breakers

[chevyx92]

Exactly how long should it take and exactly how much current does it take to trip a 20A breaker? Is it suppose to trip exactly when it has 20.1A or 21A?? Reason I ask is I was on a call recently that had me checking amp loads in a panel. I came across one 20 amp circuit that had 20.65 Amps. Is that normal? Hell I have also seen a 20 amp breaker that was pulling 26 amps (long time ago though). Now I know that was not right.

 

[mikeames]

There are many variables that go into that calculation. Its not straight forward as a 20 amp breaker trips at 20 amps. The utility transformer, distance, wire size, load type, breaker type, and time all are variables that must be known to calculate the answer to your question. My first encounter with this was when I saw a 15 amp breaker trip the 200 amp main on a house. New house no breakers in it except one 15 amp breaker used for tools. A fault occured while I was next to the panel and bamm ! 200A main tripped. This was about 7 years ago. I was beside myself and had to research it to find out why. I learned a lot thanks in part to this site.

 

[mdshunk]

The current level as well as time must be considered. It's a graph that is published for each model of breaker that takes into account time and current.

Here's the trip curves for QO breakers (page 19), as one for instance:
http://ecatalog.squared.com/pubs/Ci...rs/QO-QOB Circuit Breakers/QOT/0730CT9801.pdf

 

[splinetto]

When I first started in the trade we were doing apartment.. There was a bad splice, it was phase to phase short, when you turn the switch on.. It didn't trip the 15A breaker or the 100A outside for the unit it only would trip the 1000A main breaker for the whole building....Looking back now should we have done something at least called some one ....The guy that I was with didnt believe it so he reset the breaker and stood outside and had me flip the switch on again...

 

[brian john]

SPLIN:

Most likely this was 480/277 and the Main Switch had Ground Fault Protection, while similar in nature. For this to happen it would have to been a phase to ground at some point.

If this was not the situation the Main CB would have had to be set on minimal pick up and delay and even then I would think the 15 amp was defective.

Molded case circuit breakers are inverse time currrent devices copmprised of a thermal trip (long time) and a magnetic trip (instantaneous). The higher the current the faster the CB trips. At 60 amps a 20 amp CB may trip between 30-60 seconds (SWAG I did not check a curve) the instaneous trip for this same CB may be at 120-200 amps.

 

[templdl]

NEMA AB4, Guidelines for Inspection and Preventive Maintenance of Molded Case Circuit Breakers Used in Commercial and Industrial Applications.
Section 5, Test Procedures. Explains and illustrates how to field-test molded case circuit breakers (MCCB) in the field. When the procedure is properly set up and followed the maximum trip time for a 20at MCCB rated 250v at 300% rated current (60a) should be less than 50 seconds.
There is no way to field check the calibration of a physically thermal magnetic circuit breaker with any accuracy if one does not have a professional test set that would have the capability to duplicate the test procedure as required by UL489.
With electronic breakers most if not all have the capability of being tested by secondary current injection wiht a suitable test kit.
To address a couple of the other posts regarding tripping a breaker upstream form a fault, if there is a bolted fault where all breakers in series see that same fault. if the instantaneous magnitude of the fault is great enough to fall into the magnetic pick-up calibration of all of those breakers ir is pot luck as to which breaker will trip. There is no such thing as coordination in those cases.
Regarding the 1000a breaker that tripped, there was very little details given about the distribution system involved. However, I would assume that it was a 15at breaker on a 480Y/277 volt system where the main breaker was required to have ground fault protection and the breaker tripped on ground fault. The common magnetic trip calibration for a 1000a breaker is 100ka +-20% and usually adjustable down to 50ka. Normally there are set at the highest value.
As such I would find it highly unusual to trip a 1000a breaker before tripping a 15at breaker unless the 1000a breaker has GF protection.