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Community and Q&A

Feeder Cable and Conduit Options

Jeff Cooper | Posted in Mechanicals on
The property where I’m building has an outdoor pedestal, which I gather is called a feed-through meter main, that powered a manufactured home there many years ago.   The feeder cable for my new house will have to go underground for 7′ from the pedestal into my crawl space, then 32′ to get to my 200-amp indoor main breaker panel.  From what I’ve gathered (please correct me if I’m wrong), I could bury 4/0-4/0-4/0-2/0 aluminum MHF (mobile home feeder) cable directly in the ground to go from the pedestal into the crawl space and then encase it in conduit hanging below my floor joists to get across the crawl space and then up into the house, OR I could use four aluminum XHHW wires of the same sizes in conduit all the way.  If I had thought ahead better, I might have dug a 24″ deep trench across the crawl space so that I could bury MHF the whole way; digging a trench that deep now would be too difficult.
Southwire provides a handy conduit fill calculator, https://www.southwire.com/calculator-conduit, that tells me three 4/0 and one 2/0 XHHW wires will fill a 2″ Sch 40 PVC conduit 30.57%, well under the 40% limit, with a very low Jam Probability of 3.475382.  It doesn’t have a selection for MHF cable.  A few opinions I’ve seen elsewhere have a contrary view, saying it would be difficult to get the XHHW through 2″ conduit, but I expect that would depend largely on the length of the conduit and the number of bends.   My pedestal currently has 2″ conduit attached, with smaller wires inside, and 2″ conduit is half as expensive as 2-1/2″, so I would prefer to use it if possible.
What is your experience with getting wires or cable of these sizes through 2″ conduit, and would you go with the MHF cable or the XHHW conductors?  They’re pretty close in price.  The MHF cable would avoid the two 90-degree bends to go under my stem wall and then back up into the crawl space.
I haven’t found any type of cable that can be buried to get into my crawl space and then run without conduit above ground inside the crawl space, but if such exists, that would be great.

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Replies

  1. Expert Member
    Akos | | #1

    You are generally limited with the amount of unfused feeder you can run inside a house.

    If you want longer runs the feeder needs to be buried in concrete or be fused at the meter or where it enters the house.

  2. Jeff Cooper | | #2

    Thank you, Akos. If I understand you correctly, there's a main breaker at the pedestal where the meter is installed.

  3. Expert Member
    Zephyr7 | | #3

    There should be a main breaker at the pedestal. You're otherwise limited usually to 6-10 feet (local codes can vary) between the meter can and the electrical panel.

    I would absolutely NOT direct bury the cable. Run PVC conduit, then pull cable through that. Cost is only a little more, but you're much better off down the road having conduit in place. Bring the PVC conduit up to a pull box on the exterior of the house, then run EMT conduit from there to the panel. You could also use SEU cable from the pull box to the panel to avoid the conduit run indoors (SEU is a nometallic sheathed cable, somewhat like what massive Romex -- NMB -- would look like). You'd need to put in the correct splice terminals in that pull box though. I personally would run conduit inside the home and keep unspliced conductors from the meter/disconnect all the way to the panel within the home.

    You'll need a panel with an isolated neutral busbar. Most panels have a bonding screw that can be removed to isolate the neutral (it's usually a large green screw that goes through the neutral busbar into the steel panel to electrically connect them together), but you'll sometimes need to add the seperate ground busbar. Since you'll have a remote main breaker, you need a 4 wire feed from that breaker to the panel to keep ground and neutral seperate. You'll want to make sure you have a ground rod at the meter/disconnect, and you'll want to make sure to ground your water lines to the ground of the electrical panel in the home too.

    For 200A service, you are correct that you would need three 4/0 conductors for the two hots plus neutral, and you'd need a #2 conductor for the ground (this assumes all aluminum conductors). BE SURE you use no-ox paste on all the connections, and use both the correct paste (for aluminum connections), and the correct application (work it between the strands with a brush, don't just coat the outside) to ensure reliable connections, ESPECIALLY the outdoor connections. If you'll be contracting that work out, I'd recommend you specify those connections be made using compression lugs pre-loaded with no-ox paste. That's what I write into my specs at work when using aluminum conductors. The compression lugs are like huge crimp lugs, and they ensure a long-term reliable connection.

    You should be able to fit those wires through a 2" conduit, but you'll need to use lube. I'd use the Aquagel lube made by ideal. Don't use the old standby "yellow 77", since that stuff sets up like wax over time, making future maintenance work difficult. Yellow 77 is also a lot harder to clean up compared to the newer aquagel lube. If you want to make your life a bit easier, use 2-1/2" PVC conduit for the underground portion of the run, and 2" EMT inside. The pull box in the middle will give you a mid span pull point which will make the pull easier if pulling the wire in by hand.

    Another little trick from the trades: use either "mule tape", which is a sort of flat pull rope, or actual rope of around 1/2" or so diameter to pull those cables in. Get a big Kellems grip to attach the wires to the rope, since that's the only good way to make a solid connection without also making the head of the cable fatter, which will tend to jam in the conduit with a big fill like this. The heaver rope or mule tape are less likely to cut through the conduit fittings on the ends, and they're much easier on your hands too compared to pull string.

    BTW, I'd use grounding bushings on the conduit fittings if you're going to be in tight quarters when pulling the wire. These aren't required by code here, but they are far, far more durable than regular plastic bushings and they won't split when you're pulling fat wires over them with a lot of force.

    Bill

  4. Jeff Cooper | | #4

    Many thanks for lots of detailed advice, Bill.

    What would be the advantage of EMT over PVC conduit inside the crawl space?

    1. Expert Member
      Zephyr7 | | #5

      EMT is thin wall steel conduit. It’s smaller than schedule 40 PVC conduit of the same trade size, and it’s a bit easier to pull through since it tends to be made with a slick inner coating. EMT is also more durable, and it’s conductive. The conductive part makes it a bit safer under fault conditions, and helps a little with lightning protection due to the additional reactance it provides. EMT is also far more fire resistant. EMT is better from a safety standpoint, basically, and also slightly easier to work with indoors.

      Bill

      1. Jeff Cooper | | #6

        Thank you, Bill.

        Following up on your previous advice: SEU cable sounds appealing, because if doesn't require conduit, I would limit the length of XHHN wires I'd have to pull through conduit to the short length from the pedestal into the crawl space, where I could splice the XHHN to the SEU. SEU, however, seems to be a three-wire cable, just two conductors and a neutral. How would that work?

        A pull box, as you suggested, would also let me pull through shorter lengths, but I don't want to put a hole through my rim joist or stem wall to mount it on the exterior. Would it be worth using if I mounted it in my 30" tall crawl space, or would I do better pulling through the whole length of conduit from inside the house or outdoors by the pedestal, where I would have plenty of room to stand up, etc.?

        1. Expert Member
          Zephyr7 | | #7

          Sorry, I meant to say SER cable. SEU is 2 conductors plus ground, SER is 3 conductors plus ground. That one letter makes a big difference :-D

          XHHW is what you want to use underground if you run aluminum wire. XHHW has a cross linked polyethylene jacket, and it's very tough stuff. THHN would be the nylon over PVC jacket, and is commonly used with copper conductors. Either can be run in conduit underground, but only XHHW is rated for direct burial -- although I recommend running it in conduit anyway.

          You will want to make any splices in that pull box using Polaris connectors like these:
          http://www.nsiindustries.com/UserFiles/Documents/Product/14_it_polaris17.pdf

          Don't use split bolts and tape -- Polaris connectors are far more reliable. You'll need a BIG pull box, probably 10-12 inches square and at least 4" deep (6" deep is better). I would mount it on the exterior (which means you need a 3R rated box), and I'd bring the underground run up into the bottom of the box, then exist out the back of the box, through the wall, and into the house. You're going to have to put a hole in your house somewhere, might as well use the box to conceal it. Be sure to seal things well though. I would not want to be doing the terminations in a crawl space, so I'd avoid putting the pull box there. Keep any splice locations accessible.

          Bill

          1. Expert Member
            Akos | | #8

            Instead of trying to pull big wires, I've had good luck with doing it in section.

            Install and cut your PVC conduit for the whole run but don't glue it. Take it all apart.

            Lay down the wire you want to run along the route. Now pull each section of PCV over the wire starting from about the middle and working your way towards each end. An extra set of hands makes a big difference pulling an elbow over a bundle of 4/0. Once all the conduit is over the wires, install back in place. You can do pretty long and complicated runs like this without too much trouble and you might be able to avoid a splice. Cable lube definitely makes things easier, but you can also skip it.

            For something like 4/0, I would not want to install a 4 conductor bundle, individual wires are much easier to work with.

          2. Jeff Cooper | | #9

            Thank you for more detailed advice, Bill.

            If I run XHHW from the outdoor pedestal through conduit under my stem wall and up into the crawl space, I won't have to put any holes through my walls. I like the idea of limiting the conduit to that short distance, and I was very happy to learn that SER can be run in the crawl space without conduit, but at the Home Depot page for the SER cable of that size, Southwire responded to a customer question as follows:
            Q: I need to install a service wire in the attic from my 400 Amp panel to my new 200 Sub Panel. . . .
            A: The “dwelling” ampacities given on the SE cable datasheet (see below) assumes that the product is used as a residential service conductor rather than as a feeder. When used as a feeder, 4/0-4/0-4/0-2/0 SER cable has an allowable ampacity of 180 Amps (not 200 Amps) as long as it isn’t installed in thermal insulation. If it is installed in thermal insulation, the ampacity is further reduced to 150 Amps [per NEC 338.10(B)(4)(a)]. 4/0-4/0-4/0-2/0 SER cable is not large enough for your application. . . .
            by SouthwireCustomerService|Oct 5, 2021

  5. Jeff Cooper | | #10

    Thank you, Akos. I was hoping that technique was an option, but I couldn't find anyone recommending it till you did. In the one discussion I found about the idea, everyone but the OP was very discouraging about the idea:
    https://diy.stackexchange.com/questions/215039/why-can-i-not-assemble-conduit-around-cable-but-must-pull-it-after-assembly

  6. Expert Member
    Zephyr7 | | #11

    Replying to both of Jeff's posts #9 and #10:

    The risk with piece-by-piece assembly of metal conduit is that you'll nick and damage the insulation of the wires. There is an additional risk with PVC conduit of the fumes from the PVC cement solvent damaging the insulation, especially in the case of THHN or TW wire, since the primary insulating material in both of those is PVC. I would avoid doing piece-by-piece assembly for this reason. If you want to make the pull easier, go up one size in conduit from the smallest size the code tables say will accomodate your wiring. Another trick is to use a large pully over the end of the conduit, supported on an A frame (assuming the conduit is coming straight up out of the ground). Run the rope over the pulley, tie it to your car's rear bumper, then IDLE your car to pull the cable. You will need a helper watching to do this safely, and I strongly recommend a shock absorber (I use a second rope tied off to a tree that is then loosely spiraled around the main pull rope), just in case something snaps.

    Regarding the SER rating, that's something you'd want to run by your local inspector. There is an exception in the code for residential service conductors that essentially lets you size them from the 90 degree C column of the ampacity tables. This usually means you can use one size smaller wire for the service ampacity than would otherwise be required. While you are running after a disconnect, the purpose of the cable is still to act as the primary service conductors for the house. If I was your inspector, I'd be OK with that. Check with your local building department people and see what they say. If they OK it, you're good to go. If they say "nope", then you need to go up a size, to 250kcmil aluminum here.

    There is a sneaky way to get by with 4/0 conductors, but you can't use SER cable. If you run seperate THHN or XHHW conductors, and use compression lugs (not mechanical lugs with the allen screws) that are rated for 90*C, then you can actually use the 90*C column of the ampacity tables and be OK. The reason you can't normally size 90*C rated wire from the 90*C column in the ampacity tables is that the CONNECTIONS -- the lugs -- on circuit breakers and most other devices are only rated to 75*C, which limits you to the 75*C column in the ampacity tables even though the wire itself is rated for 90*C. Compression lugs attached to bus bar are OK for 90*C (assuming the lugs are rated for 90*C, but every one I've ever seen is). I would ask your local building people first though, I think they'll be fine with SER cable used as a service entrance cable (for the 200A rating for 4/0 conductors), even though you're running from a disconnect on a pedestal to a seperate panel.

    Bill

    1. Jeff Cooper | | #12

      Super interesting and informative, Bill; thank you! It took me a minute to get what you meant by IDLE for the car: no foot on the gas pedal, but in gear. I hope nothing nearly as powerful as a car is necessary; getting my car into it would be too much for me.

      1. Expert Member
        Zephyr7 | | #13

        Yes, exactly: you want your car to just creep along at minimum speed. Idle speed is usually around 1 MPH or so, around 1 to 2 feet per second or so, which is a managable speed if you're careful. You want slow and steady, abrupt bursts of speed tend to damage things. You'll still want to use pulling lube if you try this.

        In the trades, we have a device called a "chugger", which is a wire pulling machine. It has a big funnel-shaped pulley that you wrap your pull rope around several times, then you run the chugger and control the pulling by a combination of the speed of the chugger and the tension you put on the rope coming off the pulley: tighter = more pulling force put on the rope by the chugger. The chugger ends up doing most of the work for you. You can rent these if you need one, and they're much more controllable than a car.

        For a short(ish) pull without too many bends, you can probably pull it in by hand, but you'll want a helper. To give you an idea of what is possible to pull by hand, I personally have pulled in about 900 feet of 24 strand armored fiber optic cable (similar to about a 3/0 XHHW conductor, but lighter weight) with one helper feeding on the far end, and one helper pulling on the near end with me (so two people pulling and one "pushing"). This was through an underground 1.25" HDPE duct, with four swept (large radius) 90 degree bends in it. It wasn't super easy, but it was doable. Try to keep your conduit run as straight as possible for easiest cable installation.

        Bill

        1. Jeff Cooper | | #14

          Thank you, Bill. I'll be pulling less than 40 feet with four sweep 90 bends, unless I splice to SER inside the crawl, in which case it would be less than 10 feet with two sweep 90 bends. I expect I could attach a small manual winch to an A frame if I need more pulling power.

          At https://www.southwire.com/wire-cable/building-wire/simpull-xhhw-2-wire-with-alumaflex-brand-conductors/p/11277101, Southwire says of its (presumably improved) XHHW-2 wire, "This product is designed to be installed without the application of pulling lubricant." I'm guessing the lubricant you recommended wouldn't do any harm, but please let me know if I might be mistaken.

          1. Expert Member
            Zephyr7 | | #16

            My response to Southwire's "This product is designed to be installed without the application of pulling lubricant.": hahahahahahaha! :-D

            I'm sure they tried to make the jacket more slick, and it's probably improved, but you'll likely still want lube. With your short run, and limited bends, I'd probably try without lube though -- avoid the mess if you can. Just be sure you have the lube on hand when you're ready to start the pull, and remember that the lube needs to go on the "head" of the wire when you start pulling, then along the run as you go. You can't just add lube after getting stuck. The usually way to do this is to start by squirting a bunch of lube down the conduit prior to pulling the wire (but after putting in the pull rope!), then either wiping the wire as it goes in with lube in a rag, or squirt lube on periodically. Lube works best when the entire run has lube on it.

            Lube won't hurt anything, it's designed for this purpose. The cross linked polyethylene jacket of XHHW wire is impervious to almost everything too, it's very tough stuff.

            BTW, a "swept 90" is a very different fitting from a regular pre-formed conduit 90. A swept 90 fitting for 2" PVC conduit will be a piece of conduit maybe 5 feet long or so, with the entire length formed into a very gentle 90 degree bend. We use those for fiber optic cable because the bend radius is critical (too sharp of a bend and the light will shoot out the side of the glass fibers. No, conduit alone isn't enough to do that, but you actually can demonstrate it with bare fiber optic strands, which I've done many times for high school kids). We also sometimes pull in either very large (1"+ diameter) cables, or entire bundles of smaller "innerducts". We're limited on pulling force too, so we have to be more careful. You should be OK with the smaller, cheaper, "regular" 90 degree bends.

            Bill

      2. Expert Member
        Akos | | #15

        Same here Bill, very informative.

        1. Expert Member
          Zephyr7 | | #17

          No problem.

          I should probably mention that the risk to PVC insulated wire isn't just from the fumes, but from the wire sliding and/or sitting in not-yet-cured puddles of PVC cement inside a freshly made joint. PVC cement will soften PVC, which is how it works, and it will do the same to PVC insulation at which point the insulation is able to "cold flow". This can lead to thin spots in the wire, and potential failure points in the insulation. I strongly advised against piece-by-piece assembly of PVC conduit for this reason. XHHW insulation probably won't react with the solvent, but I still wouldn't risk it. I think PVC cement uses methylethylchloride (don't trust my spelling on that :-) as a solvent, and it will dissolve a lot more than just PVC.

          With metal conduit, you can get away with it if you carefully ream all the ends first to remove burrs, and you're careful to avoid sharp bends and kinks over the edge when sliding the conduit on. I still wouldn't recommend it though. It's much better to just oversize the conduit to make the wire install easier.

          Bill

  7. Jeff Cooper | | #18

    Again all great to know, Bill; thank you!

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