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Kevin, Licensed Electrician
Category: Electrical
Satisfied Customers: 2823
Experience:  29 years as a Licensed Electrical Contractor in Illinois, 5 year college Electrical Instructor, Former Electrical Inspector, Diploma in Digital Electronics, Former Illinois Licensed Home Inspector
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Also is 18 in. Deep trench sufficient .

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Also is 18 in. Deep trench sufficient .

Hello 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!



1) Hello the expert that you requested is no longer an expert on the Just Answer site. I will be happy to assist you with your electrical question.


2) Can you expand upon your question? It appears that you wish to bury some Underground Feeder cabling or conduit or Low Voltage cabling?


3) Can you be more specific in your application and I can then provide with the National Electrical Code compliant requirements...........Thanks..........Kevin!


Customer: replied 2 years ago.

i guess only the last part of my question came thru .i want to run a servie line to our new garage . what size wires and conduit for 100amp panel . we have 200amp load center in the house . also what type wire sould i use . im going with aluminum.

Customer: replied 2 years ago.

also its underground so is 18in. deep tren ch enough for pennsylvinia

) Steve... I only rec'd the trenching portion of your question.

2) Since you are looking to install a 100 amp sub-panel in the garage, I will need the following information:


A) What is the approximate distance from your main electrical panel to the 100 amp sub-panel? I need to calculate the voltage drop.


B) Will you be using Underground Feeder cable or Conduit? I would recommend conduit instead of UF and a direct burial in the event that the feeders ever encounter problems in the future. You only want to trench the run once.


C) Will you be burying under General topsoil or a driveway or any concrete such as a patio, sidewalk, etc? I need to know your application before I can tell you if 18" is sufficient or not.


D) Do you have 2 consecutive spare spaces (either left or right hand side) in your main electrical panel for a double pole feeder circuit breaker? What size feeder breaker in the main panel will you be using, also 100 amps?

Customer: replied 2 years ago.

ok, aprox , 80 ft. from the house. yes in conduit . and i what aluminum wire . cheaper . mostly general topsoil but across our stone driveway . and yes i have two consec. spaces

Customer: replied 2 years ago.

also the 100amp. panel for the garage is a square d that used to be in our house . can i use it as the sub panel

Customer: replied 2 years ago.

also the 100amp. panel for the garage is a square d that used to be in our house . can i use it as the sub panel . yes 100amp feeder breaker

1) Thanks for the replies Steve!


2) Based on a 240 volt circuit, running 80 feet with a 100 amp load, the voltage drop = 4.3 volts or a 1.8 % voltage drop. National Electrical Code recommends a maximum of 3% VD, therefore you are good and will require 2 AWG Stranded Aluminum, Type THWN conductors. The feeder circuit from the 200 amp main panel to the sub-panel requires a 4 wire circuit, ie, 2 hots, 1 neutral and 1 equipment grounding conductor.


The 2 hots and 1 neutral will be sized as 2 AWG Stranded Aluminum, Type THWN. The equipment grounding conductor will be sized as 6 AWG Stranded Aluminum, Type THWN.


3) Since the garage is detached, the sub-panel needs to be a 100 amp Main Breaker panel and not a Main Lug panel. Detached buildings require a means of disconnect and the 100 amp breaker in the panel will serve this function.


4) Suggest using Gray electrical PVC conduit, Sch. 40 for the underground. Minimum conduit diameter for the 4 wires, requires 1 1/4" diameter for PVC. However, I would recommend to increase the conduit size to either 1 1/2" or 2" in order to make the feeder pull easier.


5) Both panels require an equipment grounding bar to land the 6 AWG equipment grounding conductor. You will also need an 8 foot ground rod at the garage. You will need an 8 AWG Stranded Copper Grounding Electrode Conductor from the ground rod to the equipment ground bar at the sub-panel.


6) DO NOT install the green ground screw or the main copper bonding jumper at the sub-panel. All subs, require the neutral bus bar to be isolated or floating from the panel metal enclosure. Your white neutrals for the branch circuits will only land on the neutral bus bar.


Any bare copper for Romex or green grounding conductors will only land on the equipment ground bar.


DO NOT inter-mix neutrals with grounds as that is an electrical code violation and a safety issue.


7) If using PVC Sch 40 conduit, 18" is sufficient under general topsoil and the driveway. Both are OK to use for the entire conduit run.


8) All receptacles in the garage will need to be GFCI protected either using a GFCI circuit breaker or GFCI receptacle. The lights do not require GFCI protection.


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.



The next time you have an electrical question, you can also request for me at:


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Steve............thank you for the positive service rating...........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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