Help with Isolated Ground Dedicated 20Amp Circuits.

Discussion in 'Home Theater Projects' started by Jeffrey_S, May 22, 2002.

  1. Jeffrey_S

    Jeffrey_S Stunt Coordinator

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    Hi all,

    I've decided to add two isolated ground dedicated circuits to my home theater. From what I've read, the correct way to do this is to use metal conduit connected to the service panel and to a metal outlet box at the other end. For my basement theater, both circuits will be approx. 15-20 foot runs. If someone with a little knowlege about isolated ground circuits could comment on the following I'd appreciate it:

    1. What kind of metal conduit should I use to shield against interference? I think they make flexible steel and aluminum. I think there are non flexible conduits as well.

    2. I'm planning on using 10ga THHN wire, one hot and one neutral twisted together. A 10ga THHN ground wire wrapped around the twisted pair in the opposite direction. These will be pulled through the metal conduit. Does this sound like the correct wire to use?

    3. What brand of isolated ground outlet would you recommend? If you know a model number that would be great too.


    I'm planning to do the installation myself and then have an electrician come in to OK the install and make the final connection at the service panel.


    What do you all think?


    Thanks,

    Jeff
     
  2. John S Smith

    John S Smith Agent

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    Jeff
    1. No conduit will provide Electromagnetic sheilding.

    2. You need two hots, two neutrals and two grounds, all 12 Ga. Don't understand what the "twisting" will accomplish.

    3. Any brand, "commercial grade".

    4. Let the electrician do the job, is it worth the hassle?

    ..john
     
  3. brucek

    brucek Second Unit

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    Jeff,

    In a home environment, an isolated ground is completely uneccessary. A dedicated circuit installed to your HT system makes that statement even more true.

    An isolated ground system is required in a commercial building because they have many miles of cable and heavy equipments such as motors and air chillers etc, that can create a rather noisy ground system that wouldn't be very compatable with any sensitive equipment that was plugged in the wall. As a result, they keep this "dirty" ground isolated to the buildings metal conduit system and run a separate safety ground to the third prong of the buildings wall receptacles.

    This requires that "three wire cable" is run instead of standard electrical "two wire" cable. The cable uses an insulated hot wire, insulated neutral wire and an insulated ground wire plus the standard bare ground wire. The receptacles used don't have continuity between the third prong and the metal case to keep these two grounds isolated. These two grounds are kept separated and are tied to different ground points. Ultimately the two ground systems are then bonded together. This keeps the third prong isolated from the dirty signiture of the case ground in the building.

    If you are running a dedicated two wire circuit in your home with a standard safety wire, what would adding the extra isolated ground wire gain you? Nothing. In a dedicated circuit the case ground is not being passed on through other multiple case connections, nor is there any parallel conduit interference. The safety wire in your dedicated circuit takes a direct path to the service panel without any other interference or connections. This is the lowest impedance point to ground.

    Simply run standard 12 gauge two wire NMD cable you can purchase at Home Depot for your dedicated 20 amp circuit(s). It's a nice idea to purchase the better industrial grade receptacles rather than the 99 cent kind, but the isolated ground type won't gain you anything in this situation.

    Generally if you're concerned about EMI/RFI interference, you would use simple local filtering to remove it. Most of these line conditioners they sell contain a good EMI/RFI filter.

    brucek
     
  4. Bill Kane

    Bill Kane Screenwriter

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    Here are some 15A and 20A Leviton Industrial "Decora" duplexes
    CLICK
     
  5. Wayne A. Pflughaupt

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    Jeffrey,
    I noticed this the last time you gave us a description of IG circuits, and I find the two ground wires puzzling. The IG circuits I’ve seen use only the single insulated ground (THHN color-coded green). Regular non-IG circuits have no ground wire at all in commercial buildings - only the hot an neutral are run. The conduit system acts as ground (tied to a central ground stake somewhere). Thus it makes sense to run a separate insulated ground for an IG circuit, but what does the extra one accomplish if the conduit is already grounded?
    Perhaps this is just a difference in the way things are done in Canado vs. the US?
    Regards,
    Wayne A. Pflughaupt
     
  6. Jeffrey_S

    Jeffrey_S Stunt Coordinator

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    Thanks for all the advise everyone!


    Brucek,

    Thanks for the detailed explanation. I see your point. I guess what I outlined above is overkill. But I would just like to get some clarification on a couple of points if you wouldn't mind:

    1. the metal conduit would not provide significant shielding. Correct?

    2. using stranded hot and neutral THHN wire that is twisted and wrapped with the ground would not provide any significant benefit. Correct?

    3. one and two above would be more than dealt with by using a Monster Power HTS3600 or some other similar unit. Correct?

    Am I understanding all this correctly? Thanks again for the advice.


    Wayne,

    What do you think about this whole issue of whether or not IG dedicated circuits do anything in residential applications? Brucek makes alot of sense.


    If you guys are in agreement that the benefit of twisting stranded THHN wire and using metal sheilding is negligible, then I am inclined to just us 10ga romex for the job.


    Jeff
     
  7. ChrisB

    ChrisB Stunt Coordinator

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    I have never understood why some places have us put in isolated ground duplex's. In order to have a true isolated circuit you need a isolated ground bar in the panel, and a isolated ground wire run outside or to a bonding point. We still install them, but I always tell people for the same price they can buy a good surge protector.

    Brucek is right, run a 12-2 romex to a plastic box(if you run pipe you could pick up some interference) and it will basically be the same as a isolated ground(just not a true one). It will be isolated all the way to your panel.


    As for doing it yourself, unless you know a electrician, you are going to pay a minimum of one hour show up time(50-75 depending on where you live) service charge(20-40) so if the circuit is not to far from the panel, and your walls are studded it might be better to just pay him to do it all. I would suggest buying the material to save the 30% markup, but we really hate that as we are used to using certain materials. If you do it yourself and have him tie it in, have him tighten down all the wires on your breakers, it that time of year where wires tend to loosen up and cause problems. Plus it will at least help get your monies worth, I usually do it when I am in someones panel, as it only takes a few minutes, and it is a great service to the customer.



    If your walls are not studded you will have to run pipe down the wall to protect the romex, here are a few tips on that-

    1. unless you have a pipe bender to bend the box offset, buy "mini's" for straps.
    2. Put a connector on the top of the pipe to act as a bushing
    3. Make sure to drill a hole in the concrete for the ground screw before mounting it
    4. Use a ground screw in the back of the box
    5. Wrap tape around your screws on the side of the plug before installing it.
    6. Straps need to be within 6" of the box then put one near the top.
     
  8. brucek

    brucek Second Unit

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    Wayne,

     
  9. Jeffrey_S

    Jeffrey_S Stunt Coordinator

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    Thanks again Brucek. That really makes things clear. I'm going to do two 20 amp romex runs using 10ga solid core wire. I'm not going to use any conduit.


    ChrisB,

    Thanks for the advice! I have a wooden framed home. I'm going to have to open a wall to run the cable. When I finished my basement about 5 years ago, I wired 22 recessed light fixtures and about 25 outlets myself using 12ga romex. I had an electrician make the panel connections but I did all the rest. Point is, I feel pretty comfortable running cable and installing some good outlets. I wish I was into hometheater when I finished my basement since it would have been great to do these dedicated circuits then. I wish I had also run surround speaker wire in the walls at that time as well.

    I will have a licensed electrician check my work and make the panel connections. Great suggestion about having all the connections in the panel tightened while its open.


    Thanks again,

    Jeff
     
  10. Wayne A. Pflughaupt

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    brucek,
    Thanks for the clarification. Maybe it’s just our local codes, or perhaps the commercial examples I’ve seen was the work of lazy electricians (not that I’ve seen hundreds of examples or anything). Wouldn’t doubt if it were the latter – I learned on one of the Forums a few months back that new code requirements include a ground wire between the outlet and the metal box (rather than rely solely on the outlet mounting points for ground). I’ve yet to see that, either!
    Jeff,
     
  11. Jeffrey_S

    Jeffrey_S Stunt Coordinator

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    Thanks again for all the help!

    Well, I'm putting together my materials to run the two 20amp circuits and I can't decide which outlets to get. Obviously they need to be 20amp outlets. The highly recommended P&S 5242 and 5262 are 15amp. Many of the recommended 20amp outlets I've seen recommended are IG outlets.

    Can someone recommend a specific outlet manufacturer and model number.


    Thanks,

    Jeff
     
  12. brucek

    brucek Second Unit

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    Thanks for the kind words Wayne. I'm sure the ground system you put in is great. Driving an 8 foot rod is indeed a pain.

    Jeff,

    I'll give you my thoughts on your post, although many may disagree. When I installed my 2 cables for my 2 dedicated circuits, I used the appropriate wire gauge required for 20 amp circuits.
    I then used high quality 15 amp receptacles and 15 amp breakers at the service panel. Perfectly legal, in fact, its better than 15 amp cable with 15 amp breakers.

    Here's my reasoning. I'm using a gauge of wire that results in low loss because of its larger size, and if I trip a 15 amp breaker when using my system, then I feel I would want more dedicated circuits anyway. If I'm continually drawing 15 amps on a circuit, then I want to split that load further and add a third dedicated line.
    Of course most peoples systems don't draw anywhere near these twin 20 amp circuits - it's usually overkill.

    My point is - if you have a 15 amp receptacle you like, then use it and breaker the line at 15 amps.

    brucek
     
  13. Jeffrey_S

    Jeffrey_S Stunt Coordinator

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    Brucek,

    I see your point. I guess my only remaining confusion lies in the fact that I will be using one of these circuits for my Rotel 1095 amp. On startup, it draws alot when charging its caps. This has caused shared 20 amp circuits to trip their breaker in my panel (sometimes even when the other outlets and recessed lighting on these circuits were not in use). Infact that is what got me into this mode of installing dedicated circuits. Would a 15 amp line be equivalent in its power supplying capabilities as compared to a 20 amp line when used for this purpose? Also, would I still benefit in any way using 10/2 romex as as compared to 12/2 if I do go with a 15 amp circuit?


    Thanks again,

    Jeff
     
  14. brucek

    brucek Second Unit

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    Jeff,

    Yeah, the 1095 is a mother of an amp for sure - very nice. I'm surprised it's tripping a 20 amp breaker, but if this is your experience, it does seem to demand a breaker of that size.
    I have 3 Bryston amps, plus a few other devices plugged into one of my dedicated 15 amp breakers, but I do sequence one of them on startup, so I guess I save a little there. You've got all 5 channels lighting up at once - no doubt it's a fair surge, but breakers tend not to trip with short instantaneous current surges - strange. Oh well, not a big deal really, since you have the larger wire, just breaker at 20 amps and go to Home Depot and buy their spec grade Leviton outlets, you can either back-wire or side-wire those with #10 I believe.

    The code where I live is perhaps a little different than where you are. Here we don't use Romex (BX), we use standard NMD7 two wire cable. #14 is 15 amp, #12 is 20 amp and #10 is 30 amp. Where you live perhaps you're supposed to use #12 for 15 amp and #10 for 20 amp - correct? - surprising, because I thought the NEC in the USA specified the same gauges as here in Canada... #10 is very large and hard to work with. If your code says 20 amp is #12 (most places are this way), then I would recommend #12 wire size.

    Either way, the answer to your question is yes, you will gain something by going with a smaller gauge (larger wire) for your circuits. The resistance of the larger wire is lower and the losses will be less.

    brucek
     
  15. ChrisB

    ChrisB Stunt Coordinator

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    Its the same here in the US, 14 is for 15 amp circuit, 12 for 20 amp circuit etc. By raising the gauge of wire you are using you can avoid voltage drop, thus less power needed to run your equipment and less heat. Since your equipment is less than 100' away, I can't see a reason to use a larger size wire. Like brucek said, there is never a problem putting a smaller breaker on a larger wire, just putting a larger breaker on smaller wire is a no no.
     
  16. EricWT

    EricWT Extra

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    I was always under the (probably false) impression that BX (AC90) cable was the same as NMD cable, but with extra shielding around it. Now I see that the bare wire inside it is touching the armour; many installation pictures I have seen show that bare wire folded back, so that it touches the mounting bolt thingy on a box, but does not actually enter the box. The shield itself is then part of 1 of the 2 grounds in an isolated ground system (the other one being the insulated wire attached to the plug, I think.) However, I plan on using BX cable for a standard circuit; I bought it because of the extra protection it would afford.

    Okay, now my question: can I use the bare copper wire in 12/2 BX (Black, white, bare) to enter my outlet boxes in the same manner as I would with NMD wire? Or should I be folding this back and letting it be clamped on the outside?

    Thanks,

    Eric
     
  17. Brian OK

    Brian OK Supporting Actor

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    Jeffrey,
    I don't see where anyone answered your last question about which 20amp duplex outlet to get.
    Well, I can highly recommend the Hubbell 20amp 5362. Here are the specs:
    Model: HBL5362BK (if BK, then it's black, but does come in grey too)
    :Straight Blade 20A 125V 2P 3W GRND SPEC Grade, back and side wired
    I own four of these (mine are crogenically treated), two for my two dedicated circuits, and two for my JRisch line conditioner (I removed the Pass&Seymour 5242's which came stock with it and dropped in the 5362's instead).
    I'm not saying cryo'd outlets are for most, but for me they are well worth the extra money. This is IME, and for my ears the only way to go. I replaced the highly touted P&S 5262A with the Hubbell 5362 (for my wall duplexes) and was simply stunned at the audible drop in the noise floor and bass response improvement. Again, YMMV.
    ...can be had for the best price I have seen in the open market .... www.dedicatedaudio.com not cheap, but it is the CHEAPEST serious audio upgrade out there IMO.
    Good Luck,
    BOK
     
  18. Matt_Doug

    Matt_Doug Stunt Coordinator

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    Jeff, if you're looking for ideas on shielded twisted pair geometries, oversized conductors and 20amp isolated ground hospital grade outlets (the oranage kind) on dedicated circuits you'll find more sypmathetic ears among the inmates at the audio asylum. We're a little more down to earth around here. BTW Its the transformer and inrush that draws all the power on start up. something to do with core saturation and magnetic field strength build up.
     
  19. Wayne A. Pflughaupt

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    Eric,

    The BX I’ve seen where you wrap the “ground” wire back around the armor, the wire was aluminum or steel, and very thin. If the bare copper ground wire in the BX you’re using is 12ga. it will be too large for wrap-around. If that’s the case then go ahead and take it to the outlet.

    Regards,
    Wayne A. Pflughaupt
     
  20. EricWT

    EricWT Extra

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    Thanks, Wayne. It is a 12 ga. bare copper ground wire.

    FWIW, I just called my electrical inspector, and he said that I should bring any ground from BX into the outlet like a normal NMD wire. Since he will be the one inspecting, that's what I will do. :) However, it makes me wonder a little if he does not know about the bonding strip vs. ground wire issue.
     

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