At least one online expert insists that the exception does not permit service heads below the point of attachment, only movement AWAY from the point of attachment.
An exception should directly apply to the rule.
The rule in this case is that service heads be ABOVE the point of attachment, so the exception permits otherwise (BELOW the point of attachment).
If the rule was, for example, that service heads be within 12" of the point of attachment, I might agree that the 24" exception would not necessarily permit the haed installed below.

Anyone else?

Redsy,

IMO the words ... "Where it is impracticable to locate the service head above the point of attachment, the service head location shall be permitted ..." should leave no doubt that it could be left up to AHJ interpretation of what is 'practicable'.

Bill

Assuming that it is agreed that it is indeed impracticable (whatever that may mean in this particular instance).
Does the exception permit you to install the head lower than the attachment point of the service drop.
I say it does.

I say it does too.
I can't see it meaning anything else.

Bill

The exception is included as a part of the NEC to provide a reference for us AHJ's to modify the rule as written. (A little common sense always helps too) In my area, a discussion with the Utility Co. Wiring Inspector prevents this subject from becoming a nightmare.
John