I²t Let-Through Energy Explained
I²t is a compact way to describe the thermal stress caused by current over time during a short circuit. Because heating is proportional to current squared, a very high current acting for a short period can produce substantial thermal energy.
What I²t represents
Protective-device manufacturers often publish pre-arcing, clearing or total I²t data for fuses and current-limiting devices. The correct value depends on the device, rated current, prospective fault current and system voltage.
Why current limitation matters
A current-limiting fuse or breaker can open before the prospective fault current reaches its natural peak. This reduces both peak let-through current and the energy delivered downstream.
Comparing with conductor withstand
For an adiabatic conductor check, standards commonly express thermal withstand in the form:
where S is conductor area and k depends on conductor material, insulation and allowable temperature rise. The exact application and constants must follow the governing wiring or equipment standard.
Equipment checks are more nuanced
Equipment short-circuit ratings can depend on certification, test conditions, specified upstream protective devices and combination ratings. A mathematical I²t comparison is not automatically a substitute for a required certified combination.
Technical reference
Apply the cable thermal check
Use the IEC cable sizing calculator to enter applicable total-clearing I²t and compare the thermal requirement with ampacity and voltage-drop requirements.