Subpanel Feed Fine Cold, Warms Under Load: Lug vs Conductor Decision Tree

Why this matters

A subpanel feeder that reads normal when the building is quiet but warms up under heavy load is telling you something specific: either a high-resistance termination (a loose, corroded, or wrong-material lug) is generating localized heat, or the feeder conductor is genuinely undersized for the load and heating along its whole length. The two have different fixes and different urgency. A bad lug is a point hazard that can be re-terminated, while an undersized feeder is a sustained over-temperature condition that degrades insulation and demands a conductor upsize or a load reduction. Misreading a hot lug as "just an undersized feeder" leads to pulling new wire when a re-termination would have fixed it; misreading an undersized feeder as a lug problem leaves a feeder cooking inside the wall. This tree separates localized termination heat from distributed conductor heating using IR mapping and voltage-drop math.

Symptom presentation

The feeder lugs at the main, the feeder conductors, or the subpanel main-lug terminals run warm to hot when the subpanel's loads peak, and cool when load drops. Voltage at the subpanel bus sags under load relative to the main panel. Breakers in the subpanel may trip on hot days, or downstream equipment may behave like it is undervolted. Whether the heat is concentrated at a termination or spread along the conductor is the diagnostic fork.

Quick checks

• Clamp feeder current under peak load and compare to the feeder ampacity for the conductor size, material, and termination temperature rating per NEC 310.16 and 110.14(C).
• IR-scan both feeder lugs at the main, the conductor run where accessible, and the subpanel main-lug terminals under load. Note where the hot spot concentrates.
• Measure voltage drop from the main panel bus to the subpanel bus under load, per leg. Compare to the conductor's expected drop for its length and load.
• Check the lug material/conductor material match (AL-CU listing), torque, and signs of corrosion or oxidation.

Isolation tree

1. IR map under load. If the heat concentrates sharply at one lug or terminal while the conductor and the other terminations stay near ambient, the fault is a termination: proceed to step 2. If the conductor itself is uniformly warm along its length and both terminations are only as hot as the conductor near them, the conductor is carrying more current than its ampacity allows: proceed to step 4.

2. Termination work. De-energize and open the hot termination. Look for a loose lug (under-torqued), a corroded or oxidized interface, an aluminum conductor in a copper-only lug, or thread/lay-in damage. Measure the voltage drop across just that lug under load before opening if access allows; a measurable drop across a single bolted connection is definitive.

3. Re-terminate to the listed torque with the correct AL/CU-rated lug and antioxidant where required for aluminum. Re-energize and re-IR under the same load. The hot spot should vanish and the across-lug voltage drop should fall to near zero. If a second termination is also hot, repeat.

4. Conductor sizing check. With heat distributed along the conductor, verify the actual conductor size and material against the feeder breaker rating and the connected load. Compute expected voltage drop for the run length; a drop well beyond the NEC 215 recommended 3 percent at the feeder confirms an undersized or overlong run. Confirm the termination temperature rating is not forcing a lower ampacity (a 90 degree C conductor on 60/75 degree C lugs is limited to the lower column per 110.14(C)).

The termination-rating trap

A surprising number of "undersized feeder" calls are actually correctly sized conductors limited by their terminations. NEC 110.14(C) requires the conductor ampacity to be taken from the column matching the lowest temperature rating among the conductor, the lug, and the equipment. A 90 degree C THHN conductor terminated on a panel and breaker rated 75 degrees C must be evaluated at the 75 degree C column, and if the lugs are only listed for 60 degrees C the conductor is held to that lower ampacity. A feeder that looks adequate at the conductor's own 90 degree C rating can be running above its allowed ampacity once the termination rating is applied, and it will heat along its length exactly as a truly undersized conductor would. Always read the breaker and lug temperature markings before declaring a conductor undersized, because the fix may be a higher-rated termination rather than larger wire, and because installing larger wire on the same low-rated lugs does not raise the limiting ampacity.

Confirming diagnosis

Confirm a termination fault by an IR hot spot localized to one lug, a measurable across-lug voltage drop under load, and both the hot spot and the drop disappearing after re-termination. Confirm an undersized conductor by uniform conductor heating, a calculated and measured voltage drop exceeding feeder targets, and load current at or above the conductor's true (temperature-corrected) ampacity. A feeder can have both faults; clear the lug first, then re-evaluate the conductor.

Remediation

• Loose/corroded/wrong-material lug: re-terminate with a listed AL/CU lug, antioxidant on aluminum, torqued to the listed value per NEC 110.14.
• Undersized or overlong feeder: upsize the conductor to carry the load within ampacity and voltage-drop targets, or reduce/manage the subpanel load.
• Temperature-rating mismatch at terminations: derate to the lowest-rated component column per 110.14(C), or upgrade terminations.

References

• NFPA 70 (NEC) Article 110.14, Electrical Connections (termination torque and temperature ratings)
• NFPA 70 (NEC) Article 215, Feeders (sizing and voltage drop)
• NFPA 70 (NEC) Article 310.16, Ampacities of Insulated Conductors
• NFPA 70 (NEC) Article 240, Overcurrent Protection
• NFPA 70B, Standard for Electrical Equipment Maintenance (infrared inspection)
• UL 486A-486B, Wire Connectors (AL/CU listing)