Conduit Fill Calculator
Add mixed conductor sizes and insulation types. The calculator totals the actual insulated conductor areas and applies the correct fill percentage automatically.
Conductors in the raceway
InsulationSizeQtyCCC Qty
Why there are two quantities: every conductor physically in the raceway counts toward conduit fill. Only conductors that count as current-carrying conductors belong in the CCC column for ampacity adjustment. Equipment grounding conductors do not normally count as CCCs; neutral treatment depends on the circuit.
Reference: NEC Chapter 9 Table 1 fill percentages; Table 4 raceway areas; Table 5 insulated-conductor areas. One conductor = 53%, two = 31%, three or more = 40%; qualifying nipples may use 60%. Verify the adopted edition and local amendments before final installation.
Box Fill Calculator
Total the volume allowances for a box containing 18–6 AWG conductors. Enter individual wires, not cables, and use the box's marked or applicable table volume.
Use the marked volume or applicable NEC 314.16(A) value. Do not use outside dimensions.
Add only a permitted marked or table-listed volume; avoid counting it twice.
Circuit conductors
Each entering wire that terminates, splices or passes through counts once. A qualifying long unbroken loop counts twice. Enter long loops separately; do not also include them in the first quantity. Pigtails wholly inside the box have no separate conductor allowance.
Grounds and bonding jumpers entering the box
Count entering equipment grounds and equipment bonding jumpers only. Internal grounding pigtails are excluded from this quantity. Up to four entering grounds share one allowance; each extra adds one quarter, at the largest entering ground size.
Devices and terminal blocks
A standard duplex receptacle or switch on one yoke counts as one yoke, with two volume allowances. For equipment wider than one device box, enter each mounting gang. Choose the largest conductor connected to that yoke. Terminal block assemblies each take one allowance at their largest terminated conductor.
Clamps and support fittings
If a wholly internal pigtail is larger than the wires above, enter its size for clamp and support-fitting allowances. The calculator uses the largest conductor present.
What counts: external clamping mechanisms, small locknuts, bushings and splicing connectors receive no separate allowance. Long unbroken loops count twice when their length is at least twice the minimum free-conductor length required by NEC 300.14.
Reference: NEC 314.16 volume allowances and counting rules, including the post-2020 grounding rule and 2023/2026 terminal block allowance. This is a volume check for ordinary boxes, not a complete installation approval. Calculate barrier compartments separately. Conduit bodies, conductors 4 AWG and larger, special luminaire exceptions and listed clamp assemblies with integral terminations need separate treatment. Verify the locally adopted edition and equipment listing.
Conductor Ampacity & Derating
Start with the conductor's insulation-temperature column, apply ambient correction and current-carrying-conductor adjustment, then check the equipment terminal limitation and the general small-conductor overcurrent limits.
Use the applicable ambient for the installation.
Adjustment begins above three CCCs.
Use the marking on the actual equipment or listing.
Optional load check
Calculator uses 125% for the continuous portion.
Important distinction: physical conduit fill percentage does not itself set the Article 310 ampacity adjustment. The thermal adjustment is based on the number of current-carrying conductors, plus ambient temperature and other applicable conditions.
Reference: NEC Table 310.16 base ampacities; 310.15 ambient correction and conductor-count adjustment; 110.14(C) terminal temperature limitations; 240.4(D) general small-conductor OCPD limits. This tool does not cover every special rule for motors, HVAC, welders, cable trays, rooftop adders, or other special installations.
Voltage Drop Calculator
A fast field estimate using conductor circular-mil area and the standard 75°C K-factor. Enter one-way distance — the calculator handles the return-path multiplier.
Use line-to-line voltage for three-phase or 240 V loads; line-to-neutral for 120 V loads.
Source to load, once. Do not double the distance.
Use the remaining allowance if an upstream feeder already consumes part of your target.
Design target: the familiar 3% branch / 5% combined figures are NEC informational-note recommendations for reasonable efficiency, not a blanket mandatory voltage-drop limit. Equipment specifications or project requirements can be stricter.
Method: single-phase VD = 2 × K × I × L ÷ CM; three-phase VD = √3 × K × I × L ÷ CM. K = 12.9 copper or 21.2 aluminum (ohm-cmil/ft, field approximation). For sensitive loads, motors, long feeders, low power factor, or engineering design, use a full impedance calculation and verify equipment operating-voltage requirements.
Ohm's Law & Electrical Formulas
Enter any two known values. The calculator solves the other two, so nobody has to remember which way the formula wheel goes after twenty years in the field.
AC quick formulas
Single / Three-Phase Current
Formula reminders
Ohm's LawV = I × R
DC / resistive powerP = V × I
1φ real powerkW = V × I × PF ÷ 1000
3φ real powerkW = √3 × V × I × PF ÷ 1000
1φ apparent powerkVA = V × I ÷ 1000
3φ apparent powerkVA = √3 × V × I ÷ 1000
Power factorPF = kW ÷ kVA
Field use: formula calculators are arithmetic aids. Motor, transformer, generator, branch-circuit, feeder, and service sizing still require the applicable NEC article, equipment nameplate data, and local requirements.