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  #1   IP: 66.190.209.39
Old July 10th, 2012, 08:20 PM
jlambe5 jlambe5 is offline
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Default Main Lug or Main Breaker to feed shop?

Hello, I'm new here and seeking some advice. First off, I'm in Louisiana. My area uses the NEC 2008 as the electrical code.

I'm planning to run power to my detached shop soon (hopefully). When we built the house, I spoke to the power co. and asked the best way to prepare for bringing power to my shop. I wanted 200A for the house and 100 or more to the shop. They sold me a Milbank 320A meter socket which has double-barrel lugs. One barrel of the lugs feeds the house. The other will feed the shop. I intend to install a 125A panel at the house. That will feed another 125A panel in the shop. I'm sure I can just feed straight to the shop, but I figure it would be convenient to be able to cut power to the line feeding the shop if ever needed. The panel in the shop will have a main breaker. What I'm unsure about is should that first panel at the house have a main breaker plus a 125A branch circuit breaker to feed the shop panel, or should it just be a main lug with a 125A branch circuit breaker feeding the shop panel. Keep in mind the panel at the house will only be feeding the shop, so there will be no other breakers/circuits in it. As per NEC and best practices, which is the correct way? I've attached a picture to hopefully explain my plan better.
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  #2   IP: 173.238.37.130
Old July 10th, 2012, 08:25 PM
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joed joed is offline
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If the panel in the house has space for more than 6 breakers then it needs to have a main. Not sure it is permitted but why not just use a 125 amp disconnect instead of a full breaker panel?
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  #3   IP: 130.76.32.218
Old July 11th, 2012, 10:12 AM
suemarkp suemarkp is offline
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The use of a double lugged 320A meter was wise, but your design is not using it effectively.

If you insist on a disconnect at the house for the shop, there are some issues. If you use just a disconnect (2 pole breaker and no other slots), it can be anywhere at the house but the wires feeding it are Service conductors so you are limited as to how far inside the house those wires can go (best to just keep it outside anyway). If you use a breaker panel, then most inspectors would assume that panel can also serve circuits at the house. That means the second panel is now part of your building disconnect and that panel will have to be "immediately adjacent" to your existing 200A panel (you'll have a "grouped disconnect" for the house consisting of a 200A main and a 125A main). That 125A panel could also be MLO as long as it never has more than 5 breakers. You are limited to 6 "grouped" disconnects per building. Your 200A main is one of them. A second 125A main would be 2. If you used a small 125A MLO panel instead, a 125A feeder to the shop would be your 2nd disconnect and the remaining slots could be used for circuits. You could have no more than 4 more circuits installed. Some inspectors would not like a panel that could allow you to have too many circuits and allow to you exceed the 6 disconnect limit.

Putting a second panel at the house also adds some grounding complications. If the second panel is a grouped disconnect with your 200A panel, you'll need to run ground pigtails to your existing ground electrode system and water pipe unless you installed a grounding system sized for the sum of the 200A and 125A conductors during your initial install. If you just add a disconnect at the house, I'm not sure how do deal with grounding. It is not serving the house, but it exists there so it needs a ground electrode system. I think it will need 2 rods and those need to tie in to your existing ground electrode system at the house with a #6 or larger copper wire.

If you put a disconnect or panel at the house, you must run 4 wires to the out building (hot-hot-neutral-ground). With a 4 wire feed, you can put the panel anywhere in the shop and the wires can go inside the shop for any length you want before terminating in the shop main panel.

The "normal" way of feeding an outbuilding from a 320A meter is to run 3 wires from the meter to your outbuilding. This is the least expensive approach, avoids the grounding issues, and also saves you a wire (that can be significant if the house-shop distance is very long). These are Service conductors, so you can't mix them with other circuits or feeders and you have to follow the rules for Service wires. They can only enter the shop so far before they terminate into the shop disconnect (in many areas it is 3' or less, but you need to check with your local inspector).

The only way you can kill power completely to the shop is to pull the meter (if you take the last approach of Service wires directly from the meter, and you have a "normal" meter and not a CT can). But that is no different than your house is now.

So what approach to take? First design issue is how loaded is your 200A house panel and do you foresee needing more than 200A at the house. If not, then with run Service conductors from the meter to the shop, or use a 125A disconnect (not panel) at the house to make a feeder to the shop. If you could use more power or circuit breaker spaces at the house, install a 125A to 200A main breaker panel there (immediately next to your 200A panel) with a 125A for the shop and whatever new house circuits you want.

Finally, watch your wire bending space in any panel if using a 125A double pole breaker. Most breaker panels don't have a lot of clearance to the side wall. There is a minimum distance requirement from an ungrounded wire on a lug to a metal wall of a panelboard. This distance changes by wire size. If you're running aluminum to the shed, you may not have enough bending space to use wire of the required size. With copper, you'll be close. A 100A would be easier. You can avoid this by using a disconnect panel, as those will have plenty of clearance, or by using a subfeed lug kit, or buying a panel with feed through lugs. The subfeed or feed through has the wires pointing down instead of sideways, and there's usually plenty of wall clearance in that direction.
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  #4   IP: 66.190.209.39
Old July 11th, 2012, 08:16 PM
jlambe5 jlambe5 is offline
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Thanks for the information. Let me provide a few more details that I left out. The length of run to the shop is about 130 ft. I already have the wire run. It is oversized for what I need, but the price was right. It is 4/0-4/0-2/0 AL and I ran a #4 CU ground along with it. I intend to reduce the wire size as needed at the panels. The 200A panel for the house is outdoor, next to the meter. I have extra room in it and shouldn't ever need more than 200A, so I don't think I need to allow room for it in the new panel.

Let's change it up a bit. I think I may downsize the shop service to 100A. Does the disconnect at the house for the shop have to be fusible or some other form of circuit protection if I have a main breaker in the shop panel? Reason being, I have a 100A non-fusible disconnect (safety switch). Can that be used as my "means of disconnect" and then feed that to the shop panel? The disconnect is a Siemens NFR323. See the attached picture. Now it is 3 phase, but I believe it can be used as single phase, correct? I'm sure 100A would be plenty for my needs. My shop is only 24x24 and I have a few woodworking tools, air compressor, and welder. Would this be a feasible or even better way to do it?
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  #5   IP: 130.76.32.197
Old July 12th, 2012, 12:13 PM
suemarkp suemarkp is offline
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First, a disconnect for the shop is not required at the house. If you want one, and are feeding it directly from the meter, then I think the only thing you're permitted to have is a "Service Disconnect" which must be labeled "Suitable for use as Service Equipment" and it must have overcurrent protection in it or immediately adjacent to it. So this doesn't buy you much and you'd probably be better off just buying a 100A or 125A disconnect which is suitable for Service Equipment use.

I'd say if you're looking for a 100 to 125A feeder to the shop, and your 200A panel has plenty of capacity, then just feed the shop from a breaker in the main panel. That will be the required feeder protection. Once you've done that, you can put your big switch after it if you want to (it would be a switch in series after the breaker, so 2 possible ways to kill power to the shop). An advantage of using that switch is you may have more available gutter space to neck that 4/0 aluminum down to whatever the lugs will take (and if it would take 4/0 directly you'd be even better off). Then you can use #2 copper from your 200A house panel to the switch which will use a smaller conduit and take less space in the 200A gutters.

Since you ran such large wire, I'd be inclined to buy a small 20 space 200A panel for your shop. It is ok to feed a 200A panel from a 100A breaker, and will make terminating and dealing with that large wire easier in the shop.
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