CT Wire Extension

This is the published version, checked on 23 April 2012.

Accuracy	Basic
Depth	Basic
Readability	Basic

Overview

There are occasions when longer current transformer (CT) lead wires are needed. If the standard 8 foot leads are not long enough, they can be extend by 100 feet or more, especially if you keep the wires away from electrically noisy equipment, such as variable speed drives (inverters).

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

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

Twisted - Our standard CT leads are twisted. Twisting is important for noise immunity, especially at 60Hz.
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 60Hz. 60Hz is much better blocked by twisting the wires. 60Hz interference is a big concern, because there is a lot of 60Hz noise and 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 300V and 600V ratings. The 300V rating should be sufficient for 120/208/240VAC panels and circuits, but 600V rated cable should be used for 277/480VAC and 347/600VAC 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 AWG, we recommend extending with 22, 20, 18 or 16 AWG cables. 22 AWG and 20 AWG will be the easiest to work with.

Manufacturer	Part Num	Voltage Rating	Wire Pairs/ Conductors	AWG	Temperature Range	UL / NEC	Supplier	Price	Diameter
Carol	C2536A.41.10	600V	1 / 2	16 AWG	-20°C to 75°C	UL 2106 NEC CL2, CM	Digi-Key	$474 / 1000	0.308"
Alpha Wire	3221 SL001	600V	1 / 2	22 AWG	-55°C to 105°C	-	Digi-Key	$189 / 100ft	0.181"
Alpha Wire	3231 SL001	600V	1 / 2	20 AWG	-55°C to 105°C	-	Digi-Key	$215 / 100ft	0.189"
Tyco / Raychem	44A1121-22-0/9-9	600V	1 / 2	22 AWG	-65°C to 105°C	-	Newark	$509 / 328ft	0.132"
Tyco / Raychem	44A1121-20-0/9-9	600V	1 / 2	20 AWG	-65°C to 105°C	-	Newark	$570 / 328ft	0.148"
Belden	9341 0101000	600V	1 / 2	18 AWG	-30°C to 90°C	UL 1277 NEC TC	Newark	$983 / 1000ft	0.276"
Belden	9841	300V	1 / 2	24 AWG	-30°C to 80°C	UL 2919 NEC CM	Allied	$393 / 500ft	0.232"
Alpha Wire	6010C SL005	300V	3 / 6	22 AWG	-20°C to 60°C	UL 2493	Digi-Key	$127 / 100ft	0.310"
Alpha Wire	6456 BK005	300V	3 / 6	22 AWG	??	-	Newark	$282 / 100ft	0.414"
Belden	9369-500-60	300V	3 / 6	18 AWG	-30°C to 105°C	UL 2493 NEC CM	Newark	$1028 / 500ft	0.298"
Belden	9553-1000-60	300V	3 / 6	18 AWG	105°C	UL NEC	Newark	$1813 / 1000ft	0.411"

Unshielded

The standard CT wires are unshielded, but have good noise rejection due to the twisting of the wires. Unfortunately, these wires are not available as a stock item and must be custom ordered.

CTS and CTT Wires

Supplier: Allied Wire
Web: http://www.awcwire.com
Phone: (800)828-9473
Part #: UL-1015 22GA BLK&WHT CUSTOM BULK WIRE - 1" TWIST.

CTB Wires

Supplier: Anixter
Web: http://www.anixter.com
Phone: (503)943-5108
Part #: 1015-22/7-9/0 Black+White 2/C Twisted .50" LH LA RoHS

Alternates

These unshielded cables should be more readily available.

Manufacturer	Part Num	Voltage Rating	Wire Pairs/ Conductors	AWG	Temperature Range	UL / NEC	Supplier	Price	Diameter
Belden	9486 0101000	600V	1 / 2	18 AWG	-30°C to 90°C	UL 1277 / NEC TC	Newark	$755 / 1000ft	0.275"

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 two #22 AWG wires. Do not use wire nuts (typically red or yellow sizes) used for electrical work; they will not make reliable connections with small #22 AWG 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.

AWG	Ohms / 1000ft	Cable Length for 0.1% Attenuation
16	4.016	2850
18	6.385	1800
20	10.15	1130
22	16.14	710
24	25.67	450

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 neglible. 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.

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