If you design or install large equipment layouts, you need to watch out for special rules covering access to the required workspaces. Don't get confused between the rules covering workspace sizing, and the other rules we address in this article on how you get in and out of these workspaces.
When the rule applies Similar to the case with workspace sizes, there is a general rule, without specific dimensions, that requires access for all equipment. Then there is a very specific rule for some large equipment. However, the scope of these two rules differs somewhat. The workspace sizing rules in Sec. 110-16(a) cover equipment you are likely to work on while energized. This is a somewhat different subgroup of electric equipment than the specific workspace access rule, which covers any equipment that includes "overcurrent devices, switching devices, or control devices," if it meets the minimum size constraints even if this equipment were interpreted as not requiring access while energized.
If the equipment is rated 1200A or more, and exceeds 6 ft in length, the specific rules apply. You must meet both conditions simultaneously. Be careful because the two measurements don't use a parallel construction, and this has caused considerable confusion. The first measurement begins at the nominal figure ("1200A or more"), and the second measurement only applies above the nominal figure ("over 6 ft wide"). Thus, a 2500A switchboard that is only 6 ft wide, exactly, does not have to meet the enhanced access requirement, but a 1200A motor control center 73 in. wide does need to comply with these provisions.
There is another issue as well. The rule applies to equipment, not to multiple equipments. For example, suppose three 400A panels and related equipment occupy 7 ft of wall space as they line up. None of the installed equipment equals or exceeds 1200A, and the enhanced access rule doesn't apply. This would be true even if you tapped the three panels off a 1200A feeder in a wireway running under the panels, which exceeded 6 ft in length. This is because the equipment that does comply with the size requirement (the wireway) doesn't meet the basic application provision ("containing overcurrent devices ...").
What the rule requires If the characteristic provisions and large-size provisions are met, you need to provide at least two means of egress from the equipment in question. The problem has been arcing burn-downs which, if they occur at one end of an equipment line-up, can and have trapped workers on the other side.
You need to be sure each means of egress is at least 2 ft wide and at least 61/2 ft high. The egress must be from the equipment subject to the rule, not just one side (usually the front) of that equipment. This may seem obvious, but we have seen drawings by major code authorities showing the second path of egress being a corridor framed by the rear of the equipment on one side and the walls of the room on the other. In other words, in order to get out of the room in a case where a burn-down began between you and the door, you needed to pass around to the rear of the faulted equipment in order to get out the door. This wouldn't work if the fault involved arc blast damage in progress at both the front and rear of the equipment at the point of the fault.
The exceptions This rule is in two paragraphs, the generic first (one entrance "of sufficient area" to all workspaces) and the second with the specific requirements for large equipment. There are two exceptions, and they only apply to the second paragraph. The first waives these provisions if there is a "continuous and unobstructed way of exit travel." We have seen this interpreted so as to basically remove the rule from the Code altogether. The argument is that since the facility will keep the workspace clear at all times, the path out will always be unobstructed without providing any secondary egress.
As a general matter, never read an exception in such a way as to remove the rule from the Code. Sec. 110-16(b) prohibits the use of workspace for storage, so the Code essentially requires clear workspace at all times. If you could rely on that workspace as a path of exit travel, the requirement for two means of egress would effectively disappear. The exception covers cases, rare in today's cramped architecture, where there is so much room there wouldn't be any possibility of entrapment.
The real action is in the second exception, which waives the double-access requirement in cases where the workspace is doubled, as in the drawing, which assumes a current rating of 1200A or more. The basic workspace-depth requirement for this equipment in Sec. 110-16(a) is 4 ft, because we have two line-ups facing each other; Sec. 110-16(a) Ex. 3 assumed not applicable. The architect has allowed double this distance, or 8 ft as shown.
There is another issue, however. Just because you can safely step back from a fault doesn't mean you won't be asphyxiated by the products of combustion. This actually happened in an Idaho potato plant, because the burn-down occurred at the end of the equipment adjacent to the room doorway. For this reason, you now (since the 1987 Code) need to be sure the equipment is at least the Table 110-16(a) workspace to the door jamb, assumed to be a grounded surface in this case.
Note that the main part of the exception refers to the Sec. 110-16(a) workspace, and the doorway setback refers to the Table 110-16(a) distance. This allows you to recalculate the distance to the doorway based on the conditions at that point. As shown in the drawing, that would be the live-parts-to-grounded-surface distance, or 31/2 ft at this voltage.
We are aware of interpretations that the width of the entry space needs to be the same as the required width of the aisle way between the equipment. For example, if the door were 2 ft wide (but still began at the corner of the room), the equipment would need to be 6 ft from the door jamb (8 ft less 2 ft for the door). We don't think either the literal text or good engineering supports so restrictive an interpretation, but you can certainly always exceed the minimum safety requirements in the Code.