I'm vaguely familiar with older farm setups where a service is installed on a pole with a central meter. In the few of these that I've seen, they've used a feed-through outdoor panel. The feed-through lugs feed the largest of the farm buildings and smaller buildings are fed from breakers in the feed-through panel. I also see plenty of manufactured homes set up this way to provide power to heat pumps, etc. That part makes perfect sense to me.
In this case, I am trying to help my neighbor who has an older farm. I know that the way they had it done originally was wrong. He has a meter and a standard raintight fused disconnect at the pole. From there, they ran 2" PVC to the house with three 4/0 aluminum for the 150 amp service in the house. This work dates back to the early 1970's. Even that part was done correctly until it came time to feed the other buildings. Since it's just a standard 200 amp disconnect, they just installed multiple lugs on the load side and used them to feed multiple buildings, some with even simple 12/2 UF cable. I talked him into getting rid of that stuff, which he did. Still, there is only one single neutral set of neutral lugs with a simple "one in, one out" capability. There is one small lug to allow for the GEC which is there. There's no way to add any more conductors to this block without multiple taps. I thought about replacing the neutral block with one containing more lugs, but then I would be getting into altering a listed assembly.
Now he's building a new greenhouse and wants to install a 60 amp panel in there (he's already installed it). I told him that we really need to run 6/3 UF cable to the panel in the house to do it properly. He's fairly insistent that we just come off of the original main 200 amp disconnect like the other ones had been done. He's trying to save money on cable, plus he's fearful of digging something up going back to the house. I've told him that I'm not willing to do this unless he invests in a feed-through panel that contains the appropriate lugs and overall design. I'm all in favor of helping him save money and I'll do what I can, but I can't see how to legally come off of the main disconnect.
I could understand if we were talking about parallel conductors or if all of them were rated for the full 200 amps, but this is not the case this time. Would this type of installation qualify for a tap rule exception? It's a lot more than 25 feet from the disconnect to the greenhouse. Again, I know that "farm tap" rules had all kinds of exceptions, but I think that those are all but gone in today's NEC.
I've never done any work on a farm, so I want to make sure that the guidance that I'm giving him against this approach is correct. If there are any exceptions, I'd sure be interested to know about them. Thanks!
The tap may comply with ('05 NEC): 240.21(B(5) "Outside Taps of Unlimited Length. Where the conductors are located outdoors of a building or structure, except at the point of load termination, and comply with all of the following conditions:." It goes on to list the conditions that must be met.
There's about 20 various "tap exceptions" 240.21 addresses them
To answer the neutral bus question you need to get you magnifying class and read all the kits listed on the panel label. There may be a kit for that panel that will expand the neutral bus ... if you can find one.