Installing a sub panel from a sub panel

Yeah I know, the title is interesting huh? Let me explain...I bought a 26 year old house and gutted it replacing just about all the wiring except for some of the ceiling light runs. The original main panel is 100 amp and because of an addition that was built, another 100 amp sub-panel was installed 18" away and next to the "main" panel. This was done by a professional electrician with the proper permits pulled and final inspection was approved about 2 years ago (the state is MI by-the-way). Oh, and he also replaced and installed new wire from the transformer on the pole to the original 100 amp house panel.
NOW MY QUESTION:
I have a barn that needs power and requires a wire run of about 75-80 feet (actual run length from box to box, not just the distance from house to barn). What I would like to do is run direct bury, #2 aluminum (4 wire) from a 100 amp breaker in the house sub-panel, underground to another 100 amp sub-panel in the barn. The electrician I hired 2 years ago said that doing so would not be a problem as long as the 2nd sub panel in the barn was properly grounded. I would like other opinions or advice on this please.
Why 100 amp instead of 60 amp in the barn you ask? Why not, besides I already have a Square D 100 amp panel I bought that was much cheaper than 60 amp panels I looked at, not to mention the expandability 100 amps offers. In the barn I plan to run lights, 20 amp power outlets, an air compressor (220), small arch welder (220), gas furnace, and typical power tools such as a table saw, drill press, etc...
Of course, if my "plans" aren't safe, than I would certainly change whatever was needed to be considered safe. That's why I'm asking for feedback from this group. In hind-sight, I really wish I would have bought a new 200 amp panel when I remodeled the house. Ah...if only I knew everything....lol
Thank you in advance, it's appreciated!
RC
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I would connect the new subpanel to your main electrical panel. The reason for this is if you have overloading problems in the future. Then you would only need to upgrade your main panel/service to 200 amps and not mess with either subpanel.
"rchtravel" wrote in message

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to hear you going 4 wire. If your careful you will not have a problem. Unless your trying to build an project and your house is all electric. Which with an 100 amp service I will bet you have some gas appliances.
Years ago I fed a residence (reasons are way to hard to explain) from a 60 amp 2 pole breaker. Unless the washer, dryer, oven were on every thing worked fine until I changed it over to the utility some 2 years later. They did trip 4-5 times those years but that is not bad. I owned a home in Phoenix, 2 a/c's electric water heater, stove, and 3 swamp coolers. I measured the conductors at the roof, pulling about 50 amps with everything on.
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rchtravel wrote:

If your service load calculations indicate that your total load, including the barn, will not exceed 100 amps then you can proceed as you outlined. You do, however, have some alternatives you might want to consider.
If you are in the US you can install a large NEMA 3R box at the meter in which you can tap off a new service lateral to your barn. Those conductors must remain entirely outside of your house. That lateral would be three wire and you would wire the barn exactly as you would a new service.
If the load exceeds the ampacity of the present service you can do a heavy up by using a new service equipment panel to supply feeders to both of your existing house panels and the new panel in the barn. Then you can supply four wire feeders to each of the three panels. This eliminates the cost of rewiring the existing circuits but you will have to separate the Equipment Grounding Conductors, if any, from the neutrals in the panels that are supplied by feeders. It may not be practical to do that if the panels are of the older type that are only suitable for use as service equipment. -- Tom H
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rchtravel wrote:

Run the 100A subpanel feeder off a 60A or 70A breaker; it will be plenty of capacity for what you are doing, the breaker will be a lot cheaper, and the feeder conductors can be a lot smaller and cheaper.
-bob
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I agree with this and would recommend taking it off the first panel or whichever panel makes the wires long enough to reach either panel should you upgrade one of them to 200A. Main reason being do you have any more neutral lugs that can take a #2 wire? Do you have proper bending space for a #2 wire? Those big neutral lugs tend to disappear quickly. If you only have #4 holes, then a 60A feeder would be good with aluminum wire.
Its OK to feed a 100A panel with a 60A feeder.
--
Mark
Kent, WA
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In addition to what the other posters have said, I would add this: Avoid direct burial of aluminum wire. Even though aluminum is approved for the purpose, if it is not installed carefully under optimum conditions the failure of the wire can be premature. If you want to use aluminum wire, use single conductors pulled into conduit such as PVC.
John Grabowski http://www.mrelectrician.tv

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