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Wanted to run my plan by the experts to make sure I’m not screwing up.

I have an approximate 180’ run from my 200 Amp Eaton Main Panel, to a proposed 125 Amp sub-Panel in my new shop.

The Run

Wire used: 2/0-2/0-2/0-1 Syracuse Quadruplex Aluminum Conductor Underground Direct Burial 600V URD from NNC - Nassau National Cable

  • From 200 Amp Main Panel, 4’ up the wall into the attic in conduit
  • Approx. 95’ through the attic not in conduit
  • 9’ down back wall in conduit
  • Into 70’ long, 24” deep trench where it is direct burial. Then into the shop to the sub panel.

Plans are to run a 125 Amp sub-Panel in the shop. Counted up everything that I would possibly have running at the same time, and if they were pulling max load per machine, it could equal up to 85 Amps.

2 Questions

  1. Have not purchased the Eaton breaker for the main panel to feed the run. Looking at one from Home Depot. (BR 125 Amp 120/240 Volts 2- Pole Circuit Breaker)
  2. Proposed Sub Panel Box, also from Home Depot. Square D - Homeline 125 Amp 30-Space 60-Circuit Indoor Main Breaker Plug-On Neutral Load Center with Cover – Comes with factory installed 125 Amp Breaker

Am I missing anything?

Thank you for your help with this.

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  • 2
    URD is 100% NOT ALLOWED inside as it's not fire rated. You do NOT want URD. Nov 28, 2023 at 17:18
  • Oh good catch @HarrisonFrith, I will revise as it's rated for duct or direct burial use only.
    – KMJ
    Nov 29, 2023 at 0:04
  • Revision in place. The indoor cable will need to be another style with appropriate splices in a junction box.
    – KMJ
    Nov 29, 2023 at 0:10
  • Thanks for the info. When I contacted NNC and told the "Expert" what I was doing, and outlined the run, this is what they recommended. Temp rating was 90C / 194F. Asked the question in a few groups also, and this is what I came up with my research. Even looked through the Q&A and did not see that. Awwwwww!!!!! What is the real danger here? Don't want to burn the house down.
    – Phil Acker
    Nov 29, 2023 at 14:04
  • The real dangers are that A) it'll catch fire and burn your house down, or, if the house catches fire, this will emit toxic smoke, and B) your inspector will see it in the attic, flag it, and make you replace it. May as well get ahead of the curve.
    – FreeMan
    Nov 29, 2023 at 17:59

2 Answers 2

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I don't see ground rods in your plan. You'll need grounding since it's a detached building.

Adding up all the loads isn't a valid load calculation. Make sure you've done one for the total house and shop.

125 amps is probably kind of overkill. You can spend less on wire if you're OK with a smaller feeder. A bigger panel can go on a smaller feeder, even a 200A panel if you want all the possible breaker spaces for future whatever. Of course if you don't mind the cost, the bigger feeder is definitely future-proof.

Were it me, I'd throw an extra conduit in the trench just in case I wanted to pull network cable in the future.

Type URD is specified for use direct burial or in ducts, but not indoors. Per the manufacturer you specified it cannot be used in attics. You will need to use appropriate splices to a different style of cabling starting at the building entrance. It's OK to run it up through a conduit to the panel or junction box.

Otherwise: your feeder is big enough, you're using the right sort of cable for between buildings, and your run doesn't seem too long or to be subject to any worrisome derating.

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  • Thanks KMJ. Did not count on the ground. I will look into that. I did run CAT 8 Ethernet cable in the same trench. Buried the feeder first, added a foot of dirt, then the CAT 8 cable. Tried to keep at least 8' of separation.
    – Phil Acker
    Nov 28, 2023 at 15:33
  • Also, do I still connect the ground cable to the Main Panel when using grounding rods?
    – Phil Acker
    Nov 28, 2023 at 15:59
  • You put the cat cable in conduit, right? Direct buried it will no longer be CAT8. Regarding ground, ground rods connect to the sub panel, ground in sub connects back to ground in main/service entrance panel, ground and neutral do not get tied together at subpanel. Tip for driving the ground rod: use a rotohammer and a ground rod driver bit, it will go very quickly.
    – KMJ
    Nov 29, 2023 at 0:02
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  • You need a disconnect in the new location - the main breaker on the subpanel takes care of that.
  • You can use a 200A panel just as easily as a 125A panel - whatever is the best deal. Often large panels will be bundled with several branch circuit breakers, and if you can use them then it makes the net cost for the panel itself lower. The only place you need a 125A breaker is at the feed in the main panel.
  • You will need a ground connected to the new panel. Normally two ground rods, but if this a truly "new" shop (as opposed to an old building that is a new-to-you shop) then an ufer ground is preferred.
  • Don't count on using "60 circuits". Treat it as 30 spaces = 30 circuits. That's because most circuits you install will need GFCI and/or AFCI, depending on NEC version and other factors.

As far as the type of wires or cable: As others have noted, standard URD cable is not allowed for indoor use. So you might think: Just splice it to a different cable for the indoor portion. The problem is that splices for larger than around 6 AWG start to get (a) very expensive and (b) require large boxes. So even though conduit adds to the cost, it is worth considering using appropriately rated wires (the wires themselves do not have to be, in fact can't be directly buried) inside conduit. There are several different conduit types with varying prices and also varying burial depths. You can use different conduit inside vs. outside, but by using the same wires throughout you avoid having any splices at all, which saves some money and avoids an important possible point of failure.

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  • Thanks Manassehkatz. Didn't factory in the ground. It's a new shop, but is more of a shed. 16'X40' on wood skids, so unless I use a post hole digger, and pour concert over rebar, might need to use grounding rod method. North Arizona area, high clay content. So no main breaker on sub panel? Do I need an AFCI for the feeder breaker on main panel, or not really required?
    – Phil Acker
    Nov 28, 2023 at 15:44
  • You don't need a main breaker on the subpanel. But you do need a disconnect switch for the building, either on the panel or before the panel. It just so happens that a "main panel" and a "subpanel" are essentially the same thing except for the "main breaker". A "main breaker" works perfectly fine as a disconnect, and when used as a disconnect can be any size >= the feed size. TL;DR use a main panel and you are 100% but don't need to limit to "exactly a 125A panel". As far as AFCI on the feeder breaker, (a) I am 99.999% sure you not need that and (b) if you did need it, Nov 28, 2023 at 15:49
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    Sorry, one more question. So putting in 2 grounding rods 10 feet apart, running #4 wire to sub panel ground bar. Running from main panel 2 hot's, 1 neutral & 1 ground. I will tie ground bar in the sub to the run ground & the grounding rod ground. -- I do not tie the ground bar & the neutral bar together at the sub panel, right? That's only at the main panel.
    – Phil Acker
    Nov 28, 2023 at 19:57
  • 1
    Exactly! You got it Nov 28, 2023 at 21:02
  • 2
    Fair warning that as a commenter pointed out type URD is not rated for use in attics and it's one of the manufacturer FAQs. nassaunationalcable.com/blogs/blog/…
    – KMJ
    Nov 29, 2023 at 0:10

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