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I've rented a space where the transformer and main panel are fenced off and I can’t access it without calling the landlord. He lives rather far away and is the only one with the key and that kind of potential downtime would be catastrophic for my business, however I do need to pull a lot of power consistently and will be pushing it as hard as I can which means I might occasionally trip on hot days.

The hypothetical setup is as follows: A new 125 A cutler hammer breaker on the service side A new 125 A square d breaker backfeeding our main sub panel (in our building) About 120 feet of underground 3/0 aluminum between the two

My question is: If our equipment pulls more that 125 A which breaker trips first, the one on the service side (outside) or the one on the load (in our building) or both?

If the answer is the one on the service side or both (or either depending on manufacturing tolerance variation or whatever) my followup question is: Is there any way we can purchase or modify our inside 125 A breaker to flip at ~123 A so that it always flips first?

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Your question depends on very many variables.

1) Are the breakers magnetic trip or thermal? Which is which?

2) If you are using thermal breakers, which breaker is exposed to the highest ambient temperature?

3) What is the current profile of your high-current loads? Relatively constant or fluctuating?

In general, it is a crap shoot as to which breaker trips first when two identically-rated breakers are in series. The breaker with the smallest amount of internal thermal heat-sinking will usually trip first but there are no guarantees.

Your best bet is to either change the breaker that you have access to down to 100 Amps or increase the feed breaker to 150 Amps. I'd have to check, but I think that your 3/0 feeder cable is rated for at least 150 Amps. Check with a qualified electrician to make sure.

  • Thanks for the information! I'm googling now but is there any easy way to tell if my breakers are thermal or magnetic trip? Which is more common. – Mauzy Virginia Jun 18 '15 at 23:52
  • Unfortunately due to the parameters I'm working with I can't upgrade the outside one or shrink the inside one. – Mauzy Virginia Jun 18 '15 at 23:53
  • My load will be mostly constant. – Mauzy Virginia Jun 18 '15 at 23:53
  • Thermal-trip breakers are by far the most common style. – Dwayne Reid Jun 18 '15 at 23:54
  • The only thing that I can suggest is that if you do indeed have thermal-trip breakers at both locations, add some cooling to the breaker that you don't have access to. Run an air conditioner in the room or whatever it takes to keep that breaker many degrees cooler than the breaker that you have access to. Note that I do NOT suggest that you heat any breaker - only cool them. – Dwayne Reid Jun 18 '15 at 23:55
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There really isn't a definite answer for which trips first (under some fault conditions a short on a circuit with a 20A breaker may trip the 200A main instead - but not every time.)

Under most conditions, a 125A breaker won't trip at 125A unless, perhaps, you pull 125A for a LONG time. How fast they trip is related to how much, and for how long, excessive current is drawn.

If you have a business case for needing it not to go out, you have a business case for a 200A feeder. Running anything right to the limit is a recipe for failure, and that includes your electrical service feed. If, for whatever reason, you cannot go to a 200A feeder, you might want to put in at minimum a current meter so that you or your employees can monitor the actual load (before turning something else on, or to know that they need to turn one off to turn another on), and on the more expensive side of things an automated load shedding system which can shut down lower-priority or non-essential loads to limit the overall draw. Generally, a 200A feeder will cost considerably less than a load-shedding system.

Alternatively, step down to a 100A main breaker on your local breaker.

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It depends on if the system was properly coordinated (code uses this term). my current plant I have a problem where a 125a load center will trip out a 800 a feeder, the trip curves were not used to set the system up ( no shorts just equipment start up) this problem doesn't happen very often so the owners don't want to spend the $ to fix it. You may run into a similar problem, we now have a start sequence on this load center starting the largest motor first once that is up to speed then the next largest after that all the smaller motors can be started at the same time. You may need to develop a starting procedure to prevent the main from tripping.

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