Application
Table 44.1
Factor "A" - Cable Or Bus Size Multiplying Factor
Percentage of Rated
Factor A
!Required) Cross-Sectional Area
(%)
50
140
60
1.25
70
115
80
1.07
90
1.02
100
1.00
125
0.99
150
0.97
zoo
0.97
Ambient Temperature
Ambient temperatures have a n even wider effect on the
rating of the breaker-cable system. While the internal sens
ing and tripping circuitry with the Power+™, MicroVersa-
Trip Plus™ and M icroVersaTrip PM™ trip units are ambient
insensitive, high-ambient temperatures may cause i nternal
components to exceed their operating temperature limits.
Low temperatures substantially increase the current-carry
ing capabilities of the breaker-cable system until other limit
ing factors occur (e.g., lubrication problems or mechanical
binding of internal parts due to d ifferential contraction). The
ambient temperature of a Power Break®
I I
breaker should not
be less tha n
-zooc
nor 70°C.
Cable with insulation ratings above
?soc
may be used,
providing it is sized to 75°C, per the N EC, or other applicable
codes.
The term "ambient" temperature a lways refers to the tem
perature of the air immediately surrounding the breaker and
not the temperature of the air outside the breaker's enclo
sure. Room or outside air temperatures only esta blish the
thermal floor to which all other heating is added.
To convert breaker ambient from room ambient, it is nec
essary to know the temperature rise within the equipment
housing the breaker. The temperature rise is a function of
several variables, including heating caused by other equip
ment, ventilation, solar heating, factors relating to group
mounting and the free surface area of the breaker's enclo
sure.
Once the device's ambient temperature is determined, select
Factor B from the table.
44
Table 44.2
Factor "B" - Ambient Temperature Rating Factor
Ambient Temperature
Maximum Wire
Factor
B
·c'
Insulating Rating
•c2
25
75
1.00
40
90
1.00
50
105
1.00
60
125
110
70
125
115
Notes:
11
Average air temperature over a 24-hour period outside the break
er's insulated case, but ir rside the enclosure. 2) Cable (wire) must be based
on 75oc ampacities per Table 3 10-16. current National Electrical Code.
Operating Frequency
All Power Break
I I
circuit breakers may be applied at their
published ratings on 50 Hz and 60 Hz power systems. At
nominal system frequencies less than 50 Hz but a bove direct
current, digital solid-state trip devices become inoperative
due to sensor saturation. On direct current systems, digital
solid-state trip units are completely inoperative. Power Break
11
circuit breakers are, therefore, not suitable for direct cur
rent applications.
System operating frequencies above 60 Hz may change the
performance and ratings of circuit breakers by heating me
tallic parts and significantly reducing interrupting capacity.
Factor "C"
-
Operating Frequency
For operating frequencies of both 50 Hz and 60 Hz, the
operating frequency rating foetor "C"
=
1.00. For operating
frequencies other than 50 Hz and 60 Hz, derating factors
"
C
"
are l isted below.
Table 44.3
Factor "C" - Frequency Rating
Frequency
•
Factor C
de
-
50-60 Hz
1.00
150-180Hz
1.07
200-240 Hz
1.25
-·--·-
300-350 Hz
140
400-450 Hz
1.51
* Power+ trip units and the energy management functions in MicroVersaTrip
PM trip units are suitable far 50/60 Hz a pplications only
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