The CAT (category) rating system refers to the location of a circuit in relation to the energy source. It may also denote the ability to withstands transient high energy surges.
Parameters:
(1) relative location in distribution path from main line
(2) phase
Relative Location of Circuit
Description
CAT Rating
from distribution line to main panel
three-phase utility connection
IV
from main panel to sub panels
three-phase distribution
III
from fixed locations to wall outlets
single-phase receptacle for local distribution
II
end device-related
secondary circuits within appliances; electronics
I
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