Main Lug or Main Breaker to feed shop?

   / Main Lug or Main Breaker to feed shop? #1  

Jarrett

Silver Member
Joined
May 24, 2004
Messages
237
Location
Holden, Louisiana
Tractor
Kubota L3130DT
Need some help here. 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 have attached a pic to help explain my plan.


shop service.jpg
 
   / Main Lug or Main Breaker to feed shop? #2  
I think the shop will require a means of disconnect at the house, which means a panel in the house to feed the shop. If you feed the shop directly from the lugs of the meter, without a panel near the meter, you will fail the "means of disconnect" test.

Savy?
 
   / Main Lug or Main Breaker to feed shop? #3  
PS

Both you diagrams seem to be the same
 
   / Main Lug or Main Breaker to feed shop? #4  
Both panels, house and shop should have disconnecting means.
 
   / Main Lug or Main Breaker to feed shop? #5  
you should definitely have a disconnect (with circuit protection) at the house, but don't need to double your circuit protection there (ie. main breaker plus branch circuit breaker at same panel). I would do a 125 or 150 amp main disconnect at the house and then the 125 amp panel at the shop.
The are two rules in the NEC (10' tap rule & 25' tap rule)that allow for taps to be less amperage than the service entrance and not have circuit protection, but I don't think you'll qualify for either. It's been a very long time since I did any of this kind of work so some things may have changed.
 
   / Main Lug or Main Breaker to feed shop?
  • Thread Starter
#6  
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.

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. 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?
shop disconnect.jpg
 
   / Main Lug or Main Breaker to feed shop? #7  
Well, at 130' run you will need some type of over current protection at the house (not just a disconnect).
Obviously, local codes precede anything I tell you. But, the run to the shop must have OC protection (either fuse or circuit breaker).
There are some instances where the OC device can be on the end of the line instead of the start (example: the main at your homes service entrance protects the meter and service drop coming from the pole, the dropout fuse at the transformer protects the transformer and main line but not each individual service drop. Also, if you had a 400A service at your home or business, you could tap off that with a 100A or 200A feed to a sub-panel and that lesser feed would be protected by the OC at the sub-panel as long as it was with-in a specified distance. 10' or 25' depending on the size of the tap)
Yours is not the case. You will obviously need some type of panel with branch circuiting at the shop. Your disconnect (as shown in pic) will work IF you had some type of OC protection with it. I know that's not what you want to hear.
You might find a trade for the 3ph disconnect and get a single phase fusible or CB type disconnect. (the 3 phase is usable BTW, I have a 3ph fused disconnect as my main, just use two of the disconnect lugs and pass the neutral & ground straight thru.)

Something like this, would be your best bet. Shop Murray 2-Circuit 2-Space 200-Amp All-in-One/Combination Main Breaker Load Center at Lowes.com
On a side note. I would think about using a larger amp disconnect since you already have the wire size for it. You may not need 200A now, but in the future???
My shop is 24 x 28, and I have to go outside to change my mind. You never know what additions you'll make in the future.
 
   / Main Lug or Main Breaker to feed shop? #8  
the main at your homes service entrance protects the meter and service drop coming from the pole

How does the panel main protect the meter and the service? They are upstream from the OCP.
 
   / Main Lug or Main Breaker to feed shop? #9  
Technically it provides little protection. It's basically an allowable rule in code, nothing more.
The rule assumes there is little or no reason for overload of the feeder in question and if the feeder is sized for the OC device it is feeding then it is also protected.
If you'll note: the drop out fuse on a power line transformer (servicing generally up to four homes) is on the primary side, and it is size considerably higher than any individual service drop is rated for. The fuse will not blow for a typical home overload situation, it pretty much takes a dead short on the service drop to do it.
The same rule applies to feeders from main panels as long as the feeder is sized for the sub-panel OC device the feeder is considered protected. You'll see this a lot in commercial applications, where on the back of a building a meter and main is located, under the main is a tray type box that feeds several sub-panel. The cabling with the tray is sized for the sub-panel it feeds and may be considerably smaller than the Main OC device. I'm not saying it makes sense, it's just allowable rules with the National Electric Code. Again local codes will supercede.
a quick search of the NEC's 10' and 25' tap rules may provide additional information.
 
   / Main Lug or Main Breaker to feed shop? #10  
Again, it has been a number of years since I've been involved in Electrical construction. Codes continually change.
It would be a good idea to consult a currently licensed electrician rather than an old fart who hasn't been licensed for some years. :thumbsup:
 
 
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