Hello Jim...........thanks for the replies.
1) In your initial question, the optional information is mentioned as a Siemens 100 amp main breaker panel. I am assuming this is the sub-panel that you would like connected for your shop? If looking to retain the 40 amp feeder breaker inside the 200 amp main panel, let me know as my answers below will change. My answers below are based on using the 100 amp main breaker sub-panel. If otherwise, let me know...........Thanks........Kevin!
2) 8 AWG Stranded copper wire is only rated as 40 amps and this wire size is too small if you are desiring to connect it to the 100 sub-panel. A 100 amp main breaker sub-panel will require 2 AWG Stranded Copper or 1/0 AWG Stranded Aluminum conductors as the feeder cables, 2 hots and 1 neutral from your 200 amp main electrical panel in the house. The equipment grounding conductor will need to be size 8 AWG Stranded copper. I would recommend to use 2 AWG Stranded copper since the distance is only 40 feet away. Therefore, the 4 conductors are as follows:
Quantity of (2) two 2 AWG Stranded copper conductors for the hots
Quantity of (1) one 2 AWG Stranded copper conductor for the neutral
Quantity of (1) one 8 AWG Stranded copper conductor for the equipment ground
The (4) four conductors need to be rated as THWN insulation for wet conditions if using a underground conduit system or Underground Feeder cable (UF).
I calculated the voltage drop using 240 volts, 40 foot distance, 100 amps as the load and 2 AWG Copper wires. The voltage drop is only .6 percent which is way less than the recommended 3% maximum voltage drop. Therefore, 2 AWG copper will work fine.
3) All sub-panels require 4 conductors, ie, 2 hots, 1 neutral and 1 equipment ground. This is an electrical code requirement.
4) You will need either the 2 AWG copper or the 1/0 Aluminum to be Underground Feeder cable rated if installing as a direct burial or you would need to install a minimum conduit diameter of 1 1/4" if not using UF cable. If considering conduit, you can either install Schedule 40 Gray electrical PVC conduit or Rigid Metal conduit. I would recommend the Schedule 40 Gray electrical PVC since this is a more cost effective solution. I would also recommend to bump up the conduit diameter to 1 1/2" in order to make the wire pull easier.
If using Rigid metal conduit, then the equipment grounding conductor is not required since the rigid metal conduit would be acting as the equipment ground. UF cable will need to be trenched a minimum of 24 inches. PVC conduit will need to be trenched a minimum of 18 inches. I would also recommend to use the PVC conduit, since you only want to trench this once. The underground conduit system is a better solution in the event of any faulty conductors or troubleshooting purposes down the road.
If removing the concentric knockouts at either panel, you will also require a bonding bushing. Use 8 AWG Stranded copper from the bonding bushing and terminate to the respective equipment ground bar at each panel. If not removing a concentric knockout, then the bonding bushing is not required.
5) The 200 amp main breaker panel will require a double pole 100 amp breaker. The 2 AWG conductors will land on the 100 amp breaker inside your main electrical panel. The 2 AWG neutral conductor will land on the main neutral bus within your main panel. The main panel will require an equipment ground bar which will land the 8 AWG equipment grounding conductor. Use some electricians tape and tape the 2 hots as red and black or red and blue. Tape the neutral as white and the equipment ground as green. The color code of the tape will identify the 4 conductors. Tape the respective conductors at each end.
6) At the sub-panel, the 2 hots get terminated on the 100 amp main breaker. Does not matter which screw of the main breaker the hots will land on. The neutral wire lands on the neutral bus bar in the sub-panel. If the sub-panel is not equipped with an equipment ground bar, you will need one. The 8 AWG equipment ground lands on the equipment ground bar inside the sub-panel.
Do not intermix the equipment ground and the white neutral in the sub on the same bus bar. They need to land on two separate bus bars for safety reasons. In all sub-panels, the neutral conductor must remain isolated from the metal cabinet enclosure. Therefore, do not install the green grounding screw or the copper main bonding jumper strap inside the sub-panel. The neutral wire must float inside the sub.
7) Connect the 8 AWG equipment ground wire from the sub-panel equipment ground bar to the existing 8 foot ground rod.
8) If using Non-metallic cable (Romex) within the shop area, the bare copper grounding conductors only land on the equipment ground bar and not on the neutral bus bar. The equipment grounds and neutrals must remain separated within a sub-panel. Bare coppers to the equipment ground bar and the white neutrals to the neutral bus bar. Do not intermix these two.
9) For any receptacles within the shop either consider installing 20 amp GFCI branch circuit breakers or use 20 amp GFCI receptacles on the LINE side and regular 15 or 20 amp duplex receptacles on the LOAD side of the GFCI receptacle. The use of a 20 amp GFCI receptacle is a more cost effective solution.
10) Within the shop, I recommend that the branch circuits for the receptacles and lighting be on separate circuit breakers.
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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