C. System Wiring and Conduit
The selection of a proper conductor is one of the most important design criteria of an isolated power system. If improper conductor insulation is chosen, the result is the same as if the capacitive leakage is raised. A good commercially available wire insulation for this application is cross-linked polyethylene, having a mineral filler instead of a carbon black filler. A minimum wall thickness of 2/64-in. should be
demanded for use in 120 V, 208 V, and 240 V applications. It is also important to specify wire with a dielectric constant of 3.5 or less, as recommended by the NEC and NFPA No. 99.
Standard Type THHN wire is definitely unsuitable. It can, however, be used for the ground conductor. The code demands that the #1 conductor in the system be color-coded orange, the #2 conductor color-coded brown, and the ground conductor color-coded green. In three-phase systems, the third conductor shall be color-coded yellow. Schneider Electric is often asked to specify manufacturers
and wire catalog numbers for the low leakage conductor. This is extremely difficult to do since the availability of these wires differs from region to region. Also, manufacturers have sometimes discontinued production of wire types that we have
recommended. The most accessible XLP wire has been low leakage wire #FR-XLP (VW-1 XHHW-2).
Recently, manufactures have rescinded their notation in their specs for XHHW and XHHW-2. This note referred to the recommendation found in the NEC (517-160) concerning the 3.5 dielectric strength. The different compounds used in the manufacturing of the insulation have changed or are subject to change, and the 3.5 dielectric strength may not be obtained. The XHHW and XHHW-2 insulation is still
the choice since it comes closest to meeting the recommendation. This is important to note since this could affect the overall installation of an isolated power system.
