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Old 08-15-2012, 12:09 PM   #1
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Garage Sub Panel


I am considering a sub panel in my garage (workshop) to simplify some home run wiring I have back to the main panel.

  • The garage is attached to the house.
  • I live in IL where EMT is mandatory (AC and NM not allowed).
  • Considering a 16 slot panel without main disconnect in it for garage.
  • Feed with a 50A breaker from main panel.
  • Use three #8 THHN (copper) wires in 1/2" EMT from main panel. (2 hots, 1 neutral)
  • No ground wire in conduit, EMT is the ground.
  • No ground rod at garage panel (attached structure)
  • 1/2" 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
Does this sounds OK?


OK to go without a main disconnect on sub panel?



Just wondering if EMT is an acceptable ground for higher amperage. Do the conduit connectors at painted knockout really make good enough connection for a 50A fault path? Just seems a potential problem if nuts don't bite through painted steel well enough.

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Old 08-15-2012, 01:30 PM   #2
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There are actually pipe size requirements per panel amperage size. I'm not sure if the nec has the same rules as the cec. Also you would need #6 I believe unless that's also an exception in the nec. Everything else looks peachy.

I generally like to run grounds inside the pipe just to take paint and set screws in couplings out of the grounding equation.

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Old 08-15-2012, 01:56 PM   #3
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Originally Posted by andrew79 View Post
I generally like to run grounds inside the pipe just to take paint and set screws in couplings out of the grounding equation.
I'm with Andrew on this one. Conduit works if installed properly and kept tight but I would feel more at ease with good ol wire.
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Old 08-15-2012, 07:14 PM   #4
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Quote:
Originally Posted by curiousB View Post
I am considering a sub panel in my garage (workshop) to simplify some home run wiring I have back to the main panel.

  • The garage is attached to the house.
  • I live in IL where EMT is mandatory (AC and NM not allowed).
  • Considering a 16 slot panel without main disconnect in it for garage.
  • Feed with a 50A breaker from main panel.
  • Use three #8 THHN (copper) wires in 1/2" EMT from main panel. (2 hots, 1 neutral)
  • No ground wire in conduit, EMT is the ground.
  • No ground rod at garage panel (attached structure)
  • 1/2" 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
Does this sounds OK?


OK to go without a main disconnect on sub panel?



Just wondering if EMT is an acceptable ground for higher amperage. Do the conduit connectors at painted knockout really make good enough connection for a 50A fault path? Just seems a potential problem if nuts don't bite through painted steel well enough.
Main not required.
Use #6 CU
Neutral has to be white
Use 3/4 pipe incase future upgrade is needed.
Everything else looks good.
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Old 08-15-2012, 07:24 PM   #5
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3/4 is minimum you can put 3 6's in. No room for more later there.
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Old 08-15-2012, 11:47 PM   #6
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Quote:
Originally Posted by electures
Use #6 CU
Use 3/4 pipe incase future upgrade is needed.
Ok I was not applying ampacity table for THHN properly. I saw #8 rated to 55A but forgot to apply the 125% factor to wire sizing.

Not sure if I go with #6 or drop back to 40A and go with # 8. I could do # 8 in 1/2" EMT which is a little easier to work with. #8 seems to be special order at my local HD so maybe easier to go with #6. Just need to rent a 3/4" tube bender then.

If I go 3/4" EMT with 50A and #6 THHN can I add a ground wire and still meet fill? Does ground have to be insulated and green or is a bare conductor inside EMT ok? Just feeling ground integrity with EMT tube is a little suspect with possibility of loose fittings and such. What size conductor for a ground wire for a 50A feeder?
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Old 08-16-2012, 06:18 AM   #7
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You need a #10 ground. Not sire about nec, here grounds don't count for fill. It will have to be insulated. I would personally use 1" if the runs any distance at all. It will make your life much easier for pulling.
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Old 08-16-2012, 06:36 AM   #8
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That wouldn't be difficult at all in a 3/4" EMT at all (assuming less than 360 of bends), however, I'm still trying to figure out why everyone is spec'ing #6's.
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Old 08-16-2012, 06:59 AM   #9
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All conductors in the raceway count towards conduit fill under the NEC.

Not all conductors count as current carrying when counting towards conductor derating.
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Old 08-16-2012, 07:00 AM   #10
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8's are only good if you use 75 degree celcius or higher rated wire for a 50a feed. I think he's decided to go with 6's because eights are a special order where he is.
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Old 08-16-2012, 07:17 AM   #11
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Originally Posted by curiousB View Post
Ok I was not applying ampacity table for THHN properly. I saw #8 rated to 55A but forgot to apply the 125% factor to wire sizing.
What 125% factor?
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Old 08-16-2012, 08:36 AM   #12
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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..

from: http://www.angelfire.com/ct2/ecology.../cableSize.htm

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Old 08-16-2012, 09:48 AM   #13
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You will not find terminals rated for 90 degrees.

Cables like NM are derated from their 90 degree rating but cannot be used at greater than the 60 degree rating.
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Old 08-16-2012, 11:20 AM   #14
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#8 is rated for 50 amps under the 75 degree column.
Most residental panels and breaker are rated for that much.
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Old 08-16-2012, 01:38 PM   #15
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Can't add much but to answer your question about the paint on the entry knockouts in the panel. The locknuts will bite into the metal if tightened properly don't worry about the paint. EMT is an excellent grounding path if installed properly.

Here is a drawing to give you a visual of what your doing just keep in mind that even though this is my drawing you should make sure it is accurate for your application. Also your using emt so apply that to the drawing even though I make note of it on the drawing. You may be required to have more than one grounding electrode like 2 ground rods at the service equipment. This depends but in my area it is common to just drive 2 ground rods to avoid the ground resistance testing.
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Last edited by Stubbie; 08-16-2012 at 02:12 PM.
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