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The Accu-CT® ACTL-1250 (and ACT-1250) family of current transformers (CTs) have maximum amperage numbers as well as a rated amps associated with each model. This article explains these numbers.

Rated Amps

Also called “rated primary current”, this is the nominal full-scale current rating for the CT. At rated amps, the CT will output its nominal output of 0.333 Vac or 1.00 Vac (with Option 1V). The accuracy of CTs is generally specified at percentages of the full scale rated current. For industry standard CTs, the range is typically from 10% to 120% of rated amps, but for Accu-CTs the range is from 1% to 120% of rated amps. The standard ACTL-1250 models have a  0.333 Vac output, so there is no rated secondary current. The ACTL-1250 family is available with a current output: contact sales for details. Note: the ACTL-1250 and ACT-1250 families look identical but the ACTL-1250 is a UL 2808 Listed device

Maximum Continuous Amps

The maximum continuous amps (called “Max Amps” on the CT label and the model tables) is the maximum continuous current a particular CT model can handle without suffering any degradation of accuracy or damage. Running the CT for an extended period of time (more than 10 seconds) at currents higher than the max amps may overheat internal components (not the winding), causing degraded accuracy or failure.

  • The Max Amps are different for different models and different options, so be sure to check the Max Amps value for the CT you are using.
  • There are no standard accuracy specifications for operation above 120% of rated amps. Generally, the CT magnetic core material will start to saturate before reaching Max Amps, so the accuracy may be poor at Max Amps.

Maximum Amps for All Models with 0.333 Vac Output

Model Rated Amps Max Amps
ACTL-1250-250 250 720
ACTL-1250-400 400 720
ACTL-1250-600 600 720

Maximum Amps for All Models with 1.0 Vac Output

Model Rated Amps Max Amps
ACTL-1250-250 Opt 1V 250 600
ACTL-1250-400 Opt 1V 400 600
ACTL-1250-600 Opt 1V 600 720

Maximum Continuous Amps – Safety Limit

All the ACTL-1250 family of current transformers have been approved for use by UL at maximum continuous currents of 720 amps. The UL test ensures that the windings will not overheat at this current and that the dielectric withstand capability is preserved. In other words, operating an ACTL-1250 CT up to 720 amps continuously will not pose a safety hazard. Exceeding 720 amps (the “Maximum Continuous Amps – Safety Limit”) for more than 10 seconds can cause overheating of internal components and of the secondary winding, potentially leading to an unsafe condition!

Maximum Fault Amps

In the IEEE C57.13 standard, this is referred to as “Short-time mechanical current rating and short-time thermal current rating”. The ACT-1250 family has not yet been tested for these limits.

Rating Factor

Described by IEEE C57.13 as “continuous thermal current rating factor (RF)”. The rating factor indicates a multiple of rated amps. For example, an RF of 1.2 means the CT meets the continuous thermal current rating factor requirements at 1.2 times (120%) the rated primary current. IEEE C57.13 has the following requirements for the rating factor:

  • The CT must meet the Transformer Correction Factor (TCF ) accuracy requirements for 100% of rated current at the RF current. For example, if the RF is 1.2, then the CT must meet the 100% accuracy requirements at 100% and 120% of rated current.
  • At the RF current, the winding temperature rise must not exceed limits established in the standard.

The ACT-1250 series of CTs have an RF of 1.2. Many of the models can achieve higher rating factors if needed, but our standard production test only includes 120% of rated current. Also, the WattNode meter’s input circuitry may saturate with CT signals larger than 120% of nominal. However, if your application needs a higher rating factor, contact CCS for more information.

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