 Previously, emergency exit signs also tended to use the intermediate base, but U.S. and Canadian rules now require long-life and energy-efficient LED lamps, which can be purchased inside a conventional Edison base bulb as a retrofit.A medium screw base should not carry more than 25 amperes current; this may limit the practical rating of low voltage lamps.E29 "Admedium" bases are used for special applications, for example UV spotlight lamps in magnetic crack detection machines.