IRC E3905-E3906 (NEC Article 314) · 14 practice questions
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Reading
Every outlet, switch, and junction point lives inside a box, and that box has to be the right size
for what goes in it, fastened so it cannot work loose, and reachable without cutting into finished
construction. This lesson works through the box fill calculation that decides whether a box is big
enough, then the support and access rules that decide whether it is installed correctly, both routine
findings on a rough-in walk.
Box fill: what counts, and how much it costs
Every outlet, device, and junction box has a listed cubic-inch volume, and everything that occupies
wiring space inside it counts against that volume as a certain number of "counts," each valued at the
volume allowance for a particular conductor size. Each 14 AWG count is 2.00 cubic inches, each 12 AWG
count is 2.25, and each 10 AWG count is 2.50. The counting rules:
Each insulated conductor that enters the box and is spliced, terminated, or simply passes through
without a splice: 1 count, at that conductor's own size.
A conductor that both originates and terminates entirely inside the box, such as a short pigtail
from a wire nut to a device, is not counted at all.
Equipment grounding conductors: up to four together take 1 count, at the largest ground present.
Under E3905.12.2.5 of the 2021 IRC, each additional grounding conductor or bonding jumper beyond four adds a quarter count at that same size.
All internal cable clamps in the box together: 1 count, at the largest conductor in the box.
Each device, such as a receptacle or switch yoke: 2 counts, at the largest conductor connected to
that device.
Each type of support fitting, such as a fixture stud or a hickey: 1 count for that type, at the largest conductor in the box.
Key numbers
Volume allowance per conductor: 14 AWG 2.00 cu in; 12 AWG 2.25 cu in; 10 AWG 2.50 cu in
Grounds: up to four together count once (largest size); each one beyond four adds 1/4 count (E3905.12.2.5)
All clamps together: 1 count; each device: 2 counts; each type of support fitting: 1 count
Free conductor at a box: at least 6 in., with 3 in. outside a box opening under 8 in.
Cable sheath extension into a nonmetallic box: at least 1/4 in.
Outlet box luminaire support without a specific higher listing: 50 lb maximum
Box setback from finished surface: 1/4 in. maximum in noncombustible material; flush or projecting in combustible material
Gaps between a box and the surrounding wall surface: 1/8 in. maximum
A worked box fill example
A device box holds two 12-2 NM cables with grounds, the box's internal cable clamps, and one duplex
receptacle. Tally the counts: 4 insulated conductors (2 hots, 2 neutrals) at 1 count each is 4; all the
equipment grounding conductors together count once; all the internal clamps together count once; and
the receptacle yoke, being a device, counts twice, for the largest conductor connected to it, 12 AWG.
That is 4 + 1 + 1 + 2, or 8 counts, all at the 12 AWG allowance of 2.25 cubic inches, for a required
minimum volume of 8 times 2.25, or 18.00 cubic inches. A standard single-gang device box listed at
18.00 cubic inches is exactly adequate; add a third cable to that same box and the count rises to 10,
requiring 22.50 cubic inches, which no longer fits, and the box has to be changed for a deeper one or a
larger multi-gang box.
Box and cable connector marking
Standard metal boxes in the sizes listed in the code's own table (a 4-inch square by 1-1/2-inch deep
box, for instance) have a known volume without any marking, because the table already establishes it.
A box outside those standard sizes, and every nonmetallic box, must be durably and legibly marked by the manufacturer with its cubic-inch capacity. Cable connectors are evaluated the same way: a connector is listed and identified
for a specific number and size of cables, and pushing more cables through a single-cable connector
than it is rated for leaves them loose, unclamped, and bearing on the sharp edge of the knockout.
Box support
A box fastened to a framing member with nails or screws through its side must have those fasteners
located within 1/4 inch of the back or the ends of the box, so the fastener stays out of the wiring
space rather than crossing through it and abrading conductor insulation as a device is pushed in. An
old-work (cut-in) box installed into existing drywall must be a type specifically designed and listed
for that application, with its integral clamps or ears bearing on the wall material for support; a box
relying only on the device's mounting screws and the cover plate to stay in place works loose over
time, and the device eventually sits proud of the wall with its conductors flexing at the terminals.
An exposed box containing no device or luminaire, not over 100 cubic inches, may be supported by
conduit alone where two or more conduits are threaded wrenchtight into the enclosure and each one is
secured within 3 feet of the box (18 inches where all the entries are on the same side); a single
conduit lets the box rotate and eventually work loose, which is why the rule specifies two or more.
Ceiling fans and heavy luminaires
An outlet box supporting a luminaire is limited to 50 pounds unless the box carries a specific
listing for a greater weight. A ceiling paddle fan is heavier still and subject to dynamic, vibrating
load rather than dead weight, so it needs a box that is specifically listed for fan support (marked as
such by the manufacturer) or independent support from the building's framing; an ordinary lightweight
ceiling box, even an 18-cubic-inch one that easily satisfies the fill calculation, is not adequate
support for a fan no matter how many screws go through the mounting ears.
Setback, gaps, and accessibility
A device box installed in a wall of noncombustible material, such as tile, may be set back from the
finished surface by no more than 1/4 inch. In a wall of combustible material, such as wood paneling,
the box must be installed flush with the finished surface or project beyond it, so that any arcing
inside the box is not directly adjacent to exposed wood. Gaps between a box and the surrounding
drywall or plaster must be repaired so that no gap exceeds 1/8 inch; an oversized cutout around an
old-work box leaves a chimney into the wall cavity that is bad for fire spread and easy to fix with
patching compound before the final inspection. Boxes themselves must remain accessible without
removing any part of the building: a box behind a removable panel, above a lift-out ceiling tile, or
reached through a scuttle hole all qualify, but a box drywalled over and taped does not, because the
next person to work on that circuit has no way to find it. Any box containing a splice must be closed
with a listed cover, faceplate, or luminaire canopy; an open box in an insulated attic leaves
failure-prone splices sitting directly against combustible insulation.
In the field
A common GCHS rehab finding is a device box that technically holds everything required by code, but
only if the conductors are jammed in at an angle. Run the box fill count before pricing a receptacle
swap or an added circuit: two 12-2 cables plus a receptacle already uses the full 18.00 cubic inches
of a standard box, so a third cable, an added ground, or a GFCI device with bulkier internal wiring
often means swapping in a deeper box rather than forcing the old one to hold more than its listed
volume. Crammed conductors are not just an inspection item; the terminations get stressed every time
the device is pushed back into the box, and heat from a loaded circuit has nowhere to dissipate.
The same tally works for any box: list every conductor, add one line for grounds, one for
clamps, and two for each device, then multiply by the size allowance.Count each allowance from the lesson example before multiplying by the volume allowance for the conductor size. The required volume here is 18.00 cubic inches.
Practice questions
1. In box fill calculations, each 12 AWG conductor count is valued at:
2.00 cubic inches
2.25 cubic inches
2.50 cubic inches
3.00 cubic inches
Answer: b. 14 AWG is 2.00 cubic inches, 12 AWG is 2.25, and 10 AWG steps up to 2.50.
2. In box fill calculations, a receptacle or switch (a device) counts as:
Zero; devices don't count
One conductor count
Two conductor counts, based on the largest conductor connected to it
Four conductor counts, one for each terminal screw and one for each mounting ear
Answer: c. Each device yoke takes a double count at the largest conductor size connected to it.
3. A short pigtail jumper that starts at a wire nut and ends at a device, entirely inside the box, counts in box fill as:
Two conductor counts, one for each end where the jumper lands on a terminal or wire nut
One conductor count
Half a count
Zero; conductors that neither enter nor leave the box are not counted
Answer: d. Only conductors entering the box from outside are counted; a jumper made up entirely inside it adds nothing.
4. A device box holds two 12-2 NM cables with grounds, internal cable clamps, and one duplex receptacle. What is the required box volume?
18.00 cubic inches
13.50 cubic inches
20.25 cubic inches
15.75 cubic inches
Answer: a. 4 conductors + 1 for all grounds + 1 for all clamps + 2 for the device is 8 counts at 2.25 cubic inches, or 18.00 cubic inches.
5. An inspector finds four 12-2 cables, a duplex receptacle, and internal clamps installed in an 18-cubic-inch box. This installation is:
Compliant, since everything physically fits inside
Over-filled: 8 conductors + 1 ground + 1 clamp + 2 device is 12 counts at 2.25, or 27 cubic inches required
Compliant if the conductors are folded neatly
Compliant if the circuit is rated 15 amperes
Answer: b. Twelve counts at 2.25 cubic inches is 27 cubic inches, half again what an 18-cubic-inch box provides.
6. All internal cable clamps within a single box, taken together, count as:
Zero
One allowance per clamp installed, each taken at the size of the conductor that clamp holds
One conductor volume allowance, based on the largest conductor in the box
Two allowances
Answer: c. All the clamps in a box together count once, at the size of the largest conductor present in that box.
7. Under the 2021 IRC (E3905.12.2.5), when must box fill include an extra equipment-grounding-conductor count beyond the usual single count for all grounds together?
Never; all grounds always count once no matter how many are present
When more than four grounding conductors or bonding jumpers enter: each one beyond four adds a quarter count
Whenever more than two grounding conductors are present: each one adds a full count at the size of the largest conductor in the box
Only in nonmetallic boxes
Answer: b. Under E3905.12.2.5, up to four equipment grounding conductors or bonding jumpers take a single count at the largest size present, and each additional one adds a quarter count at that size. Earlier editions counted all of them once regardless of number.
8. A single-gang device box is listed at 22.5 cubic inches. Where would an inspector expect to find that volume documented?
Only in the installation instructions, never on the box, which is stamped with the trade size alone
On the cover plate
It is never marked; it must always be field-calculated
Durably marked by the manufacturer on the box, or given by the code's table for a standard box size
Answer: d. Boxes outside the standard table sizes, and all nonmetallic boxes, must be durably marked with their cubic-inch capacity (E3905.12.1.2); standard metal sizes are already given in Table E3905.12.1.
9. A box fastened to a framing member with nails through its side must have those nails located:
Within 1/4 inch of the back or ends of the box, out of the wiring space
Anywhere convenient along the box sides, provided the nail heads are flush with the wall
Only at the exact center of the box
Only at the top edge of the box
Answer: a. A nail crossing the interior wiring space can abrade conductor insulation as a device is pushed in, so fasteners stay at the back or ends.
10. An old-work (cut-in) box installed into existing drywall must be:
Held in place with construction adhesive
Supported only by the device's mounting screws and the cover plate
A type designed and listed for that application, with its clamps or ears bearing on the wall material
Wedged in place with shims
Answer: c. A box relying only on the device and cover plate to stay put works loose over time; listed cut-in boxes bear on the wall material through integral clamps or ears.
11. Without a specific listing for a greater weight, an outlet box may support a luminaire weighing no more than:
25 pounds
35 pounds
70 pounds
50 pounds
Answer: d. 50 pounds is the general limit; a heavier luminaire needs independent structural support or a box specifically listed for the greater weight.
12. A rehab job is installing a ceiling paddle fan on an existing 18-cubic-inch ceiling box that is not marked for fan support. The correct approach is:
Install the fan; the box fill calculation already checks out
Replace it with a box specifically listed for fan support, or provide independent structural support, since a fan is a dynamic vibrating load a standard box is not rated for
Add extra mounting screws to compensate
Reduce the fan's weight rating by derating it 50 percent
Answer: b. Box fill volume and structural support are two separate questions; a fan needs a box marked for fan support or independent framing support regardless of how much wiring the box otherwise holds.
13. A device box installed in a wall of noncombustible material such as tile may be set back from the finished surface no more than:
1/4 inch
1/8 inch
1/2 inch
1 inch
Answer: a. In combustible material such as wood paneling, the box instead must be flush with or project beyond the finished surface.
14. An inspector finds a junction box in an accessible attic with no cover installed over its splices. The correct disposition is:
Approve it; attics are not habitable space, so covers are required only in finished rooms
Approve it if the splices are wire-nutted
Cite it; a box with splices must be closed with a cover, faceplate, or luminaire canopy
Approve it if the circuit is rated 15 amperes
Answer: c. The cover contains an arc or spark from a failed splice and keeps insulation off the conductors; an open box in an insulated attic leaves failure-prone splices exposed.
Glossary
Box fill
The calculation that totals conductor, clamp, ground, and device counts against a box's listed cubic-inch volume.
Count
The unit box fill is measured in; each item (conductor, device, clamp group, ground group, support fitting) is worth a set number of counts.
Old-work (cut-in) box
A box listed for installation into an existing finished wall, supported by integral clamps or ears bearing on the wall material.
Support fitting
A fixture stud, hickey, or similar hardware item in a box, counted separately from conductors and devices in box fill.
Setback
The distance a box sits recessed behind a finished wall surface, limited differently for combustible and noncombustible materials.
Accessible (as applied to boxes)
Reachable without removing part of the building; a panel, a lift-out tile, or a scuttle hole qualify, drywalling over a box does not.