Starting on April 10, 2017, the new rules regarding FM translators for AM stations and how far the 60 dBu protected service contour of the translator can extend will go into effect.
The 2 mV/m contour
Like how FM stations, including LPFM stations have protected service contours that designate a primary service area where there is an expectation of station reception, AM has a similar contour. The contour designates where the signal is expected to reach a 2 millivolt per meter (mV/m) field strength. The shape of the contour depends on antenna directionality as well as conductivity of the ground in the area around the station. The map to the right is KIDR, an AM station operating on 740 kHz in Phoenix with 1 kW into a directional antenna. The contours the FCC takes into consdieration for this rule is the daytime 2 mV/m contour. Most stations will have different 2 mV/m contours at night.