Voltage = (RawValue / 1024.0) * 5000; // Gets you mV
int mVperAmp = 100; // use 100 for 20A Module and 66 for 30A Module
Both lines are wrong if the Arduino supply is not exactly 5.000volt (which it rarely is).
ACSxxx sensors are ratiometric, and should be read as ratiometric sensors.
But most sketches you find on the net read them as voltage sensors,
which introduces errors if VCC is not exactly 5.000volt.
Leo..