Article 110 in the National Electrical Code (NEC) contains requirements that cover a wide variety of topics. As the scope in 110.1 states, the article provides general requirements for the examination and approval, installation and use, access to and spaces about electrical conductors and equipment. It also contains requirements that pertain to enclosures intended for personnel entry and tunnel installation. 


One well-known topic covered in Article 110 is spaces about electrical equipment. Spacing requirements for equipment rated 600 volts (V), nominal, or less are in 110.26.


Although 110.26 is only one section, many requirements are located in the six subsections. Similar requirements for spaces around electrical equipment rated over 600V, nominal, are located in 110.32 through 110.34. 


Many of the topics in Article 110 (such as spaces around electrical equipment) get a lot of consideration from engineers and electricians. Some of the topics in Article 110 do not get as much consideration—interrupting ratings, short-circuit current ratings and series ratings for circuit breakers and fuses, for example—but it does not mean they are any less important. 


As a general rule, if the available short-circuit current on the line-side terminals of the circuit breaker or fuse is higher than the overcurrent device’s interrupting rating, that particular circuit breaker or fuse shall not be installed.


As stated in 110.9, equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that is available at the equipment line terminals. In a fully rated electrical system, all overcurrent protective devices, such as fuses and circuit breakers, shall have an interrupting rating at a nominal circuit voltage equal to or greater than the short-circuit current available at the equipment line-side terminals. The equipment short-circuit current rating (SCCR) shall also be equal to or greater than the available short-circuit current at each piece of equipment (see 110.10). 


For example, the available short-­circuit current at the service of a 208Y/120V panelboard is 51 kilo-amperes (kA). The main distribution panelboard at the service is a 600-ampere (A) panelboard. The SCCR of the main distribution panelboard is 65 kA. The interrupting rating at nominal circuit voltage of the main breaker is 65 kAIC (65,000 available interrupting current). A 200A circuit breaker, also rated 65 kAIC, supplies power to a downstream panelboard located more than 200 feet from the main distribution panelboard. Because of the distance and other factors, the available short-circuit current on the line-side terminals of the 200A panelboard is only 22 kA. The SCCR of the 200A panelboard is 65 kA, but the interrupting rating at nominal circuit voltage of each circuit breaker in the panelboard is 22 kAIC. This is a fully rated electrical system because each overcurrent protective device has an interrupting rating at nominal circuit voltage equal to or greater than the short-circuit current available at the equipment line terminals (see Figure 1).


Under specific conditions, it is permissible to install downstream circuit breakers with interrupting ratings that are lower than the available short-circuit current on their line-side terminals. In accordance with 240.86, where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected on the load side of an acceptable overcurrent protective device with a higher rating, the circuit breaker shall meet the requirements specified in 240.86(A) or (B) and (C). This is referred to as “series rating,” “series combination rating” and “series-rated combination.” 


One specific condition for installing a series-rated combination system is a system that has been tested as a series-rated combination system. The series-rated system can be installed if the combination of line-side overcurrent device and load-side circuit breaker(s) is tested and marked on the end-use equipment, such as switchboards and panelboards [240.86(B)]. With this provision, the upstream overcurrent device can be a fuse or circuit breaker, but the complete system shall be tested as a series-rated combination system. 


For example, the available short-­circuit current at a service is 51 kA. As in the previous example, the SCCR of the 600A panelboard at the 65-kA service and the interrupting rating at nominal circuit voltage of all of the circuit breakers in this panelboard is 65 kAIC. This time, the 200A downstream panelboard is installed close to the main panelboard, and now the available short-circuit current on the line-side terminals is 47 kA. The interrupting rating at nominal circuit voltage of each circuit breaker in the 200A panelboard is 10 kAIC. While 10 kAIC is much lower than the available short-circuit current of 47 kA, a series-rated combination system will be installed. The combination of the main panelboard, the downstream panelboard and all installed circuit breakers has been tested and rated 65 kA. Even though the circuit breakers in the downstream panelboard have a rating of only 10 kAIC and the available short-circuit current is 47 kA, they can be installed because the system has been tested as a series-rated combination system rated for 65 kA (see Figure 2).


Where the electrical system is a tested series combination system, the electrical equipment shall be labeled in accordance with 110.22(C). Equipment enclosures for circuit breakers or fuses applied in compliance with the series combination ratings marked on the equipment by the manufacturer in accordance with 240.86(B) shall be legibly marked in the field to indicate the equipment has been applied with a series combination rating [110.22(C)]. The marking shall meet the requirements in 110.21(B), be readily visible and state the following:


Caution
Series combination system 
rated ______ amperes. 
Identified replacement 
components required. 


In the previous example, the main distribution panelboard and downstream panelboard was tested as a series-rated combination system rated for 65 kA. Therefore, the caution label required by 110.22(C) shall be installed (see Figure 3).


Another specific condition where it is permissible for the downstream circuit breaker(s) to have lower interrupting ratings than the available short-circuit current on their line-side terminals is under engineering supervision in existing installations. Section 240.86(A) contains a number of very specific provisions that shall be met for a licensed professional engineer to ensure the system is a series-rated combination system [see 240.86(A)]. An electrical system that has an interrupting rating sufficient for the equipment at the time it was installed could become insufficient over time.


For example, the electrical system in a small industrial occupancy was in compliance with the interrupting rating provisions when the system was installed in a new building 15 years ago. When this industrial plant was constructed, there were no other buildings in the area. Over time, more buildings have been built in the area. Power to a new building that is located adjacent to the existing building is now being supplied from the same utility transformer that supplied power to the original building. Because of the changes, the electric utility had to change the transformer. The available short-circuit current in the existing building is now greater than the interrupting rating of the existing electrical equipment. In this example, complying with the provisions in 240.86(A) would be permissible. 


Where a system has been engineered as a series combination system in an existing installation, the equipment enclosures shall be legibly marked in the field as directed by the engineer to indicate the equipment has been applied with a series combination. This marking requirement is in 110.22(B). This label shall state the following: 


Caution
Engineered series combination 
system rated ______ amperes. 
Identified replacement 
components required.


In accordance with 240.86(C), series ratings shall not be used where: (1) motors are connected on the load side of the higher rated overcurrent device and on the line side of the lower rated overcurrent device, and (2) the sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower rated circuit breaker. 


Next month’s column continues the discussion of requirements for electrical installations.