Terminal contact-heating equations
Contact resistance creates voltage drop and localized heat. Because current is squared, small resistance increases become important at high current.
Vcontact = I × RcPheat = I² × RcΔT = Pheat × Rθwhere:
- Rc
- Measured contact or joint resistance[Ω]
- I
- Current through the connection[A]
- Pheat
- Localized electrical loss[W]
- Rθ
- Validated thermal resistance of the connection to ambient[K/W]
- ΔT
- Estimated temperature rise[K]
Without validated thermal resistance, I²R accurately estimates electrical loss but cannot by itself predict exact terminal temperature.
How to calculate terminal heating
Enter current and measured connection resistance. The calculator uses I²R for contact loss and IR for voltage drop, then compares the result with a healthy reference at the same current.
- Measure current under stable load.
- Measure or estimate contact resistance consistently.
- Calculate voltage drop and I²R loss.
- Compare phases or a healthy reference.
- Validate temperature under real enclosure conditions.
High-current contact example
At 200 A, a 100 µΩ connection dissipates 4 W. A healthy 25 µΩ reference dissipates 1 W, so a fourfold resistance increase creates four times the heat at the same current.
P = 200² × 0.0001 = 4 W
V = 200 × 0.0001 = 20 mV
Resistance and thermal runaway
Loose hardware, poor crimping, oxide, contamination, and cycling increase contact resistance. Heat accelerates oxidation and relaxation, which can further increase resistance and produce a worsening failure cycle.
Why exact temperature is difficult
Electrical loss is calculable from I²R, but temperature also depends on terminal mass, conductor heat sinking, airflow, enclosure temperature, adjacent phases, duty cycle, and mounting. Use a validated thermal resistance or physical testing for temperature prediction.
Assumptions
- Current and contact resistance remain constant
- Resistance is the complete measured connection resistance
- Thermal resistance is an optional lumped estimate, not a tested terminal characteristic
Important Warnings
- Calculated temperature is not a certified terminal temperature; real heating depends on conductor heat sinking, cycling, airflow, enclosure, oxidation, and measurement method.
- Hot, discolored, loose, arcing, or damaged terminals require de-energized qualified inspection and corrective action.