Electrical Service Question

Discussion in 'General' started by bullockcm, Nov 25, 2012.

  1. bullockcm

    bullockcm Well-Known Member

    House we moved into 2 years ago only had a 100 amp service, I knew I would eventually build a shop and need to upgrade the service to 200 amp. That day has come. Old service was underground from the pole/transformer to the meter on the side of the house and then into the main panel in the basement. The shop cannot easily be subfed from the house and I wanted the ability to connect a generator so the new service utilizes a pedestal mounted meter socket/200 amp disconnect/8 circuit with generator breaker interlock panel. The pedestal service has already passed inspection btw and the electric company will make the service switch tomorrow. Obviously the neutral(grounded conductor) and ground are bonded in this panel per code. My question concerns how I reconnect the house and the new shop, keeping in mind these connections do not have to pass inspection & are both physically separate from the pedestal and each other...

    My understanding is current NEC code calls for 4 wire conductor(2 hot, neutral, ground) from the pedestal to both the house and the shop with a ground rod located at each building and the neutral and ground bars not bonded in each panel.

    My current "main" disconnect panel in the house of course has the neutral and ground bonded and the branch circuit grounds and neutrals are mixed on the joined bars. My understanding is that up until sometime in the last 10 years it was legal to do what I am about to propose according to the NEC code. Since the buildings are detached and I will have disconnects(breakers) at both the pedestal and buildings that I can run 3 wire(2 hots & neutral) to the house and shop, use ground rods, and bond the neutrals and grounds.

    Thoughts or recommendations? I do want a safe installation of course :D

    I have yet to find any explanation as to why the current requirement of 4 conductor plus ground rod at detached structure is better than what I am proposing other than the potential of a parallel neutral path but again there are no metallic paths between this stuff.
     
  2. burnham

    burnham Well-Known Member

    I don't have my code book at home, so I'm not 100% but I'm pretty sure about this.
     
  3. bullockcm

    bullockcm Well-Known Member

    Burnham, I have to disagree with some of what you wrote but I think you are thinking about subpanels differently, I will get back to that later though, needy toddler is getting most of my attention at the moment. Just wanted to update that I now understand what the potential risk is when using a 3 wire feed to a detached main panel. I found a diagram that it explains the hazard better than my words will but I don't have a place to upload it at this moment for sharing. In a nutshell with bonded neutral and ground(even in a main panel) if the neutral becomes loose or disconnected then you can get line voltage at any point tied to equipment ground which includes the panel enclosure, equipment such as a furnace, metal outlet boxes, etc. From Dits's recent post it sounds like issues with the neutral are more common than I would have thought.

    BTW it was 2008 when the exception was removed from the NEC code that had previously allowed for a 3 wire feed to a detached structure with no other existing metallic connection.

    Where are Todd and Darren today?
     
  4. pickled egg

    pickled egg If you don’t hear from me

    I'm in an attic, running wires. :D

    It's a double-edged sword. I don't particularly like the separately-derived system rules. If *anything*, I feel that multiple grounding conductors at each system bonded to the neutral, with a 4-wire feeder is the safest installation.

    The problem with isolated neutral/ground at a remote system is that without that low-impedance bond to the ground *at that location*, there is a likelihood of there being a potential between neutral and ground at the panel.

    I agree with having all systems bonded over a contiguous grounding connection, though. I'm glad to see them requiring 4-wire feeders for remote systems.

    What are you using for a system ground at the main pedastal?
     
  5. ToddClark

    ToddClark f'n know it all

    ^^^this^^^. a "floating neutral" has killed many folks. I can tell you first hand, that shit HURTS!

    4 wire everywhere.

    I dont have to deal with "code" much, and havent in 15 years, most of the industrial work ive been doing for the last 15 years, the code hasnt changed much, but for what you are doing, i'd be using 4 conductor for everything.
     
  6. burnham

    burnham Well-Known Member

    yeah, I did a little reading and have to call bullshit on myself.:D I'll hold out on giving advice until I get my nose back into a code book.
     
  7. bullockcm

    bullockcm Well-Known Member

    #4 ground wire in a continuous loop from the panel to 2 8' ground rods spaced 6' apart per NYSEG specifications.

    I think with what I have learned today and both you guys have backed up I will run 4 wire feeds and seperate the neutrals from the ground at both buildings.

    I appreciate the information, thanks.
     
  8. bullockcm

    bullockcm Well-Known Member

    burnham glad you went back and double checked, part of the problem is mis-information on this subject on the web. Of course changes in the code don't help. Nor does.the fact that lot's of things work they may just not be the safest.

    Darren i went back and read.what you wrote again and now I am back to being confused. it looks like both you and Todd agree the safest installation is 4 wire with ground rods at each service/panel and the ground & neutral bonded. I may be reading the code wrong but I thought if 4 wire was run that the ground & neutral needed to be separate not bonded or else the potential to energize the ground would exist.

    Can you elaborate on your suggestion?
     
  9. pickled egg

    pickled egg If you don’t hear from me

    I won't speak for Todd (Lawd knows, he can speak for hisself :D), but here's the long, drawn-out and boring basis for *my* feelingss on it.

    In an electrical system, you have the grounding conductor, and the grounded conductor. These are the ground and the neutral, respectively.

    The name "neutral" comes from the potential between the grounded conductor and earth, at the service panel. Simply put, you want zero potential between the floor you're standing on and the neutral conductor, so that in its zeal to return to earth (electricity loves that shit), *you* don't become the path. The human body is a decent conductor, but a better resistor.

    When you carry a 4-wire feeder to a remote location, you're ensuring that the neutral and ground *across the wires* has no potential (they're bonded at the main). Arguably, the ground rod you drive at the remote location *should* remove any potential between your neutral bus and earth. So your ground conductor is bonded to earth at each location, but your neutral conductor is only bonded to earth at the main. You now have a single point of failure for neutral/ground bonding, and if either neutral *or* ground connections at either end get crappy, you run a risk of potential between neutral and ground at remote locations.

    Now, the reason you're not allowed to bond ground/neutral at sub panels is that if the neutral gets crappy, you don't want a grounding conductor to be carrying current across your service feeder (especially if your grounding conductor is your conduit or duct, and not an actual wire). That concern is negated, however, by the ground rod driven at the remote location (and, in the case of your home, the bond to your water system (if your water line coming in is metal, that is)).

    So why, then, do they not permit the neutral/ground to be bonded with a locally-installed earth ground? I really don't know. From a safety standpoint, the less electrons you're forcing to make long journeys over isolated conductors for the same effect would seem the safer route to me, than to permit the electricity to take its shortest path back to earth. :confused:

    Clear as mud, huh?
     
  10. beac83

    beac83 Well-Known Member

    I think Todd and Darren have covered it, run 4 wire feeders. Use Ground rods/Grounding of the ground (grounding conductor) at each building.

    One of the reasons against bonding at each subpanel has to do with the possibility of there being differences in ground potential between the multiple grounding points. The Earth's upper crust is not an ideal conductor, and ground rods / water pipes at different locations may have different levels of conductivity to the earth.

    One example of this was a building I worked on in the 1980's in the San Fernando Valley. The older part of the building, built in the 1950's used a water main ground, the newer part, built in the 1970's used a multi-point grounding system (grounding at each building support, connected in a ring) as it did not have separate water service. there was ~ 0.100 to 0.200 volts difference between these two grounding sources, even with the two interconnected. If the two grounds were separated, the voltage difference would probably have been a few volts. At various points in the system we measured around a hundred amps flowing between these two grounds, probably related to the induced stray currents from the 138KV overhead line running about a hundred feet away from the building.

    Multiple bonds between neutrals and grounds at subpanels would have divided that current across both the neutral and the ground, thus overloading the neutral. We ended up NOT bonding the subpanels, even though it was allowed by Code at that time. It was the only way to not have to oversize the neutrals by large amounts.

    Needless to say, we had a difficult time getting the hum out of the analog video lines that ran between these two buildings, each of which contained major equipment rooms for the network.
     
    Last edited: Nov 26, 2012

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