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?
Best Answer
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.