Guide to Motor Overload Protection Questions on NEC Electrical Exams

Motor overload protection is a critical safety component in electrical systems, designed to protect motors from damage due to excessive current draw. Understanding how to properly size overload protection is essential for electricians, as improper sizing can lead to motor failure or create fire hazards. These questions frequently appear on journeyman, master, and contractor exams, testing your ability to apply NEC Article 430 correctly.

‍

Example Motor Overload Protection Questions on NEC Electrical Exams

‍

A 25 hp, 460 V, 3Ø motor with a nameplate full-load current rating of 29 amps, a 1.15 service factor, and a 50°C (122°F) ambient temperature rating requires overload protection devices rated no more than ___ amperes where separate overload protection is used.

For a 50 hp, 208 V, 3Ø motor with a nameplate full-load current rating of 147.1 and a 1.0 service factor, rated for an ambient temperature of 30°C (86°F), the maximum amperage setting that can be used for its overload protection is ___ amperes where separate overload protection is being used.

A 100 hp, 575-volt, three-phase motor with a nameplate full-load current rating of 90 amps marked service factor of 1.15 in a chemical plant fails to start due to the overload device rating. The maximum permitted rating of the overload device to allow motor starting becomes ___ amperes.

‍

How to Identify a Motor Overload Protection Question on NEC Electrical Exams

‍

Key phrases to look out for:

• "Overload protection"
• "Service factor"
• "Temperature rise"
• "Ambient temperature"
• "Motor full-load current"
• "Maximum overload device rating"
• "Motor nameplate current"
• "Starting issues" or "fails to start"

When you spot these elements:

• Confirm it's a motor overload question (not short-circuit or ground-fault protection)
• Refer to NEC section 430.32
• Pay special attention to service factor and temperature rise markings

‍

Motor Overload Protection Articles: NEC 430.32

‍

To correctly apply NEC Article 430.32, focus on these main elements:

1. Standard Protection (430.32(A)(1)):
   
   • Motors with service factor ≥ 1.15: Maximum 125% of nameplate current
   • Motors with temperature rise ≤ 40°C: Maximum 125% of nameplate current
   • All other motors: Maximum 115% of nameplate current
2. Starting/Load Issues (430.32(C)): If standard protection is insufficient for starting or carrying the load:
   
   • Motors with service factor ≥ 1.15: Maximum 140% of nameplate current
   • Motors with temperature rise ≤ 40°C: Maximum 140% of nameplate current
   • All other motors: Maximum 130% of nameplate current
3. The nameplate rating of the motor

‍

Walkthrough for a NEC Electrical Exam Motor Overload Question

‍

Question:

A 100 hp, 575-volt, three-phase motor with a nameplate full-load current rating of 90 amps and a marked service factor of 1.15 in a chemical plant fails to start due to the overload device rating. The maximum permitted rating of the overload device to allow motor starting becomes ___ amperes.

Step 1: Find Full-Load Current (FLC)

• Locate the motor nameplate current rating
• Nameplate full-load current rating: 90 amps

Step 2: Determine Applicable Multiplier

• Motor has starting issues, so refer to 430.32(C)
• Service factor is 1.15 (≥ 1.15)
• This qualifies for the 140% allowance

Step 3: Calculate Maximum Overload Protection

• Maximum rating ≤ FLC × multiplier
• 90 amperes × 1.40 = 126 amperes

Step 4: Verify Final Answer

• Confirm service factor qualifies for 140%
• Check that no temperature limitations apply
• Final answer: 126 amperes

Additional Tips:

1. Always check if there are starting issues first
2. For starting issues:
   
   • Look for service factor ≥ 1.15 OR temperature rise ≤ 40°C to allow 140%
   • If neither condition is met, limit to 130%
3. Remember that Table 430.250 is specifically for three-phase motors
4. When in doubt, use the more conservative multiplier

‍