Purpose of distance X in exhibit

[hhsting]

For large equipment 1200A or more 110.26(c)(2)(b) shows in exhibit below.

I can understand the double working space is needed and its purpose but what is the purpose of that distance “X” in the Exhibit below that is also mentioned in NEC 2017 Section 110.26(c)(2)(b)? Why have that anyone know?

 

[Carultch]

Part b of 110.26(C)(2) specifies the minimum distance X to qualify for the exception. They use X instead of an actual number, because it depends on what you look up in the working space depth table.
It shall be located such that the distance from the equipment to the nearest edge of the entrance is not less than the minimum clear distance specified in Table 110.26(A)(1) for equipment operating at that voltage and in that condition.

Considering your graphic as an example, suppose the voltage is 277/480V, and the layout is as shown in that image. This is over 250V to ground, and Condition 3 for common aisle. Looking up the corresponding distance from that table, that gives us 4 feet. The common aisle condition with doubled working space for the lack of a second exit, will mean 8 feet clear between the two switchboards.

If distance X on our drawing were less than 4 feet, then we wouldn't qualify for the exception that permits only one entrance/egress. We'd need another exit on the opposite side of the working space, or at least a way to step out of the working space on the other side. If distance X on our drawing were 4 feet or greater, we'd be good to go with just one exit.

 

[hhsting]

Part b of 110.26(C)(2) specifies the minimum distance X to qualify. They use X instead of an actual number, because it depends on what you look up in the working space depth table.
It shall be located such that the distance from the equipment to the nearest edge of the entrance is not less than the minimum clear distance specified in Table 110.26(A)(1) for equipment operating at that voltage and in that condition.

Considering your graphic as an example, suppose the voltage is 277/480V, and the layout is as shown in that image. This is over 250V to ground, and Condition 3 for common aisle. Looking up the distance in that table, that gives us 4 feet. The common aisle condition with doubled working space for the lack of a second exit, which means there would be 8 feet clear between the two switchboards.

If distance X on our drawing were less than 4 feet, then we wouldn't qualify for the exception that permits only one entrance/egress. We'd need another exit on the opposite side of the working space, or at least a way to step out of the working space on the other side. If distance X on our drawing were 4 feet or greater, we'd be good to go with just one exit.

I know X is from table 110.26(A)(1) and yes I know its exception to one exit but that is not my question.

For example Working space is for well working space for worker so then what is the purpose of X? X is not working space


Why have that distance X? What is the purpose of it? Is it for working space? How does it qualify as having one exit? What is the distance purpose?

 

[Carultch]

I know X is from table 110.26(A)(1) and yes I know its exception to one exit but that is not my question.

For example Working space is for well working space for worker so then what is the purpose of X? X is not working space


Why have that distance X? What is the purpose of it? Is it for working space? How does it qualify as having one exit? What is the distance purpose?

OK, that part I didn't get in your question. You were asking about the motive for this requirement, rather than an understanding of how to use this requirement. I don't know the answer myself, unfortunately.

 

[steve66]

Honestly never noticed that before, but it makes perfect sense.

I believe the idea is to not trap someone in a room. Lets assume one one switchboard has a short circuit and is arcing out the front. Also assume your doorway is some minimum width (maybe only 24"- the minimum required for an egress).

Now you may have 8' between the switchboards, but lets assume one switcboard is right at the doorway. You obviously have to go right by it to get out. If that happens to be the arcing switchboard, that's bad news.

By leaving the minimum working space between the door and either switchboard (say 4'), you are guaranteed to be able to get out without entering the working space of one switchboard or the other. In other words, you may still have to egress through one of the working spaces, but you can pick which working space to go through. If one switchboard is right at the door, you have not choice - you must go through its working space.