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Old 02-23-2010, 02:25 PM   #1
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Where to branch from on the main for a sub


I was talking to someone about my sub panel and was told that when I run the 100 amp breaker from my main that it should be off of the top breaker. The reasoning is that the lower on the bus bar it is the further the load has to go on your bar which is bad. My top breaker where I was going to run my range from. So do you think it is better to pull the 40 amp and run my sub from that slot and my range from my sub OR put the 100amp further down the bar?

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Old 02-23-2010, 02:42 PM   #2
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Where to branch from on the main for a sub


Makes no differance!

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Old 02-23-2010, 03:10 PM   #3
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Makes no differance!
I agree.
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Old 02-23-2010, 04:36 PM   #4
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Originally Posted by Abs777 View Post
The reasoning is that the lower on the bus bar it is the further the load has to go on your bar which is bad.
Well, the lower down the bus bar you go with a major circuit breaker, the more voltage drop you get in all circuits above it because of the current flow through the bus bar.

But the voltage drop is negligable. After all, if we assume the bus bar has the resistance equivilent to #4 copper, a 200 amp draw through a two foot long buss is only going to have a total voltage drop of 0.12 volts.

Now because their is some drop, it makes sense that in new construction, you put the largest loads at the top of the bus. But after the fact, it's just not worth the hassle of rewiring an entire panel for such a tiny difference.
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Old 02-24-2010, 12:14 PM   #5
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Originally Posted by HooKooDooKu View Post
Well, the lower down the bus bar you go with a major circuit breaker, the more voltage drop you get in all circuits above it because of the current flow through the bus bar.

But the voltage drop is negligable. After all, if we assume the bus bar has the resistance equivilent to #4 copper, a 200 amp draw through a two foot long buss is only going to have a total voltage drop of 0.12 volts.

Now because their is some drop, it makes sense that in new construction, you put the largest loads at the top of the bus. But after the fact, it's just not worth the hassle of rewiring an entire panel for such a tiny difference.
Did someone at Home Depot tell you that.
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Old 02-24-2010, 01:33 PM   #6
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Originally Posted by J. V. View Post
Did someone at Home Depot tell you that.
Tell me what?

I've just done some simple math, and I qualified where my numbers are coming from (I assumed you could find for yourself that #4 AWG copper has .000292 ohms of resistance per foot).

Beyond that, I've had a contractor tell me he likes the largest loads at the top of the panel, and I can only think of a few reasons you would want to put the largest loads at the top (assuming the panel is not upside down and all wires enter through the top).

1. Reduce the amount of voltage drop through the bus
2. Save a few pennies in materials by having the larger wires be the shortest in the box.
3. Save overall space in the box by limiting larger wires to the top (though I'm not sure what that buys you).
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Old 02-24-2010, 01:50 PM   #7
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Where to branch from on the main for a sub


I always thought it had to do with reducing heat in the load center. Further the high voltage/amp runs up the bars, the more heat generated, which could also potentially impact the breakers inbetween. No science, just what I've heard and what I've thought through......

...but I'm very interested to hear what the experts say on this!
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Old 02-24-2010, 02:08 PM   #8
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I do not doubt your numbers. And I understand not all contractors do things the same. I just do not see the importance of your statements regarding voltage drop.
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Old 02-24-2010, 03:00 PM   #9
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I do not doubt your numbers. And I understand not all contractors do things the same. I just do not see the importance of your statements regarding voltage drop.
That was sort of my point...

There is indeed a voltage drop, and I've tried to show a worst than real world worst case just to put a number on it. As as you can see, that number is so small as to not be important.
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Old 02-24-2010, 03:06 PM   #10
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Originally Posted by Itsdanf View Post
I always thought it had to do with reducing heat in the load center. ...
Well, when you have a voltage drop, the voltage is in essence lost to heat. With the sample numbers given, 0.12 volts at 200 amps translates into 24 watts. But then this was a worst than worst case.
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Old 02-24-2010, 06:38 PM   #11
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Where to branch from on the main for a sub


Didn't some of the older panels have the fins on the bus bars arranged that only the slots lower down had alternating legs of the 240 volt power supply where a double wide breaker for 240 volts would span both legs?
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Old 02-24-2010, 07:40 PM   #12
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Originally Posted by HooKooDooKu View Post

There is indeed a voltage drop, and I've tried to show a worst than real world worst case just to put a number on it. As as you can see, that number is so small as to not be important.
Exactly, which is why it DOES NOT matter where you put a feeder or branch circuit breaker.

Actually, your numbers above are off. Why would you assume a buss equivalent to #4 carrying a 200A load??? I would say a panel buss has far less resistance than a #4 conductor.
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Old 02-24-2010, 11:52 PM   #13
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Actually, your numbers above are off. Why would you assume a buss equivalent to #4 carrying a 200A load??? I would say a panel buss has far less resistance than a #4 conductor.
I would agree.

I didn't know what the size of the bus was, but figured it couldn't be smaller than (i.e. MORE resistance) than #4. But it does give you something of a gross order of magnitude guestimate. After all, if you drop the size to #1, the resistance is only cut in half.

Now drop the current to 100A (a plausable draw if you source a sub-panel from the bottom), you cut the numbers in half again. So perhaps more real world values would be about a 0.03 volts drop and 3 watts of heat.
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Old 02-25-2010, 01:31 AM   #14
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Where to branch from on the main for a sub


Yeah, and if you're worried about a loss of 3 watts to heat in your panel because you put your breaker a few inches down the bus, let's not even get into the total loss of all the wiring around your house! I'd say in a fully loaded house with a large amount of electrical equipment, drawing heavy loads, you could easily be losing the equivalent of a 60 watt bulb - or even more on long wire runs - to heat across all your wiring!



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