1) John....... thanks for the replies
2) I would suggest installing Schedule 40 Gray electrical PVC conduit to connect the buildings. You can use Underground Feeder cable, but I always recommend using conduit if possible. You only want to trench and dig this one time. You can also use Rigid Metallic conduit. If using Sch 40 PVC and it is subject to damage above grade, you will need to transition to either Sch 80 PVC or Rigid Metallic conduit. Sch 40 Gray electrical PVC is not rated for areas subject to possible damage.
3) The conductors from the main panel to the sub-panel require a 4-wire circuit, ie, 2 hots, 1 neutral and 1 equipment grounding conductor. If using Rigid Metallic conduit, then no need for a separate equipment grounding conductor since the Rigid Metallic conduit acts as the Equipment Grounding Conductor.
4) For a 125 amp feeder circuit, you will require either 1/0 AWG Stranded Copper or 1/0 Stranded Aluminum, type THWN. The 1/0's will be used for the 2 hot conductors and the 1 neutral conductor. The voltage drop for a 20 foot run using 1/0 AWG is next to nothing and you are OK there.
5) For a quantity of (3) three 1/0's and (1) one 6 AWG equipment grounding conductor, a minimum of 1 1/2" diameter Sch 40 Gray PVC will be required for the conduit fill ratio of the 4 conductors.
6) At the main panel, you will also need an equipment grounding bar if using the 6 AWG equipment grounding conductor. The 6 AWG EGC only gets connected to the equipment ground bar and not the neutral bus bar. Tape the end of the 6 AWG using green electrician's tape at each end to identify this as a ground. Tape the 2 phase conductors at both ends with black and red electricians tape to identify these as the hots. Tape the neutral at each end with white electricians tape to identify this as the neutral. You'll need (1) one 125 amp double pole breaker installed at the main panel.
7) The 125 amp sub-panel will need to be a main breaker panel and not a main lug panel. This will allow you to disconnect the entire detached building via the sub's 125 main disconnect breaker.
Inside the sub, the neutral bus bar does NOT get bonded with the panel's metal enclosure. You will need to remove either the sub's main bonding jumper (green grounding screw or a copper jumper strap). This isolates the neutral and the equipment grounding conductor. In any sub-panel, the neutral is always floating. The sub will also require an equipment grounding bar to land the 6 AWG equipment grounding conductor. Since a detached building, you will also need a minimum of (1) one 8 foot ground rod connected to the equipment ground bar using 6 AWG Stranded Copper and an acorn type ground clamp. Depending upon your soil conditions, you may require 2 or more 8 foot ground rods all bonded together. The total resistance of the ground rod system needs to be 25 ohms or less.
If you will be using Romex as your conductors for the building branch circuits, the bare copper Romex grounds only get landed on the sub-panel equipment ground bar and NOT on the sub-panel neutral bar. This is for safety purposes and is an NEC requirement.
8) I would not recommend paralleling the 240 volt receptacles. Each 240v receptacle should be on its own dedicated circuit breaker for safety reasons. Reason being, if the 240v receptacles are wired in parallel and more than 1 machine running concurrently, there is always the risk of overloading the circuit and tripping the breaker.
Hope this helps.........If you have any additional questions, let me know and I'll be glad to answer them for you.
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