Baby Step #1: Installation of 150' 600V underground cable

[BackwoodsEE]

The first baby step in what will be a long summer of a DIY hybrid off-grid/grid-tie installation is coming up soon: A 150' run of cable for each of four 12-panel strings, to be ground-mounted out in a clearing in my woods. I'm using a hefty 9-conductor cable of 10 AWG THHN wires rated for direct burial and wet/dry installation. It won't be direct buried, though. Rather, I'll be pulling it through 1 1/4" IMC conduit because I don't want to have to go down the full 24" depth that would be required. (Call me crazy, but I want to avoid as much as possible hitting the roots of some fairly big trees near the house end of the run.) NEC allows just 6" of depth from grade to the top of the conduit with IMC underneath this little piece of the property, where no vehicles ever go.

The conduit is pretty big, but not much bigger than required by NEC to accommodate the 0.76 inch outside diameter of the cable. I'm using the conduit as my EGC, although I have a spare conductor (8 are needed for the 4 strings) and I'll bond the green one to the equipment ground on the equipment end and the conduit body on the array end.

I'll be renting a small hydraulic bender if I can't find some 90 degree elbows in time for making the sweep upwards from the trench. I'm hoping I can adjust the overall length of the run slightly to avoid having to cut and thread the conduit before it runs into the C bodies on either end.

I've pulled a permit just for the cable installation, and am assuming I'll be calling for an inspection after the conduit is laid in the trench and then I've pulled the cable through it. (Of course before the trench is backfilled.)

The cable will be going into a dedicated steel equipment shed near but not attached to the house where all my disconnect, ground fault protection, charge controller, inverter/charger, and FLA goodies will be housed (and locked). Hopefully painting the thing all white and giving it some modest forced-air ventilation will keep things from getting too hot inside during the summertime. One nice thing about using a ground-mounted array and a "detached building that is only used to house PV equipment" is that NEC 2017 exempts you from both arc fault and rapid shutdown.

Comments welcome! (And in this forum, I say that knowing I might wind up receiving some harsh but realistic criticism of my plans. Plans are easier to change than hardware.)

[bcroe]

I do similar things here, but a combiner box at the panels reduces the underground run to much fewer, big cables.
Also I threw a direct burial 12-2 in the trench for a handy outlet at the panels, with GFI. Bruce Roe

[NEOH]

What is the Fill-Factor with 9 x 10 AWG inside the 1 1/4" IMC ?
Has your state adopted the 2017 NEC code?

[NEOH]

...One nice thing about using a ground-mounted array and a "detached building that is only used to house PV equipment" is that NEC 2017 exempts you from both arc fault and rapid shutdown...

Is Module Level Shutdown required within the Solar Array boundary in NEC 2017?

[sensij]

I'd be looking closely at the local grounding requirements. If your AHJ requires you to put in a dedicated electrode at the array (or use the ground mount system as the electrode) per 2014 NEC or just because that is how they interpret things, it should probably be bonded directly to the same electrode providing the AC EGC for the inverter. In that case, you might want to bury some copper along with the conduit. 2017 NEC should remove the requirement for the electrode at the array, but again, you might want to check with your AHJ to see exactly what they will want.

[BackwoodsEE]

What is the Fill-Factor with 9 x 10 AWG inside the 1 1/4" IMC ?
Has your state adopted the 2017 NEC code?

WA adopts NEC 2017 on July 1. I could stick with the 2014 version by pulling my permit by the end of the month, but I think the "dedicated solar building" exception under NEC 2017 actually makes things easier.

NEC chapter 9, table 1 has two notes that are pertinent to your other question:

(5) For conductors not included in Chapter 9, such as multiconductor cables and optical fiber cables, the actual dimensions shall be used.

(9) A multiconductor cable , optical fiber cable, or flexible cord of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area.

For 1 1/4" IMC, the maximum single-conductor fill area is 0.873 in². My cable has a cross-sectional area of 0.454 in², so the conduit is quite oversized. But it's a pretty long pull at 150 ft, so I guess that's OK with me. (It looks like I could have gotten away with 1 inch IMC but not 3/4 inch.)

[BackwoodsEE]

Is Module Level Shutdown required within the Solar Array boundary in NEC 2017?

Not for a ground-mounted array where the solar equipment is not on or in a building, or in a building whose purpose is only to house solar equipment. And not until 2019 regardless, because the <80V rapid shutdown "requirement of 690.12(B)(2) shall become effective January 1, 2019."

I really hope sanity prevails before then, at least in some states' adoptions of the code, and this overreaching requirement is dropped. SMA has pointed out that the solar industry side of things is disproportionately represented in the code making process by companies that stand to gain from the module level shutdown nonsense.

[BackwoodsEE]

Well, the first of several challenges has been met. The inspector approved my 1 1/4" IMC conduit in the 150' trench (6" depth) and the cable is fine because it's rated for direct burial. I can fill in the trench now. He did say I couldn't use the set-screw connectors I had transitioning from IMC to EMT above grade to the equipment shed on one end and to a NEMA box on the other. I spent a couple of hours yesterday disassembling the wiring and EMT conduit segments in the equipment shed and replacing the connector with a rain-tight version, and feeling a bit stupid about thinking I could put a set-screw connector outside exposed to the elements.

Overall, though, I'm pleased to have gotten this part of the job done. Pulling that much cable (0.7" O.D. with about 290 degrees of total conduit bend) through that conduit was not an easy task. I wound up winding the fishtape around one of my tractor forks, very slowly raising them to pull a few more feet of tape out of the box on the array end of the conduit while maintaining tension on the reel of tape by hand, lowering the forks, winding up the tape, and repeating many times. Was sure glad I'd spent the $20 or so on a wire mesh cable-holding thingy to connect the tape to the cable.
wire-pull.jpg

I wore my logging faceshield+earmuffs while doing the pull. Didn't like the idea of that fishtape snapping under tension and springing back in my face.

Still have to swap out the set-screw connectors on the far end, as shown. That one will be easy as there is minimal conduit to pull apart and no wiring done yet.

[max2k]

Well, the first of several challenges has been met. The inspector approved my 1 1/4" IMC conduit in the 150' trench (6" depth) and the cable is fine because it's rated for direct burial. I can fill in the trench now. He did say I couldn't use the set-screw connectors I had transitioning from IMC to EMT above grade to the equipment shed on one end and to a NEMA box on the other. I spent a couple of hours yesterday disassembling the wiring and EMT conduit segments in the equipment shed and replacing the connector with a rain-tight version, and feeling a bit stupid about thinking I could put a set-screw connector outside exposed to the elements.

Overall, though, I'm pleased to have gotten this part of the job done. Pulling that much cable (0.7" O.D. with about 290 degrees of total conduit bend) through that conduit was not an easy task. I wound up winding the fishtape around one of my tractor forks, very slowly raising them to pull a few more feet of tape out of the box on the array end of the conduit while maintaining tension on the reel of tape by hand, lowering the forks, winding up the tape, and repeating many times. Was sure glad I'd spent the $20 or so on a wire mesh cable-holding thingy to connect the tape to the cable.


I wore my logging faceshield+earmuffs while doing the pull. Didn't like the idea of that fishtape snapping under tension and springing back in my face.

Still have to swap out the set-screw connectors on the far end, as shown. That one will be easy as there is minimal conduit to pull apart and no wiring done yet.

too late but I found wire pulling wax helps a lot- drops friction at least by factor of 2 or even more. You'd probably need bucket of it in your case .

[Mike90250]

too late but I found wire pulling wax helps a lot- drops friction at least by factor of 2 or even more. You'd probably need bucket of it in your case .

Yeah, fun stuff. I call it elephant snot, yellow, goopey and slippery. Sort of like waterless hand cleaner, but electrical safe.

[Sunking]

A 150' run of cable for each of four 12-panel strings, to be ground-mounted out in a clearing in my woods. I'm using a hefty 9-conductor cable of 10 AWG THHN wires rated for direct burial and wet/dry installation

OK there is nothing in the code that says you CANNOT do this, but will limit how it is used. So why a 9 conductor cable when you only need 2 maybe 3 conductors?

Here is what I am driving at. From the sound of it you might be thinking of paralleling cables to increase current capacity, lower voltage loss, or both. If that is your intent, it won't Fly. You cannot parallel conductors smaller than 1/0 AWG per NEC 310.10(H). Sounds like a stupid rule, but there is a good reason because chances are good, for whatever reason one of the parallel conductors is opened up and consequently over load the now single conductor. That can happen and you would not likely notice anything until some day you use enough current and over heat the 1 existing wire.

Just curious as that caught my eye.

[sensij]

OK there is nothing in the code that says you CANNOT do this, but will limit how it is used. So why a 9 conductor cable when you only need 2 maybe 3 conductors?

Here is what I am driving at. From the sound of it you might be thinking of paralleling cables to increase current capacity, lower voltage loss, or both. If that is your intent, it won't Fly. You cannot parallel conductors smaller than 1/0 AWG per NEC 310.10(H). Sounds like a stupid rule, but there is a good reason because chances are good, for whatever reason one of the parallel conductors is opened up and consequently over load the now single conductor. That can happen and you would not likely notice anything until some day you use enough current and over heat the 1 existing wire.

Just curious as that caught my eye.

Just jumping in, but I'm pretty sure he is running the DC+ and DC- from each of the four strings separately through the 150 ft run (using 8 conductors) and either combining them or running to independent mppt in the equipment shed.

[foo1bar]

Yeah, fun stuff. I call it elephant snot, yellow, goopey and slippery.

The wire lube I used was clear.
Searching for "wire lube" brought up Ideal ClearGlide which looks like what I remember for the bottle I used.

I agree that the consistency was sort of like those gel hand cleaners. (But obviously it'd be a bad idea to try to substitute that)

I also found you can get a 50-gal drum of it on amazon.

[J.P.M.]

The wire lube I used was clear.

I also found you can get a 50-gal drum of it on amazon.

Obviously your wire lube snot came from elephants that don't have allergies. My money's on it came from a zoo animal.

I have it on good authority that the 50 gal. drum stuff comes from elephants in the wild, I.E., organic and not taking artificial hormones or allergy medication. Probably not vaccinated as infants either.

[BackwoodsEE]

too late but I found wire pulling wax helps a lot- drops friction at least by factor of 2 or even more. You'd probably need bucket of it in your case .

I went through a full quart of wire pulling lube. My kid was feeding it in at the equipment shed and I instructed him to see that half of it was gone when half the wire was.