Dual service conductor routing

Old 12-02-09, 02:46 PM
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Dual service conductor routing

Hi Guys,

Great forum - based on the thread counts, maybe it should be the electrical and plumbing forum!? While googling around for my various electrical questions, I came upon many, many threads from this forum.

For my new construction (residential), I am requesting 400A service, mostly so that I can run a 200A load center for the main part of the house and also have a 100A or 200A load center in the basement to supply heatpump strip heaters, elec water heater, arc welder, air complressors, etc. So I'll come out of the meter base with two sets of service conductors, one for each load center.

Because of placement limitations between the meter base and the two load centers, I will need two 200A fused disconnects. I'd like to place these immediately inside the structure instead of outside for both aesthetic and cost reasons (NEMA1 vs NEMA3). But outside would be OK. Either way, they will be "grouped together" per NEC. And they will be no more than 2 ft inside the structure. I had heard somewhere that if inside, they need to be within a certain distance of an exterior door for fire department access, but I can't find this in the 2005 NEC. Is it a requirement in the 08? If inside, they WILL be in an "accessible location" in the basement.

If I am interpreting the NEC correctly, the right place to bond my ground and neutral is in the meter base. So now with the new 2008 NEC, I understand that I will need to run FOUR service conductors to each of the disconnects. This where my plan begins to have issues! The meter base as supplied by the EMC has only two knockouts in the bottom (max 4 in) - one will be for the incoming service lateral, so I have to do all of my load side through the remaining 4 in. So now I have 8 (6 current-carrying) conductors in one 4 in conduit, which I believe I need to derate to 80% (so I need even bigger 4/0 cu conductors)! How can I route these conductors to the two disconnects? I am having trouble finding any 4" conduit fittings/tees. Does this need to be done all in auxilliary gutters and nipples? I have to come out of the bottom of the meter base, and have about 4 ft max of vertical space before I have to bore through the concrete wall, and then split the 8 conductors to the two disconnects.

Thank for any advice!

- Jay
Old 12-02-09, 03:37 PM
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Welcome Jay,

What you read about the FD requirements for the disconnect must be a local thing. It is not in the NEC. The NEC only wants the unfused cable length inside to be "as short as practical". Normally this is just inside the wall to the first means of disconnect. If disconnects are installed you can place the panels anywhere you meet the clearance requirements except for clothes closets and bathrooms. On a side note the disconnects do not need to contain fuses, a breaker would suffice.

Local power companies may not allow the neutral bond to occur in the socket. You would need to check with them. It can occur in the disconnect, if installed, or inside the panels if no disconnect ahead of the panel is installed.

You only need to feed a 3 conductor cable to the first means of disconnect. Around here that can be type SE-U cable. If disconnects are installed you need a 4 wire cable between the disconnect and panel, but can still use 3 conductors to feed the disco from the meter socket.

I would take 2 tails out of the socket, 1 to each panel.

I am sure this will generate more questions so ask away.
Old 12-02-09, 03:57 PM
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Under the tap rules found in 240.20(B)(1) you may be allowed to run two sets of 200 amp wire (2/0 copper) to your disconnects. You could install a gutter above your 200 amp disconnects and then put a pipe down to each one and run your wires into them.
Old 12-02-09, 04:39 PM
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The dual service conductors from the meter base is assuming that the meter base has dual lugs on it, right? Why not just bring the service lateral in through an LL, LR or even an LB into the upper side of the meter base and then use 2 inch conduits (and LBs) from the bottom of the meter base into the basement? Since the supply connections are at the top of the meter base this would sure make the routing of the conductors inside the meter base a lot easier I would think.
Old 12-02-09, 11:01 PM
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Hello Jay

What your doing requires a bit of expertise and knowledge before you can start opening the code book and take guesses at what it is saying. You need to know what it is required in your situation. Most of what you have said in your post is inaccurate because you simply don't quite understand the language. If you have never done this and it is going to be inspected you don't stand much of a chance getting thru the first time and will likely get a red tag.

To clear a few things up. The place you bond your neutral and ground is the service equipment. So this will take place in the 200 amp disconnects, not the meter base. The meter base is bonded by the factory.

As PCBOSS said you will run parallel sets of entrance conductors... 3 wires from the meter (it has double lugs)to each service disconnect. These disconnects will have a neutral provision and a main bonding jumper that you will install to bond the neutral and ground. From those disconnects you will run 4 wire feeders H-H-N-Grd to two main lug panels. At these panels you will not bond neutral and ground...you will need to know how to do this.

The meter base will be a class 320 for a 400 amp service.. meaning that it can carry 80% of 400 amps continuously. The meter base will have double lugs on the load side and my provide bypass lugs or lever..if it doesn't have the lugs and they usually do not come with them you will have to purchase them separately.

You will not have 8 conductors 6 current carrying. You will have 6 conductors and 4 current carrying. Unless you have 3 phase service the neutral of the service entrance sets is not counted as current carrying. Yes you would derate for 4 current carriers but you will not have to upsize unless the derated value is less than the calculated load that you must provide to the inspector and POCO to validate the need for a 400 amp service...or if the calculated load is greater than the ampacity of the conductors using table 310.16. Otherwise 2/0 copper or 4/0 al paralleled will meet all requirements for a 400 amp service. Simply put if using 2/0 cu the calculated load cannot exceed 175 amps and if 4/0 al it cannot exceed 180 amps. This assuming both panels are rated 200 amps with 200 amp disconnects ahead of them.

You will not use a 200 amp disconnect ahead of a 100 amp main lug panel like you mentioned using for the basement. You will install a 100 amp service rated disconnect ahead of it.

It would be better to just have two 200 amp panels and two 200 amp service disconnects..

Your grounding electrode conductor to the ufer or water pipe is sized to table 250.66 and in your case would be #2 copper and #6 to the ground rods. This is explained in the footnotes below the table.

Your distance is so short consider 2/0 copper instead of 4/0 al this will reduce your conduit size and make the fittings more likely to be found at the big box. 2 1/2 or 3 inch sch 80 for 6 2/0's.

As Toyln said a gutter will work for the disconnects but you will have to go to an electrical supply to get the metal it's a nice way to go.. You could also enter a junction box then exist it with separate runs of conduit to each disconnect. If your disconnects have an auxilary gutter, you may be allowed to pass one set of service entrance thru to the next disconnect.

Your meter can is likely a bottom feed only 3R enclosure since this is a lateral service. The poco probably wants 'line' to enter at the bottom left to allow for wire bending space.

Gotta get some sleep

Last edited by Bruto; 12-03-09 at 12:31 AM.
Old 12-03-09, 12:11 PM
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Thanks a bunch guys - I certainly will NOT take guesses at the code! That's exactly why I'm running my plans by you guys. Once I have a coherent plan, I intend to sketch it out and meet with the local inspector to get his agreement in advance of any installation work. I'm just trying to be sure that I bring him a logical plan with no code errors. He has previously inspected and passed one other residential structure I wired, but that was much smaller in scope (single 100A LC).

Thanks for the correction on the bond location. Bruto, your line diagrams in the sticky posts made this very clear. I guess it does NOT speak well of my capabilities that I misunderstood that (doh!). It's a good example of why I am checking everything twice before I even bring to my inspector.

The base that the EMC supplied includes dual load lugs, so that's not a problem.

Once I calculate my actual load, I doubt that I will exceed 175A for either LC, so hopefully 2/0 will suffice. If the basement load calc results in less than 100A, I might use a 100A LC for that, which I would of course use a 100A disconnect for. But I agree that just using 200A disconnects and 200A LCs for both would make things simple.

Can anyone spec a manuf/model of a disconnect enclosure with gutter space that would allow pass-through of one set of conductors to the second?

The meter base does have a cover for a top hub, so that might be a possibility to bring in the lateral, but I'm pretty sure the EMC will require that I use the bottom knockout since the lateral is underground. The enclosure seems to be symetrical, so I'm not sure if they care about right or left side, but I will double-check with their local engineer.

Thanks again!

- Jay
Old 12-03-09, 02:37 PM
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Go to an electrical supply or a big box (not sure of their inventory) and ask about service rated circuit breaker enclosures. Tell them you want one that is ready to go. Not one the you have to install everything into. Just about any nema 1 indoor 200 amp service rated disconnect with overcurrent protection will provide gutter for pass through. Be careful of 3R's these can be top feed only no pass through.

I tried to find a good photo of a pass thru and did find this one below of two main disconnect panels using pass thru fed from a 320 class meter base. Couldn't locate a pass thru image of two main disconnects like your wanting to do but I think this gives you the idea I was speaking to.


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