Current Transformer (CT) Wire Extension

Overview

There are occasions when longer current transformer (CT) lead wires are needed. If the standard 8 foot (2.4 meter) leads are not long enough, they can be extended to 100 feet (30 m) or more, especially if you keep the wires away from electrically noisy equipment, such as variable speed drives (inverters). To minimize power line noise from interfering with sensitive CT signals, extension wires should be run in conduit (raceway) without any power conductors.

While CTs with leads longer than 8 feet can be special ordered for an additional charge, the most common practice is to extend the leads in the field using 22 AWG to 18 AWG twisted pair wire. Distributors typically stock jacketed shield cable, with one or more twisted pairs.

If you want to extend your CT leads, there are four basic options:

• Twisted – Our standard black and white #20 AWG gauge CT leads are twisted. Twisting is important for noise immunity, especially at 50-60 Hz. This wire is available in 100 foot (30 m) coils, see the CT Wire page for details. The two conductor, #20AWG white and black twisted pair is equivalent to about one #8 AWG (0.213″ dia.)
• Shielded and twisted – Adding a shield—in addition to the twisting—can offer some additional noise immunity,  especially for long extensions. Note: for best immunity to crosstalk, the wires should be twisted in pairs.
• Non-shielded and non-twisted – Most cables are twisted, so this normally only arises if you use two single conductors. We recommend against this for most applications, because there is very little noise immunity. This can work if the conductors for each CT are kept in close contact to minimize loop area and if the CT conductors are kept away from line (mains) conductors and in their own conduit or metal raceway.
• Shielded but non-twisted – This is a bit unusual, but does occur. There are two concerns with this type of cable. First, the shield primarily blocks high frequency interference, but not 60 Hz. 60 Hz is much better blocked by twisting the wires. Interference from 60 Hz line noise is a concern because it’s the main frequency we are measuring. Second, because the wires are not twisted into pairs, there is a greater risk of crosstalk between CTs if you run more than one pair of CT wires in the same cable. If possible, use twisted pair cabling when extending CT wires.

Shielded

CT signals are susceptible to common-mode and differential-mode interference, magnetically and capacitively coupled into the wires. The twisted-pair arrangement blocks most magnetically coupled and differential noise, but does not reject common-mode, capacitively coupled noise. A shield can help block common-mode capacitively coupled noise, but is most effective if both ends are grounded. If only one end is grounded, the ungrounded end will oscillate up and down with the interfering signal, and then couple into the twisted pair wires.

Shielded cable is typically available in 300 and 600 volt ratings. The 300 Vac rating is sufficient for 120/208/240 Vac panels and circuits, but 600 V rated cable should be used for 277/480 Vac and 347/600 Vac panels.

In three phase applications to cut down on the number of cables to run, a jacketed cable with three twisted pairs and an overall shield can be used to advantage. We recommend no more than three pairs per cable.

Since our standard CT wires are 22 to 18 AWG, we recommend extending the CT leads with the same or larger gauge wire.

Unshielded

ACTL Series and other UL 2808 Listed CTs

The standard CT wires are unshielded, but have good noise rejection due to the twisting of the wires. Our standard twisted black and white CT wires are available in 100 foot (30 m) coils, see the CT Wire page for details.

CCS uses wire with the following specifications: Current Transformer Wire, 20 AWG, 16/30 Strand, Black/White twisted pair (one twist every 0.95 inches +/-15%), 105C, 600V, 0.31 VW-1  UL 1015, RoHS.

Alternates

These unshielded cables should be more readily available.

Splicing

When the 8 foot lead wires on standard stock CTs are not long enough, you can extend them by splicing on additional wire.

• You may want to shorten the original CT wires, especially if you are going to use shielded cable and ground the shield to earth ground.
• Avoid creating large loops or gaps between the white and black wires at the junction point, because this can increase electrical interference.
• Twist-on wire connectors (wire-nuts) can be used but only if they are rated for connecting the stranded CTs wires, typically #18 to #22 AWG to the gauge of the extension wire. Do not use wire nuts (typically red or yellow sizes) used for electrical work; they are too large to make reliable connections with smaller stranded wires! For reliable connections, be sure to follow the manufacturer’s instructions.
• Butt splice and other styles of crimp connections can work well if a ratcheting crimping tool specifically designed for the crimp connector is use. Caution!: low-cost, general purpose crimping tools do not always make reliable connections. After crimping, tug on the wires to make sure that they do not pull out of the crimp.
• The most reliable connection is a soldered connection with heat shrink tubing insulation. Unfortunately, solder connections are very time consuming.

Signal Attenuation

Long lengths of cable can slightly attenuate the CT signals (in addition to increasing susceptibility to noise). The following table shows the required total CT length to reduce the CT signal by 0.1% as a function of AWG, based on the WattNode CT input resistance of 23 kilo-ohms.

Effect of Interference or Crosstalk

Customers commonly ask how much error will be introduced by extending the CT leads or by using different types of cables. Unfortunately, there is no quantitative answer to these questions.

In the case of crosstalk, the amount of crosstalk depends on the following:

• Signal frequencies: if your load has strong harmonics, there could be more crosstalk.
• Cable properties: such as the capacitive and inductive coupling between adjacent conductors.
• CT source impedance: different current transformers have different winding resistances and internal burden resistors, resulting in different source impedances and different susceptibilities to interference and crosstalk.
• Cable length: the longer the cable, the more crosstalk.
• Signals on different CTs: are you monitoring a balanced three-phase load or are there completely different current signals on different CT cables?

Many of these variables are unknown and their interactions with each other are difficult to predict. With the recommended cable types (twisted pair, optionally shielded), crosstalk should be negligible. With other cables, it is impossible to predict if crosstalk will significantly affect the accuracy.

Interference is also impossible to accurately predict or quantify, because it depends on all the factors for crosstalk, as well as the amplitude and frequency of the interference, the distance from the source of the interference, and external shielding, such as conduit.

In our experience, it is easier to avoid long CT lead extensions and if you must extend the leads, to use good cables.

See Also

• WattNode Wires and Cables

Keywords: extended, lead, CT