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Kevin, Licensed Electrical Contractor
Category: Electrical
Satisfied Customers: 3663
Experience:  30 years Licensed Electrical Contractor in Illinois, Adjunct College Electrical Instructor, Former Electrical Inspector, Diploma: Digital Electronics, FCC Technician License
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The underground wire that comes from my house panel to my workshop

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The underground wire that comes from my house panel to my workshop panel is yellow. It separates from the red and black wire and connects to the ground that goes under the building to the buried ground. On my new panel purchased for the shop I have two main lugs that I assume is fro the red and black wires. Does it matter which goes to which? There is a row of connectors on the right side of the box. At the top is a green screw under that is a row of lugs. On the row of connectors do I put the white wires from my circuits and black goes on the breaker itself. I would guess the buried ground goes to the green screw and that leaves me unsure where the grounds for the circuits go. perhaps on the breaker itself? I have done some wiring before but never installed a new panel. I don't want to mess it up. name is XXXXX XXXXX I will be happy to assist you with your electrical question. My goal is to exceed your expectations on Just Answer!

Hello Jim....... before I can provide you with an answer to your questions, I need some additional information as shown below.........Thanks.........Kevin!


1) What is the wire size of the existing underground yellow wire originating from the house to the workshop panel?


1A) Do you plan on re-using that wire?


1B) Does the yellow wire have a labeling on the exterior insulation indicating NM or Non-Metallic or UF or Underground Feeder?


1C) Is this Romex cable with 4 individual conductors (black, red, white and bare copper ground wire) all installed within a yellow exterior insulation to form 1 cable?


2) Does the existing underground feeder wire contain (4) four conductors, ie, 2 hots, 1 neutral and 1 ground?


3) Is the yellow wire a direct burial or installed in conduit? If in conduit, what is the diameter and type of conduit? Gray PVC Electrical conduit, EMT, Rigid Metal?


4) How many amps will the feeder circuit for the workshop sub-panel be? Will you be using a 100 amp double pole circuit breaker in your main electrical panel to feed the Siemens sub-panel?


5) Can you provide me with the approximate distance from your main electrical panel to the workshop? I need to calculate the voltage drop based upon the feeder wire size and distance.


6) Does the shop have an existing 8 foot ground rod?

Customer: replied 4 years ago.

So sorry about the confusion. I was asking things without enough sleep.

1a) The wires coming from the house breaker are 8gague.
1b) I see nothing like that on the wire.
1c) No this is a braided wire with the black red & yellow covering over the copper wire.
2) No it is red, black and the yellow as above.
3) I see no evidence of conduit on the house or shop side.
4) The electrician who set up my 200amp service in the house has the shop hooked to a double 40 amp breaker.

5) best estimate would be about 40'

6) yes it does

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.


Otherwise, don't forget to rate me before you log Off.

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Customer: replied 4 years ago.

So am I getting that if the underground wire is 8g I need to have a heavier wire buried in the ground? SInce I will not be using heavy equipment such as a welder or big compressor would current wire be acceptable if I remove the 100 amp main breaker in the sub panel and replace with a 40 amp breaker?

1) Yes, that is correct Jim, if you want to install the 100 amp breaker in the sub-panel, then the wires need to be sized as per my previous reply.


2) If you want to use a 40 amp breaker for the sub-panel, the 8 AWG copper wires are OK, but keep in mind that the sub-panel requires 4 conductors, ie, (2 hots, 1 neutral and 1 equipment ground). If using a 40 amp breaker, you will require a quantity of (3) three 8 AWG's (2 hots and 1 neutral) and (1) one 10 AWG copper for the equipment ground. If using the 40 amp breaker in the sub, the minimum conduit size is 3/4". However, I would recommend to bump it up one size and use 1" diameter conduit to make the wire pull easier. The additional cost to increase the conduit is minimal.


3) If using a 40 amp breaker for the new sub, I would recommend that you only install a maximum of (4) four circuits. Since the new Siemens sub-panel has spaces for 20 circuits, if you load more than 4 circuits using a 40 amp breaker and the 8 AWG feeder wires, you will end up overloading the 40 amp breaker since the 8 AWG feeders will not be able to carry a load greater than 4 circuits. The 100 amp breaker is OK to max out the sub-panel using all 20 circuits. If using a 40 amp breaker in the sub, do not insert more than the 4 circuits. A 40 amp sub-panel is not meant for 20 circuits.


Jim........Thank you for the positive service rating as well as the bonus............much appreciated!


If you have any other questions, just let me know.


Take care and have a great day............Thanks...............Kevin!

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