Current Transformer Ratio and VA Burden Calculator
Select the next standard current transformer ratio from line current and calculate total secondary VA burden including copper lead losses.
Formula used
R_loop = 2 × L × 0.0175 / S · P_wire = I_sn² × R_loop · S_total = S_meter + P_wire
Current transformer secondary cable loss and total VA burden table
Calculated for copper conductors (ρ = 0.0175 Ω·mm²/m, outgoing + return 2 × L) with a 1.5 VA meter/analyzer burden. Notice that a 1 A secondary reduces cable VA loss to 1/25 of a 5 A secondary.
| One-way distance | 5 A · 2.5 mm² cable loss (VA) | 5 A · 2.5 mm² total burden (VA) | 5 A · 4 mm² cable loss (VA) | 1 A · 2.5 mm² cable loss (VA) |
|---|---|---|---|---|
| 2 m | 0.70 | 2.20 | 0.44 | 0.03 |
| 5 m | 1.75 | 3.25 | 1.09 | 0.07 |
| 8 m | 2.80 | 4.30 | 1.75 | 0.11 |
| 10 m | 3.50 | 5.00 | 2.19 | 0.14 |
| 15 m | 5.25 | 6.75 | 3.28 | 0.21 |
| 20 m | 7.00 | 8.50 | 4.38 | 0.28 |
| 30 m | 10.50 | 12.00 | 6.56 | 0.42 |
Example: 220 A line, 5 A secondary, 8 m of 2.5 mm² copper wire, 1.5 VA analyzer
- Next standard CT ratio: 250/5 A.
- Loop resistance (2 × 8 m): R = 0.1120 Ω; 5 A cable loss: P = 5² × R = 2.80 VA.
- Total burden: 1.5 + 2.80 = 4.30 VA → next standard CT rating: 5 VA.
How is current transformer (CT) VA burden calculated?
Total secondary burden is the sum of the connected instrument VA consumption and the secondary wire loss: S_total = S_device + I_s² × (2 × L × 0.0175 / S). The selected CT rated output (e.g. 5 VA, 10 VA, 15 VA) must be at least equal to S_total.
How is CT secondary wire VA loss calculated?
Multiply the round-trip copper loop resistance (R_loop = 2 × L × 0.0175 / S) by the square of the rated secondary current (5² = 25 or 1² = 1) and add the instrument VA burden.
When should a 1 A secondary CT be chosen over 5 A?
5 A CTs are standard inside a panel; when the distance between the CT and the analyzer or meter is long, a 1 A secondary cuts copper lead VA loss by a factor of 25.