TL;DR

NM Cable (Romex): Non-metallic sheathed cable for residential wiring per NEC Article 334. Not permitted in commercial construction.

By Valenke Exam Prep Team·Last updated 2026-06-02

NM Cable (Romex)

JOURNEYMAN ELECTRICIAN

Definition

Non-metallic sheathed cable for residential wiring per NEC Article 334. Not permitted in commercial construction.

Calculation Methods

Calculations for nm cable (romex):

Formula	Equation	Example
Power (DC)	P = E x I = I squared x R = E squared / R	120V x 15A = 1,800W
Transformer sizing	kVA = (V x I) / 1000 (single-phase), kVA = (V x I x 1.732) / 1000 (3-phase)	200A service at 240V: (240 x 200)/1000 = 48kVA, use 50kVA transformer

Common Errors

Common mistakes related to nm cable (romex) that electricians must avoid:

No AFCI protection: Bedrooms and living areas in new construction require AFCI per 210.12
Undersized conductor: Wire ampacity insufficient for overcurrent device rating per Table 310.16
Missing GFCI: Bathroom, kitchen, garage, outdoor, and crawl space receptacles require GFCI per 210.8
Double-tapped breaker: Two conductors on a single-pole breaker not rated for multiple conductors
Improper grounding: Bootleg ground (jumping neutral to ground at receptacle) or missing EGC

Safety Considerations

Safety requirements relevant to nm cable (romex) in electrical work:

NFPA 70E arc flash boundaries: limited approach, restricted approach, prohibited approach based on voltage and available fault current
Working space clearances per 110.26: minimum 3ft depth, 30 inch width, 6.5ft headroom for under 600V equipment
Ground fault protection of equipment required on 480Y/277V services over 1000A per 230.95

Practical Example

Electrical Calculation: Power (DC)

Formula: P = E x I = I squared x R = E squared / R

Variables: P (watts), E (volts), I (amps), R (ohms)

Example: 120V x 15A = 1,800W

Professional Standards

Professional standards for nm cable (romex) are established by the relevant certification body and regulatory agencies. Certified professionals must demonstrate competency through examination and maintain credentials through continuing education. Scope of practice is defined by state law and facility policy. Never perform tasks outside your authorized scope.

Overview

Non-metallic sheathed cable for residential wiring per NEC Article 334. Not permitted in commercial construction. NM cable applications and limitations tested on JE exams.

Relevant formula: Transformer sizing is kVA = (V x I) / 1000 (single-phase), kVA = (V x I x 1.732) / 1000 (3-phase). Variables: V=voltage, I=current, kVA=kilovolt-amperes.

Why It Matters

NM cable applications and limitations tested on JE exams.

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Frequently Asked Questions

What NEC articles cover nm cable (romex)?▼

For nm cable (romex): Article 300 (General Requirements for Wiring Methods): 300.4: protection against physical damage (nail plates). Article 314 (Outlet, Device, Pull, and Junction Boxes): 314.16: box fill calculation each conductor = volume per Table 314.16(B).

What formulas are needed to calculate nm cable (romex)?▼

For nm cable (romex): Power (3-phase): P = 1.732 x E x I x PF. Variables: P (watts), E (line voltage), I (line current), PF (power factor). Example: 480V x 20A x 0.85 x 1.732 = 14,117W.

What safety requirements apply to nm cable (romex)?▼

For nm cable (romex): NFPA 70E arc flash boundaries: limited approach, restricted approach, prohibited approach based on voltage and available fault current. PPE categories 1-4 based on incident energy level (cal/sq cm): Cat 1 = 4 cal/sq cm, Cat 4 = 40 cal/sq cm.