Electrical Box Size Calculator - Free online calculator tool. Its primary purpose is to ensure safety, compliance, and efficiency in electrical systems by calculating the required box. Kenneth Alambra is a civil engineer who specializes in fit-out construction and technical sales engineering. Both of these fields involve problem-solving and creative thinking, which he greatly enjoys. Conductor fill: Count each insulated conductor entering the box, then multiply by its cubic inch allowance. It checks straight pulls at 8 times the largest raceway trade size and checks angle and U-pulls from the governing raceway row on each wall. The calculation is based on the following standard volumes: The formula is: [ text {Box Size} = (text {Number of Wires} times 2.