Using the tables and informative annexes

A large share of master exam questions end with a number: an ampacity, a conductor size, a burial depth, a box volume, a motor current, a number of conductors in a conduit. Almost all of those numbers come from tables. The tables themselves are easy to read once you have found the right one; the difficulty is choosing the right table, reading the right column, and applying the notes and footnotes that change what the numbers mean. This lesson covers how the Code's tables are organized and a method for reading any of them, plus the informative annexes that can save minutes on calculation questions.

To respect the NFPA's copyright, the sample tables in this lesson are made up for teaching. They have the same structure as real Code tables, but the numbers are not the Code's. Use your own code book for real values.

Two kinds of tables

The Code has two families of tables. Article tables appear inside the articles and are numbered after the section that introduces them, so Table 310.16 belongs to section 310.16 and Table 250.122 belongs to section 250.122. Chapter 9 tables are collected at the end of the Code and numbered simply Table 1, Table 2 and so on, with notes that are part of the tables. Article tables answer questions about one subject; Chapter 9 tables mostly support calculations that several articles refer to, such as conduit fill and conductor properties.

A method for reading any table

Dana's practice tests show that her table mistakes fall into a few types: the wrong table, the wrong column, the wrong row, or a note she did not read. She builds a five-step routine to catch all of them.

The routine looks slow written down, but with practice it takes well under a minute.

Steps 1 and 2 are where most points are lost. Titles carry conditions. An ampacity table may apply only to a certain number of current-carrying conductors and a certain ambient temperature, with other situations handled by correction and adjustment factors in other sections. Notes and footnotes can limit values, refer you to another section, or add a condition. In Chapter 9 the notes to Table 1 are part of the table and contain rules you need for almost every conduit fill question.

Columns: material, temperature and method

Many tables have parallel sets of columns. Ampacity tables are divided by conductor material, copper on one side and aluminum or copper-clad aluminum on the other, and within each material by insulation temperature rating, typically 60, 75 and 90 degrees Celsius. Burial depth tables have a column for each wiring method. Motor tables have columns for voltage. Choosing a column is the step where field habit most often betrays a candidate: in the field, most electricians work with copper at 75 degrees, and their eyes go to that column automatically.

Rows: rounding and the words that control it

Rows are where rounding happens. A table might list conductor sizes, overcurrent device ratings or quantities, and your calculated value will often fall between two rows. The Code's wording tells you which way to go. Not less than, minimum and at least mean you go to the next larger row. Not more than, maximum and shall not exceed mean you stay at or below. A few tables, such as the motor branch-circuit protection table, are accompanied by rules that allow the next higher standard rating in some cases; those permissions are in the text of the section, not in the table.

Two common table structures need special care. In tables sized by the overcurrent device, such as the equipment grounding conductor table, each row covers a range of ratings, and the device rating, not the load, selects the row. In tables sized by the largest conductor, such as the grounding electrode conductor table, each row covers a range of conductor sizes, and for parallel conductors you use the equivalent area of the parallel set. Read the column headings carefully to see which input the table wants.

# Choosing a row when a value falls between rows ('not less than' means round up)
standard_ratings = [15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100]

def next_up(value):
    return next(r for r in standard_ratings if r >= value)

def next_down(value):
    return max(r for r in standard_ratings if r <= value)

calc = 42.5
print("Minimum device (not less than):", next_up(calc))
print("Largest device not exceeding:", next_down(calc))

Minimum device (not less than): 45
Largest device not exceeding: 40

Chapter 9 and its notes

Chapter 9 Table 1 sets the share of a conduit's cross-section that conductors may fill: one share for a single conductor, a smaller share for two conductors, and a middle share for three or more. The notes to Table 1 contain rules you will need constantly: how to count an equipment grounding conductor, when a short nipple may be filled more fully, how to handle cables, and when to round up the number of conductors. Tables 4 and 5 give the dimensions you need to work out the actual areas: Table 4 for the internal area of each trade size of each type of conduit, and Table 5 for the area of each conductor size with each insulation type. Table 8 gives the area and DC resistance of bare conductors, which you use for voltage drop.

For conductors of the same size and type, Annex C does the conduit fill arithmetic for you: it gives the maximum number of conductors of each size and insulation type in each trade size of each kind of conduit. On an exam, a question asking how many 10 AWG THHN conductors fit in a given EMT size can be answered in seconds from Annex C. Mixed sizes require the long method with Tables 4 and 5.

The informative annexes

The informative annexes after Chapter 9 are not requirements, but several of them are excellent study and exam tools. Annex D is the most valuable for master candidates: it contains worked examples of load calculations for one-family dwellings by the standard and optional methods, multifamily dwellings, stores, industrial feeders, motor circuits and more. When you are learning load calculations, working through the Annex D examples side by side with the articles is one of the best ways to learn the order of operations.

Remember that an annex example can only show one way of applying the rules; when an exam question differs from the example in a detail, the articles control. And because annexes are not requirements, an answer choice that says the annex requires something is usually wrong.

Practice: one question, every step