New dedicated lines have high noise floor.

AC Power Wiring Types (cont’d) Metal Clad (MC)is manufactured in both steel and aluminum with twisted conductors that help reduce AC magnetic fields. Although the steel jacket helps reduce AC magnetic fields, the twisting of conductors has the greatest effect on reducing these fields. Another benefit is the constant symmetry of the phase conductors with respect to the grounding conductor which greatly reduces voltage induction on the grounding wire. (NEC article:330)


Two conductor plus 1 ground MC (Metal Clad)is a good choice for Non-Isolated Ground A/V systems. MC cable contains a safety grounding conductor (wire). The three conductors in the MC cable (Line, Neutral and Ground) are uniformly twisted, reducing both induced voltages on the ground wire and radiated AC magnetic fields. The NEC article 250.118 (10)a prohibits the use of this cable for isolated ground circuits because the metal jacket is not considered a grounding conductor, and it is not rated for fault current.


Two Conductor plus 2 ground MC (Metal Clad)may be used in an Isolated Ground installation, because the cable contains two grounding conductors (one for safety ground and one for isolated ground). The conductors are twisted, but the average proximity of the hot conductor and the neutral conductor with respect to the isolated grounding conductor is not equal. Under load, this will induce a voltage along the length of the isolated ground wire, partially defeating the intent of isolation (see Ground Voltage Induction section of this paper).

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Single-Phase 120/240V electrical service is most commonly found in residences and smaller commercial buildings, and is commonly used to feed AV equipment. One key advantage that single phase has over three phase is that while harmonic currents are still present, only even order harmonics can add in the shared neutral, and they are uncommon, since the waveform would be asymmetrical. In addition, use of single-phase 120/240V can result in at least a 6dB reduction in noise floor as compared to three phase if the capacitances of the connected equipment are relatively well balanced. Furthermore, if a signal cable is connecting two pieces of ungrounded equipment powered from opposite phases, the leakage current flowing in it will increase (causing more noise) as compared to powering the equipment from the same phase.