Circuit to detect amperage?
Here is a circuit that should do what you want.   It uses the LT6100 is that it has variable gain and 0.5% accuracy.  I have used this chip in a commercial product.



If you use it with a 100 mΩ resistor, a 1A load with drop 100 mV across the resistor. Using a gain of 20, this will give a 2v output. By adjusting the voltage divider feeding the + lead of the comparator, you can set the threshold you need.

If you combine the LT6100 with a 100 mΩ 0.5% resistor, you will have 1% accuracy for the combination.

The LT6100 is available from Digi-Key for $2.36.