Master Electrician Practice
Chapter VI

Motors & Controls

Master Electrician Practice study guide with diagrams.

Motors & Controls — Master Exam Study Chapter

For the Delaware Master Electrician Exam (DE-MST) — 2023 NEC (NFPA 70)

Learning Objectives

By the end of this chapter, you will be able to:

6.Apply the general requirements of Article 430 to motor branch circuits, feeders, and controllers, including the correct use of Tables 430.247 through 430.250 for full-load currents.
7.Size motor branch-circuit conductors, short-circuit and ground-fault protection, and overload protection using the correct percentages and the next-higher-standard-size rule.
8.Calculate motor feeder demand loads per 430.24 and apply the 125% continuous load factor correctly.
9.Identify the requirements for motor controllers, disconnecting means, and control circuits, including the exceptions for cord-and-plug-connected motors.
10.Navigate the interaction between motor loads and the service/feeder calculations of Article 220, including the largest-motor 25% adder.
11.Recognize common field inspection failures and exam traps related to motor nameplate ratings versus NEC tables, dual-element fuse applications, and thermal protector requirements.

1.1 The Scope of Article 430 — Beyond the Nameplate

Article 430 is the single most referenced article for motors, yet it is also the most misunderstood. The critical distinction a master must make is between nameplate full-load current (FLC) and the table values in 430.247–430.250. The NEC tables are used for conductor sizing, short-circuit and ground-fault protection, and disconnecting means ratings. The nameplate is used for overload protection and for the actual connected load in feeder calculations.

Exam Trap: When a motor nameplate shows a current lower than the table value, you must still use the table value for branch-circuit conductor sizing. The nameplate value is only used for overload relay selection (430.32) and for the motor load in the feeder/service calculation (430.24 Exception 1 allows nameplate if it is larger than table).

1.2 Motor Branch-Circuit Conductors — 430.22

Motor Branch Conductors: 125% of FLC — NEC 430.22 Motor Branch Conductors: 125% of FLC NEC 430.6(A)(1) + 430.22(A) — 2023 NEC / NFPA 70 MOTOR NAMEPLATE 40 hp, 460 V, 3-phase Nameplate FLC: 54 A (high SF / high eff) Table 430.250 40 hp @ 460 V → FLC = 52 A Code table wins — never nameplate for conductor sizing ignore STEP 1 — FLC FLC = 52 A (Table 430.250) Not nameplate 54 A STEP 2 — 125% 52 A × 1.25 = 65 A NEC 430.22(A) No second 1.25 multiplier! STEP 3 — CONDUCTOR #6 Cu THWN — 65 A @ 75°C per Table 310.16 CONDUCTOR SIZING RESULT #6 Cu THWN — 65 A capacity 60°C col: 55 A → too small 75°C col: 65 A → ✓ correct ⚠ KEY NUANCE 125% already covers motor running current. Do NOT also apply continuous-load 125%. ✘ COMMON TRAP Nameplate 54 A × 1.25 = 67.5 A Seems bigger — but code tables win! Next: 430.52 SC/GF device sizing → M Master Electrician Practice — NEC 430.22 motor branch-circuit conductor sizing (2023 NEC / NFPA 70)

Single motor conductors must have an ampacity of 125% of the motor FLC from the appropriate table (430.22(A)). This is not a continuous load factor per se; it is a minimum sizing rule that accounts for the motor’s starting and running characteristics.

For multiple motors on one branch circuit (430.22(C)), the conductors must be sized at 125% of the largest motor FLC plus the sum of the FLCs of all other motors on that circuit.

Critical nuance: For a motor with a service factor of 1.15 or greater, or a temperature rise of 40°C or less, the branch-circuit conductors are still sized at 125% of the table FLC — the nameplate service factor does not change conductor sizing. However, the overload protection (430.32(A)(1)) may be set at 125% of nameplate for those conditions.

Supervision Point: On site, verify that the conductor ampacity is based on the 75°C column of the ampacity table (Table 310.16) when terminals are rated 75°C. A common error is using the 90°C column for the entire circuit — the 90°C column is only permitted for derating purposes, not for the final ampacity at the termination.

1.3 Overload Protection — 430.32

Overload protection is designed to protect the motor, the branch-circuit conductors, and the motor control apparatus from excessive heating due to running overloads and stalled rotors. It is not short-circuit protection.

Three tiers of overload protection:

27.Separate overload device (thermal overload relay, electronic relay): Must be set at no more than 125% of the motor nameplate full-load current for motors with a service factor of 1.15 or greater, or with a temperature rise of 40°C or less. For all other motors, the setting must not exceed 115% of nameplate (430.32(A)(1) and (A)(2)).
28.Integral thermal protector: If the motor has an approved integral thermal protector, the protector must be listed to prevent dangerous overheating due to overload and failure to start (430.32(A)(2) exception).
29.Built-in thermal protector (impedance-protected motors): For motors that cannot be overloaded (e.g., torque motors), the branch-circuit short-circuit protection may serve as the overload protection if the motor is marked "thermally protected" (430.32(B)).

The "next higher standard size" rule (430.32(C)): If the motor nameplate current multiplied by the percentage (125% or 115%) does not correspond to a standard fuse or heater size, you may go up to the next standard size. Standard sizes are listed in 240.6(A). For fuses and inverse-time circuit breakers, the next higher size is permitted. For adjustable trip relays, the next higher setting is permitted, but not to exceed the next standard size.

Exam Trap: The 125% overload rule applies to the nameplate, not the table FLC. If a motor nameplate says 10 A but Table 430.248 says 12 A, the overloads are set at 10 A × 1.25 = 12.5 A (next standard size up). The conductors are sized at 12 A × 1.25 = 15 A.

1.4 Short-Circuit and Ground-Fault Protection — 430.52

Motor SCGF: Four Devices, Four Ceilings Motor SCGF: Four Devices, Four Ceilings 50 hp, 460 V motor — FLC 65 A (Table 430.250) — NEC 2023 Art. 430 MOTOR 50 hp, 460 V FLC = 65 A Non-TD Fuse Table 430.52 300% × 65 A = 195 A 430.52(C)(1) Ex 1 Next higher std: 200 A fuse Dual-Elem TD Table 430.52 175% × 65 A = 113.75 A 430.52(C)(1) Ex 1 Next higher std: 125 A TD fuse Inverse-Time Brkr Table 430.52 250% × 65 A = 162.5 A 430.52(C)(1) Ex 1 Next higher std: 175 A breaker Inst-Trip Brkr Table 430.52 800% × 65 A = 520 A 430.52(C)(1) Ex 1 Next higher std: 600 A breaker ⚠ THE TRAP — MAXIMUMS ARE NOT TARGETS These ceilings protect the branch-circuit conductors — NOT the motor. They are the largest permitted SCGF device, not a recommendation. Sizing UP to the maximum is legal but reduces component protection. Sizing DOWN to the minimum that handles starting current is the better design practice. ✓ OVERLOAD PROTECTION (430.32) — SEPARATE AND LOWER Overload relays protect the motor from running overcurrent — sized at 115–125% of FLC (per 430.32): 65 A × 1.25 = 81.25 A → next standard size per 430.32(A)(1) Design sequence: 430.6(A) FLC → 430.22 conductor → 430.52 SCGF ceiling → 430.52(C)(1) Ex 1 selection → 430.32 overload Master Electrician Practice — NEC 430.52 Motor Branch-Circuit Short-Circuit & Ground-Fault Protection Standard ratings per 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600

This is the device that protects the branch-circuit conductors, the controller, and the motor against short circuits and ground faults. It is not sized to protect the motor itself from overloads.

Maximum permitted ratings (430.52(C)(1), Table 430.52):

Non-time-delay fuses: 300% of motor FLC (table value)
Dual-element (time-delay) fuses: 175% of motor FLC
Inverse-time circuit breaker: 250% of motor FLC (instantaneous trip breakers have different rules — see 430.52(C)(3))

The "next higher standard size" rule for SC/GF protection (430.52(C)(1) Exception 1): If the calculated value (e.g., 175% of FLC) does not correspond to a standard fuse or breaker, you may go up to the next standard size. However, if the motor will not start with that size, you may go higher, but not exceeding 400% for non-time-delay fuses, 225% for dual-element fuses, and 400% for inverse-time breakers (Exception 2(b)).

Exam Trap: The 400% maximum for non-time-delay fuses and breakers is a hard ceiling. You cannot exceed it even if the motor stalls on start. The only exception is for motors that are part of an approved assembly where the manufacturer has listed specific protection.

Supervision Point: Check that the fuse size is marked on the disconnect or the controller. A common violation is installing a 30 A fuse where a 20 A dual-element fuse is required, because the installer used the next standard size incorrectly.

1.5 Motor Controllers — 430.81 to 430.90

A motor controller is any device that governs the electric power to the motor. This can be a manual motor starter, a magnetic contactor, a variable frequency drive (VFD), or even a general-use snap switch for motors rated 2 hp or less (430.83(C)).

Controller rating (430.83): The controller must have a horsepower rating at the applied voltage that is not less than the motor's horsepower rating. For a motor rated more than 100 hp, or for a controller rated more than 100 hp, the controller may be marked "100 hp" and used for any motor up to that rating if the controller has a current rating at least equal to the motor's FLC.

Controller for a motor with a VFD (430.120): The VFD itself is the controller. It must be rated for the motor's FLC and for the specific application (constant torque vs. variable torque).

Control circuits (430.71): Control circuits (e.g., start/stop pushbutton wiring) must be protected against overcurrent. If the control circuit is tapped from the motor branch circuit, the tap must have overcurrent protection rated no more than 300% of the control circuit conductor ampacity (430.72(B)(2)). If the control circuit is supplied from a separate source, it must have its own overcurrent protection.

Exam Trap: A control transformer supplying a 120 V control circuit from a 480 V motor branch circuit must have its secondary protected at no more than 125% of the secondary conductor ampacity (430.72(C)(5)). Many installers forget the secondary protection.

1.6 Disconnecting Means — 430.101 to 430.113

Each motor must have a disconnecting means that is capable of disconnecting the motor and the controller from all ungrounded supply conductors (430.102). The disconnect must be:

Within sight of the motor and the driven machinery (430.102(A)). "Within sight" means visible and not more than 15 m (50 ft) away.
Capable of being locked in the open position (430.102(B)).
Rated at least 115% of the motor FLC (430.110(A)). For a motor with a nameplate FLC of 20 A, the disconnect must be rated at least 23 A — so a 30 A disconnect is the minimum standard size.

Exception for cord-and-plug-connected motors (430.109(C)): If the motor is cord-and-plug-connected and the plug/receptacle is within sight of the motor, the plug/receptacle may serve as the disconnecting means, provided it is rated for the motor's FLC.

Supervision Point: For a motor with a separate controller (e.g., a magnetic starter in a panel), the disconnect must be within sight of the controller and the motor. If not within sight of both, a second disconnect is required at the motor location.

1.7 Motor Feeder Sizing — 430.24 and 430.25

Motor Feeder: 125% Largest Plus the Rest — NEC 430.24 & 430.62(A) Motor Feeder: 125% Largest + 100% Rest — NEC 430.24 Feeder ampacity vs. feeder OCPD — 460V, 3-phase, 3 motors | 430.62(A) ceiling MOTOR 1 10 hp FLC = 14 A MOTOR 2 15 hp FLC = 21 A MOTOR 3 25 hp FLC = 34 A LARGEST 60 A TD 30 A TD 20 A TD #12 Cu #10 Cu #8 Cu 90 A FUSE 430.62(A) #4 Cu THWN SOURCE 460 V 3-phase FEEDER AMPACITY — NEC 430.24 I_feeder = 1.25 × I_largest + ΣI_others = 1.25(34) + 21 + 14 = 77.5 A Conductor: 75°C column → #4 Cu THWN Ampacity: 85 A ≥ 77.5 A ✓ FEEDER OCPD — NEC 430.62(A) Max = largest branch device + Σother FLCs = 60 A + 21 A + 14 A = 95 A Next standard: 90 A fuse (≤ 95 A ceiling) MASTER DEPTH NOTE Feeder OCPD (90 A) exceeds feeder ampacity (85 A) — this is deliberate. Running overloads are handled at each motor by 430.32, not the feeder device. Branch OCPD per 430.52 (TD fuses shown) Feeder conductor Feeder OCPD Motor branch ■ 430.32 overloads at motor Master Electrician Practice — NEC 430.24 feeder ampacity & 430.62(A) feeder OCPD (2023 NEC) FEEDER 125% largest + rest

Motor feeders supply multiple motors. The feeder conductors must have an ampacity of 125% of the largest motor FLC plus the sum of the FLCs of all other motors on the feeder (430.24). This is the same rule as for multiple motors on a branch circuit, but applied at the feeder level.

Important interaction with Article 220: For service and feeder calculations under 220.50, the motor load is calculated at 125% of the largest motor plus the sum of all others. However, 220.50 also requires the largest motor 25% adder — which is effectively the same calculation. The key difference is that 220.50 applies to the service calculation, while 430.24 applies to the feeder calculation. For a master, the practical result is the same: the largest motor gets a 25% adder.

Feeder short-circuit and ground-fault protection (430.62): The feeder protection must be sized at the largest branch-circuit protective device rating for any motor on the feeder, plus the sum of the FLCs of all other motors. The "next higher standard size" rule does not apply here — you must use the next lower standard size if the calculated value is not standard.

Exam Trap: If you have a feeder with a 30 A branch-circuit fuse for motor #1 and a 20 A branch-circuit fuse for motor #2, the feeder protection is 30 A + (FLC of motor #2). If that sum is 42 A, you must use a 40 A fuse (next lower), not a 45 A (next higher).

1.8 Motor Loads in Service Calculations — 220.50

For a commercial or industrial service, motor loads are added to the general lighting and receptacle loads. The motor load is the largest motor FLC × 1.25 plus the sum of all other motor FLCs. This is the same 25% adder that applies to the largest motor.

Critical distinction: For a service supplying only motors (e.g., a pump house), the service conductors are sized per 430.24, not per the general Table 220.12 lighting load. The 25% adder is already built into 430.24.

Supervision Point: When reviewing a set of plans, verify that the service calculation includes the motor 25% adder. A common omission is adding the motor FLC at 100% and forgetting the 125% factor on the largest motor.

1.9 Generators and Separately Derived Systems — 445 and 700

Generators are treated as separately derived systems when they have no direct connection to the utility source (445.10). The generator's output conductors must be sized per the generator nameplate rating, not the prime mover rating (445.13). The generator must have a disconnecting means that is capable of carrying the full load current (445.18).

Overcurrent protection for generators (445.12): Generators must be protected against overloads and short circuits. If the generator is a standalone unit, the overcurrent device must be rated at not more than 115% of the generator's rated current. If the generator is connected to a bus with other sources, the protection must be coordinated with the other sources.

Exam Trap: A generator rated 100 kW at 480 V three-phase has a rated current of approximately 120 A (100,000 ÷ (480 × 1.732)). The overcurrent device must be rated at 115% = 138 A, so a 150 A breaker is the minimum standard size. Many installers use a 125 A breaker, which is too small.

Transfer switches (700.5): For emergency systems, the transfer switch must be listed for emergency use and must be mechanically held. The generator must be able to supply the emergency load within 10 seconds of a power failure (700.12).

1.10 Variable Frequency Drives (VFDs) — 430.120 to 430.130

VFDs are now the standard method for motor speed control in commercial and industrial applications. The NEC treats a VFD as both the controller and the overload protection device.

Key requirements:

Conductors between the VFD and the motor (430.122): These conductors must have an ampacity of at least 125% of the motor FLC. However, the VFD output is not sinusoidal — it is a PWM waveform. The conductors must be rated for the VFD's output current, which may be higher than the motor FLC due to harmonics.
Overload protection (430.124): The VFD must provide overload protection for the motor. If the VFD is not provided with electronic overload protection, separate overload relays are required.
Short-circuit protection (430.128): The VFD must be protected by fuses or a circuit breaker rated per the VFD manufacturer's instructions. The VFD's input current is typically lower than the motor FLC, so the branch-circuit protection is based on the VFD rating, not the motor.

Supervision Point: Verify that the VFD is programmed for the correct motor FLC. A VFD set to 10 A for a motor with a nameplate FLC of 12 A will trip on overload during normal operation. A VFD set to 15 A will not protect the motor.

1.11 Code Navigation — Where to Find It

ConceptNEC Reference
Motor full-load currents (tables)Table 430.247 (DC), 430.248 (single-phase AC), 430.250 (three-phase AC)
Branch-circuit conductors430.22(A), 430.22(C)
Overload protection430.32, 430.36
Short-circuit/ground-fault protection430.52, Table 430.52
Standard fuse/breaker sizes240.6(A)
Controllers430.81–430.90
Disconnecting means430.101–430.113
Motor feeders430.24, 430.25, 430.62
Motor loads in service calc220.50, 220.14(C)
Generators445.10–445.18
Emergency systems700.5, 700.12
VFDs430.120–430.130
Conductor ampacity tablesTable 310.16
Continuous load factor (general)210.19(A)(1), 215.2(A)(1)

1.12 Inspection and Supervision Points

As a master electrician, you are responsible for the final sign-off. On any motor installation, verify:

90.Conductor sizing is based on the table FLC, not the nameplate, and the 75°C column is used at terminations.
91.Overload relays are set to the nameplate FLC × 1.25 (or 1.15) and are the correct heater size for the ambient temperature.
92.Branch-circuit fuses/breakers are the correct type (dual-element vs. non-time-delay) and size per Table 430.52.
93.Disconnect is within sight of the motor and controller, is lockable, and has a horsepower rating at least equal to the motor.
94.Feeder protection is sized per 430.62 — and remember, no "next higher" rule for feeders.
95.Control circuit transformer secondary is protected.
96.VFD programming matches the motor nameplate and the application.

1.13 Common Exam Traps

Nameplate vs. table: Always use the table for conductors and SC/GF protection; use the nameplate for overloads.
125% vs. 115%: 125% is for motors with SF ≥ 1.15 or temp rise ≤ 40°C; 115% is for all others.
Next higher vs. next lower: The "next higher" rule applies to branch-circuit SC/GF protection and overloads; the "next lower" rule applies to feeder protection.
Feeder SC/GF: Do not add 25% to the largest motor for the feeder SC/GF calculation — that adder is only for conductor sizing.
Generator overcurrent: 115% of the generator's rated current, not the conductor ampacity.
VFD branch circuit: The SC/GF protection is based on the VFD rating, not the motor FLC.

1.14 Summary

Mastering motors and controls requires a clear separation of the three distinct protection functions — overload, short-circuit/ground-fault, and disconnecting means — and knowing which code table or nameplate value drives each calculation. The master electrician must also understand how motor loads integrate into the broader service and feeder calculations under Article 220. By memorizing the key percentages (125%, 115%, 175%, 250%, 300%) and the exceptions for next-higher sizes, you will be prepared for the calculation-heavy questions on the Delaware Master exam. Always verify the 2023 NEC adoption date (January 1, 2026, for Delaware) and use the current tables during the open-book exam.

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