Writing numbers correctly – technical writing tips

Using numbers is an integral part of an electrical engineer’s job. It’s one thing to know how to calculate them, but another to know how to use them when writing mod packages and other technical documents. Here are a few tips to keep in mind when using numbers . . . .

Write out any number that is nine or less. Above that, print the numeral:

Four cables are required.

Seven lugs are used.

Two cables were damaged.

The cable with 13 conductors is less flexible than the cable with six.

One cable contains 72 fibers.

There are several exceptions to this, including units of measure, dates, age, percentages, and money:

6 feet

November 2, 2010

4 years old

9 percent


The first set of examples above also illustrates another rule: never begin a sentence with a number. To do so is confusing to the reader, who is expecting a capital letter.

If multiple numbers are used in a sentence, write the numerals if they are all nine or less:

The solution is to install seven breakers, five cables, three instruments, and two panels.

If any of the numbers is greater than nine, then write each number as a numeral (even the ones less than nine):

The solution is to install 14 breakers, 10 cables, 6 instruments, and 4 panels.


Hyphenate a number and its unit of measure when they describe a noun. This indicates to the reader that the number is associated with the unit and not the noun:

6-amp fuse

12-volt battery

600-volt insulation

20-amp breaker

12-conductor cable

Without a hyphen, the reader might get confused that you have 12 conductor-cables or 6 amp-fuses. The hyphen removes the ambiguity and conveys your thought clearly to the reader. However, do not use a hyphen when only describing the units. The fuse is rated for 6 amps does not require a hyphen. Neither does The cable contains 12 conductors.

Round numbers correctly when dividing them and include the correct number of digits. The standard is set by the number, either the numerator or the denominator, that contains the most digits after the decimal. Using a calculator, dividing 12.78 by 2.689 results in 4.75269617. The correct way to write the quotient is 4.753. The last digit is rounded up if the digit being dropped is five or greater, and it is left unchanged if the digit being dropped is less than five.

When two numbers appear next to each other, it’s preferred that you spell out the smaller one. 2 3-conductor cables should be rewritten as two 3-conductor cables. Similarly:


thirteen three-pole breakers

4 5-amp fuses

68 5-amp fuses

3 13,500-volt inverters


thirteen 3-pole breakers

four 5-amp fuses

sixty-eight 5-amp fuses

three 13,500-volt inverters

An exception to this rule is that you always print the numeral, even if it is smaller, when it is combined with a unit of measure to modify a noun. Thus, sixty-eight 5-amp fuses.

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