The 5 amp CT voltage is the secondary potential difference generated across the connected burden impedance when a current transformer outputs its rated 5-amp secondary current. When you clamp a standard industrial metering CT around a 400A feeder, the primary current induces a proportional secondary current (up to 5A), and the voltage you measure across the meter's terminals is entirely dictated by Ohm's Law acting on the secondary loop's total impedance. Understanding this voltage is critical because it dictates your wire sizing, meter selection, and whether your CT will saturate under heavy load.

The Physics of 5 Amp CT Voltage and Burden

A current transformer is fundamentally a constant-current source on its secondary side. It does not output a fixed voltage; it outputs a fixed current ratio (e.g., 400:5). The voltage only appears as a byproduct of pushing that current through the connected load, known in the industry as the burden.

Core Formula: Vsecondary = Isecondary × Zburden

VA Burden: VA = Isecondary² × Zburden

Think of the CT as a pump pushing a constant flow of water through a pipe; the narrower the pipe (higher burden resistance), the higher the pressure (voltage) required to maintain the 5A flow. If the required voltage exceeds the CT's magnetic core capacity, the core saturates, the current waveform clips, and your meter reads inaccurately low.

This dynamic changes how you approach a real installation. You cannot simply run any gauge of wire from the CT to the meter. The wire itself has resistance, which adds to the meter's internal impedance. If the total loop resistance pushes the required 5 amp CT voltage beyond the CT's rated VA (Volt-Amp) capacity, the system fails to meet standard instrument transformer accuracy classes defined by IEEE C57.13.

Worked Numeric Example: Calculating Secondary Voltage

Let's look at a standard jobsite installation to see how these numbers play out on the bench.

  • CT Rating: 400:5A, Metering Class, 15 VA Burden Rating
  • Meter Impedance: 0.1 Ω (internal shunt)
  • Wire Run: 20 feet of 12 AWG copper (THHN) from CT to meter and back (40 feet total loop length)

Step 1: Calculate Wire Resistance
12 AWG copper has a resistance of approximately 1.588 Ω per 1,000 feet at 20°C.
Loop resistance = (40 ft / 1000) × 1.588 Ω = 0.0635 Ω

Step 2: Calculate Total Burden (Z)
Ztotal = Zmeter + Zwire
Ztotal = 0.1 Ω + 0.0635 Ω = 0.1635 Ω

Step 3: Calculate 5 Amp CT Voltage at Full Load
At maximum primary current (400A), the secondary pushes exactly 5A.
Vsecondary = 5A × 0.1635 Ω = 0.8175 Volts

Step 4: Verify VA Rating
VA = 5² × 0.1635 = 25 × 0.1635 = 4.08 VA

Verdict: The 4.08 VA burden is well below the CT's 15 VA rating. The CT will not saturate, and the 0.81V signal is safely within the analog input limits of standard digital power meters.

Where You Meet This in Practice (And Common Confusions)

You will encounter 5A secondary CTs primarily in commercial switchgear, industrial motor control centers (MCCs), and heavy-duty solar inverter combiner boxes. They are the standard for protection relays and high-end revenue metering.

However, there is a massive point of confusion among hobbyists and junior technicians transitioning from low-voltage DC to AC mains monitoring:

The "5 Volt" Confusion

Many makers confuse a 5A secondary CT with a 5V voltage output. Standard industrial 5A CTs do not output 5 volts. They output up to 5 amps. If you connect a standard 5A CT directly to an Arduino or ESP32 analog pin expecting a 0-5V signal, you will likely destroy the microcontroller or get a reading of zero (if the pin's internal impedance is high enough to limit current). To interface a standard 5A CT with a microcontroller, you must pass the secondary current through a precision external burden resistor (e.g., a 1 Ω, 50W resistor) to generate a measurable voltage, and then use a voltage divider or op-amp to shift it into the 0-3.3V ADC range.

Conversely, hobbyist sensors like the YHDC SCT-013-030 are voltage-output CTs. They have an internal burden resistor and output 0-1V AC. They are entirely different animals from the 5A secondary metering CTs used in commercial panels.

Real-World Scenario Walkthrough: The Long-Wire Saturation Trap

Here is a classic failure mode that happens when installers ignore the physics of 5 amp CT voltage.

The Setup:
An installer is monitoring a 200A HVAC compressor using a standard 200:5A CT rated for 15 VA. The digital power monitor is located in a control room 100 feet away from the panel. To save money, the installer runs 18 AWG stranded control wire to the meter.

The Numbers:
18 AWG wire has a resistance of 6.385 Ω per 1,000 feet. The total loop length is 200 feet (100 ft out, 100 ft back).
Wire Resistance = (200 / 1000) × 6.385 = 1.277 Ω
The meter's internal shunt is 0.2 Ω.
Total Burden Z = 1.277 + 0.2 = 1.477 Ω.

When the compressor kicks on and pulls its full 200A, the CT secondary attempts to push 5A.
Required 5 amp CT voltage = 5A × 1.477 Ω = 7.385 Volts.
Required VA = 25 × 1.477 = 36.9 VA.

The Outcome:
The CT is only rated for 15 VA. The magnetic core completely saturates trying to push 5A through 1.477 Ω. The secondary current waveform flattens at the peaks. The power monitor reads 135A instead of the actual 200A. The facility manager gets a false sense of security, and the breaker sizing calculations based on that meter data are now flawed.

What Went Wrong & The Fix:
The installer treated the CT secondary like a standard 4-20mA or voltage signal loop, forgetting that 5A is a massive amount of current to push over thin, long wires.
The Fix: Upgrade the wire to 10 AWG (dropping wire resistance to ~0.2 Ω, keeping total VA under 10), OR swap the CT for a 200:1A secondary CT (which drops the VA burden by a factor of 25, allowing long runs of 18 AWG wire without saturation), OR install a local 5A-to-4-20mA transducer at the panel and run the 4-20mA signal to the control room.

FAQ: Critical Safety and Wiring Questions

Can I leave a 5A CT secondary open-circuited while the primary is energized?

Absolutely not. If the secondary circuit is open, the burden impedance becomes infinite. The CT will attempt to generate thousands of volts to push 5A across the air gap. This will result in lethal voltages at the terminal block, catastrophic insulation breakdown, and likely an explosive failure of the CT core. Always use a shorting block or a shorting switch when disconnecting a meter from an energized 5A CT.

Do I need an external burden resistor if my digital meter already has one?

No. Standard digital power meters designed for 5A inputs have internal shunts (burden resistors) precisely calibrated to their ADC ranges. Adding an external resistor in series will increase the total burden (risking saturation) and alter the current-to-voltage ratio, causing the meter to read low. Only add external burden resistors when interfacing raw CTs to oscilloscopes, microcontrollers, or high-impedance voltage inputs.

Why do protection relays use 5A CTs instead of 1A CTs?

Historically, 5A secondaries were preferred in electromechanical relays because the higher current provided more torque to physically move the relay armature and trip the breaker. Today, with solid-state and microprocessor relays, 1A secondaries are increasingly common for long wire runs because they drastically reduce the 5 amp CT voltage burden and allow for smaller wire gauges, as detailed in IEEE C57.13 standard guidelines.