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Thread: Ground Conductor size for parallel SE

  1. #1

    Default Ground Conductor size for parallel SE

    Hi again everyone,
    Thanks for all the ideas on my initial question. My rewiring project is going slowly forward. To catch you up on what I'm doing and where I am. I pulled parallel runs of 4/0 aluminum URD from a splicebox at the base of the power pole to the meter base. The power company will be running 300 kcmil aluminum from the transformer to my splice box. From the meter base, I ran 300 kcmil through a 2" conduit into a trough. In the trough I spliced a run of the 300 kcmil up to a 200 amp main breaker panel, using 2" conduit, and another splice of the 300 kcmil to a 200 amp disconnect. From the disconnect I'll be running #2 MC 65 feet to another main breaker panel. I ran a 5/8 ground rod in at the base of the meter, and used #4 copper to ground and bond all the panels, trough, and disconnect.

    That said, the electrical inspector came and looked at things, so I could at least get the trench filled back in. He started questioning everything I did, and instead of telling me what he wanted, he just kept asking me the same question. "Is that what the code book says?" I thought it was his job to either say it was OK, or tell me what he wants to change it to. But, he didn't, he just kept saying: "I don't think the code says that's what it should be." or would ask me again what the book said. SIGH....

    OK, so, the things he thought he didn't like, but wouldn't tell me how he wanted it, were the size of conduit from the meter base to the trough and breaker panels, and the size of the ground wire. He also didn't like that the ground wire was run and attached to the breaker panel in the garage, and then bonded from there to the trough and disconnect, but that's easy to fix. What I don't know, is what size conduit and what size ground wire. When I look at Annex "C", it says I was correct to use 2" for the conduit. I checked 250.66, and it says the #4 for ground was correct, but then I read #1, and wondered if I needed a larger ground wire because I ran the parallel 4/0 aluminum up to the meter base. I just figured, since the power company was only running 300 kcmil, I was fine with the #4. Again, the inspector never told me to change it, he just kept asking if it ws to code.

    So, what would you guys do? Should I go through all the trouble and change the conduit size to 2 1/2 adn the ground wire to something bigger, or should I just show him the code book to back up each of the things I did? If I do change the ground wire, what should it be upped to???

    Thanks in advance!

  2. #2
    Join Date
    Dec 2003
    Kent, WA


    First I'll ask some questions. Since you're running 300 KCMIL Al Service Entrance Conductors, I'm assuming you wanted a 250A service. Is this what you intended, or did you think you have more? The parallel 4/0 run would allow more current, but you're limited by the 300 KCMIL AL coming out of the meter to your trough. Why did you run a segment of parallel 4/0 and a segment of 300 KCMIL? Are your conduits PVC, and if so sch 40 or sch 80? What type of wire is the 300 KCMIL (THWN, XHHW, ???).

    Is your neutral also 300 KCMIL? What size wire did you run to the ground rod from the meter? Why did you run 300 KCMIL to the 200A panel and disconnect -- can their lugs take conductors that large? Why are you running a #2 (al or cu) off of a 200A disconnect? Is it going to a panel with a 100A main breaker? Can you explain exactly where the #4 ground/bonding wire is going. Much of your service equipment is bonded by the neutral. Metal conduits may need a wire run to a bonding bushing, but only if you have concentric knockouts.

    Now for answers and comments. It seems to me you've run things backwards. I would have run larger single wires from the splicebox to the meter (and something like 600 KCMIL would make sense to match parallel 4/0's). Running parallel 4/0 would be OK too if that costs less. Now, from the meter, especially since you're going to two separate panels, parallel conductors would have made a lot of sense, especially parallel 4/0 since that is what the other end of your service is. But running a 300 KCMIL cable is OK IF IT MEES YOUR AMPACITY REQUIREMENTS. Normally, you'd split bolt smaller conductors to this 300 KCMIL wire to each panel (4/0 wires if the panels are 200A).

    Your conduit, if sch 40, is maxxed out with three 300 KCMIL THWN conductors. There is no additional room if you need to run anything else through that hole. If sch 80 PVC, you've exceeded the fill unless the conduit is straight and less than 24" long. If you used wire larger than THWN, you're probably over the fill limit too. If you rip out the 300 and go to dual 4/0's in this conduit, then you'll have to use 3" conduit. Or consider using copper for these shorter runs in which case the conduit you have should be fine.

    Your grounding electrode is sized per 250.66 based on the largest service entrance conductor. Your service conductors are either single 300 KCMIL's or two 4/0, so need to see which has greater area. A 4/0 wire is 211600 CM, so two of them are 423 KCMIL. This is larger than 300 KCMIL. So for wires between 250 and 500 KCMIL, you need a #2 copper or #1/0 Al GEC. However, the GEC to a rod does not have to be larger than a #6. I'd run a #4 to the rod, and by the way, you must have two rods. If you have any metal water pipes or exposed structural steel, they must be bonded with a #2 copper or #1/0 Al wire. The bonding wires inside the trough and meterbase must be #2 copper or 1/0 Al because these are still upstream of your service disconnect. If you used all plastic conduits, then just terminating the neutral to a lug in the trough should be plenty.

    Your grouding electrode conductor can come from many places in your service equipment, but it is only to be used for the GEC and nothing else -- no bonding of enclosures with it. It is also good to run that conductor as straight as you can to the ground electrode.

    It is a little disconcerting for the inspector to be wondering aloud as to what the code book says. You're doing an advanced residential service, so he may not have been prepared for all the details. But a commercial inspector should have known about conduit sizes, GEC sizes, etc. Since he's not providing you much protection, I'd study 250.66 (actually most of 250), 310.15(B)(6), and the surrounding text to at least get the conductors sized right.
    Kent, WA

  3. #3


    Hi Mark,
    Thanks for the reply. I see I didnít clarify. The 300 KCMIL from meter to trough is copper, not alum. The 4/0 from pole to meter is aluminum. I am wanting 300 amps when all is said and done.

    >>Why did you run a segment of parallel 4/0 and a segment of 300 KCMIL?
    The parallel is direct burial, and once itís in the meter, we went to the 300 kcmil copper since it would be easier to work with in the trough.

    I used THHN stranded copper from the meter base to the trough, and ran it through an 10Ē threaded 2Ē galv. nipple. The only PVC used was up to the meter base, and in and out of the splice box, and those are 2 ĹĒ schedule 80. From the trough to the disconnect and the trough to the one panel, I used 2Ē long 2Ē galv. nipples.

    From the trough to the fused disconnect, and from the trough to the one breaker panel I mis typed before. Those runs are 2/0 copper, with the neutrals being the same size. The parallel 4/0 from the pole to the meter has a reduced neutral. The lugs and power distribution blocks are all sized correctly. (The power distribution blocks have one 300 KCMIL on one side, and the 2 2/0ís on the other side going up to the disconnect and breaker panel.

    The ground wire is #4 solid, attached to a 5/8Ē grounding rod located directly below the meter. Another #4 will be run from the trough, down to the basement and attached to the water line a couple inches from where it comes in.

    >>Why are you running a #2 (al or cu) off of a 200A disconnect?
    >>Is it going to a panel with a 100A main breaker?
    This is where it MIGHT be dicey. <G> Iíve got a 200 amp panel in the garage, a few inches up from the trough. Thatís for garage receptacles, lights, the AC compressors, etc. Then, the other 2/0 goes from trough to the 200 amp fuesd disconnect, with the MC coming out of it 65 feet to a SECOND 200 amp panel. It isnít that I am going to run 400 amps, but I need all the home runs for the lighting controls Iím putting in.

    OK, let me see if I can explain the grounding I just did. #4 solid copper from ground rod up to ground bar in garage breaker panel. Then ran #4 stranded to insulated bushings at each end of the nipples going to all boxes. (trough, disconnect, generator transfer switch, meter base, and 200 amp panel.) The other ground isnít installed yet, but will attach to the 200 amp breaker panel in the garage, and go to the water pipe.

    >>Running parallel 4/0 would be OK too if that costs less.
    Thatís the exact reason for doing the parallel 4/0. TREMENDOUS price difference here when I checked. <G>

    >>Normally, you'd split bolt smaller conductors to this 300 KCMIL wire to
    >>each panel (4/0 wires if the panels are 200A).
    Right you are, which I did, just mistyped before.

    Am I OK on the 10Ē nipple from meter base to the trough, when it comes to fill capacity? I thought I was.

    >>I'd run a #4 to the rod, and by the way, you must have two rods.
    I did run #4 to the ground rod, but the inspector here said only ONE ground rod, and the other MUST be the water pipe. I thought 2 ground rods would be fine.

    >> If you have any metal water pipes or exposed structural steel,
    >>they must be bonded with a #2 copper or #1/0 Al wire.
    >>The bonding wires inside the trough and meterbase must be #2 copper
    >>or 1/0 Al because these are still upstream of your service disconnect.
    DARN! I used #4 for bonding the insulated bushings. Does it matter that the electric company is only going to run 300 KCMIL from the pole? Can we not get by with #4 ground since the weak link is the power company run from the pole?

    >>It is a little disconcerting for the inspector to be wondering aloud
    >>as to what the code book says.
    MY SENTIMENTS EXACTLY!!!! I could not believe it. He kept quizzing me. I was rushed, put on the spot and didnít have my books with me to quote chapter and verse. <G>

    Iíve got a sheet with notes to justify what I did, but this ground wire size was a bit confusing. So, with all Iíve explained, what do you think now?


  4. #4
    Join Date
    Dec 2003
    Kent, WA


    Good, the 300 MCM copper gets you a service rated at 300 amps. I'm still confused with the size of wire to the 200A panel and 200A disconnect. You said you split bolted to the 300 MCM wire -- did you splice on 2/0 copper, 4/0 al, or something else?

    For your nipples, they are each less than 24" long so you are allowed 60% fill rather than the normal 40% fill. Three 300 MCM THHN wires in a 2" galv conduit are near maximum fill at 40%. Since you can go to 60%, you have room for more. I can't tell if your 2.5" conduits are OK because I don't know the size of the URD and whether that is individual wires or a cable. I'll assume you calculated those correctly....

    The ground rod connection is OK, except that you are required to have a second rod if you can't prove your existing rod is 25 ohms or less. The fact that you have a water pipe ground is not relevant. If you had any other type of ground (buried wire, concrete rebar electrode, etc) then an additional rod wouldn't be needed.

    The conduit bonding wires in the trough and meter base are to be sized per the largest service entrance conductor. The fact that the power company runs teeny wires upstream of you doesn't matter (but even 300 MCM aluminum requires a #2 copper bonding jumper). I did this for you before, but thought the 300 MCM was al not cu. However, you still get the same answer -- you've got to have #2 copper in those locations, not the #4 you used. Is the trough bonded to the neutral in any way (perhaps a triple lug is bolted to it)? If not, you'll need a #2 bonding wire to the metal trough. Inside the disconnect and 200A panel, a #4 conduit bonding wire is OK.

    The water pipe ground also must be this larger size (and this is regardless whether the pipe is metal in the yard dirt or not) -- if the water pipes are metal in the house, they must be grounded or bonded with a #2 copper, not the #4 you were planning.

    Your 200A panel off the 200A disconenct is definitely a problem. You either need to change the disconnect to 100A, or change the wire to 2/0 copper. You could also replace the 200A disconnect with another 200A main panel (like the other main one), and put a 100A breaker in that panel to feed the 200A panel 65 feet away via the #2 MC cable (this is probably the cheapest and most versatile solution).
    Kent, WA

  5. #5


    Sorry for the delay in replying to you. Power was out for a week here in Louisville. 115,000 people without power due to a freak 70 MPH wind storm that blew through on a week ago Tuesday. Over 3200 power lines came down. Then, another storm came through and took out 6,000 more homes 3 days ago. Being without power for 5 days put me so far behind, Iím just now getting caught back up. So, I wasnít trying to ignore your prompt reply to my questions. <G>

    >>did you splice on 2/0 copper, 4/0 al, or something else?
    Spliced on 2/0 copper to the 300.

    >>I'll assume you calculated those correctly....
    Yes, I did calculate those correctly, and my supply house said this is what they sell everyone on a regular basis. I also thought I was correct using the 2Ē nipple. Thanks for clarifying that.

    Itís interesting that you say I need an additional ground rod, along with the water pipe. This inspector was adamant that all he would require is the one rod and the water pipe. Iíll play it safe though and add the second rod at least 6 feet from the first, just to be sure a different inspector says something different. <G>

    OK, so it sounds like I need to change to #2 no matter what for both grounds. Not a problem, since I hadnít pulled the one all the way through the house to the water pipe yet. Iím only out a bit of money on the #4 I already purchased. I wasnít sure if it mattered about it being alum or copper. Good to know that the #4 between disconnect and trough is OK though. We have the trough bonded to the neutral now.

    >>Your 200A panel off the 200A disconnect is definitely a problem.
    >>You either need to change the disconnect to 100A, or change the wire
    >>to 2/0 copper.
    OK, I need to learn how to type better, or proof-read what I type better. <GRIN>
    It is 2/0 copper going from the trough to the disconnect, NOT #2, and then 2/0 going to a generator transfer switch, and 2/0 MC going the rest of the way to the 200 amp panel in the basement.

    So, it sounds like as long as I swap my ground from #4 to #2, from the ground rod to the panel/trough, add a second #2 ground to a second ground rod, and run a #2 to the water pipe, I should be fine. Good to hear!!

    One more question while I have you. I am needing more than 40 breaker spaces in the basement, and want to do two 40 space panels. Can I use two 200-amp main breaker panels, and pass through from one to the other, or is that then considered a 400 amp service? There isnít any way I could load the thing with 400 amps, but am wanting it because of all the individual home runs I need for a home automation system I am hooking in. The Square-D panel (QO series) does not offer a pass-through lug on the bottom, so I was going to put another splice block on it once it comes into the first panel, and take it over to the second one. That would give me 80 spaces, which is plenty. Or, should I swap out the 200 amp breaker in each of them, and just put a 100 amp in each panel? Or ďDĒ, none of the above? 

    Thanks Mark for the advice!

  6. #6


    You could use an NQOD (Square D) panel to get the feed through lug option (this panel takes QO or QOB breakers). Or, you could feed out of a breaker in panel #1 to the lugs in panel #2. Or, you could bring the 200A line to a wire trough, and bug the wires down to both panels. Don't know which one Mark's going to reccommend... (my personal preference would be the trough).

  7. #7


    That was a quick reply. <G>
    I looked at the NQOD panels for just that reason, but they are considerably larger and more expensive than QO ones, so I went with the QO.

    This might be dumb, but how do you get such a large wire into a breaker? Do they make some sort of lug that attaches?

    I also thought about the trough. Let me ask you this. I am recessing the two panels into a wall, with about a 6" space behind the wall, to the concrete foundation behind the wall. One panel is going to be mounted between 2 studs, and the other panel between the next 2 studs. If I use a trough, would I need to put a brace in this wall, and cut the middle stud out between the two panels, and mount the trough just below them? (I'd have to mount the panels upside down to feed them from the bottom.) This way I can run my home runs up the top and out. Do they make a trough that would allow recessed mounting? I do care how this all looks. <G>

    Lastly, is it OK to do what I intended, using 2 panels with the 200 amp main breakers in each, when I only have a 300 amp service coming into the house, and the feeder wirie is only rated for 200 amp service? Is it assumed that the fused disconnect will protect things before the feeder wire causes a problem?

    Thanks to both you and Mark! (And others here!)

  8. #8
    Join Date
    Dec 2003
    Kent, WA


    You can have as many panels as you want hanging off that 200A feeder. What matters as far as your 300A service size is what is the calculated load connected to the 2 (or more) 200A panels. If its less than 200A, then you're fine for the feeder, and if the rest of your service is 100A, then the total service being provided is adequate. If those panels only calculate to 60A, then you have a huge margin in the feeder and can have 240A of other equipment connected to your service. I know you need a bunch of breaker spaces, but I'm curious what your calculated load is on these basement panels -- I can't believe you'd need more than 60A for lighting only circuits.

    I think you can find a wireway or trough to do what you'll want, but I'd say yes you'll need to cut the center stud and put a header of some sort below it (4x4, doubled 2x4). This could complicate how you get the conduits out of the gutter and into the panels, as you may need to take a significant hunk out of the header.

    You may be better off to just put a 100A breaker in the first panel and run a #2 copper (2-3 w/gnd) cable or a 1.25" conduit to the second panel. You'll only have 100A available to the second panel (and its still OK to have a 200A main on the second panel if you want or you can go MLO).
    Kent, WA

  9. #9


    I haven't done an official calculation, because I am currently living in an identical house to the one I am remodeling, and am directly across the street. Where I am now, I have almost the same amount of stuff running on a 100 amp service, and haven't tripped a breaker once. There will be several large UPS units for all the computers. (I runabout 5-6 servers in a rack at a time, along with a few other machines as needed.) So I was fairly confident that 200 amps of service was plenty, but was more concerned about breaker spaces. Should I do an official calculation of the loads I will be running off this, even though I am confident I have plenty of headroom?

    I will look and see about a trough. If I can get one that fits, I can bolt it directly to the panels from below, using close nipples to get the conduits between the two.

    Thanks again for the help.

  10. #10



    Without performing a demand load calculation per the NEC rules found in Chapter 2 then you are only guessing. Guessing when it comes to electricity is not a good thing. Does not take that long to calculate a demand load calculation. Do you know a format to perform this demand load calculation? If not look at the top of this page in the green and yellow boxes for "demand load calc and follow the instructions. Then you will be making an informed decision.

    Good Luck


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