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Old 08-16-2012, 03:59 PM   #16
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Originally Posted by curiousB View Post
Maybe I am confusing some things. Table 310-16 shows #8 good to 55A using THHN rated for 90 degrees. I thought then I had to derate the wire by 125% per the comment below.

3. Cable Sizing. Cables shall have a 30C ( i.e. table basis) ampacity of 125% of the current determined in Step 1 (continuous current) to ensure proper operation of connected overcurrent devices. (There are no additional deratings applied yet-this is just applying the 80% rule to the cable and finding a wire size and insulation type in the relevant NEC tables..


That is for feeder sizing. You add 100% of the non continuous and 125% of the continuous loads to figure out your calculated load. If you calculate your load to be 20 Amps, but you really want a 50A sub panel for future, then you size the feeders for 50A.


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Old 08-16-2012, 04:34 PM   #17
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Originally Posted by jbfan
#8 is rated for 50 amps under the 75 degree column.
Most residental panels and breaker are rated for that much.
But we don't know what he has.
All responses based on the 2011 NEC.
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Old 08-16-2012, 05:01 PM   #18
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Originally Posted by electures View Post
But we don't know what he has.

I haven't bought it yet but I think I will go with the following:

  • The garage is attached to the house.
  • I live in IL where EMT is mandatory (AC and NM not allowed).
  • A 16 circuit panel without main disconnect in it for garage.|1
  • Feed with a 50A breaker from main panel.
  • Use three #8 THHN (copper) wires in 3/4" EMT from main panel (2 hots, 1 neutral (red, black and white)) plus one #10 green for ground.
  • As noted I will run a ground in conduit and not rely on EMT for ground to sub panel
  • No ground rod at garage panel (attached structure)
  • 3/4" EMT conduit run from main panel to garage will only be 15-20' in length.
  • Will disconnect bonding screw in sub panel isolating neutral bus from chassis ground.
  • Will use GFCI outlets in garage circuits
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Old 08-16-2012, 05:42 PM   #19
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Just a few points

If you run the ground wire you need to purchase a ground bar to install in the panel. Which lends to what are you going to do for your branch circuits. Ground wires in those also?

Your code only requires metal conduit I see no reason to run the ground wire. Just so you will know something like 60% of ground fault current will use the conduit the rest over the ground wire you install.....parallel paths hopefully that never happens.

That panel will allow a backfed main breaker if you ever chose to install one.

You have done an excellent job of researching your project ...


Last edited by Stubbie; 08-16-2012 at 05:45 PM.
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