maxed out circuits ok til a few years later

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Stevenfyeager

Senior Member
Location
United States, Indiana
Occupation
electrical contractor
Two stores this week, a Tire store and a convenience store called with breakers tripping. The one store has 3 refrigerators totaling 19.6 amps on a 20 amp breaker. (by nameplates) The other store has two tire machines 15 amps each (nameplates) on a 20 amp breaker. The tire store had someone run the circuit two years ago, and the other store probably plugged in the three refigerators a few years ago. I realize someone should have not loaded these circuits like this but why did they work for a few years, but not now? Thank you.
 
Breakers get weaker over extended periods of being at maximum capacity. Don't know if a breaker manufacturer will agree, but I've seen a lot of old circuit breakers in my day.

A new breaker MIGHT put them back in operation temporarily again, but obviously is not the correct long-term solution.

The breakers are only seeing that maximum when every device is on at the same time, so that's only part of the time, and they might just be getting lucky with a machine shutting down before the breaker trips, especially as I'm picturing a tire machine as something with a low duty cycle. Unfortunately, the only duty cycle considerations that allow you to use a larger circuit breaker than normal, that I know of, is for welding machines, per Art. 630. That said, I know a lot of shops that have put their 20A chop saw receptacle on a 30A breaker so they can bog it down when they need to. Not Code-compliant, but ...
 
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Also consider a possible change in load profile over time. The refrigerators may have dirt buildup on the condenser coils causing higher pressures and duty cycles. Could the tire shop be getting busier and running the machines more?
Regardless, it sounds like additional circuits are warranted.
 
I always hate these types of jobs it's hard trying to explain to a customer that new ckts should be installed when all they know is that " well it's been working for two years" I always think that the customer just thinks we're all the same and trying to rip them. Wich is not the case

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