CONTROL, INSTRUMENTATION, COMMUNICATION AND FIRE ALARM CABLE

Control Cable and the Shield Question

Control cable is a multi-conductor assembly for switching and status circuits rather than power: 600 volts, 18 to 10 AWG, two to sixty-one conductors, with each conductor numbered or color coded so the far end can be identified. Conductors are usually THHN, TFFN or XHHW, and the cable may or may not be tray rated.

The shield is the first question. An overall foil shield with a drain wire controls electrical noise on a circuit that carries a signal; an unshielded cable does not. Shielded cable costs more to make and is worth more as surplus, and the shield becomes visible the moment the jacket is stripped back.

• Unshielded control cable, 600 V, 18-10 AWG, numbered conductors.
• Overall foil shield with a tinned copper drain wire.
• Individual and overall shields on instrumentation pairs and triads.
• Armored control cable with interlocked or corrugated armor.
• Tray-rated and non-tray-rated versions, which are not interchangeable.

Twisted Pairs, Triads and Thermocouple Wire

Instrumentation cable is built around pairs and triads. A twisted pair carries one signal and its return; a triad carries a three-wire sensor. Each pair may have its own foil shield with a second overall shield over the bundle, and that construction is what keeps a 4-20 mA loop readable beside a drive.

Thermocouple extension wire is a different product again. Its conductors are alloys chosen to match the thermocouple rather than copper, so the wire does not create a second junction at the terminal. Color coding follows the type, and red is the negative lead in the North American convention.

Communication Cable and the Plenum Rule

Communication cable is the largest part of most low-voltage surplus: Cat5e, Cat6 and Cat6A, four twisted pairs, 24 or 23 AWG, 100 ohm impedance. The numbers are bandwidth. Cat5e is tested to 100 MHz, Cat6 to 250 MHz and Cat6A to 500 MHz, which is what carries 10 gigabit links over the 100 meter channel Cat6A is designed for.

The jacket rating decides where the cable may be installed. CMR is a riser cable for vertical shafts between floors, CMP is plenum rated for air-handling spaces, and CM is general purpose. A box of CMP is worth more than the same box of CMR, and only the print on the jacket says which one it is.

• Cat5e, 100 MHz, 24 AWG, four pair, unshielded or shielded.
• Cat6, 250 MHz, usually 23 AWG, with a center spline.
• Cat6A, 500 MHz, larger and stiffer, often shielded.
• CMR for risers, CMP for plenum spaces, CM for general use.
• 1,000 foot boxes, 500 foot spools and part boxes with cable remaining.

Fire Alarm and Class 2 Cable

Fire alarm cable is Article 760 cable, and its ratings follow the same logic as communication cable: FPLR for risers, FPLP for plenum spaces, FPL for general use. NPLF is the non-power-limited version. Sizes are 18, 16 and 14 AWG in two to four conductors, with red jackets the norm.

Class 2 and Class 3 cable carries CL2 and CL3 markings, with P and R suffixes for plenum and riser. These are power-limited circuits, and the cable is not interchangeable with 600 volt control cable even when the conductor count matches. The print settles it and nothing else does.

Photograph the Spool and the Print

Photograph the spool face and the print together. On control cable the print gives conductor count, gauge and voltage; on Cat6 it gives the category and the jacket rating. A cut end shows the shield, the drain wire and the pair separator in a single frame.

What we buy is wire and cable, not the cabinets it lands in. Transformers, switchgear, breakers, motor control centers, generators, chillers and machinery fall outside the offer. Tray-rated power cable belongs with MC and tray cable, plant and utility work is described under data centers and utilities, and the quote form accepts several photographs at once.

• The spool or box label, with footage and part number.
• The jacket print, showing type, voltage and any plenum or riser rating.
• A cut end, showing shield, drain wire and pair separator.
• The conductor count and color sequence where the print is worn.
• Whether the spool is full, part, or a remnant under 100 feet.