Use the applicable reach range and electrical code, not a mixed list of customary heights.
For installations subject to the 2010 U.S. ADA Standards, covered operable parts must be within a reach range allowed by section 308 and have the required clear floor space. An unobstructed adult forward or side reach is generally 15–48 inches above the finish floor. Reach over an obstruction has separate limits and can reduce the maximum height.
Measure the usable control or receptacle location as required by the applicable standard. A dimension to the top, bottom or center of an electrical box is not automatically the same measurement.
The ADA reach rules do not replace electrical-code requirements for receptacle locations, spacing, bathroom or outdoor protection, or equipment-specific installation. They also do not impose one universal mounting height on every private residence or Canadian project.
Have the designer or electrician apply the locally adopted electrical and accessibility requirements to the actual room, approach and obstructions. Do not use the former mixed list of customary heights, obsolete 54-inch side reach or unverified state/province rules as a code schedule.
Source: 2010 ADA Standards, sections 205 and 308–309.