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In Reply to: RE: Major ground loop problems after construction on new house posted by M-B on August 24, 2014 at 21:17:38
I had my contractor install 4 dedicated 20 Amp circuits on their own subpanel (about 50' from the outlets). Two are in the front of the room for the amps. Two are in the back of the room for the electronics and video.
Each circuit home runs hot, neutral, and ground to to the electrical panel using 10 gauge wire. The outlets are PS Audio Power Ports which are isolated ground outlets.
* Isolated ground receptacles? Is the wiring installed in metallic conduit? If conduit, was the wire pulled into the conduit after installation of the conduit? If conduit, was an individual conduit installed for each Dedicated circuit?OR,
* Is the wiring MC cable with an outer steel or aluminum spiral armor around factory installed conductors.
http://www.afcweb.com/mc-metal-clad-cables/mc-lite-aluminum-armored-cable/OR.
AC cable with an aluminum corrugated armor around factory installed conductors.
http://www.southwire.com/products/ArmorliteTypeAC.htmOR,
* Is the wiring 10-2 with ground NM-B cable? (Romex is a trade name of NM-B cable). IF 10-2 W/GRD NM-B cable the outer jacket of the cable will be orange in color.
http://www.homedepot.com/p/Southwire-250-ft-Orange-10-2-Romex-NM-B-W-G-Wire-28829055/202316274
Are all the dedicated circuits fed from the same power Line, leg, in the sub panel? All from Line 1, L1, or all fed from L2?Don't know? Quick check is to look at the 4 breakers in the sub panel that are for your audio/video equipment.
If the breakers in the sub panel are mounted horizontally they are connected to L1 and L2 down each side as follows,
You will find numbers stamped into the dead front cover that covers the breakers around the handle and face of the breakers.
>>>>>>>>>>> left side - - right side
L1 .... breaker spaces 1 ...... 2
L2 .... breaker spaces 3 ...... 4
L1 .... breaker spaces 5 ...... 6
L2 .... breaker spaces 7 ...... 8
And so on down each side of the panel
Example are your 4 dedicated circuits connected to breakers in L1 spaces 1,2,5,6 or L2 breaker L2 spaces 3,4,7,8?
A sure way is with an AC volt meter. in your case you will need a piece of wire to extend one of the test probe leads of the meter.
Measure from one of each small slot contact of each dedicated circuit duplex receptacle to the other small slot contact of all other dedicated connected connected duplex receptacles. You should measure zero volts.
Edits: 08/25/14 08/25/14 08/25/14Follow Ups:
There is a central junction box close to the room that attaches to the sub-panel via a piece of PVC that was buried ~18" in the ground (the house is on slab and very difficult to pull wires through). Flexible metal conduit was then installed from the junction box to the outlets. Much of the conduit has shared wires in them. The wires were pulled after the conduit was installed.
The metal raceway has its own ground as well per code.
The circuits share the same leg.
I will check the voltage when I get home
Flexible metal conduit was then installed from the junction box to the outlets. Much of the conduit has shared wires in them. The wires were pulled after the conduit was installed.
Induced voltage onto the safety equipment grounding conductors caused by the loosely installed current carrying conductors in the flexible metal conduit multiplied by more than one dedicated branch circuit sharing the same conduit. A true dedicated branch circuit does not share the same conduit or cable with other branch circuits.
Starting on page 31 of Link provided below.
Quote:
The “Conduit Transformer”
• This finally explains what drives 99% of all ground loops!
• Load current in line and neutral produces opposing magnetic fields
since instantaneous current flow is in opposite directions
• Imperfect cancellation magnetically induces voltage over the
length of the nearby safety ground conductor
• Strongly affected by geometry and proximity of wires
• Highest voltages with randomly positioned wires in conduit
• Lower voltages with uniform geometry of Romex®
• Voltage is directly proportional to load current, wire length, and
rate of change in current or ΔI/Δt
• Mechanism favors high-frequency harmonics of 60 Hz
• For constant current in L and N, induced voltage rises at 6 dB/octave
The “Conduit Transformer”
Wires randomly positioned in conduit
produce the worst possible results!
http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf
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