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100 AMP sub panel from 200 AMP main

Installing a 100 amp sub panel located in a detached garage about 90' from a 200 amp main. Will be running a 240v welder off the new panel,plus normal lights and other small tools. . What size THWN cable should I use? I'm looking at #3 cooper but sure if I am accounting for the drop in voltage. The cable will run halfway under the house, other half under ground. Also do I need conduit running under the house, or just when the cable need to be run underground to the garage?
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Posted 2018-06-22T16:29:52+0000  by mgagnier mgagnier
 

Hello mgagnier.  Welcome to the Community!


There are 2 issues when determining the best wire size for a given load application. 


First there is the absolute minimum wire size you can use given the circuit amperage, and thus the breaker rating.  For a 100 amp circuit, the minimum THWN copper wire size is 3 gauge.  This does NOT apply to cables like UF.


Second is the acceptable voltage drop based on the length of the circuit run.  Typically a 3% drop is the max you should accept, and less is better.  For runs of 90 feet, 3 gauge wire will give you that 3% drop.  This makes me want to recommend 2 gauge instead.


As far as conduit goes, if you are using THWN it must be enclosed in conduit.  Codes do vary by area.  Check with your local building/electrical inspector on their specific requirements for the type, size and burial depth of the conduit.


Chris.


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Posted 2018-06-22T17:58:06+0000  by Chris_HD_CHI
If I run #2 what size conduit should I use assuming 4 wires inside?
Posted 2018-06-22T20:30:33+0000  by mgagnier

Hey mgagnier,


To add what Chris already wrote and to answer your follow up question, you will need to run 4 conductors to make a proper ground for your subpanel.


Depending on what local codes dictate, you'll most likely need to run a separate ground rod/wire to the subpanel at the detached garage locally so that it's protected. I would recommend to consult a licensed electrician in your area to ensure it is up to code and just as important, pass an inspection.


Let us know if you have any further questions,

Joseph

Posted 2018-06-25T17:31:08+0000  by Joseph_HD_ATL
 
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