According to Joseph Meehan :
I believe the problem is AC current vs DC current.

I don't really know whether the usual retail/consumer grade
clamp-on meters will do DC. But the ones that auto mechanics
use _will_ do DC readings. I've watched one in use testing
a battery/charger - they're not AC only.

These meters don't care what voltage they're measuring. All they
know is magnetic field strength around an individual wire,
and that depends solely on current, not what the voltage is to some
other wire that's arbitrarily far away.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.