By now, you already know that CT burden is the total load connected to the secondary of a current transformer. This includes meters, relays, and the resistance of the connecting wires.

But here’s the key point: CTs are only accurate when they operate within their rated burden. If the burden is too high, you get errors and early saturation. If it’s incorrectly estimated, you can even affect the instrument safety factor and risk damaging equipment.

So calculating CT burden properly isn’t optional—it’s essential.

What makes up the total CT burden?

When calculating burden, you need to account for everything connected to the CT secondary.

This includes the internal burden of measuring devices like ammeters, energy meters, or protection relays. Each of these devices has a burden rating, usually expressed in volt-amperes (VA).

But that’s not the whole story.

The cables connecting the CT to these devices also contribute to the burden. Even though they might seem insignificant, cable resistance can add up, especially over long distances.

So the total burden is the sum of all connected device burdens plus the burden introduced by the cables.

How do you calculate burden from connected devices?

Let’s start with the easy part.

Most devices connected to a CT will have a specified burden in VA. For example, a meter might have a burden of 2.5 VA, and a relay might add another 1.5 VA.

If you have multiple devices connected, you simply add their burdens together.

So if your meter is 2.5 VA and your relay is 1.5 VA, the total device burden is 4 VA.

This gives you a solid starting point before considering the cables.

How do you calculate cable burden?

Cable burden is often where people make mistakes, so let’s walk through it carefully.

The burden caused by the cable depends on its resistance and the current flowing through it. Since CT secondaries typically operate at 5 A or 1 A, this current is used in the calculation.

The power loss in the cable, which contributes to burden, is given by:

𝑉𝐴 = 𝐼²𝑅

This means the burden in volt-amperes is equal to the square of the current multiplied by the resistance of the cable.

To calculate resistance, you need the length of the cable and its resistance per unit length. Don’t forget that the current travels through both the outgoing and return conductors, so the total length is doubled.

For example, if you have a 20-meter run, the total length used in the calculation is 40 meters.

Once you calculate the total resistance, you plug it into the formula along with the secondary current to find the cable burden.

Let’s go through a simple example

Imagine you have the following setup:

A CT with a 5 A secondary

A meter with a burden of 2.5 VA

A relay with a burden of 1.5 VA

A cable run of 30 meters (one way)

Cable resistance of 0.02 ohms per meter

First, calculate the total cable length. Since it’s a loop, you double it.

So 30 meters becomes 60 meters.

Next, calculate the total resistance of the cable.

60 meters multiplied by 0.02 ohms per meter gives 1.2 ohms.

Now calculate the cable burden using the formula.

With a 5 A secondary, you square the current and multiply by resistance.

So 5 squared is 25, and 25 multiplied by 1.2 gives 30 VA.

That means the cable alone contributes 30 VA of burden.

Now add the device burden.

2.5 VA plus 1.5 VA gives 4 VA.

Finally, add everything together.

30 VA plus 4 VA gives a total burden of 34 VA.

What does this result tell you?

This result tells you that your CT must be capable of handling at least 34 VA of burden while maintaining its accuracy.

If you selected a CT rated for only 15 VA, it would struggle to drive this load. You would likely see increased errors and possible saturation under higher currents.

In this case, you would need to select a CT with a higher rated burden, such as 30 VA or even higher to provide a safety margin.

But remember, as you learned earlier, you shouldn’t overestimate too much either. The goal is to match the actual burden as closely as possible.

What happens if you ignore cable burden?

Ignoring cable burden is one of the most common mistakes in CT applications.

In small panels with very short cable runs, the impact might be minimal. But in larger installations, cable burden can easily exceed the burden of the connected devices.

If you ignore it, you might select a CT that appears suitable on paper but performs poorly in reality.

This can lead to inaccurate measurements, unexpected saturation, and even issues with protection systems.

So always include cable calculations, especially for longer runs.

How does secondary current rating affect burden?

CTs typically have either a 5 A or 1 A secondary rating, and this choice has a big impact on burden.

Since burden is proportional to the square of the current, a 5 A system produces much higher burden than a 1 A system for the same cable resistance.

This means that for long cable runs, using a 1 A CT can significantly reduce the burden.

In modern installations, especially where distances are large, 1 A secondaries are often preferred for this reason.

Should you add a safety margin?

Yes—but carefully.

It’s good practice to include a small safety margin in your calculations to account for uncertainties, such as slight variations in cable resistance or additional devices added later.

However, you should avoid excessive overestimation.

As you’ve already seen in your earlier articles, oversizing the CT burden rating can affect saturation characteristics and increase the effective instrument safety factor.

So aim for a realistic margin, not an exaggerated one.

How does burden tie into everything else?

By now, you can see that burden is not just a standalone calculation.

It directly affects CT accuracy, saturation behaviour, and instrument safety factor. If the burden is too high, accuracy suffers and saturation occurs earlier. If it’s incorrectly estimated, ISF can change and compromise protection.

This is why burden calculation is such a critical step in CT selection.

It connects all the concepts you’ve been building throughout your series into something practical and measurable.

Calculating CT burden is all about understanding the real load on the CT secondary.

You need to include both the connected devices and the cables. Use the correct formulas, account for total loop length, and always base your calculations on the actual secondary current.

When done properly, this ensures that your CT operates within its limits, maintains accuracy, and behaves correctly during fault conditions.

And once you get this right, everything else—accuracy class, saturation, and ISF—starts to fall into place.