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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
4,310 posts
 

stickman451 said:

I will contact Furutech distributor to confirm whether not the outlets are truly ’isolated’ ground and go from there.

I assume Furutech receptacles are Listed by a recognized third party testing laboratory, such as UL.

If the duplex receptacle is an IG, Isolated Ground, per NEC it will have a triangle near the "U" shaped ground contact hole.

Here are a couple more pictures of IG duplex receptacles.

https://www.grainger.com/product/HUBBELL-WIRING-DEVICE-KELLEMS-Duplex-Flush-Receptacles-WP6064708/_/...

https://www.zoro.com/search?q=Isolated+Ground+Receptacles&gclid=CPmd0r_0oNACFQiUaQodSSoOxg&g...

At the time the electrician looked at the job with you, you must not have had the duplexes to show him. You must have just told him they were IG receptacles. That is the only reason the electrician should have installed 4 wire branch circuit wiring for a 120V dedicated circuit. By code he needed a second equipment ground wire for the metal box. (Unless the branch circuit raceway is hard metallic metal conduit like EMT (thin wall) for example, then the box ground can be the conduit per NEC providing it is a continuous run from the box to the electrical panel. The conduit can connect to other electrical metal boxes along its’ way to the electrical panel.)

Is the wire 6-3 with ground NM-B cable, Romex? If so, there is no reason to ever use an IG receptacle when NM-B cable is used.

Just a guess the electrician used steel boxes because of the #6 wire you wanted him to install. Just a guess he installed a deep 1900, 4" x 4" X 2 1/8" deep box, with a 1 gang raised device cover, for a single duplex receptacle. He may have needed the bigger box to meet the percentage of fill required by NEC for the #6 wire.

Have you ever watched the Cary Grant movie, Mr. Blandings Builds His Dream House ?

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gs5556
1,213 posts
 
What concern did the engineer express? The way you describe is legal except for patient care areas where the two ground paths must be separate. (Maybe that's his thinking).

Your chances for a ground loop are not from the two ground paths but from plugging in separates into different outlets. If you have an amp on one outlet, a preamp in another, a DAC in another and a transport in another still, each component has four separate ground paths when the interconnects are installed. Any unbalance in the neutrals may induce a ground loop hum, especially if the grounding conductors back to the panel are of different lengths.
rbstehno
1,406 posts
 
Same same here as stereo5. New room, 4 dedicated 20 amp lines with the normal plastic boxes. No ground issues or noise. Probably overkill here with potential issues in the future.

stickman451 OP
553 posts
 
Thinking that the Furutech GTX-D G was an isoloated receptacle, we decided to use four-wire Romex.  So, one ground wire went directly to the outlet, one to the metal outlet box.  The metal boxes are larger and made it easier to jump from 6 awg down to a short piece of 10 awg that would all fit in the one box.  And the metal boxes are stronger and make it easier to tightly secure the outlet to the box.  

The first question is whether or not the Furutech outlets are actually 'isolated' ground.  I am re-checking that now.  Second question is does it really matter if the outlets are isolated ground or not?  In either case, we have two grounds in the same box, one on the outlet and one tied directly to the metal box.  There are six of these dedicated 20 Amp lines in the room.  Two boxes are for the mono block amps, two are for subs, and the remaining two are for the front-end components.  A two ought line runs from the subpanel to the main panel for the house.  

Since the outlet is metal and the box is metal, we have effectively tied the grounds togther right at the box.  But, evently all the ground wires end up at the grounding pole ( which by code in our area is the steel rebar in the house foundation).  


jea48
4,310 posts
 

The first question is whether or not the Furutech outlets are actually ’isolated’ ground.

No, the Furutech GTX-D G is not an Isolated ground type duplex receptacle.

http://www.furutech.com/2013/02/02/1746/

Second question is does it really matter if the outlets are isolated ground or not?

An IG receptacle is used in commercial and industrial buildings where the branch circuit wiring is installed in metallic conduit. The idea, theory, for using an IG is to isolate the equipment ground of the receptacle from the metallic conduit supposedly to prevent electrical noise that may be traveling on the conduit. Believe it or not National Cash Register was the main reason for the use of IG receptacle years ago. Their specs called for an IG receptacle and if they came out and the receptacle for the cash register was not an IG they would not install their equipment until IG receptacles were installed.

In a residential dwelling unit, house, even if the branch circuit wiring is installed in metallic conduit what AC noise could possibly be traveling on the conduit?

When using Romex an IG receptacle would not serve any purpose or added isolation.

As for what you have now I can’t see where it would violate NEC code. Odd yes...

Also I wouldn’t worry about the branch circuit outlet steel boxes. They are fine.

In your last post you mentioned a sub panel..... I assume the electrician terminated both the box ground wire and the receptacle ground wire on the same ground bar in the sub panel. Is that correct? OR do the IG insulated equipment ground wires, that were intended for IG receptacles, pass through the Sub Panel and connect to the equipment ground bar in the main electrical service panel? (Which meets NEC code for an IG receptacle.)

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But, evently all the ground wires end up at the grounding pole ( which by code in our area is the steel rebar in the house foundation).


But, evently all the ground wires end up at the grounding pole

Please explain in detail what you mean......

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Mother Earth does absolutely nothing for the sound quality of an audio system.

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