Unique Subpanel Question

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Old 09-16-15, 09:10 AM
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Unique Subpanel Question

Hello everyone, this is my first time posting but have found these forums useful in the past. I have a unique problem that I'm not sure the answer to. I have recently built a new 32'x32' garage on my property. I'm at the point of adding power to it. I thought it would be simple but after investigation maybe not so much.

I have a main panel outside of my mobile home that is a Square D Meter Main service entrance device with 8 breakers and 200amp feed through lugs that feed the mobile home. Granted even though this is a 200 amp panel there is only a 100 amp main service disconnect breaker. I believe it is like this because of the mobile home subpanel has a sticker on it saying "This panelboard shall be connected by a feeder assembly having overcurrent protection rated at not more than 100 amperes". The main panel outside the mobile home subpanel is connected with about 100 feet of 2-2-2-4 aluminum SE-R wire which connects to the feed through lugs on the main panel.

Also connected to this main panel is the well pump. The well pump is connect with a 220V 20amp breaker. There is also a 20 amp 110v breaker for a plug in for heat in the water pressure tank area outside (about 3 feet away from the panel). There is a interloc kit installed on the panel for a generator. There is a 30 amp 220v breaker that runs to a generator plug right next to the main panel. The last thing in this main panel is another subpanel for a workshop/shed. There is a 30amp 110v breaker in the main panel that supplies the 100 amp subpanel in the shed.

The subpanel in the shed seems to be done correctly for the most part. The Neutral and ground are seperated and isolated. The cable connecting the main panel and the shed is only 10-2 with a ground and runs almost 200 feet. It seems to be fine though because there really is only a couple lights and a couple plug ins.

Now on to my actual question. I want to put in a 100 amp subpanel in the garage that can supply an actual 100 amps. I will be using 220v welder, 220v air compressor, furnace, lighting, garage door openers, etc. Can I run another sub panel off of this main panel? I plan on using the same 2-2-2-4 alumininum Se-R wire buried in conduit to the garage. The panels will be about 100 feet apart. Would I be overloading the main panel?

I hope I give enough information. I took a ton of pictures if anyone wants to see pictures.

Any help you can give me would be appreciated. Oh by the way I will be getting a permit but I just want to know what I'm getting into first before I get the permit.
 
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Old 09-16-15, 09:25 AM
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SE-R cannot be buried.

You need to do a demand load calculation to see if you service is large enough.
 
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Old 09-16-15, 09:43 AM
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Thank you for the quick response pcboss! So The SE-R cable can't be buried even in conduit? I could of sworn that is what is used going to my mobile home and that is buried for about 5 feet before going under the home. The contractors receipt that installed the initial service just says "Service cable 2-3w/Grd aluminium". What would you recommend for the cable?

How would I go about doing a demand load calculation?
 
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Old 09-16-15, 11:24 AM
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The contractor might have meant USE (underground service entrance) cable, but you have to be careful on this because some USE cannot go inside a building and some can. SER cannot go underground at all. The code has also changed to restrict #2 aluminum to 90A instead of 100A, so if you use that size cable for your new building, you need a 90A breaker to feed it. I like to recommend a type of USE called MHF (mobile home feeder) cable, which is allowed direct buried at 24" deep, and also allowed inside buildings if run in conduit.

A demand load calculation is a set of formulas defined by the electrical code to determine service size. If you google the term, you will find a number of guides and spreadsheets to walk you through it. You can add as many panels as you want to the service, but what matters are the things powered from them. The demand load calc helps to figure that out.
 
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Old 09-16-15, 11:44 AM
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Thank you ibpooks! So SER is definitely out. The only big box store I have around is Menards so I've been looking on there site. It looks like they have what is called 2-2-2-4 Aluminum Mobile home feeder cable that is suitable for direct burial (according to the description). Would that work for me to feed the garage? Thank you for letting me know about the 90A rating instead of 100A. Would I need a 90A breaker on the the subpanel in the garage also or could I leave that at 100A?

I did do a google search for the load calculators. I found a few. From what I can tell with out actually looking at each appliances data plate (I'm at work right now) I would be right around 70A with a service demand of 17,060VA. That seems high to me since the only real big appliance I have is an electric hot water heater. My range and dryer are propane. My other big appliances are a furnace (no A/C) and a refrigerator.
 
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Old 09-16-15, 12:00 PM
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Yes that cable will work. The Menards in my area keeps that cable in stock in the outdoor yard, but they can order it in even if they don't stock it. You will need to sleeve it in PVC conduit from the bottom of the trench up into the panels, but the horizontal run can be direct buried w/o conduit at 24"+ deep. The 90A breaker needs to be on the main panel side. The main breaker in the garage can be 100A (assuming you bought a 100A panel kit). If you're at 70A load calc, then it should be no problem adding the garage panel.
 
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Old 09-16-15, 12:29 PM
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Thank you very much! That information helps me out a lot! When I get home and do an actual load calc (not a guess) at what point would the amperage be questionable for adding the garage panel?
 
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Old 09-16-15, 12:35 PM
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100a...................................
 
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Old 09-16-15, 06:49 PM
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there is only a 100 amp main service disconnect breaker.
I think you have a problem, you only have a total of 100 amps to work with for both the mobile hoime and the shed.
 
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Old 09-17-15, 07:01 AM
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With all gas appliances and no A/C it seems reasonable to me. Certainly he should do the calc completely, but it passes the sniff test for me.
 
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