Understanding the third article of the National Electrical Code (NEC), Article 110, Requirements for Electrical Installations, is essential to having a Code-compliant installation of the electrical system. 


Article 110 has five parts. Part I covers general installation requirements. Part II covers electrical equipment with a nominal voltage of 600 volts (V) or less, and it starts with spacing requirements around electrical equipment. 


For electrical equipment likely to require examination, adjustment, servicing or maintenance while energized, the working space shall not be less than the dimensions specified in 110.26(A)(1) through (A)(3). In accordance with 110.26(A)(1), working space depth (in the direction of live parts) shall not be less than the minimum clear distances in Table 110.26(A)(1) unless meeting the requirements in 110.26(A)(1)(a), (A)(1)(b), or (A)(1)(c).


Table 110.26(A)(1) specifies minimum clear distances for working space depth. This table is divided into two rows. First-row distances are used when the electrical equipment has a nominal voltage to ground of 0–150V. Regardless of the condition in the first row, the minimum working distance depth is 3 feet. Second-row distances are used when the electrical equipment has a nominal voltage to ground of at least 151V but not more than 600V. Some systems that would be included in this row include: 480Y/277V, three-phase, 4-wire system and 480/240V, three-phase, 4-wire system. This table also includes four columns. The first column shows the two electrical systems. The remaining columns are the three conditions for the electrical equipment. The conditions are explained below Table 110.26(A)(1).


The first condition concerns where no live or grounded parts are across from the electrical equipment in which the depth of working space is being considered. If exposed live parts are on both sides of the working space but are effectively guarded by insulating materials, this first condition would also apply.


For example, a 480Y/277V, three-phase, 4-wire switchboard will be installed in an electrical room. The wall across the aisle from the switchboard will be covered in drywall. Because the drywall is not considered grounded, Condition 1 applies. The minimum working space depth for the switchboard in this installation is 3 feet (see Figure 1).


The second condition applies when grounded parts are on the other side of the working space. The second sentence in Condition 2 under Table 110.26(A)(1) clarifies some surfaces, such as a concrete and concrete-block walls. Concrete, brick or tile walls shall be considered grounded.


For example, a 480Y/277V, three-phase, 4-wire switchboard will be installed in an electrical room. The wall across the aisle from the switchboard is concrete block. Because this wall is considered grounded, Condition 2 applies. The minimum working space depth for this switchboard is 3 feet, 6 inches (see Figure 2).


The third condition applies when exposed live parts are on both sides of the working space. For example, a 480Y/277V, three-phase, 4-wire switchboard will be installed in an electrical room. Across the aisle from this switchboard will be a 208Y/120V, three-phase, 4-wire switchboard. Because live parts could be exposed in both switchboards at the same time, Condition 3 applies. The 208V switchboard needs only 3 feet of working space depth, but that is not enough working space for the 480V switchboard. The minimum working space depth for the 480V switchboard in this installation is 4 feet (see Figure 3).


The working space depth for equipment operating at 600V, nominal, or less to ground—and likely to require examination, adjustment, servicing or maintenance while energized—shall not be less than the minimum clear distances in Table 110.26(A)(1) unless meeting 110.26(A)(1)(a), (A)(1)(b) or (A)(1)(c) requirements.


The first requirement pertains to dead-front assemblies, defined by Article 100 as without having live parts exposed to a person on the equipment’s operating side. The minimum clear working space distances are not required in the back or sides of assemblies, such as dead-front switchboards, switchgear or motor control centers, where all connections and renewable or adjustable parts, such as fuses or switches, are accessible from locations other than the back or sides [110.26(A)(1)(a)].


For example, a 480Y/277V, three-phase, 4-wire switchboard will be installed in an electrical room. A transformer is on the left side, and a panelboard is on the other side of the transformer. Another panelboard is on the right side of the switchboard. No connections or renewable/adjustable parts are accessible from the sides or back. Working space is not required for this switchboard’s back or the sides (see Figure 4).


Sometimes rear access is needed at the back of enclosed electrical equipment. If it is needed for equipment operating at 600V, nominal, or less to ground and the equipment is likely to require examination, adjustment, servicing or maintenance while energized, the working space depth shall comply with Table 110.26(A)(1). If rear access is needed to work on nonelectrical parts, a minimum working space distance is required. In accordance with the last sentence of 110.26(A)(1)(a), if rear access is required to work on nonelectrical parts on the back of enclosed equipment, a minimum horizontal working space of 30 inches shall be provided.


The second subsection under 110.26(A)(1) pertains to low-voltage equipment. Where all exposed live parts operate at not greater than 30V rms, 42V peak or 60V direct current (DC), smaller working space shall be permitted but only by special permission. The authority having jurisdiction (AHJ) will need to provide more than verbal consent. Article 100 defines “special permission” as the AHJ’s written consent.


In a new building, when dead-front switchboards, switchgear, panelboards or motor control centers are located across the aisle from each other, the working space shall meet the minimum clear distances for Condition 3. Under the third condition, the minimum clear distance for equipment having a nominal voltage to ground of 150V and less is 3 feet, and the minimum clear distance for equipment having a nominal voltage to ground of 151–600V is 4 feet. In an existing building where electrical equipment is being replaced, it may be possible to reduce the minimum clear distance. In accordance with 110.26(A)(1)(c), if this is an existing building where dead-front switchboards, switchgear, panelboards or motor control centers are being replaced and are across the aisle from exposed live parts, it may be permissible to use the distance specified under ­Condition 2 instead of Condition 3. To use the distance under Condition 2, conditions of maintenance and supervision must ensure written procedures have been adopted to prohibit equipment on both sides of the aisle from being open at the same time. Qualified, authorized people must service the installation.


For example, a 480Y/277V, three-phase, 4-wire switchboard—across the aisle from a 208Y/120V, three-phase, 4-wire switchboard in an existing building—needs to be replaced. The new 480Y/277V, three-phase, 4-wire switchboard is a little deeper than the existing one. When the new switchboard is installed, the distance between the switchboard and panelboard located across the aisle will only be 3 feet, 8 inches. In a new installation, the minimum clear distance in front of the 480V switchboard is 4 feet. However, because this is an existing building, the minimum clear distance in front of the switchboard can be 3 feet, 6 inches as long as written procedures have been adopted to prohibit equipment on both sides of the aisle from being open at the same time. In addition, authorized, qualified people must service the installation (see Figure 5).


For electrical equipment having a nominal voltage to ground of 151–600V, reducing the minimum clear distance is not permitted. While this specific wording is not in the NEC, it is still true.


For example, a 208Y/120V, three-phase, 4-wire switchboard is across the aisle from a 208Y/120V, three-phase, 4-wire panelboard in an existing building. The switchboard is going to be replaced. When complying with 110.26(A)(1)(c), it is permissible to use the distance specified under Condition 2 instead of Condition 3. For this equipment, the minimum clear distance for Condition 2 is exactly the same as the minimum clear distance for Condition 3, which is 3 feet.


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