How Long You Can Run a Cable for a CT? Explained!

A current transformer is one that, in contrast to the considerably larger current flowing in the primary winding, creates a smaller, proportionate current in its secondary winding. The primary side is attached to the primary conductor transmitting the high current. Usually 5A or 1A, the reduced current on the secondary side can be securely used by relays, meters, and protective devices. In other words, instead of subjecting sensitive electronics to hundreds or thousands of amps of current, the CT reduces it to a safe, measurable level.

CT Types

There are two main types of CTs:

Ring-type CT

Shaped like a ring (circular, square, or rectangular). The primary conductor simply passes through the opening.

Wound primary CT

Has actual primary windings wrapped around the core, often used when lower current ratios are required.

CT Ratio

The CT ratio tells us the relationship between the primary current and the secondary current. Example: A 100/5 CT means 100A in the primary equals 5A in the secondary. The ratio is fixed – a 100/5 CT won’t act like a 20/1 CT just because the numbers look similar.

Accuracy Class

Every CT has an accuracy class that defines how close the secondary current is to the exact proportion of the primary. Lower class number = higher accuracy. For example:

  • – Class 0.2 → Precision metering
  • – Class 0.5 → Commercial meters
  • – Class 3 or 5 → General industrial use

Accuracy also includes the phase angle error, which is important for energy meters (kWh, power factor, etc.).

A CT is designed to step down high current levels into something much smaller and manageable, typically 1 A or 5 A on the secondary side. That small current is then fed into a meter, relay, or protection device. The CT itself has been calibrated to operate within certain parameters, and one of those parameters is the burden. Burden is simply the total load on the CT secondary, expressed in volt-amperes (VA). It includes the resistance of the CT winding, the resistance of the connecting cables, and the resistance of the connected device.

Now, the moment we start extending the cable length, the resistance of those leads increases. More resistance means more burden on the CT. If that burden exceeds the CT’s rated capacity, the CT will no longer maintain accuracy. This is why cable length is such a critical factor—too long, and your readings may become unreliable.

Measuring CTs vs Protection CTs

Measuring CTs

Designed for accuracy up to about 120–125% of rated current. Often saturate beyond this to protect meters from overload. Used with instruments like ammeters, energy meters, and recorders.

Protection CTs

Built to stay accurate even at very high fault currents (many times the rated current). Defined by Accuracy Limit Factor (ALF). Example: A CT with ALF 10 will work correctly up to 10 × rated current. Used with relays and protective devices.

Safety Note: Never Open-Circuit a CT

If the CT secondary is left open while the primary is carrying current, the secondary can develop dangerously high voltages. This can damage insulation or even cause electric shock. Always keep the secondary connected to a load or short-circuited when not in use.

Practical Points to Remember

CT specifications should include: Ratio, Burden (VA), Accuracy Class, Dimensions. Devices on the secondary are always connected in series (unlike voltage transformers). For long cable runs, check cable resistance – it adds to the CT burden.

CT Burden and Its Role in Cable Runs

Burden is the limiting factor for how long you can run a cable for a CT. Every CT is rated with a maximum allowable burden, which could be 2.5 VA, 5 VA, 10 VA, or sometimes even higher. This figure tells us how much total load the CT can handle on its secondary side without compromising its accuracy class.

The total burden is calculated by taking into account:

  • – The internal resistance of the CT itself.
  • – The resistance of the connecting leads.
  • – The input resistance of the meter, relay, or protection device.

The formula for burden in terms of the connecting cable is straightforward:

Burden (VA) = I² × R

Where:

  • – I is the secondary current (usually 1 A or 5 A).
  • – R is the resistance of the entire loop (both wires in the pair).

As you can see, the longer the cable, the higher the resistance. If you double the length, you double the resistance, and that directly affects the burden.

The burden of a CT is the total load the transformer has to drive on its secondary side. Think of it as the resistance the CT has to “push against” when supplying the current signal.

Burden is measured in volt-amperes (VA) and includes:

  • – The meter’s input burden (the load the meter presents to the CT).
  • – The resistance of the cable between the CT and the meter.
  • – Any additional losses at terminations.

If the total burden exceeds what the CT is designed for, its accuracy drops. That means your measurement system might no longer meet class accuracy requirements, which can cause big headaches — especially in billing, compliance, or performance monitoring systems.

1A and 5A Output CTs

The traditional CTs most electricians are familiar with are rated to provide either a 1A or 5A secondary output. These have been industry standards for decades.

For these CTs, there’s a simple but often overlooked rule:

The total burden of the connected system must not exceed the CT’s rated burden.

Here’s the formula:

CT Rated Burden ≥ VA of Meter + VA of Cable + VA of Terminations

In many cases, the termination losses are very small and can be ignored. But the meter and the cable always matter.

A Practical Example

Let’s run through an example to show how this works in real life.

Suppose you’re installing a 5A CT that’s going to connect to a Cube 400 meter. You decide to use 0.5mm² wire, with a 2-metre run from the CT to the meter.

Here are the numbers:

  • – The Cube 400 meter has a burden of 0.1 VA.
  • – The cable burden works out to about 3.67 VA.

So the total burden is:

0.1 VA + 3.67 VA = 3.77 VA

That means your CT needs to be rated for at least 4 VA to ensure it maintains accuracy. If you used a CT rated for only 2.5 VA, for example, the extra load would push it outside its accuracy class, and your readings would drift.

What If the CT Can’t Handle the Burden?

Sometimes you do the maths and realise the CT you’ve chosen simply can’t supply the required burden. What are your options then?

  • – Use a larger cable size. A thicker cable has lower resistance, which reduces the VA loss. This is usually the first thing to try.
  • – Shorten the cable run. If possible, move the meter closer to the CT. This isn’t always an option, especially in retrofit jobs, but it’s the most direct way to cut burden.
  • – A CT with many primary turns and a higher VA rating should be used. Increasing the secondary current while preserving accuracy can be accomplished by repeatedly looping the primary conductor through the CT. For every additional spin, the measured current doubles. Nevertheless, this can make installations more difficult and could not work well in confined areas.

In practice, the first two options are the most common. Retrofitting usually limits your choices, so planning ahead during design is always better.

0.333V Output CTs

In recent years, we’ve also seen the rise of low-voltage CTs that output 0.333V signals instead of 1A or 5A currents. These are common in modern energy meters, particularly with equipment from manufacturers like SolarEdge and Fronius.

These CTs work differently. Instead of pushing current into the secondary, they generate a very small voltage signal. The upside is that this makes them safer and easier to connect. The downside is that they are much more sensitive to electrical noise and losses.

Because of this, switchboard manufacturers often specify maximum cable lengths for these CTs. The most common limit is 10 metres. In fact, many 0.333V CTs come with pre-attached cables, which is the manufacturer’s way of saying: “Don’t extend these.”

If you absolutely must extend, you should:

  • – Use shielded cable.
  • – Keep the run as short as possible.
  • – Avoid running alongside noisy circuits like switch-mode power supplies, VSDs, or inverter cables.

So, How Long Can You Run a CT Cable?

So, how long can you actually go? There isn’t a one-size-fits-all answer, but there are typical ranges we see in practice.

For low-voltage CTs with 5 A outputs, you’re often limited to relatively short runs unless you use very thick cables. Runs of up to 10 or 15 metres are usually fine with 2.5 mm² cable. Beyond that, you may need to step up to 4 mm² or 6 mm² to keep the burden under control.

For 1 A CTs, you can generally run cables further. Because the current is lower, the I²R losses are much smaller. This is why many metering installations in Australia prefer 1 A CTs when the meter is located some distance away. With 1 A CTs, you might manage runs of 50 metres or even more, depending on cable size and burden rating.

Voltage-output CTs (such as 0.333 V types) are a different story altogether. These are very sensitive to voltage drop and electromagnetic interference. For that reason, manufacturers usually recommend keeping the leads under 10 metres and using shielded, twisted-pair cable. If you need to go further, you should consider a current-output CT instead.

Noise and Interference Considerations

Another factor we have to keep in mind is electrical noise. Long cable runs act like antennas, picking up interference from nearby equipment, switching devices, and even other cables. This is especially problematic for low-voltage CT outputs, but even 1 A and 5 A CTs can be affected if the environment is electrically noisy.

The best way to minimise this is to use twisted-pair cables and, if possible, shielded cables. Keeping CT cabling separate from power cables also helps reduce the risk of induced noise.

What Happens if You Exceed the Maximum Length?

If your CT cable is too long and the burden too high, the CT won’t operate accurately. Instead of delivering a faithful scaled-down version of the primary current, the CT will saturate. That means the secondary current won’t increase proportionally with the primary current anymore. For metering applications, this leads to inaccurate readings, and for protection applications, it can mean delayed or failed tripping, which is a serious safety concern.

It’s also worth remembering that a CT should never be left open-circuited. A long cable with high resistance starts to behave like an open circuit to the CT. This can create dangerously high voltages on the secondary side, posing both a safety risk and the possibility of damaging connected devices.

Designing for Longer Runs

If your installation requires running CT cables over long distances, there are several approaches you can take. One is to use 1 A secondary CTs instead of 5 A. The lower secondary current means lower I²R losses, which allows longer cable runs without exceeding the burden.

Another option is to place the meter or protection relay closer to the CTs, reducing the length of the secondary leads. In modern installations, we often see distributed metering equipment installed near the CTs, with data transmitted digitally over Ethernet or fibre back to a central point. This completely removes the limitation of analogue CT cabling.

Finally, you can choose CTs with higher burden ratings. Some CTs are rated at 10 VA, 15 VA, or more, which gives you more room to work with before accuracy is compromised. This is especially useful in industrial switchboards where cable runs can’t always be kept short.

Worked Example: Calculating Maximum Distance

Let’s say we have a 1000/5 A CT with a 5 VA burden rating. We want to use 2.5 mm² copper wire, which has a resistance of 7.41 Ω/km.

The maximum resistance allowable is:

VA / I² = 5 / 25 = 0.2 Ω

So our total loop resistance can’t exceed 0.2 Ω. Since the cable has a resistance of 7.41 Ω per km, that gives us:

Length = 0.2 / (7.41 × 2) × 1000 = 13.5 metres

That’s the maximum one-way length, meaning we can only run about 13.5 metres before the burden is exceeded.

If we instead use a 1 A CT, the calculation changes:

Maximum resistance = 5 / 1² = 5 Ω

Length = 5 / (7.41 × 2) × 1000 = 337 metres

That’s a huge difference and shows why 1 A CTs are often the better choice for long-distance runs.

Here’s the simple answer you can keep in your back pocket:

For 1A or 5A CTs, there’s no fixed maximum length. It all depends on the CT’s rated burden, the cable size, and the meter burden. In theory, you can run dozens of metres if you upsize the cable, but in practice, it’s always best to keep the run short and the cable thick.

For 0.333V CTs, stick to 10 metres or less, unless the manufacturer specifies otherwise. Always use shielded cable and avoid noisy environments.

Cable Resistance and Wire Gauge

The resistance of a cable is determined by its length and cross-sectional area. Thinner wires have higher resistance, while thicker wires can carry current with lower resistance. For example, a 2.5 mm² copper conductor has a resistance of around 7.41 Ω per kilometre. That means if you run 50 metres out and 50 metres back (since the CT loop is two conductors), you’ve got 100 metres of total length, which equates to about 0.74 Ω.

If you’re dealing with a 5 A secondary CT, then:

VA = I² × R = 5² × 0.74 = 18.5 VA

That’s way too high for a CT rated at 5 VA. So in this case, you’d be exceeding the CT’s burden and accuracy would be lost.

This example shows why it’s not simply about physical distance—it’s about cable size, current rating, and the CT’s burden capacity.

What is CT Impedance?

Every CT also has an internal impedance, which is listed on the datasheet. Impedance is simply the opposition to current flow inside the CT’s secondary winding.

For example, in an Accuenergy AcuCT-4161R CT, the impedance values are:

CT Rating (A)    Impedance (Ω)    Equivalent Burden (VA at 5A)
600A             4.16 Ω           104 VA
1200A            2.08 Ω           52 VA
1600A            1.56 Ω           39 VA
2000A            1.25 Ω           31 VA
3000A            0.83 Ω           21 VA
4000A            0.625 Ω          16 VA
5000A            0.5 Ω            12.5 VA

How to Calculate CT Burden Using Impedance

If you know a CT’s impedance, you can easily find its burden using this formula:

CT Burden (VA) = (Output Current²) × (Impedance)

Example:

  • – CT: 600/5A
  • – Impedance: 4.16Ω

Calculation:
(5²) × (4.16) = 104 VA

This tells you the CT itself can handle up to 104 VA of burden.

Make an informed choice when selecting CTs because larger CTs typically have higher burden ratings and lower impedance.

In Australia, CTs are widely used in switchboards, metering enclosures, and protection systems. While international guidance often refers to North American practices, the principles are universal. The key is always to check the burden rating of the CT and match it against your cable length and size. In many Australian switchboard designs, CTs are placed right next to meters to avoid long secondary runs altogether. But in larger installations like commercial complexes, shopping centres, or substations, it’s not unusual for CTs to be tens of metres away from the measuring equipment. In those cases, selecting the right CT type and cable gauge is crucial.

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