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NEC Requirement for Permanent Manual Switching Means

 
The National Electrical Code® (NEC®) requires that backup power be available for life-safety loads when backup power equipment is taken offline for service. This article summarizes the scope of the requirement.

Background Information

Because a backup power system is vital to the operations it serves, it is important to maintain, test, and repair critical power equipment so that it will provide reliable service whenever needed. However, the service activities for engine-generators and other power devices can require that technicians take them off-line to complete the work. So, what happens if an outage of the normal source occurs while the equipment is offline?

For transfer switches, a solution has been available for decades. Installing an bypass-isolation transfer switch enables facilities to route backup power around a transfer switch mechanism so that it can be taken offline without impacting power availability to downstream loads. Part 1 and Part 2 of the ASCO Power Technologies document entitled Application and Design Factors for Automatic Transfer and Bypass-Isolation Transfer Switches provides more information.

But what if a facility needs to service an engine-generator?

Some facilities operate two or more generators. Where there is sufficient generation capacity to carry the backup load when one unit is out of service, the facility can still operate without disruption. However, for facilities where taking a genset offline impacts the power availability for life safety loads, the 2017 and 2020 Editions of the NEC are clear – the facility needs to backup its backup power source.

The NEC Requirement

A requirement to provide “backup-for-the-backup” first appeared in the 2017 Edition of the NEC under Article 700.3 - Temporary Source of Power for Maintenance or Repair of the Alternate Source of Power. It states:

If the emergency system relies on a single alternate source of power, which will be disabled for maintenance or repair, the emergency system shall include permanent switching means to connect a portable or temporary alternate source of power …. The permanent switching means to connect a portable or temporary alternate source of power shall comply with the following:

(1) Connection to the portable or temporary alternate source of power shall not require modification of the permanent system wiring.


Notwithstanding the clarity of this requirement, other NEC provisions also bear on this situation. For instance , Article 700.4 in the 2017 NEC requires that “…emergency systems shall have adequate capacity and rating for all loads to be operated simultaneously.” Under Article 700.4(B), the 2020 Edition adds that systems “shall have adequate capacity in accordance with Article 220,” which addresses circuit capacity calculations. These requirements indicate that supplemental power sources should be provided as necessary to meet capacity requirements for emergency power.

A Practical Approach to Compliance

Installing a Manual Transfer Switch (MTS) and a connection panel between a permanent genset and the downstream ATS is a practical solution. This enables the connection and use of a temporary or portable generator to power the emergency load. A permanent MTS and quick-connect panel enable rapid connection of a temporary generator whenever needed.

Notably, Article 700.3(F) requires that connection of a temporary generator be made without having to rewire any portion of the backup power system. Installing the aforementioned connection panel complies with this requirement. A connection panel can be installed separately from an MTS, an arrangement that enables flexibility in locating equipment, shown below. Manual transfer switches with integrated connection panels are also available, an arrangement that simplifies installation.
 
This video further illustrates some ASCO Power Technologies solutions for complying with NEC Article 700.3(F).

Summary

Maintenance and repair activities can make generators unavailable during service events. To provide continuous backup power availability, NEC Article 700.3(F) requires facilities with a single generator to install an MTS and a means for connecting a temporary power source. Connection panels can be integrated into an MTS or installed in a separate location. For additional information, contact an ASCO Power Technologies representative.