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A Historic Perspective on Transfer Equipment
Our society depends on electricity, which drives the purchase, installation, and use of generators and other electrical equipment that provides us with convenient alternate sources of power. Transfer equipment has long been a part of the NEC Article 700 – Emergency Systems and Article 701 – Legally Required Standby Systems.

The double throw switch is an example of transfer equipment that spans the history of connecting electrical systems to alternate power sources from present to back into the 1930’s. In 1981, the NEC Article 701 (Legally Required Standby Systems) clearly defined that transfer equipment must operate automatically when installed for legally required standby systems. The 1981 NEC also introduced Article 702 – Optional Standby Systems. Article 702 provides for the continued use of manual transfer equipment for:

"…industrial and commercial buildings, farms, and residences, and to serve loads such as heating and refrigeration systems, data processing and communications systems, and industrial processes that, when stopped during any power outage, could cause discomfort, serious interruption of the process, damage to the product or process…"

Transfer Equipment, Transfer Switch – Is There a Difference?
The 1999 NEC clearly distinguishes that transfer equipment includes transfer switches and automatic transfer switches; it does not limit transfer equipment to just automatic transfer switches.

701-7. Transfer Equipment

(a) Transfer equipment, including automatic transfer switches, …

702-6. Transfer Equipment

Transfer equipment shall be suitable for the intended use and designed and installed so as to prevent the inadvertent interconnection of normal and alternate sources of supply in any operation of the transfer equipment.

Section 702-6 addresses a fundamental safety concern about transfer equipment installations. The transfer equipment must keep two electrical sources from connecting to the load at the same time. If simultaneous connection of the two electrical sources to the load is ignored, the generator may backfeed the utility system and create a hazard for utility and other personnel.

Listing agencies evaluate transfer equipment using a number of different standards, such as UL 67, UL 98, or UL 1008. UL 1008 is a comprehensive standard that meets the NEC requirements in Articles 700, 701, and 702 and is commonly used to evaluate automatic transfer switches. Transfer equipment such as double throw switches are evaluated in accordance with UL 98 and panelboards with interlocked breakers are evaluated in accordance with UL 67 for compliance with NEC Article 702.

According to NEC Section 702-4, transfer equipment for optional standby systems must be "approved for the intended use." A number of other equipment arrangements may also qualify for application in accordance with Article 702 and not be specifically Listed as "Transfer Equipment." Examples include a substation tie or a keyed interlock arrangement.

Installation of an Article 702 System
Article 702 provides guidance for installation of Optional Standby Systems. The scope of Article 702 requires a permanently installed generator or "prime mover."

702-1. Scope

The provisions of this article apply to the installation and operation of optional standby systems. The systems covered by this article consist only of those that are permanently installed in their entirety, including prime movers. [italics added]

For the thousands of portable generator systems being installed, the scope of Article 702 technically leaves the installer and inspector without an enforceable article in the NEC. However, this is the only reasonable place for us to reference when evaluating the installed system for safety. Removing the permanently installed prime mover restriction in the scope of Article 702 would be a reasonable resolution for the 2002 NEC or adding a section on portable units.

Transfer equipment that is the service entrance equipment must be marked in accordance with NEC Section 230-66 to indicate its suitability for use as service equipment. However, transfer equipment in a residential application is often installed near the service entrance equipment. A second panel allows the separation of "necessary" loads, such as heating, from convenience loads.

Section 702-5 addresses capacity and ratings of optional standby system equipment.

702-5. Capacity and Rating

…Optional standby system equipment shall be suitable for the maximum available fault current at its terminals....

Make sure the rating of the device is adequate for the available fault current from the serving utility and the generator. This section also requires the switch to have suitable electrical ratings for switching a load between two sources of supply. Transfer Equipment listed in accordance with UL 67, UL 98, or UL 1008 will fulfill this requirement.

The available fault current from the generator must also be considered in order to comply with NEC Section 702-6.

702-6. Transfer Equipment

…Transfer equipment, located on the load side of branch-circuit protection, shall be permitted to contain supplementary overcurrent protection having an interrupting rating sufficient for the available fault current that the generator can deliver.

Section 702-6 is often misunderstood. Many have misread the permission in this section to allow the installation of supplementary protection as a substitute for branch circuit protection. Any supplementary overcurrent protection must be installed on the load side of a branch circuit overcurrent protection device. NEC Section 240-10 states:

…supplementary overcurrent protection…shall not be used as a substitute for branch-circuit overcurrent devices or in place of the branch-circuit protection specified in Article 210.

Figure 1. Common Code Violation of 240-10 and 702-6 using supplementary protection.

The transfer equipment cannot switch the branch circuit protection (listed breakers or fuses) out of the circuit as this will leave the branch circuit conductors unprotected and may even remove Ground Fault Circuit Interrupter protection from the circuit if GFCI breakers are employed in the service entrance panelboard.

Other Considerations for an NEC Compliant Installation
Transfer equipment installed in a residence is often found in the garage or in an unfinished basement. The installer may employ a convenience receptacle at the transfer equipment for portable lighting. NEC Section 210-8(a) requires receptacles located in garages and unfinished basements to be GFCI protected.

Finally, the installation of transfer equipment next to the service panel also creates a possibility that installers may place a single / common neutral in the raceway between the service panel and the transfer equipment. This is clearly a violation of NEC Section 300-3(b) as the neutral for each branch circuit "shall be contained within the same raceway."

Summary
Fundamental elements have been discussed to assist in a safe and NEC compliant installation of transfer equipment in accordance with NEC Article 702. The following list provides a summary of common items that can be overlooked when reviewing the installation of the transfer equipment:

1) The transfer equipment is listed by a recognized certification body (this would include products listed to UL 1008, UL 67, UL 98, etc., as transfer equipment).

2) The transfer equipment has a short circuit current rating suitable for the available short circuit current at the panel’s installation point in the system (this is especially critical for transfer equipment located next to the service equipment). NEC 702-5, 110-10.

3) Receptacles installed as part of the transfer equipment have appropriate GFCI protection if installed in an area requiring GFCI protection (unfinished basements, garages, outbuildings, outside, etc.). NEC 210-8

4) If supplemental protectors are employed, they are always located on the load side of a branch circuit overcurrent device... even when the transfer equipment is operating from generator power. NEC 702-6.

5) Raceway interconnections between the main panel and the transfer equipment include grounded conductors for each circuit being fed from the transfer equipment. NEC 300-3(b).

Alan Manche, a registered professional engineer in the state of Kentucky, is a sr. standards engineer for Square D Company. He represents Square D across the country by participating at IAEI meetings. He also represents NEMA on CMP 8 of the NEC and the NFPA 73 Technical Committee. Alan has held positions within Square D in product design and certification for panelboards, switches, metering, busway and wire management equipment.

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