To determine how long can we run a cable for a current transformer, we need to identify these key factors.

1. 1. CT rated secondary current
2. 2. Burden rating (CT Burden)
3. 3. Cable resistance
4. 4. Cable inductance
5. 5. Number of connected devices
6. 6. Allowable accuracy error

Once we have all the above, we can now calculate the maximum cable length for the current transformer (CT).

First of all, we need to compute for the total permissible resistance of the CT secondary circuit using this formula:

To apply this, lets consider the example below.

• CT rating: 200 A / 5 A (secondary rated current = 5 A)
• CT burden rating: 10 VA
• Cable: Copper, 2.5 mm²

Step 1: Find the total permissible resistance, using the formula above.

So, the total secondary loop resistance is 0.4 Ω. We will use this later to calculate the maximum cable length for the CT. But for now, we move on the step 2.

Step 2: Look for cable resistance per meter. Based on the table below as per AS/NZS 3008, the typical copper resistance per meter for 2.5 mm² at 20 °C is about 0.00741 Ω/m.

Since the CT secondary is a loop, we need to multiply it by 2. Therefore, the effective resistance is

Step 3: Finally, we can calculate the maximum cable length using this formula:

We can now use the computed secondary loop resistance before which is 0.4 Ω.

So, what does these 27 meters mean? It means that the maximum one-way cable run is 27m before the CT accuracy starts to degrade.

More examples:

1. CT rated secondary current: 1 A
2. Burden rating (CT Burden): 15 VA
3. Cable resistance: copper, 4 mm²
4. Cable inductance: negligible (assume 0 Ω for simplicity, typical for metering CTs)
5. Number of connected devices: 1 meter with 0.1 Ω resistance
6. Allowable accuracy error: CT accuracy class 0.5 (metering)

Step 1: Find total permissible resistance

Total secondary loop resistance must not exceed 15 Ω.

Step 2: Subtract resistance of connected devices

Step 3: Calculate maximum cable length

Based on the table below as per AS/NZS 3008, the typical copper resistance per meter for 4 mm² at 20 °C is about 0.00461 Ω/m.

Since the cable has loop resistance (go and return), we multiply the one-way resistance by 2:

Calculating maximum cable length

Note: This is very long because we used a 1 A CT with a relatively high burden (15 VA) and a thick copper cable. In practice, CT cable runs are much shorter, and we usually apply a safety factor.

Step 4: Check CT accuracy

The loop resistance (including devices) is 0.1 Ω + cable. As long as total burden ≤ 15 VA, the CT will maintain its 0.5 % accuracy.