When answering Chicago Supervising electrical exam questions it is easy to automatically reach for the gauge size you’ve used for an installation hundreds of times before. Concerned that the size you normally use may be too small? Just going up a gauge size or two may not necessarily be the answer. The length of the run can result in a code permitted voltage drop that requires an even larger gauge size.
Consider the following example:
What gauge size THW conductor is required to supply a 30-amp rated load, supplied by a 120-volt single-phase source if the distance one way between source and load is 180 feet?
A. #10 THW
B. #8 THW
C. #6 THW
D. #4 THW
Normally #10-gauge wire is the size you might think of first because you know that #10 THW conductor is rated to carry 35 amps. But given the length of the run and the voltage that will be dropped as a result, can this wire carry 30 amps if length of the run is 180 feet?
In order to err on the side of caution you decide to increase the gauge size. You don’t just increase the conductor by one size by choosing #8-gauge wire, you increase it by two sizes and choose #6-gauge THW wire which is rated to carry 65 amps. You feel confident that choosing #6-gauge wire will ensure that any voltage drop due to the length of the run won’t be a problem. Let’s use the wire size formula to see if you’re right.
CM = 2 x K x D x I/VD Permitted
= 2 x 12.9? x 180 ft. x 30 amps/3.6 volts
=139,320/3.6
= 38,700 minimum area circular mil size required
According to Chicago Electrical Code 2017 edition-NFPA 70, Table 8 of Chapter 9, a #6-gauge wire has an area circular mil size of 26,240 cm which would not meet the minimum size required.
#4-gauge THW wire with a circular mil size of 41,740 cm is required.
Would you have chosen # 4-gauge wire without using the wire size equation?
Why is the number 2 in the equation?
Where does 12.9 ohms come from?
Why does the code permit a 3.6 voltage drop?
Whether you need to answer these types of questions on an electrical exam or in the field, you need to know how to apply the code and use the formulas properly. Take the guesswork out of your exam preparation. Call 888-919-3926 or click the “Register Now” tab and start preparing to pass your exam today.