This lesson is physics, but it is the physics you can feel. Take it slowly once and you will never be confused by a thermal image again.
Heat is energy moving from a warmer thing to a cooler thing. Temperature is how hot something is. They sound the same in conversation and they are not the same thing.
A sparkler burns at well over a thousand degrees and will not hurt you much. A pot of boiling water is 100 °C and will put you in hospital. The sparkler is at a high temperature but carries almost no heat energy, because there is almost nothing to it. The water is cooler and carries far more energy.
Why an electrician cares: a small hot spot and a big warm one are different problems. A tiny lug at 90 °C and a whole bus bar at 60 °C are not ranked by temperature alone.
Current through resistance makes heat. The relationship has a name and it is worth memorising in this form:
P = I² × R
Power turned into heat equals current squared, times resistance.
Read the square carefully, because it is the whole story. Double the current and the heating does not double — it goes up four times. Triple it and it goes up nine times. This is why a connection that was "a little warm" last summer can be a failure this summer after the shop added a machine.
Now the resistance half. A properly torqued connection has very low resistance. A loose one has more: less metal actually touching, so current crowds through a smaller area. Corrosion does the same. So does a dissimilar metal joint gone bad, and an over-torqued lug that has deformed and relaxed.
Put the two together. The current through the loose lug is the same as through the good lugs — it is a series circuit, it has to be. But the resistance at that one point is higher. Same I, bigger R, so more watts turned into heat at that exact spot. That is the hot spot you see in the camera.
A connection problem is heat generated at a point. The conductor either side carries it away by conduction, which is why you see a bright spot that fades over a few inches rather than a uniformly hot cable.
A load problem looks different. If a circuit is simply carrying more current than it should, the heating is spread along the whole conductor, because the resistance is spread along the whole conductor. The cable is warm end to end.
Where the heat is tells you what kind of problem it is. That one sentence is most of electrical thermography.
A 200 amp subpanel feeds an outdoor lighting contactor. The maintenance tech reports the panel "smells warm" in the evenings.
The scan, taken at night with the lighting load on, shows the A-phase breaker lug at 84 °C. The B and C lugs are at 41 and 43 °C. Ambient inside the enclosure is about 35 °C.
Compare like to like: all three phases carry the same lighting load, all three are the same breaker, same wire, same enclosure, same air. Two agree. One does not. The difference is roughly 42 °C above its neighbours.
That is a connection finding, not a load finding, because the conductor feeding that lug reads the same as the other two. The heat is at the joint.
The repair was a retorque. The cause, found on teardown, was a lug that had never been torqued to spec at install four years earlier, and had slowly relaxed. It survived four winters at partial load and started to run away when the owner added a second lighting circuit — because the current went up, and the heating went up with the square of it.
Cold can be a finding. If one phase of a three-phase load reads much cooler than the others, that phase may not be carrying current. An open fuse or a dropped phase shows up as cold, not hot. Do not only hunt for bright spots.
Ambient is not a constant. A 60 °C reading in a 20 °C electrical room and a 60 °C reading in a 45 °C rooftop enclosure are not the same finding. Always note what the surroundings are doing, because the comparison is what carries the meaning.
Take the free 15-question electrician readiness checkVoltmark Thermography Academy is Voltmark's own training program. It is not ASNT certification and it is not affiliated with, endorsed by, or sponsored by ASNT, NFPA, ICC, or any certifying body or licensing authority. Certificates issued by this program attest to completing Voltmark training and passing Voltmark examinations. Employer qualification under NFPA 70E and any written practice remains the employer's responsibility. Always follow your employer's safety program and your supervising electrician.
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