How many wires can go in an electrical box?

NEC 314.16 assigns each wire size a volume allowance (for example 2.25 cubic inches for 12 AWG). Count each hot or neutral entering the box as one, each device such as a switch or receptacle as two, all ground wires together as one, and internal cable clamps as one — the total times the allowance must not exceed the box volume.

Do pass-through wires count toward box fill?

Yes — a wire that passes through the box unbroken counts once. Only pigtails that both start and end inside the box are free.

📦 Box Fill

Is your junction or device box big enough? Count what's in it, get a clear fits / doesn't-fit answer.

Box

Plastic boxes have their volume stamped inside (e.g. "18.0 CU IN") — pick Custom and type it. Metal box sizes use the standard code volumes.

Wire size in the box Quantity

Add one row for each conductor size. Most homes use 14 AWG (15 A circuits) or 12 AWG (20 A circuits).

Count every hot and neutral that enters the box — including any that pass straight through unbroken (those count too). Don't count ground wires here (next question), and don't count pigtails that start and end inside the box.

Devices in the box

devices

Switches, receptacles/outlets, and dimmers each count as two conductor allowances. This aggregate input uses the largest conductor listed above.

Any ground wires in the box?

All the grounds together count as one wire's worth of space (that's the code rule — not one each).

Internal cable clamps?

Clamps built inside the box count as one conductor's allowance, using the largest conductor listed above. Connectors that stay outside the box don't count.

Why box fill matters

Cram too many wires into a box and connections get stressed, insulation gets nicked, and heat builds up — a classic cause of failed inspections and flickering circuits. The U.S. electrical code (NEC 314.16) assigns every wire a space allowance based on its size, and the box must have at least that much room.

The counting rules, in plain English

Each hot or neutral entering the box = 1 count — including wires that pass straight through unbroken (those count once too). Each device (switch/outlet) = 2 counts. All ground wires together = 1 count (with more than four grounds, each extra adds a ¼ count under the 2020 rules). Built-in cable clamps = 1 count. Pigtails that live entirely inside the box are free. Multiply the counts by your wire size's space allowance and compare to the box volume — we do exactly that, and show the math.

Want to know why you should trust this result? Read how VoltDrop checks its numbers.