AC foundations
RMS current
- where
- RMS current (A)
- peak current (A)
Explain this equation
Works out RMS current from a sine wave’s peak current. Use RMS for heating, cable loading and protective device ratings.
Iᵣₘₛ is the root-mean-square current: the steady value with the same heating effect. Iₚ is the peak current.
Heating goes with I², and the average of a squared sine wave is half its peak squared. Taking the square root gives peak divided by √2.
Sense check: 10 A peak is about 7.07 A RMS. Clamp meters normally read RMS.
Watch out: many electronic loads draw non-sinusoidal current even on a sine voltage. Then √2 does not apply.
For a sinusoidal current, divide its peak by √2. The RMS current is the useful quantity for resistive heating calculations.
Electronic loads can draw non-sinusoidal current even from a sinusoidal voltage supply. The √2 conversion then need not apply.
Enter your values to see the result.
Common mistake
Assuming sinusoidal voltage guarantees sinusoidal current.
Limits
Sinusoidal current centred on zero.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.