Which of the following is the minimum voltage rating for communication cables and wires?

• A. 600V
• B. 120V
• C. 300V
• D. 24V

Answer: (C)

Understanding how insulation ratings work helps here. The voltage rating on a cable isn’t the voltage the system runs on; it’s the maximum voltage the insulation can safely withstand. For building communication cables and wires, the minimum insulation rating required is 300 volts. That means these cables are built to safely handle up to 300V, plus provide a safety margin for any transients or proximity to higher‑voltage lines.

A rating of 24 volts is far too low for most building communication uses, and a rating of 120 volts still doesn’t meet the minimum standard. A 600-volt rating exists for power wiring, but for communications, the lower-but-sufficient 300-volt rating is the standard minimum. So the 300V rating is the correct minimum.