Main Breaker Tripping.. Incorrect Assumptions?

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textmi

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I designed a project with an existing panel loaded close to max amp rating. The panel is filled with HVAC loads (rooftop units with DX cooling).

It is a 460v/3ph, 200 A panel and my total FLA calculates to be 195 A total which includes 25% of the largest motor (a compressor in one of the RTUs). I did not assume any load was a continous load since HVAC loads generally arent continous load from my undersrtanding. The panel main breaker is currently tripping in the afternoon with the recent heat.

Looking at the building automation system the RTUs aren't even operating at 100% cooling capacity. The main breaker has trip settings but my understanding is you shouldn't be adjusting those settings without a coordination study.
 
Do any load checks your self.
Compare to what you calculated.
You may or may not have an issue with load.
May have a issue with a component.
Like a main breaker, buss connection or wire connection.

When ever I design build something in the field I check the actual against calculated.

Plus I know when other fingers were in my pie.
 
A 200 amp breaker will probably only have a magnetic trip adjustment, and it will just be a min/max dial, with no multiplier. Adjusting that should not require an engineered study. If it’s at minimum, bumping it up should not pose an issue. You may have several compressors kicking on at the same time that’s tripping it on startup.
 
Another thing to check for, is a bad second stage compressor. I have had quite a few of those over the years trip out ground fault mains. Have EMS force on half of the second stage compressors, if that doesn’t do it, turn those off, and force the other half on.
 
If your breaker has adjustable settings it may also have a trip history. If you don't know what function is tripping the breaker you won't know what to correct.
 
Here are what the settings are currently adjusted to. The breaker is a Siemens Sensitrip III and is ground fault protected.. I have no experience with modifying these settings. Could someone help? The panel is an MLO so this breaker is on the distribution panel in the same room.

Breaker settings.JPG
 
textmi said:
Here are what the settings are currently adjusted to. The breaker is a Siemens Sensitrip III and is ground fault protected.. I have no experience with modifying these settings. Could someone help? The panel is an MLO so this breaker is on the distribution panel in the same room.

View attachment 2566395
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Being ground fault protected, I would definitely be looking for a bad second stage compressor. Being 200 amp, there is probably not that many units, so having EMS turning on each unit at full tilt individually could locate it fairly quick.
 
Have a HVAC mechanic check the refrigerant charge, they depend on the return suction to cool the compressors.
 
mtnelect said:
Have a HVAC mechanic check the refrigerant charge, they depend on the return suction to cool the compressors.
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The individual breakers to the units aren't tripping though, only the main breaker. Although they did install two breakers which are 5 amps over the MOP... that needs to be corrected.
 
textmi said:
The individual breakers to the units aren't tripping though, only the main breaker. Although they did install two breakers which are 5 amps over the MOP... that needs to be corrected.
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Based on the settings you posted, I can believe the 200A breaker is tripping before a downstream one. Your ground fault setting is at 20% of the breaker sensor rating for 1sec. A typical motor overload relays are typically 6X For 20 sec, so many small breakers are maybe 10X for 20sec.

If the breaker sensor is 250A, set for 200A, the ground fault will trip at 50A in 1 sec, which is often well before a standard branch breaker even as small as a 15A one.
 
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jim dungar said:
Based on the settings you posted, I can believe the 200A breaker is tripping before a downstream one. Your ground fault setting is at 20% of the breaker sensor rating for 1sec. A typical motor overload relays are typically 6X For 20 sec, so many small breakers are maybe 10X for 20sec.

If the breaker sensor is 250A, set for 200A, the ground fault will trip at 50A in 1 sec, which is often well before a standard branch breaker even as small as a 15A one.
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I believe the manual states 0.1 sec, not 1 sec. It's hard to see in the picture.The largest load on the panel is a 102A MCA unit with the largest compressor LRA of 125A. The breaker is 200A so 20% would only be 40A. Isn't ground fault only for if there's a fault? (Not an instantaneous starting load). The instantaneous pickup is set at 3 so that means 600A instantaneous current before tripping?

The manual also says if the any setting lands "in between" settings then everything is overriden to the most stringent values. Maybe 3 is not an actual value and it's use the override.
 
textmi said:
I believe the manual states 0.1 sec, not 1 sec. It's hard to see in the picture.The largest load on the panel is a 102A MCA unit with the largest compressor LRA of 125A. The breaker is 200A so 20% would only be 40A. Isn't ground fault only for if there's a fault? (Not an instantaneous starting load). The instantaneous pickup is set at 3 so that means 600A instantaneous current before tripping?

The manual also says if the any setting lands "in between" settings then everything is overriden to the most stringent values. Maybe 3 is not an actual value and it's use the override.
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If you have a bad second stage compressor, you can have a ground fault.
 
Can you post a link to the instructions for that breaker.
I would like to read them if available.
Or a breaker # brand etc.

Any load reading information.
 
textmi said:
The panel main breaker is currently tripping in the afternoon with the recent heat.
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Big clue there. In heat waves, the peak heat is in the afternoon, so the peak load on the grid is as well. A slight voltage drop on mostly motor loads will result in a large increase in current. So before going off chasing phantoms, eliminate the obvious first. Put a recording meter on this and see what the current is leading up to a trip. It may be doing EXACTLY what it is supposed to be doing.
 
textmi said:
HVAC loads generally arent continous load
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That's from the very first post. That's very likely where you went wrong. HVAC loads should be considered continuous unless you have a very good reason to say that they aren't. If the units are sized correctly, they'll operate nearly continuously during the hottest parts of the day on the hottest days of the year. Also if there is an extended power outage, they'll operate continuously after the power comes back on, to get the building cool again.
 
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