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NEC Guidelines for Automatic Transfer Switches (ATS) and Emergency Power Systems

Understanding NEC Transfer Equipment and Automatic Transfer Switches (ATS)

NEC Transfer Equipment: Automatic Transfer Switches (ATS) & Emergency Power Systems

Automatic Transfer Switches (ATS) play a crucial role in emergency and standby power generation systems. Typically used in systems rated up to 1000 volts, these switches are vital for switching between power sources in the event of an outage. Here's a detailed look at the key features and NEC guidelines for ATS:

(A) General Requirements for Transfer Equipment

Transfer equipment, including automatic transfer switches, must be:

  • Automatic
  • Identified for emergency use
  • Approved by the authority having jurisdiction

Transfer equipment should be designed and installed to prevent inadvertent interconnection of normal and emergency sources. If the system allows parallel operation, it must comply with Article 705.

(B) Bypass Isolation Switches

Bypass isolation switches may be used to bypass and isolate the transfer equipment, ensuring that inadvertent parallel operation is avoided.

(C) Automatic Transfer Switches

Automatic transfer switches (ATS) must be electrically operated and mechanically held. They should also be listed for emergency system use to ensure compliance with safety regulations.

(D) Use of Transfer Equipment

Transfer equipment should supply only emergency loads. While the alternate power source can supply other loads, the emergency system transfer switch is limited to emergency loads only.

Note: Legally required standby loads (Article 701) and optional standby loads (Article 702) require separate transfer switches.

(E) Documentation of Short-Circuit Current Rating

The short-circuit current rating of the transfer equipment must be marked on the exterior of the device. This rating varies based on the type and settings of the overcurrent protection used. Field markings help ensure compliance with the safety standards.

Example: A transfer switch might be marked with multiple short-circuit current ratings, depending on the protection settings used in the installation.

Closer Look: Time-Delay Devices on ATS

The normal power source is typically a service, while the emergency power source is an automatically started engine generator set. Time-delay devices help avoid unnecessary switching and ensure smooth power transfer during outages. The time-delay is typically set at 1 second but can be adjusted based on the specifics of the utility system.

Time-Delay Settings

  • 1 second delay: Typically for quick response to brief power outages.
  • Longer delays: Used when utility reclosers or circuit breakers take more time to reset.
  • 15-minute delay: Ensures the engine generator operates under load for at least 15 minutes for reliability.
  • Cool-down period: Often set to 5 minutes to ensure proper engine performance after a load transfer.

Important: If multiple ATS are connected to the same generator, sequencing the load transfer can reduce the generator's starting capacity requirements.

Understanding these guidelines and time-delay mechanisms ensures the reliability and safety of your emergency power systems, making them compliant with NEC standards and operationally efficient.

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