rebuilt saw line. Ray Ochoa handed the motor list to Dana Whitfield with a warning that sounded almost like a riddle. "The motor nameplates are the last thing you look at," he said, "and the first thing the inspector checks." Dana knew what he meant only partly. Most of Article 430 runs on a number that is not printed on the motor at all: the full-load current from the tables at the back of the article. The nameplate current matters too, but for a different job. Getting that split right is the foundation for every other motor calculation, and it is one of the most tested ideas on master electrician exams.

This lesson explains why Article 430 uses table currents, which table applies to which motor, how to read them, the exceptions where the nameplate governs, and how exam questions try to trip you up. Later lessons use the table current to size conductors, short-circuit protection, disconnects and feeders, and use the nameplate current to size overloads.

Why the code does not trust the nameplate

A motor nameplate reports the current that particular motor drew at rated load when it was tested. Two 15-horsepower motors from different manufacturers can carry different nameplate currents, and a motor that wears out may be replaced by one with a higher current. If conductors and short-circuit protection were sized to the nameplate, a routine replacement could leave an installation undersized. The code avoids that problem by assigning a standard current to each horsepower and voltage. Section 430.6(A)(1) directs you to use the table values, not the nameplate, when you determine the ampacity of conductors, the ampere ratings of switches, and the ratings of branch-circuit short-circuit and ground-fault protection. The tables are deliberately on the high side of real motors, so a circuit sized from them will carry almost any replacement motor of that size.

The same section then makes a separate rule for overload protection. Overloads protect the motor itself from overheating, so they must be matched to the motor actually installed. Section 430.6(A)(2) says separate motor overload protection is based on the nameplate full-load current rating. The words matter on exams: the tables give full-load current, or FLC, and the nameplate gives full-load amperes, often abbreviated FLA. Many study guides use FLC for the table number and FLA for the nameplate number, and this course does the same.

The four tables

Article 430 Part XIV holds the full-load current tables. They are organized by the type of motor, and the first exam skill is picking the right one before you look up a number.

• Table 430.247 covers direct-current motors, listed by horsepower and armature voltage.
• Table 430.248 covers single-phase alternating-current motors, listed by horsepower at several voltages such as 115 and 230 volts.
• Table 430.249 covers two-phase motors, which are rare today and almost never appear on exams except as a distractor.
• Table 430.250 covers three-phase induction motors, both squirrel-cage and wound-rotor, listed by horsepower at voltages from about 115 up through 2,300 volts. It also has columns for synchronous motors run at unity power factor.

Each table has notes at the bottom. They tell you how to adjust for voltages between the listed columns, how to handle motors built for low speeds or high torques, and how the synchronous motor columns relate to power factor. Exam questions often hinge on a note, so read them at least once while you study and tab the page.

Choosing the voltage column

The tables list motor voltages, not system voltages, and the two are deliberately different. A motor built for a 480-volt system is rated 460 volts, because the motor sits at the end of a feeder and branch circuit and sees less than the nominal system voltage. Likewise a motor for a 240-volt single-phase system is rated 230 volts, a motor for a 208-volt system is rated 200 volts, and a motor for a 120-volt system is rated 115 volts. The notes to the tables tell you that the listed currents apply to the system voltages in a range around each motor voltage. For an exam, the rule of thumb is simple: find the motor voltage column that matches the system. A 480-volt system uses the 460-volt column; a 208-volt system uses the 200-volt column, and some tables list 208 volts separately.

This is the most common way exam questions trap candidates. The question says the motor is on a 480-volt system, and the candidate looks for a 480-volt column that does not exist, or uses a power formula with 480 volts instead of reading the table. The table is always the answer for a standard induction motor; never calculate full-load current from horsepower with a formula when the question is about Article 430 sizing.

Reading a value

Once you have the right table and column, reading the value is a matter of finding the horsepower row. The motors on Delgado's list give a good set of examples. This course uses the following illustrative full-load currents throughout, chosen to resemble the published values so the arithmetic in every lesson stays consistent. When you work real problems, read your own table.

Look at how much larger the current is for the same horsepower at a lower voltage. The 10-horsepower air handler at 208 volts carries more than twice the current of the 10-horsepower dust collector at 480 volts. That is why industrial plants distribute at 480 volts, and why the voltage column matters so much: picking the wrong column can change an answer by a factor of two.

When the nameplate governs instead

Section 430.6(A)(1) lists several exceptions where the table does not apply, and exam writers like them because they reverse the usual answer. The main ones, in general terms, are these.

• Multispeed motors. The tables assume a single speed, so for multispeed motors the conductors and protection are based on the highest current at any speed, taken from the nameplate, with specific rules in 430.22(B) and 430.52.
• Low-speed, high-torque and other special motors. Motors built for unusually low speeds or high torques draw more current than the tables show, so the nameplate current is used.
• Shaded-pole and permanent split-capacitor fan or blower motors. When these motors are marked with current, the marked current is used instead of the table for sizing the circuit.
• Torque motors, which are designed to stall. Their rated locked-rotor current from the nameplate is used.
• Motors in listed equipment. When a motor is part of listed equipment that is marked with its own ratings, such as an air conditioner under Article 440 or a machine under Article 670, the equipment marking governs. Lessons 6 and 11 cover those.

There is also a rule for motors rated in amperes rather than horsepower. When a motor is marked only in amperes, the marked current is used as the full-load current, and the horsepower is found from the table that corresponds to that current where a horsepower is needed, for example to choose a horsepower-rated disconnect.

Other information on the nameplate

Although the nameplate current is used only for overloads, the rest of the nameplate is essential, and Section 430.7 lists what must be on it. On exams you are expected to know what the main markings mean and where they are used.

• Service factor. A number such as 1.15 means the motor can carry 15% more than its rated load continuously without damage. Overload sizing in Lesson 4 depends on it.
• Temperature rise or insulation class. A motor marked with a temperature rise of 40 °C or less is treated like a motor with a high service factor when overloads are sized.
• Code letter. A letter from A to V indicates the locked-rotor kilovolt-amperes per horsepower, which tells you how hard the motor hits the system when it starts. Table 430.7(B) relates the letters to ranges. Locked-rotor current is used for sizing some disconnects and for checking voltage dip, not for the ordinary branch circuit calculation.
• Design letter. NEMA design letters such as B, C and D describe the speed-torque curve. Some short-circuit protection rules treat high-efficiency Design B motors differently.
• Duty. Most motors are continuous duty. Short-time or intermittent duty motors are sized with special factors in 430.22(E).

Ray's point about the inspector was practical. Gloria Haynes, Harlow's chief electrical inspector, checks overload heater or setting selections against the nameplate during a rough-in or final inspection, and she checks conductors and breakers against the table. A contractor who sizes everything from the nameplate will usually pass the overload check and fail the conductor check, and one who sizes everything from the table will set overloads too high and leave the motor unprotected.

Horsepower from current, and current from horsepower

Exam questions sometimes ask for a horsepower, for instance to select a horsepower-rated switch for a motor known only by its current. Read the table backwards: find the current in the right voltage column and take the horsepower of that row, moving to the next larger row when the current falls between rows. Another type of question asks you to compare the table current with a calculated current to show why the table is used. A calculation from horsepower needs efficiency and power factor, which vary by motor, and it always comes out below the table, which is one more reason the code uses the table.

# Illustrative motor currents used in this course (not NEC values; read your own table)
FLC = {("3ph", 460): {5: 7.6, 10: 14, 15: 21, 25: 34},
       ("3ph", 200): {10: 30.8},
       ("1ph", 230): {1: 8, 3: 17}}

def column(phase, system_volts):
    motor = {120: 115, 240: 230, 208: 200, 480: 460, 600: 575}[system_volts]
    return (phase, motor)

for name, phase, volts, hp in [("compressor", "3ph", 480, 25), ("saw motor", "3ph", 480, 15),
                               ("air handler", "3ph", 208, 10), ("hoist", "1ph", 240, 3)]:
    key = column(phase, volts)
    print(f"{name:12} {hp:>3} hp on {volts} V uses the {key[1]} V column: {FLC[key][hp]} A")

calc = 15 * 746 / (0.90 * 0.88) / (3 ** 0.5 * 460)
print("15 hp calculated from the formula:", round(calc, 1), "A versus 21 A from the table")

compressor    25 hp on 480 V uses the 460 V column: 34 A
saw motor     15 hp on 480 V uses the 460 V column: 21 A
air handler   10 hp on 208 V uses the 200 V column: 30.8 A
hoist          3 hp on 240 V uses the 230 V column: 17 A
15 hp calculated from the formula: 17.7 A versus 21 A from the table

How exam questions test this

Questions on motor currents fall into a few patterns, and recognizing them saves time on a timed exam.

• Table or nameplate. The question gives both a horsepower and a nameplate current and asks you to size a conductor, a breaker or overloads. Conductors and breakers use the table; overloads use the nameplate.
• Wrong voltage. The question names the system voltage, and you must pick the motor voltage column.
• Single-phase or three-phase. The table for a single-phase motor gives a very different value from the three-phase table at the same horsepower.
• An exception. The motor is multispeed, a shaded-pole fan with a marked current, or part of listed equipment, so the nameplate or equipment marking governs.
• Many motors. Several motors share a feeder, and you need each table current before you can apply 430.24 and 430.62 in Lessons 2 and 3.

Marcus Lee, Dana's study partner, kept a single card in his code book's tab for the motor tables: a list of the system voltages and their motor columns, and the words "table for wire and breakers, nameplate for heaters". It took ten seconds to read and prevented the most expensive mistakes.

Dana's motor schedule

Before the next site meeting Dana built a motor schedule for Delgado's new equipment, the standard tool for keeping motor calculations organized. Each row lists the load, horsepower, phase and voltage, the table full-load current, the nameplate full-load amperes when known, and columns for the results that later lessons fill in: conductor ampacity, short-circuit protection, overload setting and disconnect. A schedule like this is also how exam problems with several motors are best worked: one row at a time, with the table current entered first.