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Electrical Wiring in the Home/correct wire size for 100 amp feed to subpanel

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Expert: Master Bob - 12/19/2004


Question
hello master bob
I am trying to figure out the correct wire size to feed a 125A sub panel from a 100 A breaker in the main panel. The one way distance to the sub panel is approx. 70 ft. The entire run will be in 1 1/4" pvc conduit, with about 25 ft. buried.
The rest will be either in crawlspace or in a metal building. The ambient temperature here can be as high as 100 F in summer. The ampacity tables are at best very confusing to a lay person. Is #4 THHN acceptable, or should I use #3 THHN. I plan to run four wires, two hot, one neutral, and one ground. There will be no connection of the neutral and the ground in the sub panel or building. Can you reccommend the correct wire sizes for this application?

Thanks in advance, Dave

Answer
Hi Dave,

  Sorry....but we are fixing to leave here shortly for our Holiday vacation...so I do not have time to answer your question in detail.

 For a 100 amp feed - you will need 3 runs of #2 copper (not #3 or #4) ....although the ground can be a #4.

 Your local inspector will be able to provide more detail when you pick up the permit.

 Hope this helps...gotta run.

Best Wishes and Happy Holiday,

Bob Osgood
Master Electrician

RKO ELECTRIC

www.rkoelectric.com  
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Master Bob

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Master Electrician / Owner RKO Electric (USA Only) - can answer most of the typical residential, commercial and industrial electrical questions regarding new installations, wiring additions, lighting, switches and receptacle outlets and electrical upgrades of any nature.I will make every effort to respond to your questions as soon as possible at the end of the work day...very busy electrical company - your patience is appreciated. Stop by and see us at: www.rkoelectric.com

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Licensed Master Electrician with over 34 years licensed experience - updated with the latest NEC Codes every 3 years since 1978. Over see the daily electrical operations of two electrical companies. Also serve as a judge / editor (for Electrical Contracting Products Magazine) to help select which new products are fit and safe for electricians and the electrical market.

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2 Years Technical College - Degree in Electrical Construction & Maintenance (Dean's List) OSHA Safety Certified (10 & 30), CPR / Advanced First Aid Certified

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