When to Supplement

A new definition covering supplementary overcurrent-protective devices has been added to Article 100 in the 2005 National Electrical Code (NEC) to indicate that limited protection may be provided for specific applications or utilization equipment. The use of supplementary devices in the NEC is expanding, so a thorough understanding of what constitutes a supplementary overcurrent-protective device and where it can be used is important.

As detailed in this new definition, a supplementary overcurrent-protective device is “intended to provide limited overcurrent protection for specific applications and utilization equipment, such as luminaires (lighting fixtures) and appliances. This limited protection is in addition to the protection provided in the required branch circuit by the branch circuit overcurrent-protective device.”

Section 240.10 provides some additional general requirements relating to these supplementary devices and should be read and understood before applying the devices in the electrical circuit. It should be noted, however, where supplementary protection is used for luminaires, appliances and other electrical equipment, it must not be used as a substitute for, or in place of, any required branch-circuit overcurrent devices.

For example, where luminaires are installed on poles in a parking lot and are listed for installation on a 20-ampere branch circuit, it is not permissible to install a 50-ampere branch circuit to the poles and use a 20-ampere in-line fuse within the pole to supply the luminaires. It would be permissible to install a 20-ampere branch circuit from pole to pole with supplementary overcurrent-protective devices installed within each pole with each supplementary fuse sized to carry the current of the luminaire.

Where a fault did occur within the pole or the luminaire, the supplementary overcurrent-protective device would operate, thus ensuring the loss of the lighting from only one pole light, rather than losing all the pole lighting on the entire branch circuit.

Supplementary overcurrent-protective devices can be used for internal circuits within equipment and to provide protection for components within the equipment but again must not be a substitute for, or in place of, any required branch circuit overcurrent devices.

For example, internal heating elements within large fixed electric space heating appliances must be subdivided to limit heating element loads to no more than 48 amperes, as covered in Section 424.22(B). These loads are required to be protected by supplementary overcurrent-protective devices that are factory-installed within or on the heater. The heater manufacturer could also supply the supplementary devices for use with the heater as a separate assembly. These particular devices, while supplementary in function, must also be suitable for branch-circuit protection, unlike many other supplemental protective devices.

The last sentence in Section 240.10 clearly permits supplementary overcurrent devices to be installed in an accessible location but not in a readily accessible location. The term “accessible” in Article 100 as it applies to electrical equipment permits close approach to the device but must not be guarded by locked doors, elevation or other effective means.

The term “readily accessible” is defined as “capable of being reached quickly for operation, renewal, or inspections without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders, and so forth.”

In both the lighting poles and the fixed electric space heaters, any supplementary overcurrent-protective devices are not required to be readily accessible.

Supplementary overcurrent-protective devices are often used to protect flexible cords and cables, as covered in Article 400, and fixture wire, as covered in Article 402. Section 240.5(A) requires flexible cords and cables to be protected by an overcurrent device in accordance with the ampacity of the cord or cable as specified in Tables 400.5(A) or (B), often using supplementary devices, since many of these cords or cables do not correspond to a standard size overcurrent device. Section 240.5(B) also permits flexible cords, cables and fixture wire to be protected by a branch circuit overcurrent-protective device.

A major change in the 2005 NEC occurs within Section 240.5(B)(1) for supply cords of listed appliances or portable lamps. In the 2002 and previous editions of the NEC, this section has permitted No. 18 AWG cords to be protected at 20 amperes, No. 16 AWG at 30 amperes, 40 and 50 amperes for cords of 20-ampere capacity and above.

In the 2005 NEC, this permission has been removed and replaced with text stating that the supply cords for listed appliances or portable lamps “shall be considered to be protected when applied within the appliance or portable lamp listing requirements.” This change effectively requires the sizing to be done by the manufacturer, the listing agency and the listing standards.

Check the equipment and the listing of the equipment, as well as the NEC, for indications of the requirements for supplementary overcurrent-protective devices. EC



ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at mark.c.ode@us.ul.com


About the Author

Mark C. Ode

Fire/Life Safety Columnist and Code Contributor
Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com .

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