Brochure

23
5/98
© 1998 Square D All Rights Reserved
Thermal Unit Selection
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.
Except for ambient temperature-compensated
overload relays, an ambient temperature higher
than 40°C would lower the trip current, and a
lower temperature would increase it. This
variation is not a factor in selecting thermal units
for the average application, since most motor
ratings are based on an ambient temperature of
40°C, and motor capacity varies with temperature
in about the same proportion as the change in trip
current. Temperature-compensated relays
maintain a nearly constant trip current over a wide
range of ambient temperature, and are intended
for use where the relay, because of its location,
cannot sense changes in the motor ambient
temperature.
CALCULATION OF TRIP CURRENT FOR
AMBIENT TEMPERATURES OTHER
THAN 40°C
For a controller ambient temperature other than
40°C (104°F) trip current can be calculated by
applying a correction factor from the curve in
Figure 1. The approximate trip current for a
particular ambient temperature is the product of
(1) the multiplier M corresponding to the
temperature and (2) the 40°C trip current rating.
NOTE: Ambient temperature is the temperature
surrounding the starter enclosure. Normal
temperature rise inside the enclosure has been
taken into account in preparing the thermal unit
selection tables.
Example 2: Determine the trip current for the
motor and controller in Example 1, except the
controller is in a 30°C (86°F) ambient
temperature. From the curve in Figure 1 the
multiplier M is 1.1 at 30°C. Approximate Trip
Current is 16.0 x 1.25 x 1.1=22 Amperes.
100
80
60
40
20
0
-20
212
194
176
158
140
122
104
86
68
50
32
14
-4
0
Ambient Temperature Correction
Curve for
Thermal Overload Relays
Ambient Temp - Equiv. Degrees Farenheit
Ambient Temp - Degrees Centigrade
02
04 06
08
10 12
14