Subpanel in detached shop

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  #1  
Old 10-19-11, 09:05 AM
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Subpanel in detached shop

Hi,
I'm new to the forum here, even though I've been stalking for quite awhile researching and gaining knowledge from the good info.
I'm getting ready to install a 100amp subpanel in my detached shop, Off the 200amp main in my house. I was hoping I could get some advice on what I've come up with. I've attached a link to a rough sketch of my plans. Please feel free to critique it as thats what I'm looking for. One question that is atop my mind...... The 100 amp breaker that im adding to the main panel... does it have to go at the top above the 20 amp breakers? I know its common practice, but why exactly?
I'm sure I left out some additional info that might be needed, but hopefully the sketch will explain it enough. Thanks in advance for the help!

Here is the link
subpanel2 | Flickr - Photo Sharing!
 
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  #2  
Old 10-19-11, 09:20 AM
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The main disconnect box pictured to the right of the the main panel is not required. If you do install it, the ground and neutral must be separated exactly like a sub panel.

#2 aluminum is rated for only 90A when used for a shop subpanel, so you'll need to reduce the breaker to 90A -or- upsize to #1 aluminum.

SER cable cannot be used underground, even in conduit. Also it will be virtually impossible to pull a cable that size through conduit. It's too stiff. If you want to use cable go with USE-2 direct burial cable; or install the conduit and pull individual XHHW aluminum conductors.

The position of the 100A breaker in the panel is not important. The only restriction is that the panel label will give a "max stab" rating which applies to breakers directly across from one another. If you max stab is 150A for example that means the largest breaker you could have across from your 100A subpanel breaker is 50A.
 
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Old 10-19-11, 09:43 AM
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WOW! thanks for the quick reply! The main disconnect box was something an engineer at my electric company (entergy)recommended. His justification was that if a fire or something was to break out at the shop, and no one was home, it would be easy access for emergency personnel to disconnect the power to the shop. I already have the box, it only has one bus bar, so I guess I would have to add another one if I use it.

I'm glad I came here for advice. Not that I take Home depot reps for gossip, but I was told the 2-2-2-4 AL SER was for burying in conduit, and would be enough for 100amp. So if I go with #1 XHHW, can I still use #4 for the ground? Or, if I go with a 90A breaker, the one in the subpanel is the only one I would have to change right? I could still use 100A in the main and the separate disconnect if I use it?

Just checked my main panel. It has max stab rating of 200A, so I'm good there. It will be alot easier to just add the breaker than moving everything down.

Thanks again!
 
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Old 10-19-11, 11:15 AM
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You will probably find it cheaper to find a main breaker panel and install it in the shop instead of adding a 100 amp 2 pole breaker to a main lug panel. As long as the overcurrent protection is correctly sized at the house the main in the garage is just acting as a disconnect. It would not need to match the feeder breaker size, but just needs to be rated at least as high as the feeder.
 
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Old 10-19-11, 12:05 PM
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I already have a 100amp panel. It's an eaton setup that came with the 100A breaker, 30A, and 5 20a breakers. I'll probably just replace the 100A with a 90A cause for some reason the electrical supply houses around here are worthless for any wire except 2-2-2-4 direct burial. And only found one place that has it, and Only has 116'. Good thing I don't need more than that.
 
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Old 10-19-11, 12:24 PM
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Originally Posted by highspeeddirt View Post
His justification was that if a fire or something was to break out at the shop, and no one was home, it would be easy access for emergency personnel to disconnect the power to the shop.
Nah, that's silly. The fire crew would have no way of knowing which breaker did what, if it was labelled correctly, if it was even wired correctly, or maybe that breaker is the one that failed and caused the fire. If there was any kind of electrical hazard they wouldn't take any chances and would simply cut your service off at the meter or the pole.

I already have the box, it only has one bus bar, so I guess I would have to add another one if I use it.
I wouldn't use it -- not required by code, just another thing to go wrong.

Or, if I go with a 90A breaker, the one in the subpanel is the only one I would have to change right? I could still use 100A in the main and the separate disconnect if I use it?
Leave the 100A main breaker in the shop subpanel as a local building disconnect. Use a 90A double-pole breaker in the main panel to feed the shop.

Unless you have some massive machinery in the shop or several employees working at the same time you'll never notice the difference between 100A and 90A.
 
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Old 10-19-11, 04:50 PM
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Sounds like 2-2-2-4 direct burial and 90A breaker without the extra disconnect might be my route! Just a couple more questions... Some have mentioned concern about aluminum inside the the house in the main panel. (which I forgot was another reason that engineer gave me for the extra disconnect) should I be cconcerned about it? Also, had some mention direct burial instead of running in cconduit was bad cause of how aluminum corrodes, as well as having no protection as it would in conduit. How much of an issue is this?
I really appreciate the help guys!
 
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Old 10-19-11, 05:05 PM
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The AL underground should not be an issue with the insulation intact. The AL inside the house or the panel should have no effect on the garage feeder.

The direct buried cable should be at the proper depth so damage should not be an issue. You could bury a warning tape above the feeder.
 
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Old 10-20-11, 06:45 AM
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Thanks for all the help! I really do appreciate it!
 
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Old 10-20-11, 09:17 AM
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Originally Posted by highspeeddirt View Post
should I be cconcerned about it?
No, it's okay.

as well as having no protection as it would in conduit. How much of an issue is this?
Get your trench a good 26-28" deep, make sure to remove sharp rocks from the backfill and the cable will last a good long time. If you have rocky soil, fill in a couple inches of sand around the cable to protect from rocks.

You could run it in conduit if you really want, but I would go significantly oversized, like 3" pipe.
 
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Old 10-20-11, 06:08 PM
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IF the fire dept wanted to kill the power to the shop, they would just cut the meter seal and jerk the meter. I also feel the outside disconnect is unneeded.

When terminating the Aluminum wire be sure to use anti-oxidation goop (paste). You can get a small tube at the big box stores. This is one brand: Noalox Anti-Oxidant Compound

While you have the trench open, you may want to run a PVC conduit 12" up from the wire for network cable, phone and/or cable TV. Something like a 1" or larger should do you pretty well.
.
 
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