A current is made of two components, a DC component of I1=3 amp and an AC component given by I2=42sinωt amp; then the reading of the hot-wire ammeter (rms value) is:
(4)
I1=3A, I2=42sinωt
Inet=I1+I2=(3+42sinωt)
Irms=32+(42)2×12=5A