Junction Box Fill Calculator (NEC 314.16)
Ensure your junction boxes aren't over-filled and meet code.
1Select Box
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2Conductors Entering Box
Conductor Rules
Do not count pigtails (conductors that do not leave the box). Only count conductors that originate outside the box.
18 AWG1.50 in³ each
16 AWG1.75 in³ each
14 AWG2.00 in³ each
12 AWG2.25 in³ each
10 AWG2.50 in³ each
8 AWG3.00 in³ each
6 AWG5.00 in³ each
3Add-ons
Counts as 1 volume allowance
Support Fittings Luminaire Studs / Hickeys
Fixture Studs/Hickeys
Counts as 1 volume allowance per type.
Devices (Yokes) Switches, Receptacles (2 vol each)
Device Fill
Each yoke (switch, receptacle, pilot light) counts as 2 volume allowances.
Ground Wires Equipment Grounding Conductors
Equipment Grounds
All grounds usually count as 1 volume allowance total (unless >4 in 2020 NEC).
Box Empty
-Fill
Add conductors or devices
Required Vol -
Total Calculated
The sum of all conductor and device volume allowances.
Box Capacity -
Available Volume
Total volume of Box + Extension Rings.
Volume Breakdown
Conductors0.00in³
Internal Clamps0.00in³
Support Fittings0.00in³
Devices (Yokes)0.00in³
Grounds0.00in³
Total0.00in³
NEC 314.16 Rules
- Clamps: 1 volume allowance total (based on largest wire).
- Devices: 2 volume allowances per yoke.
- Grounds: 1 vol for first 4, plus 1/4 vol for each additional.
Popular box fill Configurations
Box Volume Calculation
Vreq = (Ncond × Vwire) + Vclamp + Vfit + Vdev + Vgrnd
NEC 314.16 Box Fill
NEC 314.16(A) and (B) specify the volume allowances required for conductors, internal clamps, support fittings, devices (like switches/receptacles), and equipment grounding conductors.
Frequently Asked Questions
Do wire nuts count for box fill?
No. Wire connectors (wire nuts) and pigtails that do not leave the box are generally not counted towards box fill calculations.
How much volume does a device count as?
A standard device (switch or receptacle) counts as a double volume allowance based on the largest conductor connected to it (2 × conductor volume).
Do clamps count if they are on the outside?
External clamps do not count. Only internal cable clamps supplied with the box count as a single volume allowance, regardless of how many clamps are present.