Electrical Boxes

• Round boxes are disallowed where locknuts/bushings connect conduits/connectors to the box side.
  • Exception 1: Nonmetallic boxes are allowed with metal raceways/cables if internal bonding is provided between all entries.
  • Exception 2: Listed nonmetallic boxes with integral bonding and provision for attaching an equipment bonding jumper are permitted with metal raceways/cables.
• Nonmetallic boxes are suitable for open wiring on insulators, concealed knob-and-tube wiring, nonmetallic-sheathed cables, flexible cords, and nonmetallic raceways.
• Metal boxes need grounding/bonding as per Article 250, Parts I, IV, V, VI, VII, and X, unless 250.112(I) permits otherwise.
  • Exception: Longer screws are acceptable if the end is protected by an approved method.
• Field-installed screws/fasteners in wiring spaces must be manufacturer-provided/specified or meet requirements.
• In damp/wet locations, boxes, conduit bodies, outlet box hoods, and fittings must prevent moisture entry/accumulation.
• Boxes, conduit bodies, outlet box hoods, and fittings in wet locations must be listed for wet locations.
• Approved drainage openings (3 mm to 6 mm diameter) can be field-installed in boxes/conduit bodies listed for damp/wet locations.
• Listed drain fittings can have larger openings if installed per manufacturer's instructions.

Number of Conductors

• Refer to 430.12 for motor/generator terminal housing volume requirements.
• Boxes/conduit bodies enclosing 4 AWG or larger conductors must also comply with 314.28.
• Outlet and device boxes must also comply with 314.24.
• Boxes/conduit bodies must be sized to ensure adequate free space for all enclosed conductors.
• Box volume, calculated per 314.16(A), must be at least equal to the fill calculation in 314.16(B).
• Minimum volume for conduit bodies is calculated in 314.16(C).
  • Where no volume allowances are required by 314.16(B)(2) through (B)(6).

Box Volume

• Wiring enclosure volume is the total volume of assembled sections (plaster rings, domed covers, extension rings, etc.), marked with their volume or from Table 314.16(A).
• If a box has securely installed barriers, the volume is apportioned to each space. Metal barriers take up 8.2 cm3, nonmetallic barriers take up 16.4 cm3.
• Boxes 1650 cm3 or less (excluding Table 314.16(A) boxes) and nonmetallic boxes must be durably and legibly marked with their volume(s) by the manufacturer.
• Boxes in Table 314.16(A) with volumes exceeding the designated volume, they can have their volume marked as required.
• Volumes in 314.16(B)(1) through (B)(6) are added together.
• No allowance is needed for small fittings like locknuts and bushings.
• Each space within a box with a barrier is calculated separately.
  • Exception: Equipment grounding conductors (up to four), fixture wires smaller than 14 AWG, or both, can be omitted from calculations if entering a box from a domed luminaire/canopy and terminating within.

Conductor Fill

• Each conductor originating outside the box and terminating/splicing within is counted once.
• Each conductor passing through the box without splice/termination is counted once.
• Each loop/coil of unbroken conductor (at least twice the minimum length per 300.14) is counted twice.
• Conductor fill is calculated using Table 314.16(B)(1).
• Conductors entirely within the box are not counted.
• With one or more internal cable clamps, use a single volume allowance from Table 314.16(B)(1) based on the largest conductor.
• No allowance is needed for cable connectors with clamping mechanisms outside the box.
• For luminaire studs/hickeys, use a single volume allowance per type of fitting based on the largest conductor.
• For each yoke/strap with devices/equipment, use a double volume allowance based on the largest conductor connected to the device(s)/equipment.
• Devices/equipment wider than a single 50 mm device box (Table 314.16(A)) require double volume allowances for each gang needed for mounting.
• Up to four equipment grounding conductors require a single volume allowance based on the largest equipment grounding conductor.
• Each additional equipment grounding conductor requires a volume allowance based on the largest equipment grounding conductor..
• A single volume allowance is required for each terminal block assembly based on the largest conductor(s) terminated to the assembly.

Splices, Taps and Devices

• Conduit bodies with splices, taps, or devices must be durably and legibly marked with their volume by the manufacturer.
• The maximum number of conductors is calculated per 314.16(B).
• Conduit bodies must be rigidly and securely supported.
• For messenger-supported wiring, open wiring on insulators, or concealed knob-and-tube wiring, conductors must enter the box through individual holes.

Metal Boxes

• In metal boxes/conduit bodies with conductors unprotected by flexible tubing, openings must have insulating bushings.
• With flexible tubing, extend the tubing from the last insulating support to at least 6 mm inside the box/conduit body and 6 mm beyond any cable clamp.
• The wiring method must be secured to the box or conduit body.
• Boxes/conduit bodies used as pull or junction boxes must comply with 314.28(A) through (E).

Exception

• Terminal housings provided with motors are exempt.
• Boxes/conduit bodies used as pull or junction boxes must comply with 314.28(A) through (E).
• Exception: Terminal housings supplied with motors.