Jim Dollard has an extensive background in codes and standards. Send questions about the National Electrical Code (NEC) to Jim at email@example.com. Answers are based on the 2020 NEC.
When installing rigid metal conduit (RMC) underground, the NEC requires use of corrosion protection approved for the condition. How do I comply? What is approved corrosion protection?
“Approved” is defined in Article 100 as acceptable to the authority having jurisdiction (AHJ). More detail is provided in 344.10(B)(1), which permits RMC to be installed in concrete, direct contact with the earth or in areas subject to severe corrosive influences where corrosion protection is approved for the condition. Typically, an AHJ will rely on local experience and the area or venue involved. For example, areas near landfills and refineries generally contain corrosive soils. Additionally, a soil-resistivity test can be performed. Additional corrosion protection will not always be required. Where it is, it can be a factory-applied coating, field-applied paints or tape wraps approved for the purpose. These paints or wraps contain bitumen, an asphalt-like material.
When do requirements apply?
In my state, we are still working under the 2017 NEC. Did the requirements for fuses and arc-energy reduction become effective in 2020, or do we wait until our state adopts the 2020 NEC?
In all states and municipalities where the 2017 edition of the NEC has been adopted, Section 240.67 now applies. This requirement in the 2017 Code included a last sentence in the parent text that states it becomes effective Jan. 1, 2020. This delayed implementation was necessary to allow the electrical industry to develop products to meet the new requirement. Where fuses rated 1,200A or higher are installed, they must have a clearing time of 0.07 seconds or less at the available arcing current, or one of the arc-energy-reduction methods provided must be installed.
Standard size per 240.6
Section 240.4(B) permits us to round up to the next higher standard overcurrent device rating under given conditions. So, a 600 kcmil copper conductor rated at 420A could be protected at 500A. Where an adjustable electronic-trip, molded-case circuit breaker is applied, can we protect it at 450A? We have been kicking this question around for some time, but still get different answers from electrical inspectors. Some say it is OK; others tell us to protect at 500A. In some cases, the adjustable settings don’t allow an exact 500A setting.
Several NEC requirements apply here. The permissive requirements of 240.4(B) apply only where the overcurrent protective device (OCPD) is rated at 800A or less. In your question, an adjustable electronic-trip, molded-case circuit breaker is being applied. The general rule (240.6(B)) for determining the rating of an adjustable-trip circuit breaker with an external means for adjusting the long time pickup setting, is that the rating of the circuit breaker is equal to the maximum setting possible. However, where a circuit breaker has restricted access , the requirements of 240.6(C) allow the amp rating of the circuit breaker to be equal to the long-time, pick-up setting.
As you stated, Section 240.4(B) permits the next higher standard OCPD rating above the ampacity of the conductors being protected, provided all of the requirements are met. It does not mandate that protection be equal to a standard amp rating as seen in 240.6; it simply permits protection as high as the next standard size OCPD.
In summary, 240.4(B) would permit a conductor rated at 420A to be protected at 500A, provided all of the requirements are met. Protecting the conductor at a lower value (a nonstandard OCPD size) than the next higher standard size is not specifically addressed but would be permitted.
This is a good example of where a public input could increase the clarity and usability of the NEC . For example, the parent text of 240.4(B) could be modified as follows: “Overcurrent protection not exceeding the next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met.” Get involved and submit public inputs for the 2023 NEC !
For information on the submission process, read the Code Insider column on page 60.
Service conductor protection
Where is the requirement for OCPDs to be capable of clearing available fault current for services? We have to field-mark the available fault current. The requirement in Article 230 is simply for overload protection. Are we protecting the utility-owned conductors?
The requirement for OCPDs to have an interrupting rating equal to or in excess of the available fault current is in Section 110.9. This is a general rule for all equipment intended to interrupt current at fault levels and is not limited to service OCPDs. The requirement in 110.24 is to field-label service equipment with the available fault current and the date that the fault-current calculation was performed.
The field-labeled value of available fault current on service equipment must be less than the interrupting rating of the service OCPD. This field marking enables the inspector to verify that the requirements of 110.9 are met. Additionally, if the available fault current is increased due to the utility installing a larger transformer, the field label documents that the installation was compliant when installed.
The NEC applies only to premises wiring, i.e., everything downstream of the service point. NEC requirements are not intended to protect utility-owned conductors. Service conductors are required to have overload protection, in accordance with 230.90. The service conductors will terminate in an OCPD, which can provide overload protection for the service conductors, but cannot provide short-circuit or ground-fault protection.
Freezers on an emergency circuits
An owner wants us to supply multiple freezers in a commercial kitchen with emergency circuits. Apparently, they lost all their frozen food during a weather-related event a few years ago. Is this permitted?
No. Part IV of Article 700 contains requirements for emergency system circuits for lighting/power. Section 700.15 clearly prohibits appliances, other than those required for emergency use, to be supplied by emergency circuits. An appliance would be permitted to be supplied by an emergency circuit where it is considered to be essential to life safety. For example, a patient-care related appliance, could be supplied by an emergency circuit. The freezers can easily be protected by an optional standby system.
Labeling emergency disconnects
How do we label emergency disconnects required for one- and two-family dwelling unit services? This is all new to us and each inspector wants the labels to look different and to be in different locations.
Section 230.85 provides three options to meet the requirement for all service conductors to terminate in an emergency disconnect. The required marking must be in accordance with the option chosen by the installer. For example, in one option, the installer may place the “Emergency Disconnect” marking on the outside of service equipment and place the “Service Disconnect” marking on the inside of the enclosure next to the service disconnecting means. All of the markings must comply with 110.21(B). This requires permanently affixed markings (not handwritten) that will withstand the environment, using effective words, colors, symbols or any combination. An informational note sends the Code user to ANSI Z535.4-2011, “ Product Safety Signs and Labels ,” which provides guidelines for font sizes, words, colors, symbols and location requirements. The National Electrical Manufacturers Association (NEMA) has developed a free “ Emergency Disconnect Marking Guide” (NEMA EDM P1-2019). It provides suggested markings and locations for installers and inspectors to comply with 230.85.