What a Burden Resistor Does
A current transformer produces a secondary current proportional to the primary current. Most electronic measurement circuits, however, read a voltage. The burden resistor converts that secondary current into a voltage the ADC can digitise, and its value simultaneously sets the measurement range, the phase error and the point at which the core begins to saturate. Choosing it is the most consequential decision in a CT-based sensing design.
This guide covers the calculation, the accuracy trade-offs, and the practical checklist for burden resistors in energy meters, motor drives, EV chargers and protection relays.
The Basic Relationship
For a CT with turns ratio 1:N, the secondary current is:
Isec = Ipri / N
and the voltage across a burden resistor Rb is:
Vb = Isec · Rb = (Ipri / N) · Rb
The resistor must dissipate P = Isec² · Rb continuously. Use at least a 2× margin on power rating, and prefer low-temperature-coefficient metal film or thin film parts (25 ppm/°C or better), because the burden value directly scales the measurement gain.
Example: 1:2000 CT at 20 A Primary
| Quantity | Value |
|---|---|
| Turns ratio | 1:2000 |
| Primary current (rated) | 20 A RMS |
| Secondary current | 10 mA RMS |
| ADC full-scale (differential) | 1.0 V peak |
| Required Vb at rated current | 0.9 V peak (headroom for 10 % overload) |
| Burden resistor | 0.9 / (0.010 × 1.414) ≈ 64 Ω |
| Dissipation at rated current | 6.4 mW → use 0.25 W, 0.1 % |
For differential-input ADCs, split the burden into two equal resistors (Rb/2 each, one on each secondary leg) and place a small differential RC filter across them. This keeps the common-mode voltage centred and suppresses switching noise without loading the CT asymmetrically.
The Accuracy Trade-off
The burden resistor is part of the CT’s error budget, and a larger burden is not always better:
- Too small: the voltage is buried in ADC noise and offset drift; resolution suffers, and the CT’s own magnetising current and core loss become a larger fraction of the signal.
- Too large: the core can saturate at high primary current, phase error grows rapidly, and the CT no longer behaves as a current source.
The CT datasheet usually quotes accuracy class at a specified burden (often 10 Ω, 100 Ω or a VA figure such as 2.5 VA). Exceeding that burden invalidates the stated class. For revenue metering, stay at or below the rated burden; for protection CTs, the rated burden defines the accuracy limit factor, and a lower burden extends the linear range.
Phase error matters most at low power factor. A 1° phase shift at PF 0.5 introduces roughly 1 % measurement error on active power. Precision metering CTs achieve 10–30 arc-minutes only when the burden is close to the calibration value.
Saturation, Knee Point and Burden
Every CT has a saturation curve: the secondary current tracks the primary up to a knee point, then flattens. The knee point voltage Vk is roughly:
Vk ≈ 4.44 · f · Nsec · Ae · Bsat
If the burden voltage would exceed Vk at the maximum primary current, the CT saturates and the reading under-reports. Two design responses are possible: reduce the burden resistor (sacrificing resolution), or specify a larger core with higher Vk. For fault-current and short-circuit measurement, an oversized burden is a common cause of seemingly “broken” CTs.
Design Checklist
| Item | Check |
|---|---|
| Burden value | Matches ADC full scale with 10–20 % headroom at rated current |
| Accuracy class | CT quoted at this burden, not at a much lower calibration burden |
| Power rating | ≥ 2× dissipation, and derate for 85 °C ambient |
| Temperature coefficient | ≤ 50 ppm/°C for class 1 metering; ≤ 25 ppm for class 0.5 |
| Saturation | Vb at 10–20× rated current still below the knee point |
| Filtering | RC low-pass with corner at 2–5 kHz for 50/60 Hz metering |
| Layout | Burden close to the CT, twisted pair, star ground at the ADC |
| Safety | Never open the secondary of an energised CT — use a shorting terminal block |
Buried Burden and PCB-Mount CTs
Miniature PCB-mount CTs for smart meters often integrate the burden resistor inside the housing. This is convenient but removes your ability to trim the gain. If your meter needs per-unit calibration, either select a CT without a built-in burden or stage the gain in the ADC’s programmable amplifier. For split-core installations, the burden usually lives on the measuring board and the connecting cable resistance must be added to the burden calculation — a 2 m cable with 0.5 Ω loop resistance is a meaningful error at 10 mA secondary current.
Selecting the Right CT
Shaanxi SiRui Technology produces PCB-mount, miniature and split-core current transformers with ratios from 1:1000 to 1:5000, accuracy classes 0.1 to 1, and specified burden values documented per lot. Our metering CTs are wound on high-permeability nanocrystalline or permalloy cores to keep phase error low at light load, which is where modern electricity meters spend most of their life.
Related reading: CT accuracy class: metering vs protection · CT saturation: causes and effects · Choosing a CT for energy metering
About Shaanxi SiRui Technology Co., Ltd.
Current transformers, magnetic cores, power inductors and high-frequency transformers for metering, EV charging and industrial drives.
Siri Yuan, Sales Manager · Email: sales@sirui-tech.com · Web: www.sirui-tech.com
No. 2555, Chanba Avenue, Chanba Ecological Zone, Xi’an, Shaanxi Province