Accurately sizing motor overload relays is critical for protecting motors and ensuring compliance with NEC standards. This calculation prevents motor damage due to excessive current or thermal stress.
This article covers the NEC requirements, formulas, tables, and practical examples for calculating motor overload relay settings. It provides a comprehensive guide for engineers and electricians.
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Common Motor Full Load Currents and Overload Relay Settings According to NEC
The National Electrical Code (NEC) provides guidelines for sizing motor overload relays based on motor full load current (FLC). The overload relay setting is typically 115% to 125% of the motor FLC to allow for normal starting currents while protecting against sustained overloads.
| Motor HP | Voltage (V) | Full Load Current (A) | Overload Relay Setting (A) @ 115% | Overload Relay Setting (A) @ 125% |
|---|---|---|---|---|
| 1 | 230 | 5.0 | 5.75 | 6.25 |
| 1 | 460 | 2.5 | 2.88 | 3.13 |
| 5 | 230 | 14.0 | 16.1 | 17.5 |
| 5 | 460 | 7.0 | 8.05 | 8.75 |
| 10 | 230 | 28.0 | 32.2 | 35.0 |
| 10 | 460 | 14.0 | 16.1 | 17.5 |
| 15 | 230 | 40.0 | 46.0 | 50.0 |
| 15 | 460 | 20.0 | 23.0 | 25.0 |
| 25 | 230 | 60.0 | 69.0 | 75.0 |
| 25 | 460 | 30.0 | 34.5 | 37.5 |
Key Formulas for Motor Overload Relay Calculation According to NEC
Understanding the formulas behind motor overload relay settings is essential for accurate protection and NEC compliance. Below are the primary formulas used in calculations.
1. Full Load Current (FLC) Reference
The NEC provides standard FLC values for motors based on horsepower and voltage. These values are used as a baseline for relay settings.
2. Overload Relay Setting Calculation
The overload relay setting is typically set as a percentage of the motor’s full load current to allow for normal starting currents and transient conditions.
Overload Relay Setting (A) = Motor Full Load Current (A) × Setting Factor
- Motor Full Load Current (A): The rated current drawn by the motor at full load, from NEC tables or motor nameplate.
- Setting Factor: Typically between 1.15 (115%) and 1.25 (125%) depending on motor type and application.
3. Motor Full Load Current Calculation (If Nameplate Data is Unavailable)
When motor nameplate data is unavailable, the full load current can be estimated using the formula:
FLC (A) = (Motor Power (W)) / (√3 × Voltage (V) × Power Factor × Efficiency)
- Motor Power (W): Motor output power in watts (HP × 746).
- Voltage (V): Supply voltage (line-to-line for 3-phase motors).
- Power Factor (PF): Typically 0.85 to 0.95 for induction motors.
- Efficiency (η): Usually between 0.85 and 0.95 depending on motor design.
4. Overcurrent Protection Device Sizing
NEC Article 430 also specifies sizing for short-circuit and ground-fault protective devices, which must be coordinated with overload relay settings.
Maximum Overcurrent Device Rating (A) = Motor FLC × 250%
This 250% factor is a maximum allowed value for short-circuit protection devices, ensuring they do not trip during normal motor starting currents.
Detailed Real-World Examples of Motor Overload Relay Calculations
Example 1: Calculating Overload Relay Setting for a 10 HP, 460V, 3-Phase Motor
A 10 HP, 460V, 3-phase motor has a nameplate full load current of 14A. Determine the appropriate overload relay setting according to NEC guidelines.
- Step 1: Identify the motor full load current (FLC) from the nameplate or NEC Table 430.250: 14A.
- Step 2: Choose the setting factor. For standard motors, 115% (1.15) is typical.
- Step 3: Calculate the overload relay setting:
Overload Relay Setting = 14A × 1.15 = 16.1A
The overload relay should be set to approximately 16.1A to protect the motor while allowing for normal starting currents.
Example 2: Estimating Overload Relay Setting for a 15 HP Motor Without Nameplate Data
A 15 HP motor operates at 230V, 3-phase, with no nameplate data available. Assume a power factor of 0.9 and efficiency of 0.9. Calculate the overload relay setting.
- Step 1: Convert horsepower to watts:
Motor Power (W) = 15 HP × 746 = 11,190 W
- Step 2: Calculate the full load current (FLC):
FLC = 11,190 / (√3 × 230 × 0.9 × 0.9) ≈ 31.7 A
- Step 3: Select the overload relay setting factor (115%):
Overload Relay Setting = 31.7 × 1.15 ≈ 36.5 A
The overload relay should be set to approximately 36.5A to ensure proper motor protection.
Additional Technical Considerations for Motor Overload Relay Calculations
Beyond basic calculations, several factors influence the selection and setting of motor overload relays to ensure optimal protection and NEC compliance.
- Motor Starting Current: Motors draw 5 to 7 times full load current during startup. Overload relays must tolerate this without nuisance tripping.
- Ambient Temperature Compensation: Overload relays are temperature sensitive. Adjustments or compensation may be required in high ambient temperature environments.
- Motor Service Factor: Motors with a service factor greater than 1.0 can tolerate overloads better, potentially allowing higher relay settings.
- Type of Motor: Different motor types (e.g., squirrel cage, wound rotor) have varying starting and running characteristics affecting relay settings.
- NEC Article 430 Compliance: Ensure all calculations and device selections comply with NEC Article 430, which governs motor circuits and protection.
Responsive Table: NEC Motor Full Load Currents for Various HP and Voltages
| Horsepower (HP) | Voltage (V) | Full Load Current (A) | Typical Overload Relay Setting (A) |
|---|---|---|---|
| 0.5 | 230 | 2.1 | 2.4 |
| 0.5 | 460 | 1.1 | 1.3 |
| 3 | 230 | 9.0 | 10.4 |
| 3 | 460 | 4.5 | 5.2 |
| 7.5 | 230 | 18.0 | 20.7 |
| 7.5 | 460 | 9.0 | 10.4 |
| 20 | 230 | 50.0 | 57.5 |
| 20 | 460 | 25.0 | 28.8 |
Understanding NEC Article 430 for Motor Overload Protection
NEC Article 430 governs the installation, protection, and control of motors. It specifies requirements for overload protection devices to prevent motor damage due to overheating.
- Section 430.32: Requires overload protection for motors to prevent damage from sustained overload currents.
- Section 430.32(B): Specifies that overload relays must be set at not more than 115% of the motor full load current unless otherwise permitted.
- Section 430.52: Details sizing of short-circuit and ground-fault protective devices, which must coordinate with overload protection.
- Section 430.122: Addresses the use of inverse time circuit breakers as overload protection under certain conditions.
For detailed NEC guidelines, refer to the official NEC documentation: NFPA NEC Standards.
Summary of Best Practices for Motor Overload Relay Calculations
- Always use motor nameplate data or NEC tables for accurate full load current values.
- Set overload relays between 115% and 125% of motor full load current, depending on motor type and application.
- Consider ambient temperature and service factor adjustments to avoid nuisance tripping or insufficient protection.
- Coordinate overload relay settings with short-circuit and ground-fault protective devices per NEC Article 430.
- Use AI calculators or software tools to streamline calculations and ensure compliance.
Proper motor overload relay calculation is essential for motor longevity, safety, and NEC compliance. This guide provides the technical foundation and practical tools for accurate sizing.