QOU345 View Datasheet(PDF) - Unspecified
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QOU345 Datasheet PDF : 32 Pages
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QOU Miniature Circuit Breakers and Switches
Application Information
Ambient Temperature Rating
To meet the requirements of UL 489 and the Canadian Standards Association, thermal-magnetic
circuit breakers are designed, built and calibrated for use on 50/60 Hz ac systems in a 104° F (40° C)
ambient temperature. Time/current characteristic trip curves are drawn from actual test data that meets
UL 489 testing requirements.
The ambient temperature is the temperature of the air surrounding the circuit breaker. Thermal-
magnetic circuit breakers are temperature-sensitive devices, and their rated continuous current
carrying capacity is based on a UL specified 104° F (40° C) calibration temperature. The ambient
temperature can affect the performance characteristics of the circuit breaker. Thus, when applying a
circuit breaker at temperatures other than 104° F (40° C), it is necessary to determine the circuit
breaker’s actual current carrying capacity under this condition. Further, it may be necessary to rerate
the circuit breaker to compensate for these ambient conditions. See Figure 3:
Thermal-magnetic circuit breakers use bimetal strips that bend in response to temperature changes.
Current flowing through the circuit breaker creates most of the heat that causes the tripping action. The
ambient temperature surrounding the circuit breaker either adds to or subtracts from this available
heat. Conductors are sized using the ampacity rerating factors shown on the bottom of NEC Table
310-16 when designing systems for ambient temperatures other than 40° C.
Rerating of Thermal-magnetic Circuit Breakers for Ambient Conditions
Square D thermal-magnetic circuit breakers are to be applied in ambient temperatures within the range
of 14° F to 140° F (-10° C to 60° C). Use the following rerating guidelines:
• Ambient Temperatures Between 77° F and 104° F (25° C and 40° C):
— No rerating is necessary.
• Ambient Temperatures Between 14° F and 75° F (-10° C and 24° C):
— Thermal-magnetic circuit breakers operating within this ambient temperature range will carry
more than their continuous current rating without tripping. Conductor and equipment damage
can result if they are not in the same low ambient environment as the circuit breaker.
— Nuisance tripping will not be a problem. However, if closer protection of the equipment and
conductor is required, the increased current carrying capacity of the circuit breaker at the lower
ambient temperature should be taken into consideration.
• Ambient Temperatures Between 106° F and 140° F (41° C and 60° C):
— Thermal-magnetic circuit breakers operating within this ambient temperature range will carry
less than their continuous current rating and must be carefully selected to prevent nuisance
tripping.
To determine the continuous current carrying capacity of a thermal-magnetic circuit breaker at an
ambient temperature other than 104° F (40° C), perform the following steps:
1. Choose the ambient rerating curve in Figure 3 for the specific amperage rating of the circuit breaker
you wish to apply. Note that the curve crosses the 104° F (40° C) ambient temperature line at the
circuit breaker’s rated continuous current carrying capacity (Circuit Breaker Handle Rating on the
curve).
2. Follow this curve to the appropriate ambient temperature.
3. Read the adjusted continuous current carrying capacity at this point (on the left axis).
4. Add in any other applicable factors, such as continuous loading, per the NEC requirement.
09/2005
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