Does it all fit in the box?
Devices count twice. All the grounds count once between them. Pigtails count for nothing. Those three rules are most of the errors, and this applies them to the volumes you supply.
Box fill, counted properly
The volume allowances come from your book. The counting is what this is for — and the counting is the part that gets it wrong.
Find the volume allowance per conductor for each gauge in your box-fill table (NEC 314.16(B) or the equivalent). It is a short column — one figure per wire size — and every field marked below wants a number from it. This page never assumes one.
Conductors entering and leaving
One allowance each. A conductor that begins and ends inside the box — a pigtail — is not counted at all, so leave those out.
Devices on straps
Each device counts twice its allowance, not once.
Equipment grounds
All of them together count as one, at the largest.
Internal clamps and the box
What is taking up the space
The counting rules this applies
Each rule below is a counting rule from 314.16(B), not a number this page invents. The volume figures are all yours.
- Each conductor entering and leaving: one allowance, at its own size.
- A conductor beginning and ending in the box: nothing. Pigtails are free.
- A conductor passing through unbroken: one, if it is short. If it is long enough to be spliced — twice the length needed for a splice, generally 12 inches free — it counts as two.
- All equipment grounds together: one allowance, at the largest ground present. Not one each.
- All internal clamps together: one allowance, at the largest conductor in the box. Clamps outside the box count for nothing.
- Each strap-mounted device: two allowances, at the largest conductor connected to it.
Cable connectors themselves do not count under current editions, though older ones treated them differently — another reason to check what your jurisdiction has actually adopted.
The two rules that get misremembered
Almost every box-fill mistake is one of two things, and they pull in opposite directions.
A device counts twice. A single receptacle on a strap consumes two conductor allowances, not one. People count it once because it looks like one thing. On a small box with a couple of cables this single error is often the whole difference between fitting and not.
All the grounds count once. Three, four, five grounding conductors in a box together consume one allowance at the largest of them — not one each. People count them individually, which overstates the fill and leads to buying a deeper box than the job needs. It is the safer error of the two, but it is still an error, and on a retrofit it can mean cutting drywall you did not need to cut.
Together these mean the intuitive count is often wrong in both directions at once, which is why a box that "obviously fits" sometimes does not.
Why pigtails are free
The allowance exists to reserve room for a conductor's bulk and for the bending it has to do to get where it is going. A pigtail that starts and finishes inside the box adds bulk but no entry, no exit and no bend radius through the wall of the box, and the code does not count it. This surprises people, and it is worth knowing because pigtailing is often the thing you do to avoid a fill problem — splitting a device off its own short lead rather than landing two cables on one strap.
Volume is not the only limit
A box that passes on volume can still be the wrong box. Conductor bending space at terminals, the depth needed behind a device that sits proud, and the box's own listing for the number of cable entries are separate requirements that a fill calculation says nothing about. Passing here means you have cleared one hurdle, not all of them.