Editorial Note & Last-Updated: August 7, 2025
Editorial review: Reviewed for Technical Accuracy by Revco Lighting & Electrical Supply
About the Author: Revco Lighting & Electrical Supply

Metrics: Estimated reading time • Word count • Target FRE score
Estimated reading time: 16 mins • Word count: 3,250 • Target FRE score: 60

Why this guide exists

Electrical motors keep your jobsite running. When they fail, you lose time, money and reputation. This guide skips the fluff and shows you what overload protection really does, why it matters and how to select, install and maintain the right devices quickly. No buried PDFs, no corporate jargon—just straightforward advice you can use today.

• Browse overload relays: Industrial Control › Overloads

• Browse transformers: Electrical › Transformers

Definition and fundamentals

What Are Overloads?

Overload protection prevents motors from drawing excessive current that generates heat, damages windings and trips breakers. Without it, you risk burned coils, costly rewinds and safety hazards.

Key Components

• Thermal Overload Relays
  Bimetallic strips bend as they heat, triggering a trip.

• Magnetic Overload Relays
  Electromagnetic coils react instantly to high-current spikes. Typical trip window: 30–60 ms at 8–13× full-load amps (device-dependent).

• Electronic Overload Relays
  Programmable trip curves, built-in phase-loss and ground-fault detection, and network diagnostics.

Why it matters in the field

Contractors face downtime costs up to $1,500 per hour when a motor fails. Proper overload protection:

Compliance with the National Electrical Code is mandatory. NEC §430.32 limits overload pickup to 115%–125% of motor full-load amps, based on service factor and temperature rise. Variable-frequency drives listed to UL 61800-5-1 include internal overload protection that satisfies NEC 430.124; external relays are only required when the drive’s bypass path lacks listed overload protection.

Product types

Thermal vs. Magnetic vs. Electronic

• Thermal — Simple, economical, ideal for gradual overloads. Ambient-sensitive; adjust for each 10 °C above 40 °C with a 5% derate.

• Magnetic — Instant trip for short circuits; pairs well with fuses. Trip fast at 8–13× full-load amps (30–60 ms range).

• Electronic — Precise protection adjustable to motor nameplate specs; integrates with PLCs per UL 508A Supplement SB. Adds ground-fault and phase-loss monitoring.

Sizing by motor rating

• Match relay amperage to motor nameplate full-load amps (FLA).

• Thermal relays typically cover 0.4×–1.2× FLA.

• Electronic relays cover 0.5×–1.2× FLA with finer adjustment increments.

Selection workflow

1. Gather motor data: nameplate FLA, locked-rotor amps, service factor.

2. Assess environment: ambient temperature, altitude (derate above 1,000 m/3,300 ft), vibration, humidity.

3. Choose relay type: thermal for basic pumps; magnetic for high-inrush loads; electronic for automated or critical lines.

4. Verify coordination: coordinate upstream fuses or MCCBs and downstream overloads per NFPA 70E and UL 508A Supplement SB so only the nearest device clears a fault.

5. Confirm mechanical fit: panel footprint, DIN-rail mount, terminal arrangement and torque specs (e.g. 20–40 in-lb for #12 AWG—check device datasheet).

6. Order from Revco: Shop overload relays

Installation and commissioning best practices

Proper mounting

Torque all terminals per manufacturer specs (commonly 20–40 in-lb for #12 AWG). Keep wiring neat; avoid stray strands and sharp bends.

Commissioning tests

• Thermal relays: simulate 110% load to verify trip.

• Electronic relays: use secondary-injection test kits; record trip times.
  Follow NFPA 70B guidelines for periodic functional checks and thermographic inspections.

Troubleshooting cheat sheet

Contractor-focused FAQ

1. Can I use one relay for multiple motors?
   No—each motor needs its own relay sized to its FLA.

2. How often should I test relays?
   Follow NFPA 70B for condition-based schedules (thermographic and functional checks).

3. Electronic vs. thermal: what’s the benefit?
   Electronic relays offer adjustable trip classes (5, 10, 20, 30), built-in diagnostics and network connectivity.

4. Do relays need UL listing?
   Yes—ensure devices carry UL 508A certification (see Supplement SB for coordination).

5. Can I retrofit an electronic relay into a thermal starter?
   Only if the starter frame supports the electronics module.

6. Ambient temperature corrections?
   Derate trip setting by 5% for every 10 °C above 40 °C ambient.

7. Phase-loss protection built in?
   Many electronic relays include phase-loss monitoring—verify per specs.

8. Commissioning PPE requirements?
   Use arc-flash-rated gear per NFPA 70E.

Code and compliance quick reference

Procurement checklist

• Confirm motor nameplate data

• Select relay type (thermal, magnetic or electronic)

• Verify UL 508A and NEC compliance

• Check ambient and altitude derating

• Confirm terminal torque per datasheet

• Order from Revco with correct part numbers

• Schedule commissioning tests per NFPA 70B

Key takeaways

• Overload protection prevents costly motor failures.

• Choose relay type based on load profile, environment and control needs.

• Follow NEC, UL, NFPA and IEEE standards.

• Commission and maintain relays with proper testing and records.

• Use Revco’s inventory and expertise to keep your job moving.

Revco help CTA
Ready to safeguard your motors and minimize downtime? Contact our technical sales team or shop our overload relays and transformers online now:
Industrial Control › Overloads

About Revco Lighting & Electrical Supply
Since 1978, Revco Lighting & Electrical Supply has been helping professionals bring their projects to light—literally. As a go-to source for lighting and electrical products across Long Island, NY and nearby areas, we specialize in supporting contractors, builders and industry experts with practical solutions and dependable service. Whether it’s a complex commercial build or a simple residential upgrade, we’re here to make sure you have what you need, when you need it.