CT Ratio, Secondary Current & Burden Calculator
Primary ↔ secondary current by CT ratio, and total burden (VA, Ω) from round-trip leads and the meter against the CT rating. For 1 A and 5 A secondaries.
Never open the secondary
Opening a CT secondary while current flows forces all the primary current through the exciting branch and produces very high voltage peaks at the secondary terminals. A voltmeter may barely show them, but they can break down insulation and are dangerous to people; ABB's guide says an open secondary should be treated as a serious accident and, where larger CTs may be opened, a protective circuit should be permanently connected to the secondary terminals (p.26). The same guide notes that the secondary circuit is grounded to keep the voltage at meters safe (p.5).
Formulas
- CT ratio = rated primary ÷ rated secondary (e.g. 200/5 → 40)
- Secondary = primary ÷ ratio, primary = secondary × ratio
- Burden VA = I² × R, meter Ω = meter VA ÷ (rated secondary)²
- Round-trip lead = ρ × (2 × one-way length) ÷ area, copper ρ20 = 1/58 Ω·mm²/m
- RCF = actual ratio ÷ marked ratio; RCF > 1 means the meter reads low → reading × RCF
Lead burden by size and distance (copper, 20 °C)
| Size | One way | Round-trip Ω | VA at 5 A | VA at 1 A |
|---|---|---|---|---|
| 1.5 mm² | 5 m | 0.115 | 2.87 | 0.115 |
| 1.5 mm² | 10 m | 0.230 | 5.75 | 0.230 |
| 1.5 mm² | 20 m | 0.460 | 11.49 | 0.460 |
| 1.5 mm² | 50 m | 1.149 | 28.74 | 1.149 |
| 2.5 mm² | 5 m | 0.069 | 1.72 | 0.069 |
| 2.5 mm² | 10 m | 0.138 | 3.45 | 0.138 |
| 2.5 mm² | 20 m | 0.276 | 6.90 | 0.276 |
| 2.5 mm² | 50 m | 0.690 | 17.24 | 0.690 |
| 4 mm² | 5 m | 0.043 | 1.08 | 0.043 |
| 4 mm² | 10 m | 0.086 | 2.16 | 0.086 |
| 4 mm² | 20 m | 0.172 | 4.31 | 0.172 |
| 4 mm² | 50 m | 0.431 | 10.78 | 0.431 |
On the same wire a 1 A secondary has 1/25 of the burden (I²R) of a 5 A one. The longer the run, the more it matters.
Standard burdens (IEEE C57.13, 5 A secondary) — ABB guide p.13
| Designation | Impedance Ω | VA at 5 A | Power factor |
|---|---|---|---|
| B-0.1 | 0.1 | 2.5 | 0.9 |
| B-0.2 | 0.2 | 5.0 | 0.9 |
| B-0.5 | 0.5 | 12.5 | 0.9 |
| B-0.9 | 0.9 | 22.5 | 0.9 |
| B-1.8 | 1.8 | 45.0 | 0.9 |
The metering burdens from the same guide (IEEE, 60 Hz). For a CT not rated 5 A, multiply the ohms by (5 ÷ rated current)²; the VA at rated current stays the same — 2.5 VA on a 1 A CT is 2.5 Ω. IEC rated burdens and accuracy-class error limits are not shown: we have not checked the IEC 61869-2 text.
Where it is used
- Setting a power meter's CT ratio: enter the rated primary and secondary (e.g. 200/5) and the meter shows the real current. In the LS ELECTRIC GIMAC1000 and Rootech Accura device maps, measurements already include the PT/CT ratio, so do not multiply values read over Modbus again.
- Choosing a CT: see what percentage of the rated primary your normal load is («Of rated primary» above). An oversized CT gives a small secondary current at the bottom of the meter's range.
- Long secondary runs easily push lead burden past the CT's rating. Use thicker wire, put the meter closer, or use a 1 A secondary CT.
FAQ
What does a 200/5 CT ratio mean?
200 A in the primary gives 5 A in the secondary; the ratio is 40. 150 A primary gives 3.75 A secondary.
How do I calculate CT burden?
Add every resistance in the secondary circuit and multiply by the square of the rated secondary current (VA = I² × R). Convert the meter's input burden (VA) to ohms, and count the leads both ways. 2.5 mm² copper, 10 m one way, is about 0.138 Ω round trip: about 3.45 VA at 5 A.
How do I check CT ratio error?
Divide the actual ratio (primary and secondary measured together) by the marked ratio: that is the ratio correction factor (RCF). As in ABB's example, 20.2 : 1 actual against 20 : 1 marked is RCF 1.01, a 1 % ratio error, and the meter reads 1 % low.
1 A or 5 A secondary?
ABB's guide gives typical CT outputs of 1–5 A (p.4). On the same wire a 1 A secondary has 1/25 of the lead burden of 5 A, which helps on long runs. Check which input your meter takes first.
Guide on CT ratio settings, burden and open secondaries: CT ratio, secondary current and burden basics. For kVA and power factor from a meter's current and power, use the Three-Phase Power Calculator. For wire resistance and voltage drop by size, use the Voltage Drop & Wire Size Calculator.
Basis
CT ratio, RCF, standard burdens and the open-secondary warning follow ABB «Instrument Transformers — Technical Information and Application Guide» (1VAP420003-TG, December 2004 Rev. A) pp.4, 5, 10, 13, 26. VA = I²R is a definition; copper resistivity follows IEC 60028 (1/58 Ω·mm²/m, 0.00393 /°C) as in the voltage drop calculator. Meter input burdens come from each meter's datasheet.