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I am adding a small office shed (10x12') to my property, and need electrical power to run a mini-split HVAC system, plus a few LED light fixtures, and plugs for my laptop, a printer, and maybe a small speaker (Sonos-type).

Shed will be roughly 65 feet from the nearest corner of the (2-story) house.

Will the NEC (and its adopted clauses for Arlington, VA) allow me to run my own aerial drop from the top corner of my house over to the shed?

I would install a mast on the shed to terminate the aerial drop, which would be about 65' from house to shed.

I loathe the thought of trenching & placing conduit due either to the likely destruction of a large tree (at least 25% of the roots would be cut) or the need to cut a 2'-wide swath through a slate-on-gravel patio.

My property is small (75' wide), my house is smaller (25' wide), and with the tree and the hardscaping, there are no good options for trenching. (Unless I could get one of those industrial conduit placers that the telecoms use which essentially tunnel underground for 30-50' at a time).

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    Not exactly the same situation, but I think a relevant answer already exists here: diy.stackexchange.com/a/214538/126832
    – CJC
    Mar 2, 2021 at 15:53
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    There is no "US electrical code". The NEC is a spec code and it's up to each state, county and even city to determine how much of it they adopt, which version they adopt, and any changes to it they care to make. Edit your question to indicate your exact city so those who know (or can look up) these things will be able to best help you.
    – FreeMan
    Mar 2, 2021 at 15:53
  • With thanks to CJC, I've looked at the other post and found that messenger supported wiring is likely an option for me. I updated the post to show that I'm asking about Arlington VA so that anyone in this county might respond with specific code insight for Arlington County, VA.
    – JB_wonders
    Mar 2, 2021 at 17:48
  • You might want to review NEC Article 225 Outside Circuits at nfpa.org/codes-and-standards/all-codes-and-standards/… , Part I is only a couple pages. Mar 2, 2021 at 18:20
  • Are you planning to put a subpanel in at the shed, or simply have a disconnecting means with a couple of branch circuits? Mar 3, 2021 at 1:13

2 Answers 2

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Virginia is on the 2014 code according to the NEC adoption map. NEC 225.18.1 thru 5 provide clearance requirements.

10’ is acceptable for foot traffic only area.

12’ if there is a driveway.

18’ if there will be truck traffic.

65’ is not excessively long for a single span of an aluminum feeder but I would not try copper. The weight causes more sag thus more tension is required to maintain your minimum height. Aluminum feeder will cost less also.

We don’t know your total load but a messenger wire probably be enough for 65’ with aluminum. If you go to 1/4” aluminum messenger cable (not a single wire) you now have a ground that can be used with your 3 insulated conductors sunlight resistant type or a multi conductor cable.

Edit: I double checked and these references will be helpful. 6awg aluminum can span that distance NEC 225.6 I do much larger runs so I am used to messenger wires. If you connect below the roof you need to be 3’ away from any window openings 230.9.A. Above the roof check out 230.4 for those clearances.

And remember make your run at least 6” taller at the lowest sag point because a hot day the cable may sag that much it really can.

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  • What is "messenger cable"? Mar 2, 2021 at 21:58
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    @JimStewart -- a "messenger" wire is a mechanically robust (typically aluminum with steel reinforcing strands aka ACSR, although you could use galvanized steel for a non-current-carrying messenger) wire that is used to support other wires or cables Mar 3, 2021 at 1:19
  • Three phase is correct I did not specify this because I do it often. Galvanized steel single wire is not a grounding conductor but a 7 wire aluminum cable usually is (the messenger in both cases is supporting the other conductors).
    – Ed Beal
    Mar 3, 2021 at 3:46
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Yes, the NEC lets you do that

The NEC has absolutely no problem with letting you have your very own overhead cable run. This is called messenger supported wiring, and can be done using by lashing a UF cable to a separate messenger wire, as per NEC Table 396.10(A). However, a more conventional approach to this is to use an overhead multiplex cable, much like the ones used for service drops, although with an extra wire included since you're running a feeder, not a service. It's also somewhat cheaper to use the multiplex cable, as a 6-6-6-6 "Chola" quadruplex costs about the same per foot as 12/3 UF-B, and the latter requires a separate messenger wire.

So, we'll start with that for our 65' run. Since we're dealing with an "open multiconductor cable", the NEC 225.18 minimum clearance of 10' from the finished grade or sidewalk to the bottom of the cable sag applies, provided you aren't crossing a driveway. (If you do have to go over the driveway, then you have to increase the clearance to 12'.)

So, given our quadruplex cable from above at 0.162lbs/ft and about 0.625" diameter, the sag and tension calculations from the NESC as restated on pp. 33-35 of Southwire's installation manual, and the fact that we're dealing with "Heavy" ice loading as per the NESC's ice/wind loading figures and your location, we get an ice loading of 0.698lb/ft, a wind loading of 0.542lb/ft, and a resulting cable load weight of 1.32lb/ft. From there, we can use that, your 65' span, and a 1.5' sag to get a loaded tension of 465 lbs, which is well under 50% of the 1190lb load rating on our "Chola" cable's integral 6AWG ACSR messenger. We then add 6" to this to provide a margin for heat sag.

As to getting up to 12' or more above ground...

Obviously, you'll need a mast with a weatherhead at the shed; this can be done using a single stick of 1.25" RMC with some 1.25" two-hole straps, a 1" nipple, a 1.25" to 1" reducer bushing, and a 1" LL or LR body at the shed end of the circuit, along with a 1.25" threaded or clamp-type weatherhead, 4AWG Al XHHW-2 for the in-mast wires, some Burndy AGSKIT2 or equivalent dual-rated Al9Cu aboveground splices for the hots and neutral, a clamp-type wireholder for anchoring the quadruplex cable's integral messenger to the mast mechanically, and an Ilsco GTA-2-2 and Ilsco AGC-1 or equivalent dual-rated tap connector and pipe clamp to allow a 4AWG Al jumper to be used to bond the messenger to the mast. Don't forget to form a drip loop in the conductors extending out of the weatherhead before splicing them to the overhead wires!

At the house, you can either use a similar mast and weatherhead, or SER cable formed into a gooseneck along with a beefy eyebolt (remember that loaded tension from before, as well as the angle the drop is making) to secure the messenger to a stud. Either way, you'll need the same parts for splicing the mast or SER wires to the overhead wires as you used at the shed, and you'll also want to remember the drip loop here, too.

As to ampacity, we're in luck: the 4AWG Al mast wires are the limiting factor here, permitting 65A at 75°C. This is because we can run the overhead quadruplex at its 69A 90°C rating thanks to the 110.14(C)(2) separately installed pressure connector provisions applying to this sort of run. A 60A or 70A breaker can be used for this feeder if you're putting a panel and grounding electrodes in at the shed; if you wish to dispense with said panel, you're limited to a 20A, 2-pole breaker so that you can treat this as a (fat) multi-wire branch circuit instead.

And wiring up the shed...

You have two options as to how to wire the shed; either you can put a simple non-fused pullout (air conditioner) disconnect in and treat the circuit to the shed as a branch circuit, or you can put a panel and grounding electrodes in at the shed and thus have a full feeder running to the shed. The disconnect is somewhat simpler and cheaper than the panel, but severely constrains how much power you can provide at the shed compared to the panel approach, as mentioned above. You'll also need to provide a 3-port, 14-4, dual-rated insulated mechanical connector for the neutral connections at the disconnect if you go that route since an A/C disconnect only provides a single, bonded block. From there, you'll have two 20A, 120V branch circuits; one for the lights and receptacles, the other for a 120V heat pump.

If you do decide to go with a panel...

If you go with a panel, you'll want to go big here; in fact, a 24- or 30-space, 100 or 125A, NEMA 3R (outdoor rated) main breaker panel is not at all out of place in this application, as the main breaker simply provides the outbuilding's disconnecting means. You'll need to fit a separate grounding bar of the appropriate type for your new panel and land the incoming grounding wires on that atop pulling the bonding screw from the neutral, of course, since this is a 4-wire feeder.

Speaking of grounding, that panel means that you'll need to take some 6AWG copper and run it from the grounding bar, out the bottom of the panel, to a pair of 8' ground rods driven 8' or so apart. This provides a grounding electrode system for your shed so that wayward natural electricity (such as lightning-induced transients) can get returned to Mother Nature, while the feeder equipment grounding conductor returns errant utility electricity back to the utility via the neutral-ground bond in your main panel.

Note also that if you go with the outdoor panel route described above, you can simply use a 1.25" bolt-on hub atop the breaker panel instead of using the LL or LR body, reducer, nipple, and locknuts to bring the mast into panel from the side.

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  • The NEC doesn’t require a messenger support #8 aluminum is good for 50’ and #6 can go longer without messenger support I provided the code reference the separation of the grounding electrodes is 6’ or more per NEC 250.53.A.3 not 8’.
    – Ed Beal
    Mar 4, 2021 at 17:03
  • @EdBeal -- 6' is indeed the minimum, I suggest 8' because one can drive the first rod, then use the second rod as their measuring stick to get far enough away from the first :) And yes, when I'm talking about a "messenger" here, I'm referring to the integral bare ACSR messenger in the quadruplex Mar 4, 2021 at 19:48

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