How do you size an electrical cable per NEC?

Cable sizing per NEC involves three checks: (1) Ampacity - the cable must carry the full load current with appropriate derating factors for temperature, conduit fill, and continuous load per NEC Table 310.16; (2) Voltage drop - typically limited to 3% for branch circuits and 5% total per NEC 210.19 Informational Note; (3) Short circuit withstand - the cable must survive fault current for the clearing time of the protective device.

What is the maximum voltage drop allowed by NEC?

NEC 210.19 Informational Note No. 4 recommends a maximum of 3% voltage drop for branch circuits and 5% total (feeder plus branch circuit combined). While these are recommendations rather than mandatory requirements, most engineers and inspectors treat 3% as the practical maximum for branch circuits in industrial facilities.

Why do motors require 125% ampacity sizing?

NEC 430.22 requires that branch circuit conductors supplying a single motor have an ampacity not less than 125% of the motor full-load current. This accounts for motor starting inrush, service factor, and temperature rise during continuous operation. The 125% factor ensures cables do not overheat during sustained motor operation.

What additional requirements apply to cable sizing in hazardous areas?

In Class I Division 1 and Division 2 hazardous areas per NEC 500-503, cables must be installed in rigid metal conduit or MI cable, with explosion-proof seals at boundaries. Cable types must be rated for the environment (THHN/THWN-2 or XHHW-2). API RP 14F provides additional guidance for cable installation in petroleum facilities including minimum cable sizes and routing requirements.

Electrical Cable Sizing Calculator | NEC Ampacity & Voltage Drop

Load Information

Load Type

Motor Horsepower

System Voltage

Phase

Electrical Parameters

Power Factor

Typical: 0.80-0.90 (motors), 1.0 (resistive heaters)

Motor Efficiency

Maximum Voltage Drop Allowed

NEC recommends 3% branch, 5% total

Cable Run

Cable Length (One-Way)

Conductor Material

Insulation Type

Installation Conditions

Installation Method

Circuits in Conduit

Ambient Temperature

°F

NEC base: 86°F (30°C). Higher temps reduce ampacity.

Area Classification

NEC Cable Sizing Overview

Three-Step Sizing Process
1. Ampacity: Cable must carry full load current with derating factors applied (NEC Table 310.16).
2. Voltage Drop: Must stay within 3% branch / 5% total limit.
3. Short Circuit: Cable I²t must exceed protective device clearing time.

Key NEC References:
Art. 310: Conductors & ampacity tables
Art. 430: Motor circuits (125% FLA)
Art. 215: Feeders
Art. 240: Overcurrent protection

Hazardous Area Considerations:
Class I Div 1 requires rigid metal conduit with explosion-proof fittings. Class I Div 2 permits more wiring methods but still requires sealed fittings at classification boundaries. API RP 14F provides specific guidance for petroleum production facilities.

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Understand NEC cable sizing methodology, voltage drop calculations, ampacity derating, and hazardous area wiring requirements

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Results

Selected Cable Size

AWG/MCM

Full Load Current (FLA) -

Design Current (Derated) -

Cable Ampacity (Derated) -

Voltage Drop -

Voltage Drop (%) -

Conduit Size -

Breaker / Fuse Size -

Cable Resistance -

Cable Reactance -

Short Circuit Capacity -

NEC Reference -

Formula

VD = √3 × L × I × R_eff / 1000

VD = Voltage drop (volts)

L = One-way cable length (ft)

I = Full load current (amps)

R_eff = R·cosθ + X·sinθ (Ω/1000ft)

FLA = HP × 746 / (√3 × V × η × PF)

Standards & References

NEC (NFPA 70) Art. 310
Conductors for General Wiring — Ampacity Tables
NEC Art. 430
Motors, Motor Circuits — 125% FLA Sizing
NEC Art. 215 / 240
Feeders and Overcurrent Protection
NEC Table 9
AC Resistance and Reactance for Cables
IEEE 141 (Red Book)
Electric Power Distribution for Industrial Plants
API RP 14F
Electrical Installations in Petroleum Facilities

Engineering Notes

Motor circuits: Size conductors at 125% of FLA per NEC 430.22
Voltage drop: NEC recommends max 3% branch, 5% total (informational)
Temperature derating: Ambient > 86°F reduces ampacity per Table 310.15(B)(1)
Conduit fill: More than 3 current-carrying conductors requires derating per Table 310.15(C)(1)
Hazardous areas: Class I Div 1 requires rigid metal conduit, explosion-proof fittings
Aluminum: Typically requires 1-2 sizes larger than copper for equivalent ampacity

Quick Reference — Common Motor FLA

10 HP, 480V, 3φ → FLA ≈ 14 A
25 HP, 480V, 3φ → FLA ≈ 34 A
50 HP, 480V, 3φ → FLA ≈ 65 A
100 HP, 480V, 3φ → FLA ≈ 124 A
200 HP, 480V, 3φ → FLA ≈ 240 A

Electrical Cable Sizing Calculator