Adding another subpanel/building...Can this work?

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Old 11-19-20, 07:41 PM
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Adding another subpanel/building...Can this work?

Earlier this year, based on the advice of many here, I ran electrical to a shop (let's call it "A"). It has 100A of service and I ran 90' of 2AWG conductors and a 6AWG ground. Now, due to some changes in the way we need to do things, I may need to add another building. The thing is, the best place to grab power would really be off of the 100A subpanel in "A" and run more wire to "B". It should be no more than 130'. The thing is, I don't know if I can safely and legally do this and I need help with wire size, if so. Here is a diagram:

MAIN PANEL --------90' 2/2/2/6AWG----->BUILDING A 100A Subpanel -----------130' ???????------ BUILDING B 100A subpanel


Facts with this setup is that Building A and Building B will not generally be used at the same time.....and B will never have a washer dryer/fridges since those live in A. Building B will be 30x50 and have lighting/plugs and welder provisions...

Can I do this? Is this legal? Is it SAFE? If so, what wire size should I go with? It would be really swell if I can get this to work because its an easy clean trench job.
 
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Old 11-19-20, 08:33 PM
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Sure that will work. You are limited to the 100A that supplies the first shop.
You need to run at least the same size wiring and slightly larger if you are anticipating large loads.
Is the welder the only large load in the second shop ?
 
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Old 11-20-20, 05:27 AM
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Note that you have a total run of 220 feet from the main panel to building B. At the very least, the wire size from A to B has to be at least as heavy as would be needed to combat voltage drop compared with a home run from the main panel to B.

Unlike for water lines, if the "first half" of the run (from main to A) is marginal, you can usually make it up with an even heavier line for the "second half" (from A to B), to achieve the desired 4% or less total voltage drop. (This strategy fails only if the total load is so great that the system would fail with building B next to building A i.e. no long "second half".)

You will need to do load analyses for the entire complex, for buildings A and B together, and for just building B. Use the "subpanel load analysis" sample rules also at the back of the NEC code book, for the second two of these analyses.
 
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Old 11-20-20, 06:23 AM
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Download the Southwire app called “voltage drop”. It does all the math for u.
 
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Old 11-20-20, 07:33 AM
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Yes. The welder would be the heaviest load in “B”. It’s possible a fridge or freezer could be added in there but odds are it would be moved from A and into B just like the welder. All lighting is going to be LED. My air compressor is 110v so o guess a 220V could be put into service instead. But again, most of the items that are in A would not be duplicated in B but rather moved. I do have a couple of additional barns that may be dropped off of the subpanel of B but those, at this point, would be relegated to purely general lighting and a strategically placed outlet or do. Pure storage and animal feeding. I’d like to keep all of the heavy electrical confined to B.

Based on my wiring calculator, if I’ve done this right, I have a 1.46% drop from my main panel to A. I can come off of A and go to B with the same size 2awg wire for an additional 1.94% drop making it 3.4% drop total. Or I can go up to 1awg to B and reduce that to 1.59% for a total of 3.05%.

im leaning toward upsizing as, much like building A, I don’t know what may need to be hung off of this line in the future. I’d rather have oversized and overpriced wire in the ground being unused for 30 years than need it next year and not have made it suitable. Is that stupid ?


 
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Old 11-20-20, 09:11 AM
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This one is not stupid unless all of: two expert electricians, the NEC (load analyses), and a crystal ball all said one and the same thing and you did something else.
 
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Old 11-20-20, 03:28 PM
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If the #2 you used is aluminum it's only approved for up to 90A, not 100A.
 
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Old 11-21-20, 04:43 AM
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Just some food for thought: Voltage drop is determined by the actual load, not the rating of the panels.
 
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Old 11-21-20, 09:06 AM
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All wire that I used is copper.

Also, unless I misunderstand, voltage drop is a function of inherent resistance of a copper conductor. In other words, if a load requests 100A, the wire cannot deliver 100A because of basic physics. At any rate, we won't be remotely close to requesting 100 A. So far, 25A is about all I've been able to muster. LOL.
 
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Old 11-21-20, 11:37 AM
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<<< voltage drop is a function of the inherent resistance of a conductor >>>
Yes
<<< If a load requests 100Ak the wire cannot deliver >>>
Generally no.

If the load requests (attempts to draw) X amperes, then X amperes will flow unless the wire burns out or the power supply falters trying to deliver X amperes. In the process the voltage measured at the load will drop due to the resistance of the wires. Faced with a lesser voltage than expected, the load may attempt to draw a different number of amps which may be greater or less.

It is often possible although almost always undesirable to provide the desired voltage to the load through inadequate (too thin( wires by increasing the source voltage.When there is significant voltage drop in the wires, there will be noticeable fluctuations in voltage depending on the amps drawn at any given moment.

To be exact, across any two points in a circuit at any given moment the voltage measured equals the amperes flowing multiplied by the resistance of that portion of the circuit (Ohm's Law). Nitpicking: Where alternating current is involved, ordinary voltmeters, etc. give only approximate numbers needed to verify this calculation; somewhat more complicated math using what mathematicians and scientists refer to as complex numbers is needed to be exact.
 
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Old 11-21-20, 04:41 PM
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Also, if you draw 100 amps for any length of time the overcurrent protection will likely trip. This is why circuits can typically only be loaded to 80%.
 
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