This article explains NEC electric motor protection, specifying where in the NEC the information is located so that any doubts can be addressed by reviewing each section in more detail.
The National Electrical Code (NEC) establishes that electric motors must be protected against overload, short circuits, and ground faults. This is primarily addressed in Article 430, which covers everything related to electric motors, conductors, protection devices, and control equipment.
2. Overload Protection, motor protection NEC
Overload protection safeguards the motor against currents that exceed its rated capacity for an extended period.
- Reference: Article 430.32.
- Requirement: Overload devices must be set at 115% of the motor’s full-load current for most motors, as indicated in Table 430.248 (single-phase motors), Table 430.250 (three-phase motors), or other applicable tables for specific motor types.
- A setting of 125% is used if the motor has a service factor greater than 1.15.
Example Calculation:
For a three-phase motor rated at 10 HP (7.5 kW) and 230 V:
- Full-load current: 28 A (per Table 430.250).
- Overload protection = 28 A×1.15=32.2 A
- Select a thermal relay or protective device set to 33 A.
This fulfills NEC requirements for motor overload protection.
3. Short-Circuit and Ground Fault Protection
Short-circuit and ground fault protection is provided by fuses or circuit breakers that quickly disconnect the circuit.
- Reference: Article 430.52.
- Requirement: Protection must be selected based on a specific percentage of the motor’s full-load current, depending on the type of protective device used. Typical values are:
- Non-time-delay fuses: 300% of the full-load current.
- Time-delay fuses: 175% of the full-load current.
- Circuit breakers: 250% of the full-load current.
Example Calculation:
For a three-phase motor rated at 10 HP and 230 V:
- Full-load current: 28 A.
- Time-delay fuse = 28 A×1.75=49 A28A×1.75=49A.
- Select a standard 50 A fuse.
4. Motor Feeder Conductors, motor protection NEC
Motor feeder conductors must be sized to handle at least 125% of the motor’s full-load current.
- Reference: Article 430.22.
- Requirement: Use the ampacity tables in the NEC (Table 310.16) to ensure the conductors can support the calculated current.
This ensures appropriate conductor selection—neither oversized (to save costs) nor undersized (to prevent overheating).
Example Calculation:
For a 10 HP motor (28 A) with a 125% adjustment:
- Conductor current: 28 A×1.25=35 A
- According to Table 310.16, a copper conductor with THW insulation, size #8 AWG, supports up to 40 A at 60°C and is suitable.
5. Grounding
Grounding is performed according to Article 250, which outlines the general requirements for grounding systems.
- Reference: Article 430.242 (specific to motors).
- Requirement: Select grounding conductors based on Table 250.122 and the size of the overcurrent protective device.
Example Grounding Conductor Sizing:
If the motor is protected by a 50 A fuse:
- According to Table 250.122, the grounding conductor must be #10 AWG copper.
6. Control Devices
Control devices such as manual switches or starters must comply with the requirements of Article 430.83 and must be rated to handle the motor’s starting current.
Reference Tables:
- Table 430.248: Full-load currents for single-phase motors.
- Table 430.250: Full-load currents for three-phase motors.
- Table 310.16: Ampacities of conductors based on insulation type.
- Table 250.122: Grounding conductor sizing.
For further details on electrical calculations, refer to the NEC directly for comprehensive information.
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