How this calculator works
Each countable insulated conductor receives the table volume for its gauge. All internal clamps together add one largest-conductor allowance; each listed support type adds one. A device yoke normally adds two allowances based on its connected conductor. Under the 2020 grounding rule, up to four entering grounds share one allowance; each additional ground adds a quarter. Enter conductor allowances after applying loop and pass-through counting rules. Pigtails that stay inside the box are generally excluded.
Formula
Allowances in in^3: 18 AWG=1.5; 16=1.75; 14=2; 12=2.25; 10=2.5; 8=3; 6=5. Total=sum(count x allowance)+clamps+supports+2 x yokes x device allowance+ground allowance. Ground units=1+max(0,N-4)/4 for N>0.
Worked example
Six 12 AWG conductor allowances, one yoke with 12 AWG conductors and three 12 AWG grounds require 6x2.25 + 2x2.25 + 2.25 = 20.25 in^3, without clamps or support fittings.
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Frequently asked questions
Is this a Canadian Electrical Code calculator?
No. It uses the named US NEC 2020 rules. Canadian installations require the applicable Canadian code and provincial requirements.
Are wire connectors counted separately?
Ordinary wire connectors do not get a separate allowance in this model. Special equipment, barriers, unusual devices and later code editions may need additional treatment.
What does the largest conductor field do?
It sets the common allowance for clamps and supports. Select the largest conductor actually present, including relevant conductors not counted as an entering insulated wire.