Okay, let's break down the expected temperature rise of a circuit breaker under load, considering your scenario. There isn't a single definitive answer because it depends on several factors, but we can make some educated estimates.

Key Assumptions



Proper Installation

We assume the circuit breaker is properly installed and tightened to the manufacturer's torque specifications. Loose connections are a major cause of excessive heat.

Breaker Condition

The circuit breaker is in good working condition and has not been previously overloaded or damaged.

Ambient Temperature

You'll need to define what the ambient temperature of the room is.

Enclosure Size and Ventilation

The size of the electrical panel enclosure and how well it is ventilated will affect the temperature rise. A small, tightly packed panel will retain more heat.

Conductor Type and Insulation

You specified THHN, which is good, but different insulation types have different temperature ratings. THHN is typically rated for 90°C in dry locations when properly sized for the load.

Theory and Expected Behavior


Conductor Temperature Rating

The goal is to keep the conductor's temperature below its rated limit (90°C for THHN in dry locations, but consult the NEC). If the breaker connection is good, and the conductor is properly sized for the load, the conductor should be the limiting factor, not the breaker itself.2.

Ohmic Heating (I²R Losses)

When current flows through a conductor, it generates heat due to the conductor's resistance. This is known as ohmic heating or I²R losses (current squared times resistance).3.

Heat Dissipation

The heat generated by the conductor and the circuit breaker needs to be dissipated into the surrounding environment. Factors affecting heat dissipation include: Convection: Air movement around the conductor and breaker. Conduction: Heat transfer through the conductor, breaker, and panel enclosure. Radiation: Heat radiated from the surfaces.4.

Circuit Breaker Temperature Rise

The circuit breaker itself will also generate a small amount of heat due to its internal resistance and the mechanical components. However, in a properly designed and functioning breaker, this heat generation should be relatively low.

Estimating Temperature Rise

This is where it becomes challenging to give an exact number without specific measurements and calculations. Here's a qualitative approach:

Minimal Rise with Perfect Conditions

If everything is perfect – a properly tightened connection, excellent ventilation, and a low ambient temperature (e.g., 25°C) – you might see a temperature rise of as little as

5-10°C above ambient at the breaker terminals under a continuous 20A load. More Realistic Scenario

In a more typical scenario with a moderate ambient temperature (e.g., 30°C), average ventilation, and a potentially slightly less-than-perfect connection, you might expect a temperature rise of

10-20°C above ambient. Warning Signs (Potential Problems)

A temperature rise exceeding

25-30°C above ambient under these load conditions would be cause for concern. It would suggest a loose connection, a damaged breaker, or another underlying issue.

Infrared (IR) Scanning

IR scanning is a valuable tool for identifying potential problems. Here's how to interpret the results:

Baseline

Establish a baseline temperature for similar breakers in the panel with similar loads. This provides a point of reference.

Temperature Differential

Look for significant temperature differences between similar breakers. A breaker that is significantly hotter than its neighbors warrants further investigation.

Hot Spots

Focus on the breaker terminals. Hot spots indicate a loose or corroded connection.

Important Considerations and Cautions



NEC Requirements

Always adhere to the National Electrical Code (NEC) and local codes. The NEC provides guidance on conductor sizing, ampacity derating, and other safety requirements.

Derating

If the conductors are bundled together in a conduit or cable tray, you may need to derate their ampacity to account for the increased heat buildup.

Continuous Load

A continuous load is defined as a load that operates for 3 hours or more. The NEC requires that circuit breakers and conductors be sized at 125% of the continuous load (80% de-rating). For a 20A breaker the maximum continuous load allowed would be 16 amps. Running at 20 amps continuous would technically be an over loaded condition.

Manufacturer's Specifications

Refer to the manufacturer's specifications for the circuit breaker and conductors. They may provide information on temperature limits and other relevant parameters.

Safety

Exercise caution when working with electrical equipment. De-energize the circuit before making any repairs or adjustments. If you are not qualified to work on electrical systems, consult a licensed electrician.

Thermal Imaging Limitations

IR scanning provides a surface temperature measurement. It doesn't necessarily reflect the temperature inside the conductor or the breaker.*

Emissivity

Account for the emissivity of the surfaces you are scanning. Emissivity is a measure of how well a surface radiates heat. Different materials have different emissivities, which can affect the accuracy of IR temperature measurements.

In summary: With a properly installed and sized circuit, a continuous 20A load on a 20A breaker wired with #12 THHN should result in a moderate temperature rise. Monitoring with an IR scanner can help identify potential problems, especially loose connections or overloaded breakers. If the rise is too high (above ~25-30C), investigate further. And remember, adhering to NEC guidelines and consulting with qualified professionals is always recommended. Flag for review