Ideas for Running Electrical and Fiber Optic From Underground Conduit
I am building a small detached guest house that has power and networking coming from the main house through an underground conduit. The power will be 3+1 4AWG 220v; the networking is OM-3 duplex fiber optic; and both are in the same 1-1/2″ PVC coming out of the ground. All those wires need to get inside the building and hopefully do so in a fully air-sealed manner and be reasonable otherwise.
What I’m looking for here is inspiration on how, exactly, to do this. Descriptions, photos, articles, videos… whatever. What are the great ways of accomplishing this task?
Off the top of my head, maybe I can just attach a block to the exterior and drill five small holes through it and into the house. I then mount an LB conduit box in front of said block and pass each wire through an individual hole, which can be individually sealed.
Maybe the LB conduit box isn’t big enough to drill five holes, so maybe I put a junction box there?
If I’m going to put a junction box, then would it make sense to make it a metal box and have a shut-off of some sort at that point, even though the actual sub-panel is going to be inside?
But maybe the sub-panel should just be outside and then I extend the conduit above the sub-panel through the soffit into the attic and disburse the wires from there?
Maybe the conduit coming out of the ground can just pass up the wall to the attic? Not sure if that would be aesthetically up to snuff, though.
Let me know your opinions on what makes the most sense!
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Replies
First, DO NOT share the conduit between power and communications. While techically you can do this when using fiber optic cable, it's not a good idea. I would run a seperate 1" communications conduit instead. PVC conduit is cheap (at least for now...), the expensive part is digging the trench. Run a power conduit and a data conduit and be done. A third conduit is sometimes useful to allow for remotely switching outdoor lighting too.
We don't normally air seal the interior of conduits commerically. Underground conduits that enter below grade should have a water seal (I like expanding rubber grommets, but they're not made for smaller conduits with multiple cables, unfortunately). The usual way to deal with underground conduits is to bring them up above grade on the exterior of the building, run them into the bottom of a pull box, and then go from the back of that box through the wall to whatever is inside the building. You can use an LB for this transition, but it's usually easier to use a small(ish) box. For what you're planning, I'd used either an 8x8x4 or 6x6x4 box for the power conduit. Bigger boxes are easier to work with when pulling the wires.
You're not permitted to run the wires outside the conduit as individual conductors in most applications -- they are required to have the protection of the conduit. There are various ways to seal them if you really want to (I don't usually bother), I'd probably use sheets of fire stop putty myself.
For the fiber optic cable, I'd run this:
https://www.fiberinstrumentsales.com/tlc-6-fiber-mm-50um-om3-indoor-outdoor-plenum-black-4-8mm-od.html
I've installed lots, and lots of that exact cable and it's always worked well for me. It's a round cable, so you can use a small cable gland to seal it where it exits a box. I'd run it in conduit as far as you can, and only seal it right next to the patch panel. Make sure you leave a decent amount of slack on each end (I usually leave 20-50 feet). You don't need to terminate all 6 strands if you don't need them all, but it's nice to have spares and 6 strand cable costs only a little more than 2 strand cable. Don't run a jumper (patch cord) here -- that type of cable doesn't hold up well in this application. The cable at that link is the proper type to use and is intended for indoor/outdoor use like you're doing.
Note that you can probably run a smaller ground wire. 8 gauge is good up to 100A when used as a ground, for example. If you're running a smaller circuit, but are using 4 gauge wire for volt drop, you need to upsize your ground for volt drop too, but start with the size the code allows for grounding the size circuit you're using. If your outbuilding is far enough away from the main structure, you need another ground rod at the outbuilding tied into the ground bar there too.
BTW, with today's crazy copper prices, you might want to consider using aluminum wire for this run to save money. XHHW aluminum conductors will work here, and are a lot cheaper than copper even when going up a size. If you use a main-lug panel at the outbuilding, use compression lugs to terminate the wires. Using compression lugs eliminates almost all of the potential downsides to using aluminum wire.
Bill
Thank you, Bill -- that helps a lot!
The underground conduit situation is a ship that has sailed, since the trench was dug and conduit laid over a year ago. The decision to share networking and power in one conduit was very recent and saves a lot of extra work. Can you describe more why it's a bad idea to share those, though? I'm fully on board with why installing a (very conductive) Cat6 cable alongside the power cables is verboten, but I haven't heard of objections with fiber optic and power in the same conduit?
The idea to seal each individual wire comes from Matt Rinsinger, who is adamant that there is precisely one thing for any hole, since multiple of anything in one hole is very difficult to fully seal. He did have a video showing his electrical ingress, but I haven't been able to find it recently (his videos are poorly labeled and indexed). But that's a very good point that individual strands would be problematic outside of conduit. Hmm...
The 4AWG is already taking de-rating into account, due to the length of the run -- I was technically approved at 6AWG for a 60 Amp sub-panel. I even am mildly considering jumping to 2AWG for future upgradability, if it's not too egregious price difference.
And I'll look into using aluminum. I'll admit that I discount it due to horror stories I've heard about corrosion at the connection points, but if it is, indeed, much much less expensive, then learning how to do it properly might be worth it. Availability will be key, too -- I have very limited access to proper electrical supply shops so if the big box stores don't carry it, then it might as well not exist, much of the time.
There are some code rules about what can share a conduit. Telecom cables aren't permitted to share power conduits, but that's due to power cross concerns. Since fiber optic cables are non-conductive (but ONLY if you specify ALL DIELECTRIC construction! Some fiber optic cables ARE conductive, notably almost any "armored" cables), this SHOULD get you around that particular issue... but....
Code also says you can't use electrical panels as junction boxes for things not terminating in the panel. That means stuff can't "pass through" a breaker box, for example. You need a seperate pull box ahead of the electric panel.
I think you'll be OK on a technical level with the fiber optic cable in the same conduit with the power conductors, but you might want to run that by your inspector first to avoid any potential problems down the road. Make sure to have pull boxes ahead of the panels on each end so that you can seperate the fiber optic cable before it gets to the electric panel.
When pulling the cables, you'll want to pull things in a somewhat unusual way: Figure out how much slack you need on one end for the fiber optic cable to reach it's termination location, then mark that spot on the cable with some colored tape. Tape the power cable to the fiber optic cable at this spot. Now run a pull rope through the conduit, pulling enough through so that it can pull the slack in the fiber cable and the power cable together, but in a straight line. The idea is to pull the fiber into the conduit ahead of the power cable, then start the power cable into the conduit once the tape mark on the fiber is reached. The goal is that ALL cables pull through the conduit TOGETHER, and don't slide against each other. If you were, for example, to pull the power cable in and then the fiber optic cable, the moving fiber optic cable would tend to cut into the power cables and that can be a problem. It's always best to pull all cables into a conduit at the same time whenever possible. If you can't do that, try running a small microduct (small, flexible polyethylene conduit, usually given in metric sizes) along with the power cables, then pull the fiber optic cable through the microduct. This is a common technique used in the telecom world when we have to squeeze more cables into an existing underground duct bank. Those pesky road commissions just don't like us tearing up their roads over and over again...
Aluminum is/was an issue with devices like receptacles and switches, where loose connections would corrode and get hot. This is much less of an issue with larger conductors, and aluminum is commonly used between the meter can and main panel in most homes. Most outdoor utility wiring is aluminum too. What you need are proper connections. Compression lugs for aluminum wire are pre-loaded with antooxidant (no-ox) paste, and the compression process makes a gas-tight seal that won't corrode. This is the only way I permit aluminum wire terminations to be made on my commerical projects. If you are terminating one end into a circuit breaker, you won't have that option on that end, but you can make things better by using no-ox paste before connecting the wire to the breaker. No-ox paste needs to be worked in between the individual strands of the cable too, coating only the outside isn't really enough.
You're likely to find aluminum wire will be half as much as copper, possibly less. The big box stores usually have 2-2-2-4 aluminum URD cable, sometimes sold as "mobile home feeder cable". This would be a good fit for your project. You usually want to pull individual conductors into conduit, but the twisted URD cable will work too, it's just a little more difficult to work with.
Note that if you go with cable larger than #4 in a 60A breaker, check to see that the lugs on the breaker will be able to handle the larger wire. The max wire size the breaker can handled will be printed on it near the lug.
If you can tell me the length of the run I can run volt drop calculations for you for a few different wire sizes.
Bill
Bill, thank you for the veritable wealth of knowledge you've given me! There's a lot to absorb there. In particular, I was indeed planning on passing the fiber optic cable through the junction box, so you've just saved me a failed inspection and a re-do.
The fiber optic cable I got appears to be entirely non-metallic. And yeah, I was planning on pulling it and the power cables at the same time. Step 1: Use a vacuum to pull a light pull cord through. Step 2: Use the pull cord to pull a 1/2" poly strip (with length markers). Step 3: Attach the poly strip to the power cables and fiber optic and pull using a staggering amount of lube.
I'll definitely be looking at aluminum very closely now. My run is 190 ft. According to the Southwire Voltage Drop Calculator, I'm looking at 4AWG copper or 2AWG aluminum for 60 Amps. If I did want to potentially upgrade to 100 Amp service in the future, I'd want 1AWG copper or 2/0 aluminum. Hmm... not sure I wanted to go quite that big.
You can go through a junction box, but you can't go through a panel (circuit breaker box, etc.).
We call that "poly strip" "mule tape" in the trades. You can buy it surprisingly cheaply on Amazon, and lots of other places. I typically buy 5,000 foot rolls. We use it for long pulls in outdoor ducts.
I recommend when you run your first string to start with small 1/16" nylon twine. I tie a tissue on the end so that it catches more air in the conduit. A vacuum will work well to pull it in, and pulling works better than pushing here (use the vacuum to suck, not to blow).
I recommend limiting your use of lube to what is necassary. Too much lube just makes a mess. Use CLEAR LUBE! I like the "aqua gel" stuff Ideal makes. DO NOT use "yellow 77", that stuff sets up like wax and will lock your cables in place so you'll have a major problem if you ever need to pull them out to replace them. I'd stay away from the blue lube too. We used that once. My electrician that day was bald, and was working overhead. We found out the blue dye stains your skin. It took several weeks and a lot of extra showers to get the blue out. We called him "Smurf" for that period of time :-)
Copper will be smaller for the same ampacity due to the slightly lower resistance. My advice is to plan your loads carefully, and don't go bigger than you need. Since you're going to have to use 4 wire service here, your 1.5" conduit limits you, and 2/0 cable isn't going to fit.
I recommend you rent a "chugger" (wire pulling machine) for this pull. These are motorized wire pulling machines that you bolt to the floor that do most of the work for you. With an underground conduit, you'll probably need a roller and a rack so that the mule tape can come up straight out of the conduit, go over the roller, then to the chugger. If you are using PVC conduit, the mule tape will cut through the side of the PVC conduit if you pull over the edge on the end. I would screw a sacrifical plastic bushing on the threads at the end during the pull, then replace it afterwards because it will probably get broken while you're pulling. I have sometimes used grounding bushings, which are a metal bushing with a plastic insert, for pulling in cable just because they are more durable. You can get metal scoop-like devices to provide a rounded surface to pull over the ends, but I've never used those.
When you setup your pull, I would empty maybe 1/2 the bottle of lube into the conduit, and then make a big blob in an old rag. Have a helper slather that lube onto the cable as you pull it into the conduit. The initial blob you put into the conduit will tend to push ahead of the cable bundle and lube the pipe, the stuff you slather on the cable itself will help to keep it going smoothly. Wipe off as much as you can at the far end so that it doesn't get all over inside your panels.
BTW, if you are going to be terminating the fiber cable in the field, you want to seperate out maybe 8-10 inches of the yellow kevlar strength member and pull only that -- not the fiber strands themselves. If you pull the fiber strands too, you might break them. Commerically we use kellems grips on larger cables, but on smaller cables it's OK to just tie off the kevlar and pull that directly as long as you leave the fiber strands loose inside the cable. If you're using a pre-terminated cable, BE SURE you use a pulling sock over the terminations, and since you're using lube, seal them up with the good-quality 3M colored electrical tape (called "phase tape" in the trades). The way to do this is to wrap a layer over the connectors and a bit of the jacket with the sticky side out, then go over that with another layer of tape sticky side down (towards the cable), getting a bit of the jacket and covering all of the first layer of tape. This will seal the ends so that lube doesn't get into them, and the "sticky side out" layer protects the connectors from any adhesive residue. When you're done, clean them with 90+ percent isopropyl alchohol to get any remaining residue off before you connect them. Fiber wants to be clean. Lube will mess up the connectors, and attract dirt, and then you will have too much attenuation and a bad connection.
Bill
> The idea to seal each individual wire comes from Matt Rinsinger, who is adamant that there is precisely one thing for any hole, since multiple of anything in one hole is very difficult to fully seal.
In a perfect world, sure. In the real world, some things have to be protected in a certain way and the air tightness is secondary. If your overall air tightness is such that you don't think duct seal, foam, or some other combination of squishy and goopy things seals things well enough, I bet you you could go find a handful of other leaks in the envelope to make up for the new one you're introducing.
Depending on gauge, aluminum should be readily available at a big box store.