After replacing the main panel board of our 2.5HP split type aircon. We started to experience tripping of our MAIN 150 Amp Circuit Breaker (CB1). This 150 Amp Circuit Breaker is outside our house (at the Electrical Post) which is standard in our Country. I wanted to have the Circuit Breaker trip to happen inside our house so I installed a similar 150 Amp (CB2) (same brand) inside our house. However, when the trip happens it still happens at the outside circuit breaker (CB2). The tripping is intermittent it only happens when certain appliances are operated at certain conditions. We have isolated that when we a. use the 2.5 HP aircon tripping happens b. use the Water Heater (at highest level (level 3). It causes tripping. However, the tripping always happens at CB1 not at CB2?. After the initial rewiring I was expecting that the tripping should happen at the inside CB.

The 2.5hp aircon is connected to 30amp CB which is connected to a 100amp sub-panel CB then connected to the inside 150amp main CB before being connected to the 150amp electrical post CB. But always tripping happens at the 150amp electrical post CB.

The Water Heater is also connected to 32 amp CB which is connected to a 60amp sub-panel CB then connected to a 100amp sub-panel CB then connected to the inside 150amp CB before being connected to the 150amp electrical post CB.

The Water Heater and 2.5HP aircon are on not on the same sub-panel which are both 100amp.

But always the tripping happens at the outside 150Amp CB outside the house.

Questions: a. Why is the tripping happening at the outside 150amp CB not at the inside 150amp CB? b. Why are not the inside CB not tripping: 30amp, 100amp, 150amp?

  • Where are you on this planet? – ThreePhaseEel Aug 1 '20 at 0:28
  • Also, does the tripping happen immediately when one of the culprit appliances is turned on, or after the appliance has been running for a little while? – ThreePhaseEel Aug 1 '20 at 1:10
  • Do any of these breakers have a TEST button on them? – Harper - Reinstate Monica Aug 1 '20 at 5:52
  • Thanks for your answers. To answer some of your questions - a. I live in the Philippines b. Breaker Trips not at the start but a few minutes after the device is running. c. No TEST BUTTON in Circuit Breaker. – Nesty Aug 1 '20 at 10:40
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    Something def. is off here. It doesn't seem like the loads mentioned should come anywhere near tripping a 150a breaker. Getting an "amp clamp" hand held digital meter is the only way you'll be able to track this down. With absolutely no prejudice intended and not knowing your layout, is there any chance someone else is stealing power? Just asking bc something is really off here. – George Anderson Aug 1 '20 at 14:04

A: Breaker trip ratings aren't exact. They have to fit a certain profile, but there are tolerances in manufacturing that will cause one circuit breaker to trip earlier than another equally marked breaker, even from the same production line. Both fit within the prescribed limits and both get labelled with the same amp rating. This is the first reason why a pair of breakers with the same amp rating will trip according to their own invisible preferences rather than yours.

B: You've identified 62 amps of breaker capacity corresponding to the air conditioner and water heater. We don't know whether those actually pull those levels of current -- their actual consumption should probably be only 80% of the breaker value (or even less). These two loads should amount to about 1/3 of the 150 amp breaker's rating. It's worth noting that start current for the air conditioning compressor could be substantially higher, but circuit breakers are designed to not trip instantly specifically to deal with startup currents.

Are there other loads also running which haven't been accounted? It could be that the sum of all loads distributed across all those panels works fine, but the addition of those two big loads approaches the limit of that 150 amp breaker.

It's also possible that there's just something wrong with that breaker and it trips at much lower current than it should. Breakers can fail.

If you can acquire the use of a clamp-on ammeter you'll be able to check the currents on the various circuits and feeders. That will reveal whether you have an overcurrent problem or a faulty breaker.

  • Thanks for your answers. I am still wondering why the new 150 Ampere CB installed inside the house is not tripping but the 150 Ampere CB outside the house trips. We are still investigating if there is an issue in the line connecting the 2 150 Ampere Breakers. – Nesty Aug 1 '20 at 10:32
  • @Nesty You are missing the point others have made about the new vs. old breaker. They said it is likely the old one is wearing out and tripping more easily than the new one. – George Anderson Aug 1 '20 at 13:30

Once a breaker trips multiple times, it starts to take less and less to make it trip again. The bi-metal strips used to sense an overload by bending inside of the breaker develop what's called a "thermal memory" in that they stop returning all the way to their original position when they cool down, so they need less load to make them trip again. Replace the main breaker.

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I have to jump to a conclusion that you added the indoor breaker because the outdoor one tripped numerous times, this could make the tripping mechanism more sensitive.

But it could be ambient heat, maybe a loose connection creating extra heat, or just a difference within manufacturing tolerance or an manufacturing alteration that resulted in an different actual trip point within the tolerance. I would guess most manufacturers probably have a tighter tolerance that allowed by NEMA, but if you look at the "unknown band" for 1.5x the rating you can see NEMA specs require tripping from about 2 to 5 minutes. (Sorry about the image being so large.)

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