A current transformer cable is the specialized secondary wiring that connects a current transformer (CT) to metering or protective relaying equipment, sized specifically to limit circuit burden and prevent dangerous high-voltage saturation if the circuit opens. When you install a CT on a 400A feeder, the primary current is massive, but the secondary side outputs a strictly proportional 5A or 1A. The cable carrying this secondary current is not just a passive conduit; its resistance directly dictates measurement accuracy, relay tripping times, and jobsite safety.

The Physics of CT Secondary Wiring: What It Changes in a Circuit

To understand why CT cable sizing matters, you have to recognize that a current transformer acts as a current source, not a voltage source. It forces a specific current (e.g., 5A) through the entire secondary loop, regardless of the resistance it encounters. The total impedance of this loop—including the relay coils, the meter shunts, and the current transformer cable itself—is called the burden.

What does the cable change in a real installation? It adds series resistance to the burden. If you use a cable that is too long or too thin (high AWG number), the resistance spikes. When the total burden exceeds the CT’s Volt-Ampere (VA) rating, the CT’s iron core saturates. During a fault condition, a saturated CT flattens the secondary current waveform. The protective relay sees a truncated signal, assumes the fault is smaller than it actually is, and fails to trip the breaker in time. In revenue metering, saturation simply means you are under-billing for power consumption.

CRITICAL SAFETY RULE: Never open-circuit a CT secondary while primary current is flowing. Because the CT tries to force current across an infinite resistance (air), the secondary voltage can spike to several thousand volts. This will instantly destroy the CT insulation, cause a lethal arc flash, and electrocute anyone touching the terminals. Always use shorting terminal blocks when disconnecting a meter.

Current Transformer Cable Sizing and Burden Limits

Sizing the cable is an exercise in budgeting resistance. The IEEE C57.13 standard dictates the accuracy classes and burden limits for instrument transformers. Most standard 5A secondary CTs are rated for 15 VA or 30 VA. We calculate the maximum allowable loop impedance using the formula: Z = VA / I².

Below is a reference table for copper cable resistance and maximum physical run lengths. Note that "physical distance" accounts for the out-and-back nature of a two-wire loop (total wire length is 2x the physical distance).

AWG Size Resistance (Ω/1000ft at 75°C) Max Physical Distance (ft) for 5A / 15VA CT Max Physical Distance (ft) for 1A / 15VA CT Typical Application
14 AWG 3.14 Ω 79 ft 1,990 ft Short panel runs, tight MCCs
12 AWG 1.98 Ω 126 ft 3,156 ft Standard switchgear, revenue metering
10 AWG 1.24 Ω 201 ft 5,040 ft Long runs to remote relay panels
8 AWG 0.78 Ω 320 ft 8,012 ft High-accuracy revenue metering

Worked Numeric Example: Sizing a 12 AWG Run

Let’s say you are wiring a 400:5A CT with a 15 VA rating to a protective relay located in a separate room.

  • Max Total Burden (Z): 15 VA / (5A)² = 0.6 Ω
  • Relay Internal Burden: 0.1 Ω (from the relay datasheet)
  • Remaining Budget for Cable: 0.6 Ω - 0.1 Ω = 0.5 Ω

If you choose 12 AWG copper (rated in the 75°C column per NEC Article 310), the resistance is 1.98 Ω per 1,000 ft.
Total allowable wire length = (0.5 Ω / 1.98 Ω) × 1000 = 252.5 ft.
Because the current must travel to the relay and return, the maximum physical distance between the CT and the relay is 252.5 / 2 = 126.25 feet. If your conduit run is 140 feet, 12 AWG will saturate the CT during a fault. You must step up to 10 AWG.

Pro Tip: If your physical run exceeds the limits for 5A secondaries, specify a 1A secondary CT instead. Because burden losses scale with the square of the current (I²R), a 1A system reduces cable heating and voltage drop by a factor of 25, allowing you to run 12 AWG wire for thousands of feet without saturating the core.

Where You Meet Current Transformer Cables in Practice

You will encounter CT secondary wiring in any facility that monitors or protects high-current AC circuits. Common installations include:

  • Switchgear and Motor Control Centers (MCCs): Here, CTs feed digital multifunction relays (like the Schweitzer SEL-751 or ABB REF615). The cable is typically 12 AWG or 10 AWG THHN pulled in dedicated conduits to prevent inductive coupling from the primary busbars.
  • Solar Inverter Combiner Boxes: Utility-scale solar uses window-type CTs to monitor string currents. Because the runs back to the data acquisition system (DAQ) can be hundreds of feet, installers almost exclusively use 1A secondary CTs with 12 AWG shielded twisted-pair cable (like Belden 8770) to reject electromagnetic interference (EMI).
  • Revenue Metering Cabinets: For utility billing, accuracy is paramount. Installers use 10 AWG or 8 AWG wire to keep the burden as close to zero as possible, ensuring the meter captures every watt-hour.
  • Variable Frequency Drives (VFDs): Input and output CTs monitor motor current. Here, the cable must be heavily shielded because the VFD’s PWM switching noise can induce massive common-mode voltages in unshielded secondary leads.

For most commercial and industrial applications, standard 600V-rated THHN/THWN-2 in a dedicated metallic conduit is acceptable. However, in high-noise environments (near VFDs or arc furnaces), you must use a shielded, twisted-pair instrument cable with the shield grounded at one end only to prevent ground loops.

Common Confusions and Critical Mistakes

Even experienced technicians make mistakes when dealing with instrument transformers. Here is what people commonly confuse with CT wiring, and how to avoid the pitfalls.

1. Confusing CT Wiring with PT (Potential Transformer) Wiring

A Potential Transformer (PT or VT) steps down voltage (e.g., 4160V to 120V). It acts as a voltage source. PT secondary wiring can be small gauge (14 or 16 AWG) because the metering device has high impedance and draws almost no current. Furthermore, an open circuit on a PT is perfectly safe. A CT is the exact opposite. It requires low resistance (thick wire) and an open circuit is lethal. Never use PT wiring practices on a CT loop.

2. Confusing the Primary Conductor with the CT Cable

When using a "donut" or window-type CT, the massive primary cable (e.g., 500 kcmil) passes directly through the center of the CT. Beginners sometimes refer to this primary feeder as the "CT cable." In electrical engineering, the CT cable strictly refers to the secondary leads (the 12 AWG wires connected to the CT's X1 and X2 terminals) that carry the stepped-down signal to the panel.

3. Forgetting the Shorting Block

If you need to swap out an energy meter, you cannot just pull the wires off the back of the meter while the load is running. Doing so opens the CT secondary. Every professional CT installation must include a shorting terminal block (such as those made by Weidmüller or Phoenix Contact) wired in series before the meter. Before removing the meter, you slide the shorting links into place, bypassing the meter and keeping the CT secondary safely closed. For more on proper termination and safety practices, refer to manufacturer guidelines like the ABB Instrument Transformer manuals.

Getting the current transformer cable right is a balancing act between physical distance, wire gauge, and the VA rating of your specific CT. Calculate your burden, respect the 75°C ampacity and resistance columns, and never, ever leave a secondary lead floating.