Breaker tripping under Amp rating

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nizak

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What would cause a breaker to trip when the current is 20% less than the breaker rating?

Breaker in question is a QO 2150. Consistently tripping at 118-120 amps after operating for approximately 10 minutes.

Panel is located outdoors, air temp averaging 70/75 degrees.

Thanks
 
Put your hand on the breaker. If it feels to hot, it is. Shut the power off, remove it. Look at all the connection points. Is a side of the breaker discolored?

You are stretching the limits of that breaker for continuous loading especially if it’s in the sunshine.
 
Been in service for almost 10 years. All connections are tight, jaws are still tight on the buss stabs .
 
Don't assume that the connections are tight things are not getting hot. Three weeks ago I diagnosed a tripping breaker that was in service since 1972. The wire attached to it, while nice and tight, was severely nicked and was getting hot under load.
 
1. Measure Current
2. Measure Fall of Potential across CB.
3. Do as noted above in regards to temperature.
4. Resolve issue or replace CB.
 
I've had branch breakers develop weak connections in heavily loaded panels and heat the main bus enough it trips a main breaker. Look for discolored bus, or even melted plastic bus supports.
 
K8MHZ said:
My understanding is that an RMS meter won't be accurate if there are harmonics present.
Click to expand...

Unfortunately, the term "True RMS" has lost credibility due to bottom feeder meter mfrs just using it as a "marketing term". My rule of thumb is that if you spent less than $500 for a meter, trying to read harmonic current with it is a waste of time. I used to say $1,000, but people here have pointed out some good meters from Fluke that are around $500 (can't remember the numbers now). I had a Fluke 77 for years that was great at seeing RMS current coming from soft starters and SCR controllers that had chopped sine waves, but it didn't see harmonic currents worth a damn. I didn't know that until I got a Fluke 438 scope meter and compared them. The 77 showed me what a panel meter showed me which I believe was fundamental current (60Hz) only, the 438 showed me about 40% more current in higher order harmonics, which is likely seen by the bi-metal thermal trip devices inside of the breaker.

In days gone by, you may have only needed to think about harmonics when you had more than 30% of you total current being non-linear loads, which wasn't very often. But now, virtually EVERYTHING we install is a non-linear load, defined as anything with a power supply or drive. So when lighting was incandescent or had magnetic ballasts, that was linear, now we have CFL, LED, HID with solid state ballasts, computer power supplies, UPSs, VFDs, ECMs, servos, DC power supplies etc etc. ALL of those are now non-linear devices that create harmonics. Get used to having to deal with this...
 
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Do you think this is a new problem or maybe has been there forever and it is only recently that this current is being experienced?

Molded case breakers are often inaccurate and are almost never primary injection tested for accuracy due to cost. Did you replace it and get the same result?
 
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