circuit breaker not tripping?

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Angel Electric

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I had a call Where the customer was using a coffee maker and a waffle iron which as bath are on ,they drag 24.9 amps. The breaker is a 20amp sqaureD bout in type qo. Customer wants to know why circuit breaker is not tripping. I dont know at What range it should've... Any experience with the threshhold of circuit breakers will be greatly appreciated....
 
It all depends on how long the breaker is subjected to that load. I would expect a 20 amp breaker to trip within a minute or two if subjected to a 24.9 amp load. If not, it might be time for a replacement.
 
A typical residential circuit breaker is an inverse time circuit breaker. This means that the amount of time that it takes for the circuit breaker to trip is determined by the amount of current flowing through the circuit breaker. This is known as a breaker trip curve. Manufactures can provide you with the trip curve data if you check on-line or ask them.

For a standard 20 amp circuit breaker it make take a couple of minutes to trip at 24.9 amps. I have gone on service calls where a 20 amp circuit breaker has been carrying a 21 or 22 amp and never tripped.

Chris
 
A circuit should not be intentionally overloaded with the expectation that the OCPD will trip.

Regarding the OP, a breaker is calibrated at 40degC ambient. The actual ambient temperature may ne 25-30degC and as such starts out cooler which shifts the trip curve to the right, that is the breakers takes londer to trip. Without looking at the actual trip curve it difficult to assess when the breaker should trip under that application.
ANSI/NEMA AB 4 Publication which is the "GUIDELINES FOR INSPECTION AND PREVENTIVE MAINTENANCE OF MOLDED CASE CIRCUIT BREAKERS USED IN COMMERCIAL AND INDUSTRIAL APPLICATIONS" provides a means for field testing breakers.

Section 6.5 INVERSE-TIME OVER CURRENT TRIP TEST incluses the proceedure. Table 3 VALUES FOR INVERSE TIME TRIP TEST (At 300% of Rated Continuous Current of Circuit Breaker)
At 300% breakers rated up to 30at 250v or less should trip in less than 50sec.

With consideration of the OP's example there are too many variables to come to a conclusion but review a typical TC curve for a ar breaker should provide a good idea as to the trippng characteristics of a 20a breaker.
It is important to remember that the curve is based upon 40degC ambient.
You are considering 125% load. Look along the bottom or top scale and mark 125% and draw a vertical line betweem the marks. you will note the two dark lines that curve down and to the right which are marker min and max. which represents the breaker calibration allowance at 40degC.
Following the verical line that you drew where does it cross these two trip curve lines? Your line may be half way between the two at the top and cross the min trip curve at 100 sec and never cross the max trip curve. This means that should the breaker have been calibrated at the minimum it would trip in 100 sec and if calibrated at the maximum it would never trip. Remember that this is based upon a 40degC calibration.
However, note the dotted trip curve lines shown to the right of the 1 min time along the left "Maximum Single-Pole Trip Times at 25?C, Single-pole test data at 25?C based on NEMA procedures for verifying performance of molded case circuit breakers."This is an approximation as to where the max trip time turve will be at 25?C.

As such you will see that the chances of a breaker tripping with a load a 125% of its rating is slim to none.
 

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Also, fuses and circuit breakers exhibit similar performance for overloads to about 35%; neither one trips very fast, if at all, in typical installations.
 
The UL standard permits an OCPD to carry 134% of the device rating forever. It has to trip in no more than 60 minutes for 135% load.
 
don_resqcapt19 said:
The UL standard permits an OCPD to carry 134% of the device rating forever. It has to trip in no more than 60 minutes for 135% load.
Click to expand...
Hence the often whined about "conservative" nature of wire ampacity ratings in the NEC...

Here is the pdf link to the QO120 trip curve. At 24.9A, you are at 124.5% of rated current. If you plot that on the horizontal axis (125% is the first light vertical line to the right of 1.0 multiples of rated current), then go straight up, you will see that the time-to-trip is going to be somewhere between 250 seconds (4 minutes+) and infinity, as in what Don just said.
 
I cannot view that curve with clarity, but a standard residential molded case CB will carry 300% of it's rated ampacity for 30-60 seconds.
 
So, basically, the whole sizing your conductors based on OCPDs or vise versa is just a crock of $&@#? Or are breaker trip curves taken into account by CMPs when they make the rules?
 
brian john said:
I cannot view that curve with clarity, but a standard residential molded case CB will carry 300% of it's rated ampacity for 30-60 seconds.
Click to expand...

As stated in my post "ANSI/NEMA AB 4 Publication which is the "GUIDELINES FOR INSPECTION AND PREVENTIVE MAINTENANCE OF MOLDED CASE CIRCUIT BREAKERS USED IN COMMERCIAL AND INDUSTRIAL APPLICATIONS" provides a means for field testing breakers.

Section 6.5 INVERSE-TIME OVER CURRENT TRIP TEST incluses the proceedure. Table 3 VALUES FOR INVERSE TIME TRIP TEST (At 300% of Rated Continuous Current of Circuit Breaker)
At 300% breakers rated up to 30at 250v or less should trip in less than 50sec"

Also, the curve that I posted should be fairly clear and readable. It should be typical curve that would be very similar to those of other breaker manufacturers.
 
Using the chart for Square D QO breakers posted by Pete in post #3 25 amps may or may not ever trip a 20 amp QO breaker, which gives it 300 seconds to infinity (300 seconds is 5 minutes)

And even at 30 amps a 20 amp QO breaker will take between 23 seconds to almost 125 seconds and thats over two minutes.

And yes the code making panel has taken these trip curves into account when sizing conductors.
 
Jraef said:
Hence the often whined about "conservative" nature of wire ampacity ratings in the NEC...

Here is the pdf link to the QO120 trip curve. At 24.9A, you are at 124.5% of rated current. If you plot that on the horizontal axis (125% is the first light vertical line to the right of 1.0 multiples of rated current), then go straight up, you will see that the time-to-trip is going to be somewhere between 250 seconds (4 minutes+) and infinity, as in what Don just said.
Click to expand...

the link you provided is for QOD's which has a bit more conservative trip curve and as these are 2 and 3 pole they are only 240 volt rated, look at the trip curve in the link to post #3

Kind of strange as in the link it list QO-QOB breakers but the trip curve is much more higher as 30 amps starts at 70 seconds and goes to 3000 seconds which is much higher then the other PDF? and 1.25 goes from 300 to infinity?
 
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hurk27 said:
the link you provided is for QOD's which has a bit more conservative trip curve and as these are 2 and 3 pole they are only 240 volt rated, look at the trip curve in the link to post #3

Kind of strange as in the link it list QO-QOB breakers but the trip curve is much more higher as 30 amps starts at 70 seconds and goes to 3000 seconds which is much higher then the other PDF? and 1.25 goes from 300 to infinity?
Click to expand...

One link was for QOD 110A breakers with 22kAIC rating, the other curve was for QO 20A breakers with 10kAIC.
 
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