All of the chapters in the National Electrical Code (NEC) contain tables, but not all of the chapters contain articles. Chapter 9 does not contain any articles; this last chapter only contains tables. The first of two articles in Chapter 1 is Article 100, Definitions. The second article is Article 110, Requirements for Electrical Installations. Article 110 contains five parts. Part III contains general installation requirements for electrical systems rated over 1,000 volts (V), nominal.
Working space provisions for electrical equipment rated over 1,000V, nominal, are in 110.34(A). Except as elsewhere required or permitted in this Code, equipment likely to require examination, adjustment, servicing or maintenance while energized shall have clear working space in the direction of access to live parts of the electrical equipment and shall be not less than specified in Table 110.34(A). Table 110.34(A) specifies minimum clear distances for depth of working spaces. This table includes four columns, the first of which shows the five electrical systems. The next three columns are the three conditions for the electrical equipment. The conditions are explained below Table 110.34(A).
All of the conditions start with the phrase “exposed live parts.” Electrical equipment such as safety switches and panelboards will have exposed live parts when doors are open or covers are removed, but the distances are to be measured from the front of the equipment. This table also is divided into five rows. The top row distances are used when the electrical equipment has a nominal voltage of 1,001–2,500V to ground. Last month’s column covered the first condition under Table 110.34(A).
The second condition applies when there are grounded parts on the other side of the working space. To help eliminate any confusion about concrete, brick and tile walls, the second sentence in Condition 2 states that concrete, brick or tile walls shall be considered as grounded.
For example, a 4,160Y/2,400V, three-phase, 4-wire switchgear will be installed in an electrical room. The wall across the aisle from this switchgear will be a concrete-block wall. The row selected for the equipment depends on the nominal voltage to ground. It seems like the minimum clear distance for a 4,160V, Y-connected, 4-wire system would be specified in the row for 2,501–9,000V, but the second row is not the correct row. For this system, the voltage to ground is 2,400V. Therefore, the depth of working space for this switchgear is found in the top row. Because the concrete-block wall is considered as grounded, Condition 2 applies. Therefore, the minimum depth of working space for the switchgear in this installation is 4 feet (see Figure 1).
The third condition under Table 110.34(A) applies when there are exposed live parts on both sides of the working space. For example, a 4,160Y/2,400V, three-phase, 4-wire switchgear will be installed in an electrical room. Across the aisle will be a 480Y/277V, three-phase, 4-wire switchboard. Because live parts could be exposed across the aisle from the switchgear, Condition 3 under Table 110.34(A) applies. The minimum depth of working space for the 4,160Y/2,400V, three-phase, 4-wire switchgear in this installation is 5 feet. The switchboard also requires working space, but the minimum depth of working space for the 480Y/277V, three-phase, 4-wire switchboard is only 4 feet [Table 110.26(A)(1)]. Despite this minimum depth, the minimum working space required for the 4,160Y/2,400V, three-phase, 4-wire switchgear in this installation is 5 feet (see Figure 2).
There are four additional rows besides the first one. As previously mentioned, the row selected for the equipment depends on the nominal voltage to ground. An example of one system where the minimum clear depths would be selected from the second row is a 12,470V, Y-connected system. The voltage to ground for this system is 7,200V. The range of nominal voltage to ground in the second row is 2,501V to 9,000V. The distances for the working space in the second row are 4 feet for Condition 1, 5 feet for Condition 2, and 6 feet for Condition 3. The distances for the working space depths in Table 110.34(A) range from 3 feet to 12 feet.
Sometimes, rear access is needed at the back of enclosed electrical equipment. If rear access is needed for equipment operating at over 1,000V, nominal, and the equipment is likely to require examination, adjustment, servicing or maintenance while energized, the depth of the working space shall comply with Table 110.34(A). For equipment not likely to require examination, adjustment, servicing or maintenance while energized, there is an exception in 110.34(A). If there are no renewable or adjustable parts in back of equipment such as switchgear or control assemblies, working space is not required.
Fuses and switches are examples of renewable and adjustable parts. Also, if all connections are accessible from locations other than the back, no working space is required.
With some equipment, access behind the equipment is needed so work can be performed on nonelectrical parts. If rear access is not needed to work on electrical parts, but it is needed to work on nonelectrical parts, a minimum working space distance is required. In accordance with the last sentence of the exception under 110.34(A), where rear access is needed in the back of enclosed equipment to work on nonelectrical parts, a minimum working space of 30 inches horizontally shall be provided.
For example, a 4,160Y/2,400V, three-phase switchgear will be installed in an electrical room. With this switchgear, access will be needed in the back so work can be performed on nonelectrical parts. Because of the nonelectrical work that will be performed on the back of this enclosed equipment, at least 30 inches of working space is required behind this switchgear (see Figure 3).
Next month’s column continues the discussion of electrical installation requirements.