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Breaker trip

3K views 24 replies 10 participants last post by  Leah Frances 
#1 ·
Hello. I have an 80 amp circuit. Dual pole 80 amp breaker, number 4 wire, 50 foot run. I am running 67 amps on this circuit. Everything works fine until it gets hot out side. Then the breaker starts tripping. If I reset it later, when it has cooled down outside it works fine. What would be the proper fix for this?
 
#3 ·
Circuit breakers trip via two mechanisms. On very high current faults, there is a magnetic device that trips the breaker instantly. On lower-current faults, a little element inside heats up and trips the breaker. If the breaker is artificially heated to too high of a temperature, the thermal trip mechanism can activate from the heat without there being an overcurrent. That sounds like what's happening. This is almost always caused by a loose connection on the breaker. It may be the breaker's connection to the panel bus, or a wire connection to the breaker. Remove the breaker and look for any signs of overheating on the bus tab or the fingers on the back of the breaker. Make sure it seats snugly on the bus when you reinstall it. If it feels loose or shows signs of heating, sand the bus tab clean (POWER OFF!) and replace the breaker. Also check the connections to your cable. Loosen and re-torque the connections, using anti-oxidant if it's aluminum.
 
#6 ·
If the load is continuous, then the breaker and wire should be sized for 125% of 67A, or 84A. However, an 80A breaker is one which will carry 80A forever without tripping. Only a current above 80A will trip it. Tripping at 67A is abnormal and indicates a heating problem.
 
#7 ·
There's the $64 dollar question. :icon_rolleyes:

The cable is NOT tripping the breaker. What the cable is connected to is. :whistling2:
 
#9 ·
Hey guys, thanks for all the responses. The wire is 4 3 romex ( copper). I checked all connections and the are tight. The bus gar does not look abnormal in any way. I am running only lighting on this circuit. Digital ballasts. The main pannel where the breaker is, is on the east side of the house an gets hit by the sun all day. As soon as it gets above 85 outside it trips. So is this whole line to small for 67 amps?
 
#13 ·
... The main pannel where the breaker is, is on the east side of the house an gets hit by the sun all day. As soon as it gets above 85 outside it trips. So is this whole line to small for 67 amps?
You have to account for ambient temperature correction, it would seem.

The sunlight is heating up the guts of your panel, making the breaker think it is overloaded, even though it may not be. Since many breakers have thermal trip mechanisms, this may be your culprit. That, and a weak breaker, since it is carrying a heavy continuous load to begin with.

Did you replace the breaker? Is it possible to split that circuit into more, smaller circuits?
 
#12 ·
If you are using an ordinary ammeter to measure the 67 amps, that only measures the peak of the waveform and divides by root(2) to get the rms. A ballast creates significant harmonic content. Measure the amps with a true-RMS meter and see what you get. The true-RMS value is what the CB responds to.

Mark
 
#17 ·
If you've replaced the breaker and gotten the same result, then I would conclude the load is probably above 80A RMS and your meter is not displaying it properly. You'll need to measure the actual load, or at the very least add up the current ratings of all the devices on the circuit. You'll probably need a feeder and breaker rated for more than 100A, since your existing load seems to be more than 80% of that already.
 
#23 ·
Depending on whether his circuit is drawing the measured 67A (probably inaccurately low) or something more than 80A, this is between 16kW and 20kW+. That is one HECK of a greenhouse. That's the sort of thing the power company will undoubtedly notice and report to the DEA. Even if this is not a grow op, I think he can expect to attract some law enforcement attention with this kind of power draw at a residence.
 
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