One of the most rewarding feeling in life, as a future tradesman, is when you study real hard, stay up late for months reading countless books on the National Electrical Code®. Heck, you may even binge watch endless youtube videos to prepare for an upcoming electrical exam and start to really understand the code, you're rightfully pumping with pride. Then it happens, you visit the water cooler at work and ask a code question to an experienced electrician who you consider knowledgeable and your world comes crashing down on you. Was all that learning wasted?
This article is written to show a classic response when using an industry trade "facebook-type" forum. An illustration, similar to the one shown below, was introduced to the group of presumably licensed and well qualified electricians for comment. The responses and harassment this author received would frankly shock you. From "It can't be solved" to "What a terrible question" and many more that are too graphic in nature to present in this article. In fact, the illustration depicted in the exercise was from one of the leading electrical educators in the country and very commonly seen on electrical exams all across the country, as stated by the author of the published illustration. So I created the illustrations below for a technically exhaustive look at how to solve this common exam illustration.
When a student is preparing for an exam, good instructors take them through phases of learning where they have to solve for what they see based on the information provided because there is a distinct difference from an exam based question and real world question. The illustration provides raceways entries into a pull box from all available sides. The conductors are 4 AWG and Larger and plausibly can be routed in straight or angle directions so the students must adapt and account for that plausibility. However, as commonly seen on electrical exams we are not showing the actual conductors, how they may be routed, or what size they are for this basic exercise. Why? Because typically a basic electrical exam will not have questions that incorporate many possibly outcomes.
So accordingly, the student will look at the pull box and quickly observe only the information presented. A pull box with raceway entries on all sides, the student will ask themselves, what and how would I calculate the minimum NEC® compliant pull box dimensions based on what is presented? Here is where it gets interesting.
One user on Facebooks "Electricians Only" type forums stated " Impossible to answer without knowing if there are angle or straight pulls and which raceways those pulls are coming out of..". This is where we began to examine those statements from a student learning the NEC® perspective as opposed to an experienced yet be it closed minded approach from those well rooted in the trade for many years.
Upon examination, we have raceway entries on all available sides of this illustrated pull box. It is critical to remember that students taking an electrical exam, be it PSI or other nationally accepted licensing exam, have to ignore the "Real World" and simply answer the question as presented. The student has to endeavor to solve for the minimum size pull box based solely on the information provided utilizing sections 314.28(A)(1) and (A)(2) of the National Electrical Code®.
Since no conductor sizes exist, other than the 4 AWG and larger demanded by 314.28(A), the student would look at this illustration and utilize a common sense approach by assuming both Straight Pulls and Angle Pulls are plausible. In this exam only, purely fictional installation attempt to solve for "X" and "Y" as requested. Remember, the student is asked to solve for dimensions "X" and "Y" only and no other dimension.
Interesting Fact- There are other dimensions that can play a role, such as for "Z" where raceways enclosing the same conductor are used distances between those raceways have to be accounted for, that is for later in this article since only "X" and "Y" are originally requested.
LETS BEGIN THE MATH
Left Side X Value - Section 314.28(A)(1) states that the length of the box shall not be less than eight times the trade size (we will use inches in our examples) of the largest raceway. In our case we have a plausible condition where a conductor could be pulled straight through the left side to the right side so this rule applies. The math for "X" doing a straight pull starting on the left is 8 x 3"= 24".
Right Side X Value - We have already established the use of 314.28(A)(1) we know that on the right side the largest raceway is a 2" so there is no possible way the straight calculation ( 8 x 2"=16") would be greater than the left side "X" calculation so the straight pull from the right side is ignored. However, the student has to perform an angle pull calculation on the right side in accordance with section 314.28(2) because based on the graphic angle pulls are plausible. Section 314.28(A)(2) states that the distance between each raceway entry in the box and the opposite wall of the box shall not be less than six times the largest raceway in the row. Where additional raceways are also in the same row their values are added to the previous calculation. The math for "X" doing an angle pull on right is 6 x 2.5" = 15" + 2.5" + 2.5" + 2" = 22".
For the "X" value the savvy student will compare the two calculations performed and select the greater of the two values. In our example the "X" value with the greatest length would the the straight pull at 24".
Top and Bottom Side Y Value - Again we look at section 314.28(A)(1) states that the length of the box shall not be less than eight times the trade size (we will use inches in our examples) of the largest raceway. In our case we have a plausible condition where a conductor could be pulled straight through from top to bottom or vice versa so this rule applies. The math for "Y" doing a straight pull starting on the bottom or top share the same size largest raceway of 2" so only one step here is needed for the straight pull calculation which is 8 x 2"= 16".
Top Side Y Value - The student now has to perform an angle pull calculation for the top in accordance with section 314.28(2) because based on the illustration angle pulls are plausible. Section 314.28(A)(2) states that the distance between each raceway entry in the box and the opposite wall of the box shall not be less than six times the largest raceway in the row. Where additional raceway entries are also in the same row their values are added to the previous calculation. The math for Top Side Y when doing an angle pull is 6 x 2" = 12" + 2" + 1.5" + 1.5" = 17".
Bottom Side Y Value - The student now has to perform an angle pull calculation for the bottom in accordance with section 314.28(2) because based on the illustration angle pulls are plausible. Section 314.28(A)(2) states that the distance between each raceway entry in the box and the opposite wall of the box shall not be less than six times the largest raceway in the row. Where additional raceway entries are also in the same row their values are added to the previous calculation. The math for Bottom Side "Y" when doing an angle pull is 6 x 2" = 12" + 2" + 1" + 1" = 16".
For the "Y" value the savvy student will compare the above calculations performed and select the greater of the resulting values. In this example the "Y" value from the straight pull compared to the angle pulls culminated in the greatest length being the top side angle pull at 17".
So the end result of this "no wires shown" pull box sizing exercise was a minimum 24" x 17" pull box. In reality, the installer is more than likely to upsize to 24" x 18" or even 24" x 20". However, we begin to creep into the "Real World" again and we must stay firmly planted in the exam test taking world.
Let me be clear about a few things at this point. There are individuals who will say the original illustration above is flawed or lacks detail. Clearly, this author disagrees and will provide an exhaustive explanation as to why. On an exam the student are always given four (4) multiple choice options to select the correct answer. With a simple "X" and "Y" question the student can solve the question with very little effort if they understand how to use the National Electrical Code®. However, the reader will see later in this article, solving for "Z" and the separation of raceway entries enclosing the same conductor is a real world situation and emphatically not an exam based situation as it pertains to the presented illustration.
Let's circle back around to that water cooler conversation. During this experiment this author endured name calling, belittling, threats, phone calls and just about anything you can imagine simply because a "thinking mans" challenge to a real exam style question was presented to a forum online. However, in keeping with all challenges and since they went that direction this author figured we would give them what they asked for.
As shown in the illustration above, the student begins to move into the "Real World" stage of solving a pull box calculation. The aforementioned statements up to this point clarified that many exams don't show the routing of the conductors. As a result the student should solve for what they see and only answer what they are asked.
Left Side Z Value- We have our base size from our previous calculation, which is X=24" x Y=17" but now we have to add the "Z" dimension. Starting from the Left Side of the pull box, using 314.28(A)(2), 6 x 3" = 18". Notice that there are two angle routes out of the 3" raceway, one to a 1.5" and one to a 2", the one that presents the most concern is the 1.5" as it is actually closer to the 3" as shown in the graphic. We will sum that one up later.
Right Side Z Value- We still have our base size from our previous calculation, which is X=24" x Y=17" but now we have to add the "Z" dimension. Moving to the Right Side of the pull box, using 314.28(A)(2), 6 x 2.5" = 15". While there are two routes out of the right side, one path from a 2.5"to a 2" and another path from a 2.5" to a 1.5" raceway. Remember the "Z" value is only calling for the separation of raceways that enclose the "same" conductor.
Now in a "Real World" scenario you would attempt to size this box in accordance with the minimum safety standard known as the National Electrical Code®. Keep in mind that minimum box sizing at this point can be achieved by ultimately lengthening the "X" or the "Y" value or even moving raceways around during your initial installation in order to achieve the minimum sizing and separation requirements of 314.28(A)(1) and (A)(2).
The sum of all the raceways in the top side are : 1.5" + 2" + 2" + 1.5 = 7"
The sum of all the raceways in the bottom side are : 1" + 2" + 2" + 1" = 5"
We started with an "X" dimension of 24", we subtract 7" from that leaving us 17". Logically speaking, in this illustration, all the raceways are amassed in the middle, top side wall. The student would have 8.5" available on each side of the outermost raceways. One method typically used to accommodate our "Z" would be to add the additional distance to the left and subsequent right side to achieve the desired separation, such as starting from the left side, 18"+7"+15" = 40" minimum for the "X" dimension in order to also provide for the "Z" dimension as well.
Closing this out on the "Z", the reader will notice that the bottom is not a concern as the previous calculations performed increased the length of "X" resulting in a more than sufficient increase to provide for all separation requirements.
In the real world, assuming only as we have presented in terms of raceway placements, the minimum dimensions of this illustrated pull box has been increased to the following minimum specifications : X = 40" and Y=17".
A final thought to the reader is presented as such. There is a distinct reason we do not ask for "Z" values on standardized electrical exams with regards to a simple illustration, such as the first one encounter in this article. It is because the actual "Real-World" solution could literally achieve the desired dimensions by increasing both the "X" or the "Y" where applicable to accommodate the rules in 314.28(A)(1) and (A)(2).
On a standardized electrical exam the student will only have four (4) multiple choice options, keeping it simple is the desired effect by the exam development committees who write these questions as this author knows very well by having served on such committees. While it may seem redundant it is always important to again remember that it is an standardized electrical exam and not a "Real World" situation. If they wanted to solve for "Z" they would provide those values and specifically seek those answers.
Here is my message to prospective tradesman learning the electrical trade by attending exam prep classes, reading books, watching videos or using other online resources to better their knowledge. Be mindful of closed minded folks who oppose your point of view or the method to which you seek your new-founded knowledge, it is simply because it differs from their own personal views. In fact, it could be that they learned the "wrong way" many years ago, refuse to see other knowledgable opinions or quite frankly they are trying to propagate that "wrong way" to you unwittingly. Don't get discouraged. Their lack of knowledge or willingness to learn should never drag you and your prospective future down. Stick to your guns and remember this quote "They Think They Know But When It Turns Nasty They Don't Really Know™" and continue with your studies...You Will Do Amazing Things.
Paul W Abernathy, CMECP®