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I am building a purpose-built shed that will support solar panels and contain all of the solar equipment such as charge controller, batteries, inverter etc. All of this is off-grid.

An A/C feeder direct burial cable will run 24 inches below ground for 140 feet from the main panel in the shed to the subpanel at the cabin (in conduit when it exits the ground). I was originally planning on running 2-2-2-4 Al URD direct burial from the shed to the cabin, but everyone seems to be out of stock of that. So now I am going to just use 2-2-4 and only have 120V service at the cabin. That is not really a problem as I do not anticipate any 240V needs at the cabin, running 2-2-2-4 was just for future-proofing. For something like a well pump, I can run 240V directly from the shed.

At the main panel in the shed, I am going to use a 30A breaker to protect and switch the feeder to the cabin subpanel. I will use a 8 foot grounding rod at the shed and bond the neutral to the ground in the main panel in the shed.

The cabin is fairly small at just around 900 sqft. I intend on just two circuits, 15A GFCI for kitchen and bath, and 15A for the rest (mainly lighting).

My current questions are related to the subpanel at the cabin.

  1. Do I need a main breaker in the subpanel? The subpanel is in another structure, so NEC 225.31 says that there must be a "means ... for disconnecting all ungrounded conductors". But I have seen references to a "6 handle rule". Are the two breakers in the subpanel sufficient for 225.31?

  2. How do I handle grounding in the subpanel? I thought that I saw somewhere that I would need another grounding rod there and that I should not bond the neutral to the ground at the subpanel. But I cannot find where I read that.

  3. Should the subpanel be mounted in the interior or exterior? This is in central Colorado if it makes a difference.

  4. What is the easiest way to read the NEC online?

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    No, don't do 2-2-4 and 120V! You'll regret it later. Look for alternative wires, such as MH feeder, or burying conduit and using individual wires. Also remember your trenching needs to account for cable or conduit thickness, so 24"+wire thickness for direct or 18"+conduit OD for conduit. Oct 5 '21 at 22:32
  • Is it possible to do 2-2-4 with a separate 2-gauge conductor? I already have the trench down to 25".
    – tschutter
    Oct 5 '21 at 23:01
  • No afraid not.... Oct 5 '21 at 23:05
  • Why not? 2-2-2-4 and 2-2-4 both seem to be just loose bundles of individual insulated conductors. I would pick a direct burial related 2-gauge conductor.
    – tschutter
    Oct 5 '21 at 23:26
  • I am reluctant to advise it because inspectors cab be really funny with it. But if you tilt your head when you read 300.5, it'd be possible to have a separate safety ground if it's all either in conduit or direct burial. Can you live with the implications of a #4 neutral? Code: 300.5 (I) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors shall be installed in the same raceway or cable or shall be installed in close proximity in the same trench. Oct 6 '21 at 0:03
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2-2-2-4 is fantastic stuff and a regular recommendation from me. Heck the stuff is the price of #10 copper and carries 90A instead of 30A, what's not to like?

I have seen several prominent home off-grid power projects on Youtube fail hideously, because they got "copper-itis" and ran the #10 copper they could afford for a low voltage DC circuit that needed much larger wire. If they had run the aluminum feeder they could have afforded for the price of that #10Cu, their project would have worked!

Do you need a main breaker in the subpanel?

Well, you need a main disconnect of no more than 6 throws.

If you don't want a main breaker panel, this is a good argument in favor of getting a nice big panel that isn't HOMeline*. Because if you can avoid using tandems, you can turn 3 breakers into 1 throw using a handle-tie intended for 3-phase circuits. (Those ties don't work on tandems). So theoretical 18 circuit limit. Also most larger panels are convertible, so you can just go with a real main breaker if you exceed that.

You can put two 120V circuits on a 2-pole breaker, but then you get the undesirable effect of common trip, so one circuit overloading knocks out the other. So if you want to double-stuff a small panel, you can use dual 240V quadplexes...if you want to go that way, favor Eaton, as they make non-common-trip quads. But that trick only limits you to 12 circuits.

What do you do with ground rods?

Totally separate systems. The neutral comes from the neutral feeder to the neutral bar to the circuit neutrals.

The ground comes from the grounding rods AND the ground wire (different jobs) to the ground bar to the circuit grounds.

They are not bonded together. Two neutral-ground equipotential bonds would cause all sorts of mayhem.

Subpanel on the exterior?

no, interior is just fine. Must have standard working space kept clear at all times, so select an area that people will tend to normally keep clear, like a hallway or threshold.

Does the disconnect need to be outside? No.

What is the easiest way to read NEC online?

cringe

Trying to use NEC as a learning document is a bad, bad idea. And it says so right in Article 90.

You can sign up for an account at https://nfpa.org which gives free, crippled access (i.e. their lame UI, not a PDF) to the NEC text. however, it is behind a "signup" wall, so a bit of a privacy loss, and yet another password to keep track of.

Rules here prohibit discussing any scanned PDFs that may exist. The National Fire Protection Association is a non-profit 501(c)(3) public charity.

Really, an electrical education needs two things: first, a general book on the topic to be a well-rounded primer, i.e. to tell you which questions to ask... and then, a search engine or sources like us to deep drill into particular questions.

99% of the time you will find articles on mikeholt.com, electricallicenserenewal(?).com, ElectricianTalk, or here that will cover your question. Ugol's Law: you're not the only one.

Wire size

I think it's brilliant to choose #2 aluminum. You can run that as high as 90A.

#4 aluminum is good to 65A and can be breakered at 70A since 65A breakers are not made.

I recommend breakering it for 60A. The reason is, 60A is the smallest breaker that will accept #2 wires directly.

As far as 120V vs 240V, I strongly recommend being 240V ready. Since you are direct burial you ony get one shot at this. If all you can get is 2-2-4, I would either run 2” PVC conduit so you can put in any wire you like... including fiber optic by the way. Or I would throw a 6Al or 8Cu wire in the trench for use as a ground.

Use the 2-2 as hot-neutral for now. If you go 240V later, rejumper it so the 2-2 are the hots and 4 is neutral.



* Square D also sells the industrial grade "QO" panel line. HOMeline, is their, well, home line... and as such, is not offered in 3-phase models. Therefore no 3-phase handle ties.

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  • Interesting application of Ugol's Law...
    – TylerH
    Oct 6 '21 at 13:43
  • Do I need a main breaker in the subpanel? I cannot stress enough that this is a small cabin. It has two big rooms and a small half bath. I am starting with only 2 circuits, and I really do not see it growing beyond that. Just to make sure I understand correctly, if there are 6 or fewer circuits in the panel each with their own breaker and none are tied together, then I should not need a main breaker. Correct?
    – tschutter
    Oct 6 '21 at 23:26
  • What do you do with ground rods? I think I understand how to do the grounding, except in regards to the grounding rods. It appears that two grounding rods are the standard now. So I should be using 4, two at the shed and two at the cabin? And if there are two grounding rods at a structure, how are they connected to the grounding bar in that panel? One ground wire from the first grounding rod to the second grounding rod and then a ground wire from the second grounding rod to the grounding bar? Or a ground wire from each grounding rod to the grounding bar?
    – tschutter
    Oct 6 '21 at 23:29
  • Subpanel on the exterior? Actually, an exterior panel will be much easier to install than an interior one. Better panel access, easier to run circuits to where they need to go. Is there any reason that will not work?
    – tschutter
    Oct 6 '21 at 23:29
  • What is the easiest way to read NEC online? I was not thinking of using the NEC as a learning document, but it would be helpful to read the actual text when random Internet people refer to it, just to make sure that my understanding is correct. But I think you gave me enough hints to find what I need.
    – tschutter
    Oct 6 '21 at 23:30
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Try 2-2-4-6 or 2-2-2-4 MHF instead

If your mind is set on #2 Al for the hots, then I can fix your cable problem quite simply: get MHF, not URD. Mobile home feeder cable can be used for everything URD can and then some (it's tri-rated USE-2/RHH/RHW-2), and is easier to find than quadruplex URD, although not available in a full selection of sizes.

As to the subpanel disconnect? Just get a subpanel with a main breaker in it

The easiest way to handle the need for a subpanel disconnecting means is to get a subpanel that's fitted with a main breaker. This may sound like an unneeded extra cost now, but makes upgrading your cabin to something resembling a full 100A residential service in the future trivial. Interior vs exterior doesn't matter so much, though, as long as the panel is located where the feeder enters the cabin and is rated correctly for where it's at.

You read correctly on the grounding

Your source on the grounding of subpanels in detached structures is spot on: you need to fit a separate grounding bar to the subpanel and bring the structure grounding electrode conductor from that building's ground rods to it, while removing the bond screw that would connect neutral to ground in a main panel. Speaking of ground rods, you'll want to drive a second one at the shed and connect it to the first one, by the way, in order to guarantee that you have a compliant grounding electrode.

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  • I have not seen any available MHF, but I probably have not looked hard enough. None of the home supply stores seems to carry it.
    – tschutter
    Oct 6 '21 at 23:33
  • @tschutter that's very odd that they wouldn't -- it's a very standard commodity product. Perhaps they dont' stock it locally because your area bans mobile/manufactured housing or something else silly like that? (Also, have you looked online?) Oct 7 '21 at 0:06

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