Space between cables must be equal to one cable diameter -- 11 x 1. Total cable tray width required is 12. A cable tray calculator is a design tool that helps you figure out the right tray width and make sure that the planned number of cables fits within the allowable fill limitations. Cable tray fill capacity is governed by electrical codes (typically NEC Article 392) which. vides a comparison between cable tray and cable bus for power distribution systems. The comparison includes various eneral considerations on both products, highlighting pros and cons of both systems. The capacity does not depend on size only but also on cable type, diameter, and allowable fill capacity to allow safe and efficient operation.