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Old 05-05-2011, 01:25 AM   #1
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Breaker trip


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?

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Old 05-05-2011, 01:50 AM   #2
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Breaker trip


What type of wire, beyond AWG is it?

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Old 05-05-2011, 02:57 AM   #3
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Breaker trip


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.
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Old 05-05-2011, 06:41 AM   #4
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Breaker trip


An 80 Amp breaker is not sufficient to handle a 67 Amp continuous load.

Now you know why.
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Old 05-05-2011, 06:45 AM   #5
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Breaker trip


What is this circuit feeding?
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Old 05-05-2011, 06:48 AM   #6
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Quote:
Originally Posted by kbsparky View Post
An 80 Amp breaker is not sufficient to handle a 67 Amp continuous load.

Now you know why.
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.
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Old 05-05-2011, 07:00 AM   #7
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Originally Posted by NJMarine View Post
What is this circuit feeding?
There's the $64 dollar question.

The cable is NOT tripping the breaker. What the cable is connected to is.
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Old 05-05-2011, 07:01 AM   #8
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Quote:
Originally Posted by mpoulton View Post
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.
KB, this is correct. It is NOT tripping because there is 67A on an 80A breaker.
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Old 05-05-2011, 10:59 AM   #9
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Breaker trip


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?
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Old 05-05-2011, 01:44 PM   #10
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Breaker trip


Quote:
Originally Posted by sirsparksalot View Post
What type of wire, beyond AWG is it?
Quote:
Originally Posted by Scof1212
Hey guys, thanks for all the responses. The wire is 4 3 romex ( copper).
My question was based on OP's answer above. It's Romex (or, NM-B), which is listed at 60C in the ampacity chart. BUT, I suppose this isn't a factor, since 60C = 140F.

Feel free to dog me, cause I'm still learning
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Old 05-05-2011, 01:51 PM   #11
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Breaker trip


Quote:
Originally Posted by sirsparksalot View Post
My question was based on OP's answer above. It's Romex (or, NM-B), which is listed at 60C in the ampacity chart. BUT, I suppose this isn't a factor, since 60C = 140F.

Feel free to dog me, cause I'm still learning
However, #4 NM-B (Romex) is listed ashandling only 70 amps, isn't this correct?

I'm using the ampacity chart from Cerro Wire: http://www.cerrowire.com/default.aspx?id=46
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Old 05-05-2011, 04:10 PM   #12
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Breaker trip


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.

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Old 05-05-2011, 05:28 PM   #13
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Breaker trip


Quote:
Originally Posted by Scof1212 View Post
... 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?
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Old 05-05-2011, 05:32 PM   #14
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Breaker trip


Quote:
Originally Posted by Speedy Petey View Post
KB, this is correct. It is NOT tripping because there is 67A on an 80A breaker.
In theory, perhaps. Real-life experience has shown that a high continuous load such as this can, over time, cause a breaker to weaken and eventually trip out. Add in some hot sunlight and voil! Nuisance tripping. As the OP has discovered.

I would never connect such a lighting load in this manner, as problems would eventually develop ....
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Last edited by kbsparky; 05-05-2011 at 08:11 PM.
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Old 05-05-2011, 05:58 PM   #15
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Breaker trip


Yeah, I don't have a true Rms meter. So is the overall consensus that I shoul upgrade to 100 amp breaker and 2 3 romex? Or will that not cut it either? Or should I go get one of those meters first?

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