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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.



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,


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