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Old 06-22-2012, 02:39 PM   #1
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another "help me with a subpanel" topic


I've been conducting research on this forum but I still have a few qeustions I'd like to run by you guys and gals. I really want this install to future proof.

I am thinking about wiring my 10ft x 20ft detached shed for power. I am going to use a 100 amp subpanel which will be fed from either a 60 or 100 amp double throw breaker from my 200amp main panel. I will need to run about 30-40 ft of romex from the main panel to a junction box in the attic there I will splice it to either #6 or #3 THHN/THWN (depending on wether I use a 60 amp or 100 amp breaker), then run another 30ft of 2 hots and 1 neutral of THHN/THWN through a 1 1/4 inch PVC conduit which will be buried 18 inches underground.

I will use #8 wire as a ground near the subpanel which I will attach to 2 grounding rods at least 6ft apart. I will need to isolate the neutrals from the ground as the installation will be in a detached building with it's own ground. I will installing eight 20 amp receptcales for power on a 20 amp breaker, the first receptacle will be a 20 amp GFCI receptacle and the rest will be wired in series. Five sets of 48 inch flourescent shop lights on another 20 amp GFCI circuit breaker. Lastly I might install a 30 or 50 amp 240v receptacle in case I want to buy a small welder in the future which will protected by either a 30 or 50 amp breaker depending on what plug I decide to go with.

I will be doing the installation and applying for the permit, but I will be hiring an electrician to connect my wiring to the main breaker and inspect my work.

I am currently reading the NEC guide and I am not an electrician so some of the things I want to do might be wrong.

What size romex do I use for the 30 ft run from the main breaker to the junction box in the attic, assuming I will use the 100amp breaker or do I just use the THHN/THWN in a conduit from the main breaker?

Is #3 THHN/THWN big enough for 100 amps?

Is there anything you see wrong with my plan?

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Old 06-22-2012, 07:07 PM   #2
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another "help me with a subpanel" topic


You will need to run 4 wires to the sub. The sub must have the ground and neutral seperated withing, and the neutral must not be bonded to the sub, but the ground buss must be. And for 100 amps you will need #2 wires for the hots and neutral and #6 for the ground from the main panel. What are you doing in that little shed that requires 100 amps? And I believe you will need 2 grounding rods at the shed as well. Get ready for a boat load of information coming your way from the professionals who migrate here.

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Old 06-22-2012, 07:23 PM   #3
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another "help me with a subpanel" topic


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You will need to run 4 wires to the sub. The sub must have the ground and neutral seperated withing, and the neutral must not be bonded to the sub, but the ground buss must be. And for 100 amps you will need #2 wires for the hots and neutral and #6 for the ground from the main panel. What are you doing in that little shed that requires 100 amps? And I believe you will need 2 grounding rods at the shed as well. Get ready for a boat load of information coming your way from the professionals who migrate here.
Can you explain to me why you say I'll need to use #2 instead #3? Is it because the wire has to be able to hold 80% of the rated load? I don't really need 100 amps at the shed just 60 amps but I want to use wiring for 100 amp in case I need it in the future. Would I be able to add a 20 amp CB for receptacles, a 20 amp CB for lights, and a 30 amp 240v CB for a welder to a 60 amp feed from the main panel? If I can do that I will just go with the 60 amp. Would I be able to buy all black wire and just use colored tape to mark the color of the wire? All my tools are 15amp power tools and I'll never use them at the same time.
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Old 06-22-2012, 07:49 PM   #4
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another "help me with a subpanel" topic


Because #2 is rated for 100amps, #3 is not. If you don't need 100 amps, use #4 and install an 80amp breaker.
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Old 06-22-2012, 07:52 PM   #5
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Because #2 is rated for 100amps, #3 is not. If you don't need 100 amps, use #4 and install an 80amp breaker.

I am liking this idea
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Old 06-22-2012, 07:58 PM   #6
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another "help me with a subpanel" topic


Print this out for future reference. Wire ampacity chart
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Old 06-23-2012, 11:41 AM   #7
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another "help me with a subpanel" topic


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Originally Posted by Missouri Bound View Post
Because #2 is rated for 100amps, #3 is not....
Says WHO? By your own admission in the chart that you referenced #3 THWN is certainly rated for 100 Amps!

I would install #3 hots, #4 neutral and a #8 ground for such an installation.
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Old 06-23-2012, 12:09 PM   #8
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Look at it again...THHN is rated for 85 amps. Unless you are looking at service capacities...which this installation is NOT.
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Old 06-23-2012, 12:32 PM   #9
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You need a vision exam.

TW, UF-B, and NM-B are rated at 85 Amps in that chart.

RHW, THW, THWN, XHHW, SE, and USE are rated at 100 Amps.

YOU LOOK AGAIN
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Last edited by kbsparky; 06-23-2012 at 12:41 PM. Reason: Added screen shot of chart
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Old 06-23-2012, 01:24 PM   #10
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I apologize, KB.....somehow I got fixated on the aluminum chart.
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Old 06-23-2012, 01:39 PM   #11
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Even on the aluminum side of the chart, you would be required to use a #1 conductor for a 100 Amp feeder.

Since you can't use the 90 degree column size if you terminate those wires on a regular circuit breaker or load center's lugs. They are only rated for 75 degrees max.
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Old 06-23-2012, 03:15 PM   #12
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another "help me with a subpanel" topic


Quote:
Originally Posted by Missouri Bound View Post
You will need to run 4 wires to the sub. The sub must have the ground and neutral seperated withing, and the neutral must not be bonded to the sub, but the ground buss must be. And for 100 amps you will need #2 wires for the hots and neutral and #6 for the ground from the main panel. What are you doing in that little shed that requires 100 amps? And I believe you will need 2 grounding rods at the shed as well. Get ready for a boat load of information coming your way from the professionals who migrate here.
There is absolutely no code requirement for two ground rods.
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Old 06-23-2012, 05:02 PM   #13
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another "help me with a subpanel" topic


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There is absolutely no code requirement for two ground rods.
Correct.....if you can show that your resistance is 25 ohms or less.....or something like that....

To the OP....why so much power? I would be willing to bet that 30/240Vac would be plenty....you have 240Vac is you need it...but I doubt you will ever use more than 30A in your 'shed'.

I have a lot of power tools in my garage along with a compressor....I have never tripped the old 30A breaker (except for the one time I drove a ground rod through the conduit...alcohol was NOT involved).....I now have it on a 50A....but I really doubt I will ever get close to it (especially if I keep the ground rods in check).
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Old 06-23-2012, 05:07 PM   #14
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Quote:
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There is absolutely no code requirement for two ground rods.
Much easier (and cheaper) to install 2 ground rods than to provide test results that prove the resistance is less than 25 ohms.
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Old 06-24-2012, 06:18 AM   #15
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another "help me with a subpanel" topic


Quote:
Originally Posted by md2lgyk View Post
There is absolutely no code requirement for two ground rods.

While that may be true, it is much cheaper to provide two ground rods than to try and prove one rod has a 25 ohm resistance... which I doubt you could ever get with a 100 rods.

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