Transfer
Equipment for
Installation in
Accordance with
NEC Article 702 –
A Historic
Perspective on
Transfer Equipment
Our society depends on
electricity, which drives the
purchase, installation, and use of
Optional Standby Systems
generators and other electrical
equipment that provides us with
convenient alternate sources of
power. Transfer equipment has
by Alan Manche
Square D Company
long been a part of the NEC
Article 700 – Emergency Systems
and Article 701 – Legally Required
Standby Systems.
84 November/December IAEI NEWS
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.
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.
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
IAEI NEWS 1999 November/December 85
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.
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 equip-
ment. 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
Fig. 1.Common Code Violation of 240-10 and 702-6 using Supplementary Protection
86 November/December IAEI NEWS
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.