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I have a 100 amp sub in garage fed by a 50 amp from main house panel. What size wire guage is needed and what size conduit? can it be PVC? I'm in Sonoma county Ca.

  • How long will the wires have to be to reach from the main panel to the panel in the garage? Is it an attached, or detached garage? What size breaker will be in the main panel to protect the wires feeding the garage panel? – Tester101 Sep 29 '15 at 11:55
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If you plan on replacing the 50 Amp breaker with a 100 Amp...

  1. The minimum sized conductor is #3 THHN copper which includes the 2 hot legs and one neutral.
  2. The minimum sized grounding conductor is #8 THHN insulated copper.
  3. The minimum sized grounding electrode conductor (for ground rod) is #8 bare copper.
  4. The minimum size conduit would be 1 1/2" using PVC schedule 80.

If you plan on keeping the 50 Amp breaker...

  1. The minimum sized conductor is #8 THHN copper which includes the 2 hot legs and one neutral.
  2. The minimum sized grounding conductor is #10 THHN insulated copper.
  3. The minimum sized grounding electrode conductor (for ground rod) is #8 bare copper.
  4. The minimum size conduit would be 1" using PVC schedule 80.

Notes

  1. The above figures are based on a detached garage @ 100' or less distanced from the garage to the houses main panel. An additional ground rod is required for detached garages. If the garage is not detached then an additional ground rod is not required.
  2. If detached, the subpanel would require its own main breaker and the grounds and neutrals would need to be separated. Otherwise a main lug subpanel is fine but still need to separate the grounds from the neutrals.
  3. The above examples are based off the National Electrical Code 2014. Your jurisdiction may have other requirements.

References

  • Table 310.15(B)(16) Allowable Ampacities of Insulated Conductors.
  • Table 250.122 Minimum Sized Equipment Grounding Conductor.
  • Table 250.102 (C)(1) Main Bonding Jumper
  • Table C9 Conduit fill.
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Craig, I did some search on the internet and found this info. It is best to check in the state you are living in and see what their electrical code states as it sometime varies from state to states.

Standard for 50amp is 6 gauge for copper. Use #4 for 100 amp. it cost around $200 extra for the #4 vs #6 for about a 40' run. #4 is much thicker wire to run

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    #4 THHN is only good for up to 95 Amps. You can't round up. – Kris Sep 29 '15 at 11:34
  • @Kris "Round up," that's kind of funny. And of course true. – Craig Sep 30 '15 at 6:16

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