If a coil is good it should read approximately ______ ohms for a 120v coil, and approximately _______ ohms for a 24v coil.

• A. 50 Ohms and 12 Ohms
• B. 200 Ohms and 2 Ohms
• C. 100 Ohms and 6 Ohms
• D. 10 Ohms and 60 Ohms

Answer: (C)

The idea being tested is that a coil’s resistance is matched to its rated voltage so the coil draws the proper current to energize without overheating. Use Ohm’s law: current equals voltage divided by resistance, and power equals V times I (or V^2 divided by R). For a coil meant to run on 120 V, a resistance around 100 ohms yields about 1.2 A of current and roughly 144 watts of power, which is a typical amount of energy the coil can handle when energizing. For a 24 V coil, a low resistance around 6 ohms gives about 4 A and about 96 watts, enough to energize efficiently at the lower voltage without excessive heating. Because coils come with manufacturing tolerances, these values are approximate, but they reflect the expected range for a healthy coil. If the resistance were far from these values, the coil could either overheat from too much current or fail to energize due to too little current.