Jim Dollard has an extensive background in codes and standards. If you have a query about the National Electrical Code (NEC) , Jim will help you solve it. Send questions to firstname.lastname@example.org. Answers are based on the 2020 NEC .
Is that definition correct?
Recently, I was part of a discussion on the NEC definition of voltage to ground. In my opinion an ungrounded system always has a voltage to ground that may fluctuate based on circuit conditions. The definition currently states, for ungrounded circuits, the voltage to ground is the greatest voltage between the given conductor and any other conductor of the circuit. A voltage detector will prove this is not true. What do you think?
The NEC definition provides clarity in multiple Code requirements. It is not based on values that an individual may get when using a test instrument to measure voltage to ground. For a detailed answer, we will discuss how this definition affects the proper application of overcurrent protective devices (OCPD).
When an OCPD voltage rating is misapplied, it still functions and an installer may think all is well. When an improperly voltage-rated OCPD attempts to interrupt an overcurrent, it may explode or—as Code-making panel members commonly say, it will “rapidly disassemble.” Straight-rated and slash-rated voltage ratings exist for OCPDs. Fuses are all straight-voltage-rated. Circuit breakers, however, are slash-rated or straight-rated.
An example is a 208Y/120 volt (V) slash-voltage rating. This means the device is intended to open more than one pole in a fault at line voltage (208V), and the “voltage to ground” rating (120V) is the maximum voltage for opening a single pole in a fault. Slash-rated circuit breakers (most common) are not intended to open phase-to-phase voltage faults across one pole. Where an ungrounded system exists, there is a possibility for a full phase-to-phase voltage fault to occur across only one pole of a circuit breaker. This is why straight-rated circuit breakers are required in an ungrounded system.
Consider a three-phase, 3-wire, 480V ungrounded feeder at 400 amperes (A), installed in EMT. If a fork lift driver hits the EMT and creates a fault from the EMT to A phase, nothing happens, but we have a grounded system. If B or C phase go to ground, there is a full voltage fault at 480V across a single pole of a circuit breaker or a single fuse and a straight-rated device is required to open the fault.
In a 2020 NEC changes class for CEUs, the chief inspector said, while the state would adopt the 2020 NEC, electrical inspectors would not be enforcing the installation of dwelling-unit emergency disconnects on service upgrades. I have concerns about installing an outdoor disconnect on new construction but not on a service replacement right across the street. Does this rule apply to service upgrades?
Yes, all service conductors installed in a service upgrade or new installation supplying one- and two-family dwelling units must terminate in an emergency disconnect installed in a readily accessible outdoor location as required by 230.85. All of the requirements of Article 230 and other applicable NEC requirements including, but not limited to, Article 250 apply to a service upgrade. Your concerns are valid.
As an installer, you are required to meet the requirements of the NEC as adopted into regulation in your state or municipality. In many jurisdictions, the NEC is amended when it is adopted or through legislation at a later date. That is an option for your area and the chief inspector can initiate such legislation. Without a formal amendment modifying the requirements of 230.85, an emergency disconnect is required.
Where an installer ignores the requirements of the NEC and there is a subsequent loss of property, an injury or fatality, and the lack of NEC compliance is a contributing factor, it is likely third-party civil suits will be filed. If that occurs, it is extremely unlikely that the inspector will assume all responsibility based on their personal opinion and the fact that they told you to ignore NEC requirements.
We were required by an owner to install cable lashing where 2,000A feeders were installed. In the past, we always grouped the conductors with Ty-Raps for a neat install. In this case, we were supplied with webbing, nylon rope and specific instructions. Is that required in the NEC?
Cable lashing is typically required by manufacturers in switchboards, switchgear, etc., and the applicable NEC requirement would be 110.3(B), which requires equipment that is listed, labeled or both to be installed and used in accordance with any instructions included in the listing or labeling. Manufacturers’ instructions may differ, but one refers to requirements for lashing of all conductors protected at and above 800A and for all conductors protected by devices with short-circuit current ratings over 65,000A.
Electrical distribution equipment such as switchboards and switchgear are provided with a short-circuit rating. The higher the equipment short-circuit rating, the greater the capability of the equipment to withstand tremendous magnetic stresses during a fault.
Cable lashing is required to stabilize the circuit conductors to reduce movement and potential damage if a fault occurs. Strong magnetic fields are created when a fault occurs and can result in rapid attraction and repulsion of the circuit conductors. This may cause the circuit conductors to whip back and forth inside the equipment, creating damage to the equipment, the terminations and the conductors.
Several NEC requirements address protection from magnetic stress on conductors, including, but not limited to: 110.36 which requires insulators and other conductor attachments used as supports to be capable of safely withstanding the maximum magnetic forces that may occur in a fault and 392.20(C), which requires single conductors to be securely bound in circuit groups to prevent excessive movement due to fault-current magnetic forces.
There are multiple requirements spread throughout the NEC for current-limiting OCPDs. One example we are dealing with right now is in 706.31(D) for energy-storage systems. No definition exists for “current limiting” in Article 100, and the definition in 240.2 is so watered down that any OCPD could be considered current-limiting. What is a current-limiting OCPD?
Section 240.2 defines a current-limiting OCPD as one that, when operating in its current-limiting range, reduces the current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor having comparable impedance. I agree with you, this definition compares the capability of an OCPD to a solid conductor and may create confusion. The definition is clear as mud.
In my opinion, one of the key reasons that this definition is written so vague is because it has to address both fuse and circuit-breaker overcurrent protective devices. These devices are very different with regard to what makes them current limiting. The one thing that all current limiting OCPDs have in common is that they must be marked current limiting as required by the applicable product standards.
So let’s look at those standards for each type of OCPD to understand their capabilities. UL 248-1 addresses current-limiting fuses, defining it as one that limits the clearing time at rated voltage to an interval equal to or less than the first major or symmetrical current-loop duration and limits the peak current to a value less than the available peak current. UL 489 addresses molded-case circuit breakers, and this standard defines a current-limiting circuit breaker as one that does not employ a fusible element and, when operating within its current-limiting range, limits the let-through I2t to a value less than the I2t of a half-cycle wave of the symmetrical prospective current. These definitions are technical in nature but do provide the Code user with more information.
The net takeaway is that where the NEC requires a current-limiting OCPD, it must be marked current-limiting and be applied properly. I agree, the definition of current-limiting OCPD needs serious work and should be moved into Article 100 in the 2023 NEC revision cycle.