How many volume allowances must be taken for each device installed in an outlet box?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (B)

In box-fill calculations, every device mounted in the box uses up space equivalent to two conductors. The device’s yoke or strap counts as two conductor-volume units toward the total, regardless of the wire gauge. So for each device installed, you must add two volume allowances to the box-fill calculation. A duplex receptacle is one device, but it still accounts for two volumes. The actual cubic inches required for those conductors depend on wire size (for example, #14 uses 2.0 cubic inches per conductor), but the device itself consistently adds two volume units.