Grounding Metal Outlet Boxes and Isolated Ground Receptacles
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.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 ? . |
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. |
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). |
