Two important 2017 NEC changes have roots up north
BY JEFFREY SARGENT
A PAIR OF HIGHLY VISIBLE CHANGES in the 2017 National Electrical Code® (NEC®) were influenced by requirements in the Canadian Electrical Code (CEC). One of these changes affects how supply conductors to dwelling units in apartment buildings and other multifamily units are sized, and the other results in a change in the construction of panelboards.
Section 310.15(B)(7) of the NEC allows service and feeder conductors supplying dwelling units to carry more of the calculated load than in non-residential applications. For example, a 4 AWG copper conductor with 75 degree C insulation is an 85 ampere wire. Applying the permitted 0.83 factor bumps the conductor ampacity to 100 amperes.
However, this increase has been limited to use with single-phase, 120/240 volt, 3-wire systems since it was first included in the 1956 NEC. The impact of this limitation results in larger individual dwelling feeder conductors where the supply is a three-phase, 208Y/120 volt, 4-wire system, commonly used for multifamily construction.
The supply system characteristic is the physics-based reason for the ampacity allowance. In a three-wire service or feeder circuit supplied from a three-wire, single-phase system, the common neutral conductor carries only the imbalance of current from the two line conductors. This results in just two current-carrying conductors contributing heat within the raceway or cable. The same does not hold true with a three-wire dwelling feeder supplied from a three-phase, four-wire system. The 2017 NEC change permits the use of this ampacity increase for single- phase 120/240 volts and for three-phase 208Y/120 volts. Part of the substantiation used to support it was that the reduced sizes had been successfully used with these supply characteristics in Canada.
While some may contend that the reason this has worked in Canada is because of lower ambient temperatures, the real reason lies in the residential load profile, which is not significantly different from that on the U.S. side of the border. Experience has demonstrated that conductor sizes based on load calculation requirements have provided good service without excessive heating due to the diversity inherent to residential power usage. General lighting and appliance branch circuits in a dwelling are not treated as continuous loads. Cooking loads cycle and generally do not last for long periods of time. Rarely do the supply conductors carry the current of the full calculated load; in those infrequent instances where they do, it is only for brief periods.
The second change will initially impact manufacturers of panelboards, commonly referred to as breaker or fuse panels. They will have to comply with revised product standards (such as UL 67) that require the line terminals of this equipment to be isolated from accidental contact. The term used in the NEC requirement in 408.3(A) is “barrier,” and the type of barrier will be a function of the manufacturing standard. This requirement applies to panelboards used as service equipment because the line terminals typically do not have switches or other disconnecting means that can be used to de-energize the terminals.
Canada has long required that the line terminals and conductors of service panels be isolated from the open wiring space containing branch circuit and/or feeder conductors supplied from the service panelboard. This concept will now be employed within U.S. electrical equipment, making it safer for electricians and other maintenance personnel to work within these enclosures when following all of the necessary safe work practices specified in NFPA 70E, Electrical Safety in the Workplace.
It should be stressed that this isolation provides a degree of shock protection but is not intended to provide protection against arc-flash incidents. A risk assessment of the hazards associated with any justified energized tasks performed within a service panelboard is necessary to establish the requisite level of personal protective equipment.