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Electrical Subpanel ?? New Construction

OracleDesign | Posted in Energy, Heating & Insulation on November 12, 2002 06:59am

I’m building a moderately large home that will have an electrical subpanel located about 90 feet from the main service panel to service the opposite (higher load) end of the house. I plan to provide 3/0 (copper) four conductor cables from the main service panel (or meter) to the 200 amp service subpanel via 2″ under-slab conduit.

I have a few questions.

Ã…¸ Should the subpanel feed be split off the meter and separately protected before being directed to the subpanel? I plan to have the subpanel wired as a main panel that will have a 200A main breaker.

Ã…¸ Should the ground conductor from main to subpanel be sized the same as the hots and neutral?

Ã…¸ Should I be providing a separate or supplemental grounding system from the subpanel?

And, before anyone cautions me, I will be isolating the grounding bus from the neutral bus on the subpanel. Any input or advice would be appreciated.

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  1. User avater
    BillHartmann | Nov 12, 2002 09:57pm | #1

    There are a couple of issues here.

    First what is the total electrical demand. That will determine if you can get by with a total service entrance of 200 amps in which case you can run the sub-pannel off the main or if you need a larger service. If you need a larger service then in most case the ecomical method would be to add a second disconnect and parallel the two feeds at the meter.

    Note, this is an area that often varies by local custom. You need to work with both your local inspector and the local power company about what is acceptable in your area.

    Also if the majority of the load is at the sub-pannel you might be able to treat the run as service entrance and use 2/0 cable. This is new with the 2002 code.

    The ground for a 200 amp service is #6.

    1. brownbagg | Nov 13, 2002 04:55am | #2

      i ran (4) 3/0 through a 2 inch conduit in my attic for the main, only adviced I can give you is run a three or four inch conduit. i had living hell with pulling wire.

      1. booch | Nov 13, 2002 04:09pm | #7

        When you have to pull thru many bends buy teflon coated wire (THHN) and use the wire lube next time. Ideal and several other companies sell it in 5 gallon buckets to bedstand sized bottles.(electrical humor) It still can be hell but these options make it the easiest."The time has come," the Walrus said, "To talk of many things: Of shoes--and ships-- and sealing wax--Of cabbages-- and kings, and why the sea is boiling hot-- and whether pigs have wings"

        1. User avater
          BillHartmann | Nov 13, 2002 06:11pm | #8

          THHN is nylon coated.

          I don't belive that they are any "commonly available" teflon coated wire.

          But there are some solid teflon insulation. The ones that I have used where #30, but I never tried pulling that through a 2" conduit <G>.

          1. booch | Nov 13, 2002 07:57pm | #9

            Sorry, You are right. The Thhn is nylon jacketed PVC insulated. Tf Or Tff would have the teflon coat. The second would be with more flexible stranding. "The time has come," the Walrus said, "To talk of many things: Of shoes--and ships-- and sealing wax--Of cabbages-- and kings, and why the sea is boiling hot-- and whether pigs have wings"

          2. MajorWool | Nov 14, 2002 05:50am | #14

            In the good old days, you could buy surplus silver coated stranded copper with teflon insulation wire. We used it to build high end tube audio amplifiers. Getting the insulation off that stuff was hell, but it never left the wire on its own, and when you are pushing 525V DC with a buttload of capacitance behind it, you really want the insulation to stay in place. IIRC, 100 ft of 14 gauge was $20 in the early 80's. Probably no more left as this was stuff surplussed from the space program.

          3. 4Lorn1 | Nov 14, 2002 06:53am | #15

            They still make that stuff. Sometimes short length of it are use on flybacks in some TVs and on some specialized circuits in both the space program and nuclear plants. Silver coated stranded, without the special insulation, is also used by certain audiophiles for speaker wire. All of it pretty high end stuff.

          4. User avater
            OracleDesign | Nov 14, 2002 07:47am | #16

            Here is what has been planned. I requested 320-amp service from the rural utility (I’m in the country). They indicated they would run kcmil 350 to the meter (160 ft. from pole w/ transformer above). The home is 4,675 sqr. ft. of living area.

            I had planned to split off the meter to two 200A breaker panels. One feed would be to the over-sized service entrance side of the house with a 200 amp service panel and the other split from the meter to a 200A over protection device (within 10 inches of meter) then run 90 ft. of 3/0 copper through 2" under-slab conduit (with lube & two 90 deg. elbows) to the 200A subpanel that would again be protected by a 200A breaker before distribution. Whew, that’s a long sentence.

            The subpanel would service the kitchen, laundry room, garage, three bedrooms, three bath rooms, media room, two living rooms, breakfast room, dining room, 50 amp breaker tankless water heater, and a 350 sqr. ft. playroom. I could have the three heat pump units (ttl. 6.5 tons (Texas)) feed from the main panel at the service entrance to off-set the subpanel load. I don’t know what their load would be off-hand, but the condensers would be on the service entrance side of the house.

            Thanks to everyone for the replies and the wealth of information.

            Is it correct that I can down size the Neutral 90 ft. feed to the subpanel by one size smaller? Say to 2/0? Oh, what about providing a supplemental ground from the subpanel?

            Thanks Again!!

          5. User avater
            BillHartmann | Nov 14, 2002 05:28pm | #17

            I just received a new Mouser catalog yesterday and I looked it in.

            Alpha has a UL1213 teflon insulated wire ranging from #28 to #14. $75.20 for a 100 ft spool and #16 for $51.91.

            By comparison their standard PVC hookup wire is $13.06 for #16 the largest size that they list.

        2. 4Lorn1 | Nov 14, 2002 12:40am | #11

          I think your problem was more related to the "many bends" as opposed to the wire size or the actual pipe fill. The code limits you to 360 degrees, 4-90s, in a single run. Most of the more experienced electricians will go to great lengths to keep it to 3 or fewer 90s and install a LB or pull box after that. If four 90s are absolutely necessary it is common to go up one trade size. Failing that plan on bringing extra help and expanding your vocabulary. Power pullers, even the low tech shark rods, a metal pole with a pulley and a hand wench, or winch as the case may be, help greatly.

          Lubricants. "There is always time for lubricants." Ok. this singular high spot in a 'D' grade movie should be the motto for wire pullers. Hydrolube, relabeled as Hotlube for completely different purposes, Yellow77 are all good options. In a pinch just adding water can help. At least one utility I know uses Calgon and water. The Calgon, not being certified not to attack the insulation, would not be my first choice. Ivory flakes will help get the wire in but it will cause corrosion of metallic conduit and can glue the wires in place when it dries. Replacing or upgrading wires becomes very difficult.

          All of this planning, preparation and lubrication won't get you far if you don't know how to make a good head. Tight, tapered and utterly smooth it can make the difference between an easy pull and the nightmares of a jammed feed, dropped or damaged conductors.

          Edited for levity and spelling.

          Edited 11/13/2002 5:49:51 PM ET by 4LORN1

          1. User avater
            MikeS | Nov 14, 2002 01:29am | #12

            I was in a medical research lab and saw some of the lab techs trying to pull some instrumentation cable thru a conduit. I suggested that they get some lubricant made just for pulling wires. Later in the day I saw one of the techs and asked if they were successful, he said yep the KY Jelly worked great. :)  I expressed my concerns that KY Jelly probably should not have been used.

            Mike

            It's O.k. to think out of the box,           Just don't walk off of the plank!

            Edited 11/13/2002 5:51:28 PM ET by Mike S

          2. booch | Nov 14, 2002 02:43am | #13

            I was waiting for this... Why do they call it Jelly when it is really Jam?

            And does it really come from Kentucky?"The time has come," the Walrus said, "To talk of many things: Of shoes--and ships-- and sealing wax--Of cabbages-- and kings, and why the sea is boiling hot-- and whether pigs have wings"

  2. fireball | Nov 13, 2002 04:56am | #3

    Oracledesign,

    Without too many exceptions, a building can only have one service. When you say you want to have a subpanel wired as a main panel, or a second panelboard paralelled after the metering, I'd think your inspector is going to say you're violating the "one service" rule. You can have a higher capacity subpanel at a distant location, but the feeder should come from an overcurrent device in the main service panel. Just figure out your potential load and then size the pieces and parts accordingly.

    The size of your equipment grounding conductor is determined by the size of the overcurrent device ahead of it, if you look at Table 250.122. Bill's right, for a 200 amp circuit breaker, you'd pull a #6.

    No separate grounding system needed. Just land the ground going out in your feeder(an equipment grounding conductor) on the ground bar in your main panel.

    Barry

    1. User avater
      BillHartmann | Nov 13, 2002 05:55am | #5

      Barry

      While you can only have 1 service you can have up to 6 disconnects located at that service.

      He *may* have been thinking of that. He could have one local pannel with a main disconnect. And have another pannel which is just a disconnect which then feeds the sub-pannel. The feeds to the two disconnects are then paralleled at the meter. That way you can get 300-400 amp service without buying 400 amp pannels.

      1. 4Lorn1 | Nov 13, 2002 08:18am | #6

        In most cases a second 200A panel is overkill. Even the huge McMansions with everyone home often draw only 100A. On the other hand a panel that is under used will run cooler, last longer and save a small amount of money by way of avoiding I square R costs associated with the heat generated by loaded conductors. This, before the house is finished, is the perfect time to add capacity as it will cost pennies on the dollar compared  with old work.

        If you really want to go with two 200A panels I would mount two 200A fused disconnects at the meter and run feeders from them to the panels. You will need a 400A meter base to feed them. Coordinating with the utility about the transformer capacity would be a good idea before setting the plan in concrete. As would asking about the impedance of the transformer that is, or will be, carrying the service. If you are close the AIC rating of the disconnects and the fuses selected may need to change a bit.

        If you do go with fused disconnects make sure the electrician leaves a couple sets of spares. Finding large time delay fuses at 2AM Christmas morning can be a pain and lead to serious consequences if someone was to install a field expedient fuse substitute. Just make sure they are stored in a clean and dry cabinet close to the disconnects. A wet fuse can ruin your whole afternoon. I have used a .50cal ammo can spray painted grey and tapconed base to the wall to store spares, notes to the older daughter and the incriminating negatives after payment. 

        Locate the whole assembly, meter can, two disconnects and the ammo can, on the least used side of the house and away from entrances, to prevent trash can cohabitation, and hidden behind a tasteful brick wall with plenty of room. Feed the service and the subs underground and your in business.

        1. fireball | Nov 15, 2002 03:56am | #18

           I guess I've never seen a 400 amp residential service split up that way before.Almost all of my work anymore is commercial.So,is a 400 amp residential service metered with C.T.'s ? Or is it a monster meter that can actually carry 400 amps? Are the load terminals set up for multiple conductors that you want to feed your two disconnects with? Where do you bond the neutral to the grounding electrode conductor at when you have multiple disconnecting enclosures?

          I wouldn't be crazy about disconnect switches on the outside of my house though.Unless you lock them in the closed position(another set of problems),you're vulnerable to somebody with bad intent.

          And hey,are you actually forlorn?Just asking.

          Barry 

          1. 4Lorn1 | Nov 15, 2002 06:33am | #19

            The utility might want to install a larger, commercial and/or a bypass, meter. Entirely their call. They operate with a different set of rules. The chances that the meter will ever see anything close to 400A, discounting a bolted short, is pretty slim. The lugs on most larger meter bases are easily changed out double lugs to accommodate the two feeds and neutrals needed.

            Two 200A fused disconnects are not that expensive when the overall cost of a house is considered. Time delay fuses should provide enough coordination so that the 200A breakers, made necessary by the six motion rule, would trip before the fuses blow. In an age where everything is protected by breakers I still like fuses. They, IMHO, are not subject to the mechanical problems that can, rarely, effect breakers. While I occasionally find breakers that fail on I have never seen a fuses that has failed on. As long as the fuses are kept dry and not used to hammer tent pegs they are extremely reliable.

            Even when entire panels have been installed in rough neighborhoods and left unlocked I have never heard of any being used for evil intent. It seems to be far more common plot element in horror films than reality. If a person wanted to cut power to a house pulling the meter would, in most cases, not be difficult. Most utilities have meter shields and specialized locking devices that they can install. For the most part these are used for locations where meters have ben tampered with or power stolen in the past not to protect the power supply to the house. All that said the disconnects all seem to have methods to allow the use of a padlock to prevent tampering.

          2. MajorWool | Nov 15, 2002 11:59am | #20

            A simple alarm switch to trigger when the meter panel opens will suffice. I also have a big masterlock on mine, and I'm sure the guy sent to cut power by the fire department brings a big bolt cutter with him. In addition, my telco box is now a good 12 ft off the ground, and phone comes into the attic. If they want to cut the alarm out, they'll have to bring their own ladder. A simple way to rip someone off is to shut down power to the house at the start of their vacation or a long weekend, and then come back when the alarm system has run out of juice. With a 2-500mA draw, most alarms will be powered down in a day.

          3. User avater
            BillHartmann | Nov 15, 2002 07:51pm | #21

            This is always an interestiang discusion when it comes up.

            It is not really a code issue as much as local practices issues, both by the power company, by local codes, and by different building construction practices.

            I am in the Kansas City area and you would almost never find anything outside other than the meter. Climate is to bad.

            In other areas the whole distribution pannel is outside.

            In some ares they require at least an external disconnect, but not in others.

            BTW, I remember reading that in South CA where they have outdoor pannels that in some of the older neighborhoods there have been problems with people stealing the breakers. Those are for the obsolete brands where replacements sometimes run $50 or more.

            "The chances that the meter will ever see anything close to 400A"

            Yes, it I would not be surprised for it to see signicant more than 200 amps.

            He has 6.5 toms of heat pumps. I am guessing about 50 amps, possible more if he has auxillery resitance heating. 50 amps for a hot tub. 50 for a stove, 30 for a dryer and another 30 for DHW.

            And that is before you get to the refierator, dishwashers, microwave, and all of the convinnce outlets.

            Now the saving grace is that in most cases they would not all be operating at the same time or if they did only for a short time, but I would not wwant the main to trip say once a month when the magic combination got hit.

            Where I live we have an interesting setup. It is a small lake that was developed in 1927 as "rural" summer cabins. Some cabins where built and some big house where build. Some lots stayed open for a long time. Many of the cabins have been remodeled and added onto several times. Other torm down for replacements. So electrical service are all over the map.

            About 15 years ago the sewer system was replaced with a pressure grinder pumps system. Those have a burried holding tank and grider pump at each house. They did not want to go into the houses and was afraid of working with the existing pannels. So they would with the power company and tapped into the meter boxes. Where the control panel was within a couple of feet of the meter they put the overload in the control box. Otherwise they mounted a 20 amp overload/disconnect next to the meter box.

          4. brucepirger | Dec 03, 2002 02:18am | #22

            I will give a strong support to increasing your conduit size. Not sure if it is too late...

            Stepping to 3" or even 4" will only cost you a few extra bucks...like maybe $100 tops. And you'll be ever so happy you did.

            I put in about a mile of conduit at my place...about 800' of 4" for two primary lines (about 1" in diameter each). Folks laughed a bit...but was that nice to pull in...by hand...800' easy.

            Now, putting my 4/0 triplex in a 2" conduit was more of a bitch...and I had to push it through, at the end, about 50' with a 45 and a 90. WOW, wished big time I went with 3".

            If it's in concrete, it's cast there forever. A little more now might make ya very very brilliant later.

  3. JLBatey | Nov 13, 2002 05:12am | #4

    The wire to the subpanel must be protected by an overcurrent device in the main service panel.  Do not tap off the line side of the main breaker in the main service panel or tap off the meter base lugs and run the 90 feet or so to the subpanel.  The reason is simple.  The subpanel wire will have no overcurrent protection.  You always want to keep the distance from the meter to the first overcurrent device as short as possible.

    If the wire shorts out going to the subpanel and there is no overcurrent protection (other than on the transformer feeding your house), you are asking for a fire and your conduit will not necessarily protect you.  I have seen rigid steel conduit literally vaporize.  You can end up with a big electric arc welder if you do not protect this subpanel wire with an overcurrent device in the main panel.

    I hope this helps.

  4. mikemahan | Nov 13, 2002 09:24pm | #10

    You should think seriously about parallel conductors.  In other words 2-100 amp conductors = 1-200 amp conductor.  I don't have the new code handy, but in general you can use parallel conductors in sizes above 1/0.  Runs must be the same length, the same material, the same size, and be terminated the same.  You can run 2 conduits as long as they are essentially the same length. The advantages are handling smaller wire and smaller conduit.  The actual amount of metal in parallel runs of wire is less for the same amperage, therefore usually cheaper.

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