Motors, Transformers & Overcurrent Protection

Learning Objectives

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

4.Identify the correct NEC Article for motors, transformers, and overcurrent protection devices.

5.Apply the general requirements for sizing branch-circuit, feeder, and short-circuit/ground-fault protection for motors.

6.Calculate transformer primary and secondary overcurrent protection using the correct percentages and standard ampere ratings.

7.Distinguish between overload protection and short-circuit/ground-fault protection for motors.

8.Navigate the NEC tables for full-load currents (FLC) and standard fuse/breaker sizes.

9.Recognize common field installation errors and exam traps related to these topics.

1.1 The Scope of Articles 430 and 450

The NEC treats motors and transformers as distinct equipment with unique protection requirements. Article 430 governs all motor circuits and controllers, while Article 450 governs transformers and transformer vaults. Overcurrent protection is covered generally in Article 240, but specific applications for motors and transformers override these general rules.

A critical distinction: a motor has two separate protection needs. Overload protection (thermal) protects the motor from sustained overcurrent due to mechanical overload or stalled rotor. Short-circuit and ground-fault protection protects the conductors and equipment from high-level fault currents. These are not interchangeable. Overload devices are sized based on the motor's nameplate full-load current (FLC), while short-circuit devices are sized based on the motor's FLC from the NEC tables, not the nameplate.

1.2 Motor Full-Load Current: Tables vs. Nameplate

For all calculations involving motor branch circuits, feeders, and short-circuit protection, you must use the NEC Tables 430.247 through 430.250 to determine the motor's full-load current. These tables list FLC values for DC motors (Table 430.247), single-phase AC motors (Table 430.248), and three-phase AC motors (Tables 430.250). The nameplate current is used only for sizing overload relays and for adjusting the motor's actual thermal protection.

Exam Trap: A question will give you a motor nameplate current of 18 A, but the table value for that horsepower and voltage is 22 A. You must use 22 A for conductor sizing and short-circuit protection. Using the nameplate value will result in an undersized circuit.

1.3 Motor Branch-Circuit Conductors (430.22)

Branch-circuit conductors supplying a single motor must have an ampacity of not less than 125% of the motor's FLC as determined from the tables. This 125% factor accounts for the continuous nature of the motor load.

Example: A 10 HP, 208 V, three-phase motor has a table FLC of 30.8 A. The minimum conductor ampacity is 30.8 A × 1.25 = 38.5 A. You would select a conductor with an ampacity of at least 38.5 A, typically a #8 AWG copper at 75°C (rated 50 A).

For motors with a service factor of 1.15 or greater, or a temperature rise of 40°C or less, the conductors are still sized at 125% of the table FLC. The higher service factor only affects overload protection, not conductor sizing.

1.4 Motor Overload Protection (430.32)

Overload devices must protect the motor, motor control apparatus, and motor branch-circuit conductors against excessive heating. The primary rule is that the overload device must be sized at not more than 115% of the motor nameplate current for motors with a service factor of 1.15 or more, or a temperature rise of 40°C or less. For all other motors, the limit is 125% of the nameplate current.

If the chosen overload size is insufficient to start the motor (e.g., it trips during acceleration), the NEC permits a higher rating, but it cannot exceed 140% of the nameplate current for motors with a service factor of 1.15 or higher, or 130% for others. This is a common source of confusion; the initial sizing is 115%/125%, and the maximum permitted is 140%/130%.

Field Point: A journeyman will typically set overload relays based on the motor nameplate, but must verify the service factor. If a motor has a service factor of 1.0, the overloads are set at 115% of nameplate, not 125%.

1.5 Short-Circuit and Ground-Fault Protection (430.52)

This is the most calculation-heavy section for motors. The branch-circuit short-circuit and ground-fault protective device (typically a fuse or circuit breaker) must be sized based on a percentage of the motor's table FLC. The percentages vary by device type:

Non-time-delay fuses: 300% of FLC

Dual-element (time-delay) fuses: 175% of FLC

Inverse-time circuit breakers: 250% of FLC

Instantaneous-trip breakers: 800% of FLC (for motors other than Design B), or 1300% for Design B motors.

If the calculated value does not correspond to a standard fuse or breaker size (Table 240.6(A)), you may round up to the next standard size, but only if the motor can start without nuisance tripping. However, there is a hard cap: the maximum permitted size is 400% of FLC for non-time-delay fuses and instantaneous-trip breakers, and 225% of FLC for time-delay fuses and inverse-time breakers.

Exam Trap: A question will ask for the maximum permitted fuse size. You must first calculate the standard percentage (e.g., 175% of FLC), then round up to the next standard size, but you cannot exceed the 225% cap. If 175% of FLC is 40 A, you can use a 45 A fuse (next standard size). But if 175% of FLC is 60 A, you cannot use a 70 A fuse if the cap is 225% of FLC (which is 77 A) — you can, but you must check the cap. The cap is the absolute maximum.

1.6 Motor Feeder Conductors and Protection (430.24, 430.62)

A feeder supplying two or more motors must have an ampacity of not less than 125% of the largest motor FLC plus the sum of the FLCs of all other motors on that feeder. This is a simple addition problem, but the largest motor must be identified correctly.

The feeder short-circuit and ground-fault protective device is sized based on the largest branch-circuit protective device permitted for the largest motor, plus the sum of the FLCs of the other motors. This is a two-step process: first, calculate the maximum branch-circuit device for the largest motor (using the percentages in 430.52), then add the FLCs of all other motors.

Example: Feeder supplies Motor A (FLC 20 A) and Motor B (FLC 15 A). Motor A uses a time-delay fuse at 175% = 35 A (next standard size 35 A). Motor B uses a time-delay fuse at 175% = 26.25 A (next standard size 30 A). The feeder fuse is 35 A + 15 A = 50 A.

1.7 Transformer Overcurrent Protection (450.3)

Transformer protection is simpler than motor protection, but has its own traps. The primary rule is that transformers must be protected on the primary side only, unless specific conditions require secondary protection. The size of the primary overcurrent device depends on whether the transformer has primary and secondary protection or primary-only protection.

For a transformer with primary-only protection, the primary device must be sized at not more than 125% of the rated primary current for transformers over 600 V, and not more than 125% for transformers 600 V or less. If 125% does not correspond to a standard size, you may round up to the next standard size, but the maximum is 250% for primary-only protection.

For a transformer with both primary and secondary protection, the primary device can be sized at not more than 250% of the rated primary current, and the secondary device at not more than 125% of the rated secondary current. This is the most common configuration in commercial work.

Calculation: To find the transformer current, use the formula: I = (kVA × 1000) / (E × 1.732) for three-phase, or I = (kVA × 1000) / E for single-phase. You must know the transformer voltage and kVA rating.

Example: A 75 kVA, 480 V three-phase transformer has a primary current of (75 × 1000) / (480 × 1.732) = 90.2 A. With primary and secondary protection, the primary device can be 250% of 90.2 A = 225.5 A. You would select a 225 A fuse or breaker (standard size). The secondary protection would be based on the secondary voltage.

1.8 Standard Ampere Ratings (Table 240.6(A))

All overcurrent protection devices must be sized to standard ratings unless otherwise permitted. The standard ratings include: 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, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 A. You must memorize the common ones (15 through 800) for the exam.

Exam Trap: When rounding up, you may only round to the next standard size, not an arbitrary value. If your calculated value is 42 A, you can use a 45 A device, not a 50 A device unless the 45 A is not standard (it is).

1.9 Code Navigation: Where to Look

Article 240: General overcurrent protection; Table 240.6(A) for standard sizes.

Article 430: Motors, motor circuits, and controllers.

430.6: FLC vs. nameplate (use tables for calculations).

Tables 430.247–430.250: FLC values.

430.22: Branch-circuit conductor sizing (125%).

430.24: Feeder conductor sizing (largest motor + others).

430.32: Overload protection (115%/125% of nameplate).

430.52: Short-circuit and ground-fault protection (percentages of FLC).

430.62: Feeder short-circuit protection.

Article 450: Transformers.

450.3: Overcurrent protection for transformers.

Table 240.6(A): Standard fuse and breaker sizes.

1.10 Practical Field Points

Always check the motor nameplate for service factor. A motor with a 1.15 SF can have overloads set at 125% of nameplate; a motor with a 1.0 SF must be set at 115%. This is a common cause of nuisance tripping.

When replacing a motor, verify the FLC table value. A motor with a higher horsepower will require larger conductors and a larger breaker, even if the physical size is similar.

For transformers, the primary protection is often a fused disconnect. You must size the fuses based on the transformer kVA and the primary voltage, not the secondary load.

Never use a motor's nameplate current for conductor sizing. This is the most common code violation found in the field.

When installing a dual-element fuse for a motor, check the motor starting current. If the fuse trips on startup, you may need to increase the size, but you cannot exceed the 225% cap for time-delay fuses.

1.11 Common Exam Traps

76.Using nameplate FLC instead of table FLC. Always use the tables for conductor and short-circuit protection.

77.Forgetting the 125% factor for continuous loads. Motor branch circuits are continuous loads by definition.

78.Rounding up when not permitted. You can round up for short-circuit protection, but not for conductor sizing or overload protection (except in specific cases).

79.Confusing overload and short-circuit protection. Overloads are sized on nameplate; short-circuit devices are sized on table FLC.

80.Misapplying the 250% transformer rule. The 250% rule only applies when there is both primary and secondary protection. If primary-only, the limit is 125% (or 250% with rounding up, but only for transformers over 600 V).

81.Forgetting the secondary protection requirement. For transformers over 600 V, secondary protection is required unless the transformer is protected by a primary device sized at 125% or less.

82.Using the wrong voltage for transformer calculations. Always use the line-to-line voltage for three-phase transformers, not the line-to-neutral voltage.

1.12 Summary of Key Formulas

Motor branch conductor: FLC (table) × 1.25

Motor feeder conductor: (Largest motor FLC × 1.25) + (Sum of other motor FLCs)

Motor overload (SF ≥ 1.15): Nameplate × 1.25 (max 1.40)

Motor overload (SF < 1.15): Nameplate × 1.15 (max 1.30)

Motor short-circuit (time-delay fuse): FLC (table) × 1.75, round up, max 2.25 × FLC

Motor short-circuit (inverse-time breaker): FLC (table) × 2.50, round up, max 4.00 × FLC

Transformer primary current (3-phase): (kVA × 1000) / (E × 1.732)

Transformer primary protection (with secondary): Primary current × 2.50

Transformer secondary protection: Secondary current × 1.25

1.13 Final Exam Strategy

When you encounter a motor or transformer calculation on the exam, follow this sequence:

97.Identify the equipment type (motor or transformer).

98.Locate the correct FLC or current using the appropriate table or formula.

99.Determine which protection device you are sizing (overload, short-circuit, primary, or secondary).

100.Apply the correct percentage.

101.Round to the nearest standard size if permitted.

102.Check the maximum cap (e.g., 225% or 400%).

Do not skip the cap check. Many questions are designed to see if you know the absolute maximum, not just the standard calculation. If your calculated value exceeds the cap, you must use the cap value, not the calculated value.

Finally, remember that the NEC is a minimum standard. In the field, you may choose to install larger conductors or smaller breakers for operational reasons, but you must never go below the minimums calculated here. The exam will always test the minimum requirements.

Motors, Transformers & Overcurrent Protection