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Old 02-19-2011, 01:20 AM   #1
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240 Amp subpanel hooked to a 40 amp breaker


This is driving me nuts. I'm in my dad's house. He has a subpanel and we've been getting tons of blackouts from a double pole 40 amp circuit breaker flipping all the time.(does the double pole mean it's 80 amp? It only has one switch with the number 40 on it). So I followed the wires to see where it went. Turns out it's the main power to the bus bars to another subpanel. Adding up the amps on the subpanel's circuit breakers it totals 240 amps. Am I missing something or did the electrician that did this totally screw up.

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Old 02-19-2011, 06:22 AM   #2
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240 Amp subpanel hooked to a 40 amp breaker


Call an electrician to find out why the 2pole 40 is tripping.

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Old 02-19-2011, 06:28 AM   #3
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240 Amp subpanel hooked to a 40 amp breaker


The total of the breaker amps has nothing to do with the panel rating. It is rare to have everything on and at full breaker power at one time. But if the main on the subpanel is lny 40A, you could definitely be overloading the main. It is a 40A breaker. Is the subpanel breaker 40A, or is the feed from the other panel 40A??

It may not have been an electrician that installed the panel. Does it have an inspection sticker on it?? Homeowners often makes additions to the electrical without knowing what is correct, or maybe wrong. Then not having it inspected to meet code.
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Old 02-19-2011, 06:56 AM   #4
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240 Amp subpanel hooked to a 40 amp breaker


What is the size of the feed over to the subpanel? If it is #8 then you have to upgrade the feed or keep the total subpanel usage under 40 amps on each side of the 120/240 volt line (up to 80 amps of 120 volt usage altogether).
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Last edited by AllanJ; 02-19-2011 at 06:58 AM.
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Old 02-19-2011, 08:30 AM   #5
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240 Amp subpanel hooked to a 40 amp breaker


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Originally Posted by msgrappling View Post
This is driving me nuts. I'm in my dad's house. He has a subpanel and we've been getting tons of blackouts from a double pole 40 amp circuit breaker flipping all the time.(does the double pole mean it's 80 amp? It only has one switch with the number 40 on it). So I followed the wires to see where it went. Turns out it's the main power to the bus bars to another subpanel. Adding up the amps on the subpanel's circuit breakers it totals 240 amps. Am I missing something or did the electrician that did this totally screw up.
Just to make sure we understand the wiring.
There is a main panel breaker feeding a sub panel.
This sub panel feeds another sub panel.

What size breaker in the main panel feeds the first sub?
What size breaker in the first sub feeds the second sub?
Which breaker exactly is tripping?

If I read your post correctly the second sub has 240 amps of breakers which would mean something like twelve 20 amp circuits. That's a lot of circuits for a 40 amp feed.

Last edited by a7ecorsair; 02-19-2011 at 08:48 AM.
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Old 02-19-2011, 01:50 PM   #6
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240 Amp subpanel hooked to a 40 amp breaker


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The total of the breaker amps has nothing to do with the panel rating. It is rare to have everything on and at full breaker power at one time. But if the main on the subpanel is lny 40A, you could definitely be overloading the main. It is a 40A breaker. Is the subpanel breaker 40A, or is the feed from the other panel 40A??

It may not have been an electrician that installed the panel. Does it have an inspection sticker on it?? Homeowners often makes additions to the electrical without knowing what is correct, or maybe wrong. Then not having it inspected to meet code.
The 3-wire feed from the main panel to the subpanel is coming off of one 40 amp 2pole breaker. the 3rd wire that is in the same conduit is hooked up to the neutral or ground bar (by the way,,,which is it, neutral or ground?
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Old 02-19-2011, 01:58 PM   #7
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240 Amp subpanel hooked to a 40 amp breaker


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Originally Posted by msgrappling View Post
The 3-wire feed from the main panel to the subpanel is coming off of one 40 amp 2pole breaker. the 3rd wire that is in the same conduit is hooked up to the neutral or ground bar (by the way,,,which is it, neutral or ground?
In the sub panel the third wire should be to the neutral bar with all the white wires. The neutral bar should not be connected to the panel case. There should also be a ground bus bar. This bar should be mounted directly to the panel case. Is there metal conduit all the way from the main to this sub?
What about the second sub panel that you mentioned?
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Old 02-19-2011, 02:11 PM   #8
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240 Amp subpanel hooked to a 40 amp breaker


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In the sub panel the third wire should be to the neutral bar with all the white wires. The neutral bar should not be connected to the panel case. There should also be a ground bus bar. This bar should be mounted directly to the panel case. Is there metal conduit all the way from the main to this sub?
What about the second sub panel that you mentioned?
There is metal conduit from the main to the sub but there is no second sub panel. Also there's no main breaker on the sub panel. I don't know if that's weird or not. It's just those three wire from the main connecting to hot bus bars and the neutral.
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Old 02-19-2011, 02:12 PM   #9
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240 Amp subpanel hooked to a 40 amp breaker


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What is the size of the feed over to the subpanel? If it is #8 then you have to upgrade the feed or keep the total subpanel usage under 40 amps on each side of the 120/240 volt line (up to 80 amps of 120 volt usage altogether).
Looks like #8. It's about 1/4" thick
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Old 02-19-2011, 02:16 PM   #10
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240 Amp subpanel hooked to a 40 amp breaker


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Originally Posted by a7ecorsair View Post
Just to make sure we understand the wiring.
There is a main panel breaker feeding a sub panel.
This sub panel feeds another sub panel.

What size breaker in the main panel feeds the first sub?
What size breaker in the first sub feeds the second sub?
Which breaker exactly is tripping?

If I read your post correctly the second sub has 240 amps of breakers which would mean something like twelve 20 amp circuits. That's a lot of circuits for a 40 amp feed.
Sorry for the misconception. There is only one subpanel coming off the main. the breaker from the main feeding the sub is a 2pole 40Amp. The breaker that's tripping is the main 2pole 40amp.
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Old 02-19-2011, 03:07 PM   #11
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240 Amp subpanel hooked to a 40 amp breaker


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Sorry for the misconception. There is only one subpanel coming off the main. the breaker from the main feeding the sub is a 2pole 40Amp. The breaker that's tripping is the main 2pole 40amp.
OK. Now, how many circuit or breakers are in the sub panel? Does the sub panel have any two pole breakers or are they all 15 and 20 amp singles?
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Old 02-19-2011, 03:20 PM   #12
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240 Amp subpanel hooked to a 40 amp breaker


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OK. Now, how many circuit or breakers are in the sub panel? Does the sub panel have any two pole breakers or are they all 15 and 20 amp singles?
There's 12 20 amp singles.
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Old 02-19-2011, 04:21 PM   #13
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240 Amp subpanel hooked to a 40 amp breaker


It sounds like you are simply overloading the 40 amp feeder to the subpanel. If so, you would have to install a larger feeder and breaker to that subpanel to stop the problem.

while the fact you have 12- 20 amp breakers in itself isn't a problem, the fact that apparently those 12 circuits are attempting to draw a total of more than 40 amps.
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Old 02-19-2011, 04:29 PM   #14
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240 Amp subpanel hooked to a 40 amp breaker


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There's 12 20 amp singles.
There is your problem. You have 12 - 20 amp 120 volt circuits drawing against one 240 volt 40 amp breaker. This 2 pole breaker can supply 40 amps to each 120 volt bus. If the 12 circuits were perfectly divided between the two bus bars and each drawing less than 40 amps the breaker won't trip but as soon a either side exceeds its 40 amp load, the breaker will trip and you will lose both sides.
Are the 12 breakers equally divided between the two sides of the sub panel - 6 on each side?

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Old 02-20-2011, 06:20 AM   #15
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240 Amp subpanel hooked to a 40 amp breaker


Buy an amp meter take readings.

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