CALCULATION OF TRIP CURRENT
RATING
Trip Current Rating – Trip current rating is a
nominal value which approximates the minimum
current to trip an overload relay in an ambient
temperature, outside of the enclosure, of 40°C
(104°F). In all selection tables, except Class 8198
the trip current rating is 1.25 times the minimum
full load current shown for the thermal unit
selected. For Class 8198, the trip current rating is
1.15 times the minimum full load current. This
applies to bimetallic overload relays with the trip
adjustment set at 100 percent.
Calculation Procedure
1. Use the selection table for the specific
controller involved.
2. Find the minimum motor full load current listed
for the thermal unit in question.
3. Multiply that current by 1.25 (1.15 for Class
8198). The result is the trip current rating.
Example 1: Determine the thermal unit selection
and trip current rating for thermal units in a Class
8536 Type SCG3 Size 1 magnetic starter used to
control a three-phase, 1.15 service factor motor
with a full load current of 17.0 Amperes, where the
motor and controller are both located in a 40°C
(104°F) ambient temperature.
1. From Table 13 the proper selection is B32.
2. The minimum motor full load current is 16.0
Amperes.
3. Trip current rating is 16.0 x 1.25= 20.0
Amperes.
Protection Level is the relationship between trip
current rating and full load current. Protection
level, in percent, is the trip current rating divided
by the motor full load current times 100. In
Example 1 the protection level for the B32 thermal
unit is: 20.0/17.0 x 100=118%.
National Electrical Code, Section 430-32, allows a
maximum protection level of 125% for the motor in
the above example.
Minimum Trip Current (also called ultimate
current) may vary from the trip current rating
value, since ratings are established under
standardized test conditions. Factors which
influence variations include: the number of
thermal units installed, enclosure size, proximity
to heat producing devices, size of conductors
installed, ambient (room) temperature, and
others.
