Why It's Getting Hot: Resistance and Load Decision Tree

Why this matters

Heat is energy you did not want, showing up where work is being lost. A connection, motor, bearing, or pump that runs hotter than it should is telling you something is wrong, and heat is also a fire and failure risk in its own right. The two root causes are almost always the same family: too much resistance in the path, or too much load on the part. Learn to tell which, and a hot spot stops being a mystery and becomes a clear repair.

The two questions that split every overheating problem

Almost every "why is this hot" call comes down to:

1. Is current flowing through resistance it should not have? (a bad connection, a corroded lug, an undersized wire, dragging mechanical contact). Heat = current squared times resistance. Add resistance where current flows and you make a heater.
2. Is the part being asked to do more work than it is rated for? (overcurrent, overloaded motor, restricted flow, low voltage forcing high current, no cooling).

Start by deciding which side you are on.

Start here: is the heat at a connection or in the whole part?

• If the heat is a localized hot spot at a terminal, splice, lug, or contact: this is a resistance problem at that connection. High-resistance connections cook themselves. Suspect loose, corroded, or undersized.
• If the entire part is uniformly hot (a whole motor, a whole transformer, a whole pump): this is a load or cooling problem. The part is working too hard or cannot shed its heat.
• If the heat is at a moving interface (a bearing, a bushing, a seal): this is mechanical, from friction. Think lubrication, alignment, and wear.

Resistance-side branch (localized hot connection)

Electrical work: de-energize and verify dead before touching.

• Discoloration or a burnt smell confirms heat has been building.
• Tug-test and inspect the connection. Loose or corroded? Clean it, remake it, retorque to spec.
• Measure voltage drop across the connection under load (see the voltage-drop article). A real drop across one spot is the resistance making the heat.
• Check wire gauge against the load. An undersized conductor heats along its whole length, not just at a point.

Load-side branch (whole part hot)

• Measure current draw and compare to the nameplate rating. Over-rating means overload. (See the current-draw article.)
• If current is high, find why: a mechanical bind dragging a motor, a restriction making a pump push harder, a dirty filter or blocked airflow forcing the system to fight, or low supply voltage (a motor pulls more current at low voltage to make the same power).
• Check cooling. A part within its load rating still overheats if its airflow is blocked, its fins are caked, its fan is dead, or it sits in a hot enclosure. Restore cooling.
• Check duty cycle. A part rated for intermittent use will overheat if it is cycling far more than designed (short-cycling, stuck-on control). Fix the control.

Friction branch (hot moving interface)

• If a bearing or bushing is hot: it is under-lubricated, contaminated, misaligned, or worn. Re-lube if serviceable; if it is dry and damaged, replace it. Misalignment loads one side and heats it (see the alignment article).
• If a seal or packing is hot: it is over-tightened or running dry. Adjust or replace.

Confirm the fix

Re-run the unit and re-check the temperature once it stabilizes. The hot spot should normalize. If a connection was the cause, voltage drop across it should now read near zero. If load was the cause, current draw should sit within the nameplate rating. A part that is still hot after the obvious fix has a second cause; do not stop at the first find.

Why heat is never the disease

Heat is a symptom. Cooling the part without finding the resistance or the load just delays the failure. Always name the root: this connection had resistance, this motor was overloaded, this bearing ran dry. Fix that, and the heat takes care of itself.

References

• NFPA 70 (National Electrical Code) - conductor ampacity and connection requirements.
• OSHA 29 CFR 1910.147 - lockout/tagout before contact.
• Manufacturer documentation - rated current, duty cycle, and cooling requirements.
• See related: Voltage Drop, Measuring Current Draw Against the Rating, Alignment and Why Things Wear Out Early.