NEC Cable Sizing & Voltage Drop

electrical

Sizes electrical conductors per NEC ampacity tables with ambient temperature and conduit fill derating. Calculates voltage drop, selects conduit size, and determines grounding conductor size.

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Load Amps*

Design load current in amperes. Use the continuous load (or 125% of continuous per NEC 210.20) as appropriate for the circuit. Typical: 15-50A residential, 50-400A commercial, 400-4000A industrial.

Voltage

System voltage in volts (line-to-line for 3-phase, line-to-neutral for 1-phase). Common values: 120, 208, 240, 277, 480, 600. If omitted, defaults to 480V (common commercial/industrial).

Phases

Number of phases. Affects voltage drop formula: single-phase uses 2L factor, three-phase uses sqrt(3)*L. If omitted, defaults to 3-phase.

Distance Ft*

One-way circuit length from source to load in feet. Voltage drop is calculated over this distance (not round-trip; the formula accounts for return path). Typical: 50-200 ft for branch circuits, 200-1000 ft for feeders.

Conductor Material

Conductor material. Copper has higher ampacity and lower resistance per size but costs more. Aluminum requires upsizing ~2 AWG for equivalent ampacity. If omitted, defaults to copper.

Conduit Type

Conduit or raceway type. Affects conduit fill area per NEC Chapter 9 Table 4. EMT is most common for commercial; PVC for underground or corrosive environments. If omitted, defaults to EMT.

Insulation Temp Rating

Conductor insulation temperature rating in degrees C. 75C (THWN) is standard for most terminations. 90C (THWN-2/XHHW-2) allows higher base ampacity but is often derated to 75C at terminals. If omitted, defaults to 75C.

Ambient Temp C

Expected ambient temperature around the conductors in degrees C. NEC 310.16 base ampacities assume 30C; higher ambient requires a correction factor per NEC 310.15(B)(1). Typical: 30 C indoor, 40-50 C rooftop/attic, 20-25 C underground. If omitted, defaults to 30C (no temperature derating).

Num Current Carrying

Number of current-carrying conductors in the raceway. 3 or fewer = no adjustment. 4-6 = 80%, 7-9 = 70%, 10-20 = 50% per NEC 310.15(C)(1). Neutrals carrying only unbalanced current are excluded. If omitted, defaults to 3 (no conduit fill derating).

Power Factor

Load power factor (cos theta). Affects voltage drop via effective impedance (R*cos + X*sin). Typical: 0.80-0.90 for motors, ~1.0 for resistive loads. If omitted, defaults to 0.85.

Max Voltage Drop Pct

Maximum allowable voltage drop as a percentage of system voltage. NEC recommends 3% for branch circuits, 2% for feeders, 5% total. Conductor is upsized if VD exceeds this limit.

Ocpd Rating A

Overcurrent protective device (breaker/fuse) rating in amperes. Used to select equipment grounding conductor per NEC 250.122. If omitted, estimated as next standard size above 125% of load.

Reference data

This calculator pulls live values from the following engineering reference tables in the database.

nec cable ampacity
nec voltage drop
nec conduit raceway
nec grounding

About this calculator

NEC Cable Sizing & Voltage Drop is part of the Engineering Database calculator suite. Each tool runs deterministic calculations against authoritative reference data and code-compliant methods, so results are reproducible and traceable to source standards.

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