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Recently, manufactures have rescinded their notation in their specs for XHHW and XHHW-2.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.
Avoid the use of wire pulling compound since it increases the capacitive coupling. The code no longer
allows wire pulling compound to be used in conduits for isolated power systems. This compound is
usually unnecessary, because most of the runs on an isolated system are short. Occasionally, difficulty
occurs in circuits feeding portable equipment since these conductors are somewhat larger. These
difficult runs can be anticipated and provided for by using oversized conduits to ease the situation.
Obviously, conduits must be dry or the leakage characteristics designed into the system will suffer.
During construction, keep conduit ends capped so they remain free from moisture. The specifications
should state that, if moisture accidentally enters the conduits, they must be swabbed and thoroughly
dried before conductors are pulled. Use minimum fills for conduits; this results in a better random lay of
the conductors within the conduit, which further reduces the capacitive coupling.
The table below shows the approximate expected hazard currents per foot of power conductor, using the
various wiring schemes described in the preceding paragraphs.The consulting engineer can use this
table to estimate the system hazard current at the design stages. Values given are approximate;
variations in humidity, conduit moisture content, conduit fill, and wire insulation will give different results.