pressure wire

A pressure wire is a single conductor that runs from a measurement point in an electrical distribution system back to a central monitoring station, typically a voltmeter or voltage indicator mounted on a control panel or switchboard. Each wire connects to a different location or circuit, allowing the operator to read the voltage present at that point without interrupting service. The term "pressure" reflects older electrical vocabulary for voltage or electrical potential.

In practice, pressure wires are bundled together in cable trays or conduit alongside power conductors, though kept physically separated to prevent interference. A typical industrial panel might have six to twelve pressure wires feeding voltages from different bus sections, transformer secondaries, or critical loads back to a central selector switch and voltmeter assembly. The operator can then switch the voltmeter input to monitor any location in sequence, or in modern panels, a multipoint voltmeter reads all of them simultaneously.

The gauge and insulation rating of pressure wire must match the voltage it carries. A wire monitoring a 480 volt three-phase bus requires different insulation than one tapping a 120 volt control circuit. Typical installations use 14 AWG to 10 AWG stranded copper with 600 volt or 1000 volt rated insulation, depending on the system design. The wire itself carries negligible current, since a voltmeter has very high impedance, so voltage drop over long runs is not a practical concern.

Common problems and maintenance

Pressure wires fail when their insulation degrades from heat, moisture, or mechanical abrasion, leading to false or intermittent voltage readings. A broken pressure wire may go unnoticed until an operator needs that reading, causing delays in diagnostics. Testing is straightforward: disconnect the wire at the panel and measure its resistance to ground; infinite resistance indicates intact insulation. Cracked insulation near termination points is the most frequent failure mode, especially in switchrooms with high ambient temperature or condensation.

The pressure wire network is distinct from control wires, signal wires, or communications cables, though all may share the same conduit system. Its role is purely informational: it carries no control signal and no load current. In older installations, pressure wires were sometimes spliced inline at junction boxes, a practice now discouraged in favor of running continuous conductor from source to panel to eliminate points of failure.