Welcome to ECN firstoff.
We/I always measure from the front of the enclosure. If it has doors, the door must swing open a full 90 degrees minimum. I mention this, as I have come upon a few installs where there is 3'6" of clearance, but you cannot open the door, as it is 48".
There are special circumstances where a lesser clearance has been allowed, by special permission on a few retrofit installs, where space was a physical imposibility.
[This message has been edited by HotLine1 (edited 03-05-2003).]