We buy VFD cable.

Symmetrical geometry and the shield that makes it work

The three conductors are laid symmetrically, each at the same distance from the centre of the cable. That geometry keeps the magnetic fields of the three phases balanced and cancels most of what would otherwise radiate. A three conductor cable built like a feeder does not do this, which is why drive cable is a separate product rather than a selection from the power cable range.

The shield is the second half of the job. It is usually a combination of foil and braid, with a drain wire, bonded at both ends so the noise has a path back to the drive rather than into adjacent signal cable. Shield coverage is the number that matters, and a torn or incomplete shield is the first thing a buyer looks at.

The ground conductor is sized for the fault path, not for the load. In a drive circuit the ground also carries common mode current, so it is a full conductor rather than a reduced one, and its continuity along the length is part of what makes the cable usable. A reduced ground is a common shortcut on site and a common reason a reel is graded lower.

• Three power conductors plus a full-sized ground
• Foil and braid shield with a drain wire
• XLPE or EPR insulation, PVC or low-smoke jacket
• Sizes 16 AWG to 4/0 in copper

One kV and two kV: where each turns up

Drive cable is rated at 1 kV or 2 kV rather than the 600 V of ordinary building cable. The higher rating is there because switching edges produce reflected waves that can double the peak voltage at the motor terminals, and a long run makes it worse. A 2 kV construction buys headroom on the longest runs.

In practice 1 kV covers most plant motor work: pumps, fans, conveyors and the smaller skids. Two kV appears on larger drives, on long cable runs and on equipment specified by an engineering firm with an eye on the reflected wave question. Both are bought, and the rating is read from the jacket print rather than assumed.

The distinction matters to the review because the two do not substitute freely. A 2 kV reel answers a question that a 1 kV reel cannot, and a plant replacing a long run will not accept the lower rating however much length is on the reel. Read the class from the print, not from the reel size.

If the jacket print is legible, photograph it. The voltage rating and the shield type are both printed along the length.

Motor upgrades and the drive cable they leave

The usual source is a motor upgrade. A drive is added to a motor that ran across the line, a motor is replaced with a different frame, a pump skid is rebuilt and the cable no longer reaches. The electricians pull the old cable and order new, and one of the two ends up surplus.

Cooling tower work produces a steady trickle, because the motors sit far from the drives and the runs are long. Compressor rooms, chiller plants and boiler feed applications do the same. Each of them leaves partial reels in a store room, tagged with a project name that no longer means anything to anyone.

Project closeouts supply the cleanest lots. Cable bought for a scope that was descoped sits on a reel with the mill wrap still on it, and it is the easiest material in this category to place. Removed cable is more common and more variable, and the terminations at each end usually say how it lived.

Sending a drive cable lot with the right detail

Send the voltage rating, the size, the approximate length and whether the cable is new surplus or was pulled from service. Photograph the jacket print and the cut end, and if the cable has terminations still fitted, include a shot of those as well. The lug style often confirms the size.

Condition questions are the same as any shielded cable. Water under the jacket, a crushed or kinked section, and a shield that has been cut back beyond the jacket all affect the grade. Cable that was pulled out of a tray and coiled without a bend radius in mind tends to show it near the ends.

Loading is by reel or by coil. Drive cable in the larger sizes comes on a wooden reel that needs a forklift, and the smaller sizes arrive as coils that two people can lift. Say which you have, and whether a truck can get to the reel or the reel has to be brought out to the gate.

• Voltage rating: 1 kV or 2 kV
• Conductor size and copper construction
• Length, and whether it is one run or several pieces
• New surplus or removed from service
• Shield and jacket condition at the ends