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Old 10-07-2019, 05:13 PM   #1
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Default Two 100amp breakers on same leg?

Hi everyone, I'm a complete novice compared to you all, and I figured you could make quick work of this question:

I'm in the process of identifying/labelling circuits in the house we bought last year, since the old labels aren't overly accurate or descriptive. When opening up the panel, I see that there are two 100amp breakers.. not one on each leg, but on top of each other.

Can anyone here give me a brief description of my setup?

Here is an image of my panel,
https://photos.app.goo.gl/w3XKDoWJyYWiHi176





Last edited by Dennis Alwon; 10-07-2019 at 07:32 PM.
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Old 10-07-2019, 05:52 PM   #2
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You need to see an electrician.
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Old 10-07-2019, 05:56 PM   #3
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This site is for electrical pros only. Please post on... https://www.diychatroom.com/
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Old 10-07-2019, 06:47 PM   #4
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OK well, perhaps someone here is willing to spare 2 minutes to look at the picture. Thanks in advance.
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Old 10-07-2019, 06:48 PM   #5
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I have no idea what is going on.

It seems like someone backfed a 100A breaker to feed a larger sized split-bus panel. The feeder looks newer than the rest of the wiring.

Get an electrician in there to replace the panel.
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Old 10-07-2019, 06:50 PM   #6
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OK well, perhaps someone here is willing to spare 2 minutes to look at the picture. Thanks in advance.
While you're trying to upload the picture, please read the user sign up agreement before asking why you can't upload pictures.
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Old 10-07-2019, 07:35 PM   #7
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Now I am that much closer to having seen everything.
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Old 10-07-2019, 07:39 PM   #8
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For lack of any other explanation I must agree it is a split buss. The wires that go behind the buss must be connected to the lower part of the buss. The main breaker is not really necessary at all- it's redundant in this case
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Old 10-07-2019, 07:40 PM   #9
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I will leave it open for discussion purposes
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Old 10-07-2019, 08:06 PM   #10
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I will leave it open for discussion purposes
Thanks Dennis ... I'm curious now too. Op is not DIY'ing just wants to map his circuits.

My best guess is a split panel, and the top 100A breaker is just to feed the 40A to the right of it ??

Weird how the 100A (which should feed the buss) is looped around to the top MLO terms.
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Old 10-07-2019, 08:14 PM   #11
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Thanks Dennis ... I'm curious now too. Op is not DIY'ing just wants to map his circuits.

My best guess is a split panel, and the top 100A breaker is just to feed the 40A to the right of it ??

Weird how the 100A (which should feed the buss) is looped around to the top MLO terms.
It’s not feeding the lugs at the top. Those conductors coming out of the large 100 Amp breaker Loop up and then back down behind all the breakers to the lower part of the panel. You can see the conductor in the back if you look underneath the large 100 amp breaker. That is the factory way to feed the lower half of the split bus panel.

That small 100 amp breaker is feeding the entire panel.
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Old 10-07-2019, 08:18 PM   #12
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Edit: Hax beat me to it.
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Old 10-07-2019, 09:09 PM   #13
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If the long 100amp breaker was always there with the wires looped around and down to feed the lower buss, and the short 100 amp breaker is now the Main for the whole panel, is it possible that someone came in and didn’t understand how a split buss panel works?

In other words a home inspector or inexperienced electrician saw the original set up and said “this is an illegal setup”. “It needs a main breaker”. Not realizing that the top breakers grouped together were all main service disconnects. By the way, is there anything marked on the cover as a main disconnect? Usually the “Main service disconnect” is a sticker on the breaker, and the circuit directory has the loads description. I only see a furnace sticker.
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Old 10-07-2019, 09:10 PM   #14
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It’s not feeding the lugs at the top. Those conductors coming out of the large 100 Amp breaker Loop up and then back down behind all the breakers to the lower part of the panel. You can see the conductor in the back if you look underneath the large 100 amp breaker. That is the factory way to feed the lower half of the split bus panel.

That small 100 amp breaker is feeding the entire panel.
Your eyes are better than mine ... I had to look 10 times and then I saw the top conductors going behind. I thought it was on the top at first look.
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Old 10-07-2019, 09:16 PM   #15
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If the long 100amp breaker was always there with the wires looped around and down to feed the lower buss, and the short 100 amp breaker is now the Main for the whole panel, is it possible that someone came in and didn’t understand how a split buss panel works?

In other words a home inspector or inexperienced electrician saw the original set up and said “this is an illegal setup”. “It needs a main breaker”. Not realizing that the top breakers grouped together were all main service disconnects. By the way, is there anything marked on the cover as a main disconnect? Usually the “Main service disconnect” is a sticker on the breaker, and the circuit directory has the loads description. I only see a furnace sticker.
Those conductors feeding the panel look new and smaller than what would be run through that pipe. For some reason I have a feeling that the original feed was 150 or 200 A and landed on the main lugs. Then for some reason they changed it to a 100 and feed and backfed the 100 amp breaker.
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Old 10-07-2019, 09:22 PM   #16
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The other day someone showed a picture of a new Square D generator ready panel. How is that panel legal. It looked like the old split buss panels. I could see if there were two feeds in the panel. But this has one buss being fed from another buss in the same enclosure?
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Old 10-07-2019, 11:05 PM   #17
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The second DP breaker at the upper right ( poles 6,8) could be omitted if the bottom bus was fed via the top ex-Mains lugs.

Logically, poles (2,4) are being back-fed. They energize the top buss. This includes energizing the MLO bus extension seen at the top.

It's not evident, but I suspect that poles (6,8) are off of a reduced breaker, say 50-60 Amps.

Then the wiring would make sense.

This MLO set-up probably has a disconnecting means up the line.

So the 100 Amp breaker really is redundant -- as noted above.
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Old 10-07-2019, 11:53 PM   #18
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My take is that this is a split bus panel that has been converted into a main breaker panel. Other than being a little confusing to the uninitiated, I think it's a good way to avoid ripping out what is otherwise a functional panel. All the split bus panels I've ever seen had no way to disconnect them besides pulling the meter and needed to be ripped out. This little workaround was a money saver.
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Old 10-08-2019, 12:53 AM   #19
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That main breaker looks like a 125A. Anything over 100A on SQ goes to this frame size
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Old 10-08-2019, 01:56 AM   #20
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That main breaker looks like a 125A. Anything over 100A on SQ goes to this frame size
When this panel was originally installed, anything 70 and higher was the long style.
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