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Discussion Starter #1
Hello everyone. I am putting in a 200 amp Square D panel. The panel is in a utility room, and there is a main shutoff on the back of the house. I have a couple of questions about the cables that to the panel from that main shutoff.

1. My existing service cables are 2/0 aluminum. I read that a 200 amp panel should get either 2/0 copper or 4/0 aluminum. I find that kind of confusing, because isn't copper a better conductor than aluminum? And isn't 2/0 bigger than 4/0? I'm confused why a better conductor would need the bigger cable. But I suppose my bigger question is, is 4/0 aluminum a minimum standard, and would 2/0 aluminum be ok?

2. I noticed on my panel that the neutral lug is slightly smaller than the two hot lugs. Does this mean the neutral cable ought to be smaller as well? If not, why is it smaller?

The panel is a Square D QO plug on neutral load center.

Thank you very much for any thoughts. I really appreciate it.
 

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1\0 is the size larger than #1. As the numbers go up, you are dealing with large wires.

So 4/0 is bigger than 2/0.
 

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2/0 copper is allowed for main panel feeders in dwellings. 4/0 aluminum is equivalent and allowed. 4/0 is bigger than 2/0.
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Discussion Starter #4
Ok. Thanks. I mistakenly thought it was the other way around, like 10 gauge vs. 12 gauge. What scale or unit of measure is used for those service cables? Just trying to learn here. Thanks.

So, looks like my existing service entrance cables are undersized for aluminum and need to go....

Also, does anyone know the answer to my second question, about the neutral lug in the panel?

Thanks again.
 

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Discussion Starter #6
I'll have to double check the day after tomorrow---I'll be away for 24 hours tomorrow.... But I'm pretty sure the cable from the meter to the outside disconnect is 2/0 aluminum.

The full arrangement on the back of the house looks like this----there are four boxes. The one box is where the cable comes up out of the ground. To the left and right of that box are two meters----the house is a duplex. The meter on the right feeds into the fourth box, which is the disconnect for one side of the double.

In the box that receives the utility cable from the ground, there are metal bars that travel into the boxes with the meters.
 

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The neutral is not used on straight 240 n.v volt loads so there is no neutral current.
 

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Discussion Starter #9
I has been rainy here, so I haven't been able to open up the boxes on the back of the house to look at the cable size. I think it's going to rain more today.

Does anyone know, have 2/0 copper or 4/0 aluminum always been the standard for a 200 amp service? Has my 2/0 aluminum cable always been undersized? Or was it considered the standard at one time? If it was the standard at one time, does anyone know why it changed?
 

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How old is the existing service? Are you sure the conductors are aluminum and not tinned copper? You still will need for conductors if there is a disco outside unless there is a metallic conduit adjoining the disco and the load center inside.
 

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Discussion Starter #11
The service dates from the mid 80s. How would I tell if it was tinned copper and not aluminum? When you cut it, it is silver like aluminum through and through.

I have also discovered that it's not 2/0 cable. I thought that's what the sheathing said, but only so much of the cable is accessible to read the markings, and the markings are somewhat smudged. I took a scrap from the end of it to the home center to compare to 2/0 and 4/0, and it is definitely bigger than 2/0, and looks to be ever so slightly smaller than 4/0. Although it looks closer to 4/0 than 2/0. Did they ever use 3/0 for service cables?

There are just three cables, not four. All the same diameter. There is no grey jacket around all three of them. They are individual cables that enter the panel through conduit that runs through the attic. Two hots and a neutral. No separate ground. The ground and neutral were bonded in the panel, and that was it.
 

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Discussion Starter #12
I remembered a pic I took of the inside of the cutoff box not long ago. I was able to zoom in on the cables. I don't know how to attach the image here---it wants a URL?

Anyway, the notation says, "CAPWIRE AL 2/0 AWG TYPE USE OR RHW OR RHH 600V," and there's a little more that I can't read. I'm almost positive it says 2/0. But this cable is noticeably bigger than the 2/0 SER at the home center. Not even close.

All of the cable back there at the rear of the house with the various boxes appears to be the same.
 

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It sounds li,e your existing service was not 200 amp rated. Can you read the existing breaker handle?
 

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Discussion Starter #14
The main breaker on the back of the house says 200A. So does the one in the panel.

I know the cable says 2/0. But it is obviously larger than the 2/0 cable I compared it to at the home center. I got curious, because I started to wonder just how big 4/0 cable would be, if the 2/0 at my place was as large as it is. So I thought I'd snip a bit off and take it with me to compare.

The old panel is a 200A Crouse Hinds panel with 24 breaker spaces. It is a very small house---about 800 sq ft; but even so, the panel was pretty close to maxed out. The place had baseboard heaters, and the panel took 1/2" breakers, so there were a number of those.
 

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Discussion Starter #17
Yep that looks like the same panel, with the neutral off to the far right, bonded to the ground.

I figured out my confusion. The cable is 2/0. I compared it again to the cables at the home center and stood there asking myself, What am I missing? The 2/0 cable at the home center? It wasn't 2/0-----it was 2. They don't even stock any 2/0. So yeah, my 2/0 cable was not even close to the same size as 2 cable. And the 4/0? Yeah, it's bigger than my 2/0. Not by a ton, but yeah it's bigger. Mystery solved. The cable that goes from the shutoff to the panel is 2/0 aluminum and needs upgraded to 4/0.

Can you do that at the shutoff without pulling the meter? Replace the cable from beneath the lugs that are shut off, when the lines coming in on the other side of the breaker are hot? I wouldn't think it would be different than working on circuits in your panel with the main breaker shut off. But I figure I ought to ask. I accord this stuff a healthy respect. My goal is to be self-sufficient. Not dead.
 

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Discussion Starter #19
Did you have any problems getting the old lugs to unscrew? Here is a picture of the shutoff on the back of my house. The cables going out the top of the box need updated with bigger cables. But I don't want to pull the meter if I don't have to.

 

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Discussion Starter #20
Here is the full arrangement of stuff on the back of the house. The meter to the left is for the second half of the duplex. Its power is not shut off. The shutoff in the above picture for the side of the duplex I am working on is the box on the far right, barely in the picture.
 
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