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Grounding Metal Outlet Boxes and Isolated Ground Receptacles

Just installed six really nice separate dedicated 20 amp lines (with 6 awg) for my new dedicated listening room.  Decided to use heavy metal outlet boxes so that I could make my six Furutech Receptacles as strong and sturdy as possible.  I also had my electrician run 4-wire 6 agw to the boxes so that we could have one ground wire to the metal receptacle box and then a separate ground wire to the isolated ground screw on the Furutech.

I sent some photos of the setup to a friend of mine (who just happens to be an electrical engineer) and he raised an the issue that since my Furutech Receptacles are metal and they will be screwed into the metal box with metal screws, then we have effectively now tied the two separate grounds together!  Help!  Is that a bad idea?   Is having the box and receptacle setup in this way going to cause issues once my gear is in place; ground loop hums, etc?






stickman451
jea48
4,562 posts
 
bflowers,

IF the dedicated branch circuit wiring for the power amp is braided (hot, neutral, an IG ground conductors braided) together that might be the problem. Think of it as an 8ft shielded power cord. ( 3 braided conductors inside a steel outer shield) With the braid basically you have 1 hot ungrounded conductor, (120V above ground potential), and 2 conductors at ground potential. (The neutral, the grounded conductor, and the equipment ground, the grounding conductor.)
Most power amps do not like shielded power cords. My experience it sucks the air from the sound of the amp.

For a couple hundred dollars labor plus material you could find out for sure if my theory is correct.

Have your electrician pull out the braided wire from the conduit that feeds the Boulder 3060 power amp.

You didn’t say the size of the conduits used.... I am going to assume they are 1/2" EMT. https://www.google.com/shopping/product/15351473063369031769?lsf=seller:8740,store:11486162617141277...
Or possibly 1/2" flexible metal conduit.

For the new wire that will be pulled back in the conduit use solid core, not stranded, THHN/THWN. (Stranded wire can smear the sound)

Ask the electrician to slightly, to moderately, twist the hot and neutral conductors together the entire length of the conduit run. The IG equipment ground conductor will be pulled straight along side the hot and neutral twisted pair.


Read pages 16 through 35. Mainly 31 through 35.

https://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

jea48
4,562 posts
 
bflowers said,
 In metal conduits. Single run of 12G braided wire in each conduit. Run from subpanel is <8 ft in all cases. Probably 20 or so lines coming of the subpanel. Don't know about the interconnects. There are 2 legs, but all my equipment fits on one leg.

There are 2 legs, but all my equipment fits on one leg
I just learned from almarg the amp is 240V.  Hopefully almarg will read your posts on this thread. He may have some thoughts.
Jim

 
almarg's avatar
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almarg
9,670 posts
 
Jim, thanks for calling this thread thread to my attention, via your post in the directional interconnects thread. My response there on 12-6-2016, near the bottom of page 3 if posts are sorted most recent last, may be of interest to some here.

Stickman’s wiring may already have been finalized, but FWIW re:
Jea48 11-14-2016

I would wonder if the floating insulated wire might act as an antenna and pick up air born RFI. I would probably connect the end at the panel to the ground bar. Nothing wrong with that as far as a code issue. Ask your electrician if he has a problem with it.

It would be nice to hear from Al, (almarg), what he thinks. Would he connect the end of the spare insulated wire at the electrical panel to the ground bar, or leave it floating above ground?

I agree. Antenna and RFI effects are always hard to predict, of course, and it might not matter either way. But grounding the wire at the panel, in addition to perhaps reducing RFI pickup, would add a bit of capacitance between the AC conductors and ground that I suppose might provide at least a slight benefit in terms of filtering RF noise that may be present on the incoming AC.

Regarding Bflowers issue, the only thoughts that occur to me beyond what Jim has said are:

1)It would probably be a good idea to measure the AC voltages (120 volts on each leg, relative to neutral, and the 240 volt difference between the legs), during typical listening hours. If practical it would be preferable to do that while the system is powered up and playing music at normal volumes.

2)Regarding:
Bflowers 12-1-2016
BTW, had serious ground loops in the old room, but the Boulder gear was fairly impervious to it. I demoed a single ended Lamm amp and it buzzed like a fiend!
Not sure which Lamm model you tried, but I see that the currently produced models that are described as single-ended provide unbalanced inputs on their XLR connectors. Which if connected to the balanced outputs of your Boulder preamp via an XLR cable would short the inverted signal on XLR pin 3 to the circuit grounds of both the amp and the preamp. Given the relatively low output impedance of the Boulder preamp (spec’d at 100 ohms, which probably corresponds to 50 ohms per leg), that could conceivably have resulted in the buzzing you heard. See this thread for an example of a situation in which exactly that occurred, although very different equipment was involved.

Also, while very unlikely it is conceivable that the output stage of the preamp which drives XLR pin 3 on each channel might have suffered some damage as a result of having that signal shorted to ground. So to be sure, subsequent to the Lamm audition did you put the older Boulder amp back in place, and find the system to perform the same as previously?

Good luck. Regards,
-- Al

Post removed 
jea48
4,562 posts
 

First, I assume cdrc’s comments are directed at stickman451.

Second I believe the electrician stickman451 hired is a licensed electrician. I determined that to be the case by the posts of the OP when describing the wiring methods and materials used for the installation. From this side of the monitor reading the OP’s posts I do not know the quality of workmanship of the install. I assume it is fine.


cdrc said:

I don’t know if it is a bad idea but I do not think it is good. I also would doubt what you outlined is within code not knowing which state you live in.

The wiring method and materials used in stickman451 posts, for the installation, meets NEC.

What amperage breakers are you using?

I believe stickman said the branch circuit breakers are 20 amp. NEC code, for a NEMA 20 amp rated receptacle says a 20 amp branch circuit breaker must be used. Minimum size branch circuit wire used is #12awg. NEC code is bare minimum. NEC code is not intended to be used as a design manual.

I would DEFINATELY run a separate wire to bond your metal boxes and emt raceway.

At the time of his posting of this thread he has a box ground. He actually at the time of his posting had 2 equipment ground wires at each metal outlet box. One for the box ground and the other for, what stickman451 thought at the time, the IG (Isolated Ground) terminal of his duplex receptacles. Go back and read my responses to him on the subject.

For your Furutech receptacle ground wires I would braid them as you see in my sub panel pic. This will firthur reduce your noise floor a great amount from this simple trick.

That is not a braid. The green equipment grounding conductors are twisted together in a spiral. They are not braided, weaved together with one another.

To braid, simply install one end of your ground wires in a drill chuck, tighten the chuck and run the drill. Get a nice tight braid, the difference will be magical.

You might get that pass an electrical inspector beings they are equipment grounding conductors, but I would be willing to bet he would red flag the job if you used that method twisting the HOT and Neutral conductors together that way.

//

Here are a few things I found looking at your pictures where I believe you could improve the SQ, sound quality, of your audio system.

* Get rid of the steel ferrous EMT conduit and steel boxes.

* For exposed work use MC cable with an outer aluminum armor instead, if possible. Solid core wire only! Stranded wire will smear the sound. Boxes could be malleable aluminum boxes.

(In your case, from looking at your pictures, you could have built a box out below the surface mount electrical panel. The box out could have a removable cover if you thought you needed access for future installs or changes. With this method you could of used NM-B sheathed cable, (Romex is a Trade Name). Plastic boxes, for your branch circuit outlets receptacles. Providing NM cable is approved by the AHJ in your area.) The installation would meet NEC.

* Avoid STEEL ferrous receptacle cover plates! Even low grade stainless steel cover plates will have ferrous impurities in the stainless steel. Use a strong magnet to check for ferrous materials. A steel receptacle cover plate kills the SQ of an audio system. See my post above on the subject. By the way the late Al Sekela was an EE.

In your case you could pull the raised 1900 duplex receptacle cover plates and replace them with a 1900 raised device cover plate. Fasten the cover to the 1900 box. Support the back strap of the duplex receptacle to the device cover. Install a P&S non-breakable nylon cover on the duplex receptacle.

Example of:

http://www.homedepot.com/p/4-in-Square-Single-Device-Mud-Ring-Raised-1-2-in-50-Pack-8772/202056884

] http://www.homedepot.com/p/4-in-Square-Box-Cover-2-Devices-Case-of-25-52C17-25R/202595756 [


* Technically a true dedicated branch circuit never shares the same conduit with other branch circuits. Not only can a voltage be induced from the current carrying hot and neutral conductors to other circuits sharing the same conduit but also the AC noise of the current carrying conductors.

* At least for audio equipment if conduit is used, twist the hot and neutral conductors together in a slightly to moderate spiral twist the entire length of the branch circuit wiring. Just install the equipment grounding conductor straight along side the hot and neutral twisted pair.

* For 120V audio equipment that is connected together by wire interconnects all audio equipment should be fed from the same Line, leg, in the electrical panel. ALL from L1 or all from L2. NOT from both.

Jim

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