I had the privilege of being the guest speaker at a recent contractors meeting, and to my surprise, I was asked an unusual question by a well-known local contractor during the question-and-answer period. He wanted to know how the National Electrical Code (NEC) defines and addresses the difference between branch circuit and supplementary protection devices when used in an electrical installation. I had to be careful because both the contractor and inspector were looking at me and wondering how I was going to answer.

Pertinent definitions

I began by defining a branch circuit and supplementary protection device as listed in Article 100 of the 2008 NEC.

Revised Article 100 defines a branch circuit as a device capable of providing protection for service, feeder and branch circuits and equipment over a full range of overcurrents between its rated current and its interrupting rating. Branch circuit overcurrent protective devices are provided with interrupting ratings appropriate for the intended use but no less than 5,000 amperes.

Article 100 defines supplementary protection as a device intended to provide limited overcurrent protection for specific applications and utilization equipment, such as luminaires and appliances. This limited protection is in addition to the protection provided in the required branch circuit by the branch circuit overcurrent protective device.

I began answering the question by referring the contractor to review Section 424.3(B) of the 2008 NEC, which states that fixed electrical space--heating equipment shall be considered a continuous load. Article 100 defines a continuous load as a load that operates for three hours or more.

Contractor’s interpretation

The contractor designed and had his electrician run what he called a feeder circuit to a heating unit and terminated the conductors to a panelboard, enclosing three, two-pole circuit breakers that protected a 25-kilowatt electric heating unit with a 1.5-amp fan motor. The main body of his question was the conductors from the service panelboard to the panelboard located next to the heating unit.Were they considered a branch circuit or a feeder?

He said he calculated the size of the conductors based on 220.51 because he considered this section to outline the requirements of the NEC. In other words, he sized the conductors at 100 percent of the total load of the heating unit, based on the language in Section 220.51.

Inspector’s interpretation

The inspector had rejected the installation by his interpretation that these conductors were classified as a branch circuit and not a feeder and that the protective devices in the panelboard installed at the heating unit were considered supplementary. He said conductors and devices must be designed and sized at least 125 percent of the heating unit load.

Author’s interpretation of installation

As referenced in Section 424.3(B), the contractor must calculate a continuous load in accordance with 210.19(A)(1) and 210.20(A) and size the conductors and overcurrent protection devices as outlined in those sections.

Section 210.19(A)(1) clearly reads that branch circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch circuit size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous loads.

Note that Section 210.20(A) requires the overcurrent protection device to be brought along at 125 percent of the continuous load plus the noncontinuous load, same as the conductors. NEC Section 210.20(A) reads, “Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.”

For the final nail in the coffin, Section 430.24, Ex. 2, requires the ampacity of the conductors supplying motor-operated fixed electric space-heating equipment to comply with Section 424.3(B).

Therefore, the conductors routed from the service equipment to the panelboard are considered a branch circuit and not a feeder and must be sized as such.

If sized appropriately and overload protection is provided, overcurrent protection devices installed in the panel-board enclosure must be classified as supplementary devices and used solely to protect the heating elements and fan motor from short-circuits and ground-fault conditions.

I informed the contractor that the inspector’s interpretation was valid because Sections 210.19(A)(1), 210.20(A), 220.51, 424.3(B), and 430.24, Ex. 2, do not permit the conductors supplying the sub fed panelboard (housing the three, two-pole circuits) to be calculated at 100 percent of the heating unit load.

STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.