Current transformer (CT) burden is the total impedance, expressed in ohms or volt-amperes (VA), connected across the secondary terminals of a current transformer. When this burden exceeds the CT's design rating, the core saturates, which fundamentally changes the circuit by introducing severe ratio errors and phase shifts that blind protective relays and corrupt energy metering. Hobbyists and junior technicians most commonly confuse total CT burden with the physical "burden resistor" soldered inside a voltage-output CT, or they mistakenly apply voltage-transformer logic (where high impedance is good) to current transformers (where low impedance is mandatory).
The Physics of CT Burden and Core Saturation
A current transformer operates on the principle of ampere-turn balance. The primary current creates a magnetic flux in the core, which induces a secondary current that flows through the connected burden to create an opposing flux. In an ideal world, the secondary current is a perfect scaled replica of the primary current. In reality, a small portion of the primary ampere-turns must be diverted to magnetize the core itself—this is the excitation current.
As the impedance of the secondary circuit (the burden) increases, the secondary voltage must also increase to push the rated current through that impedance (per Ohm's Law, V = I × Z). This higher voltage demands more magnetic flux in the core. If the burden is too high, the core reaches its magnetic saturation limit (the "knee-point voltage" defined in IEEE C57.13 and IEC 61869-2 standards).
Think of magnetic flux like a traffic jam on a single-lane bridge. The primary current is sending cars (magnetic flux) onto the bridge. The secondary current is supposed to send an equal number of cars off the bridge in the opposite direction to keep the bridge clear. If the burden is too high, the secondary cars can't move fast enough. The bridge (the core) jams up (saturates), and the primary current's energy has nowhere to go but into generating extreme heat and voltage spikes.
Worked Numeric Example: Calculating Maximum Allowable Burden
To ensure accurate metering and reliable relay tripping, the total installed burden must be strictly less than the CT's rated burden. Manufacturers rate CT burden in Volt-Amperes (VA) at a specific secondary current, usually 5A or 1A.
Let's calculate the maximum allowable impedance for a standard panel-mount CT with a 15 VA rating and a 5A secondary:
- Zmax = 15 VA / (5A)²
- Zmax = 15 / 25 = 0.60 Ohms
This 0.60 Ohm limit must accommodate the meter's internal impedance plus the resistance of the secondary wiring. The wire loop includes both the outgoing and return conductors. Here is the resistance for common copper wire sizes at 25°C:
| Wire Gauge (AWG) | Resistance per 1,000 ft (Ohms) | Max One-Way Run Length at 0.4Ω Loop Limit |
|---|---|---|
| 14 AWG | 2.525 Ω | ~79 feet |
| 12 AWG | 1.588 Ω | ~125 feet |
| 10 AWG | 0.999 Ω | ~200 feet |
The Scenario: You are installing a 5A secondary CT in a subpanel. The digital power meter (like an Accuenergy Modbus meter) has an internal burden of 0.05 Ω. You are using 14 AWG THHN wire, and the one-way distance from the CT to the meter is 50 feet.
- Wire Loop Length: 50 ft out + 50 ft return = 100 ft total.
- Wire Resistance: (100 ft / 1000 ft) × 2.525 Ω = 0.2525 Ω.
- Meter Resistance: 0.05 Ω.
- Total Installed Burden: 0.2525 + 0.05 = 0.3025 Ω.
Because 0.3025 Ω is well below the 0.60 Ω maximum limit, this CT will operate accurately without saturating. If you had used 18 AWG wire (6.385 Ω/1000ft) for a 100-foot loop, the wire alone would add 0.63 Ω, exceeding the limit and forcing the CT into saturation, resulting in the meter reading 20% to 40% lower than the actual current.
Where You Meet CT Burden in Practice
You will encounter burden limitations whenever you interface a current transformer with measurement or protection equipment. The physical manifestation of this theory dictates how you wire your projects and panels.
DIY Energy Monitors (IoTaWatt, Emporia Vue, OpenEnergyMonitor)
In the maker space, the YHDC SCT-013 series is ubiquitous. It is vital to understand the difference between the SCT-013-000 and the SCT-013-050. The -000 model is a pure current-output CT (100A primary yields 50mA secondary). It has no internal burden resistor; you must place an external burden resistor (typically 18Ω to 22Ω for a 3.3V ADC) across the secondary leads on your PCB. The total burden is strictly what you design onto the board. The -050 model, however, has an internal burden resistor that outputs 0-1V directly. If you try to wire the -050 model through long extension cables to an IoTaWatt, the added wire resistance alters the factory-calibrated internal burden, introducing phase and amplitude errors. Always keep voltage-output CT leads as short as possible.
Commercial Panel Metering and Submetering
When installing Modbus submeters in commercial switchgear, electricians often daisy-chain multiple meters on the same CT secondary to save money. Every meter added in series increases the total burden. If a 15 VA CT is shared by three meters (0.05 Ω each) and a transducer (0.1 Ω), plus 50 feet of 14 AWG wire (0.25 Ω), the total burden hits 0.50 Ω. You are now dangerously close to the 0.60 Ω saturation threshold. In these installations, upgrading to 12 AWG or 10 AWG secondary wire, or using auxiliary CTs (summation CTs) to isolate the burdens, is mandatory practice.
Protective Relaying
For fault protection, burden takes on life-or-death importance. During a short circuit, primary current can spike to 20 times the normal rating (e.g., 4000A on a 200A breaker). The secondary current attempts to reach 100A. Pushing 100A through even a 0.5 Ω burden requires 50V. If the CT's knee-point voltage is only 30V, the CT saturates instantly, the secondary current flatlines, and the protective relay never sees the fault. This is why protection-class CTs (like IEEE C-class) are physically massive—they have larger cores to delay saturation during fault conditions.
Current Transformer Burden FAQ
What happens if the burden on a current transformer is too high?
If the burden exceeds the CT's rated VA limit, the magnetic core saturates. In metering applications, this causes the secondary current to clip, meaning your energy meter will under-report consumption, sometimes by 30% or more during peak loads. In protective relaying, saturation prevents the relay from seeing the true magnitude of a short-circuit fault, delaying or entirely preventing the breaker from tripping, which can lead to catastrophic equipment fires or arc flashes.
How do I reduce the burden on an existing CT circuit?
You have three practical options to reduce total impedance. First, increase the wire gauge of the secondary leads; swapping 14 AWG for 10 AWG cuts wire resistance by roughly 60%. Second, shorten the physical distance between the CT and the metering device by relocating the meter or using a local junction box with a 4-20mA transducer. Third, if the CT is tapped (e.g., a 200/400/600A multi-ratio CT), using a lower primary tap ratio increases the secondary turns, which inherently increases the CT's VA capability and knee-point voltage, allowing it to drive a higher burden without saturating.
Does the internal burden resistor count toward total CT burden?
Yes. If you are using a voltage-output CT (like the SCT-013-050 or a panel meter with an internal shunt), the internal resistor is the primary component of the burden. The total circuit burden is the sum of the internal resistor's impedance plus the resistance of the secondary wires and any external connections. Adding long wires to a voltage-output CT changes the voltage divider ratio, which is why manufacturers specify a maximum lead length (often under 10 feet) for pre-burdened CTs.
Why do 1A secondary CTs have higher VA ratings than 5A CTs?
Because burden impedance limits are calculated as VA / I², a 1A secondary CT yields a much higher allowable ohmic burden for the same VA rating. For a 15 VA rating, a 5A CT allows 0.6 Ω of total impedance, while a 1A CT allows 15 Ω. This makes 1A secondary CTs ideal for long-distance runs in large substations or sprawling solar farms, where the wire resistance would easily saturate a 5A CT. The trade-off is that 1A CTs require more secondary turns of finer wire, making them slightly more expensive to manufacture and requiring meters specifically configured for 1A inputs.
