TL;DR

Resistance: Opposition to current flow in ohms (Ω). Determined by material, cross-section, length, and temperature.

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

Resistance

IBEW APPRENTICESHIP JOURNEYMAN ELECTRICIAN MASTER ELECTRICIAN

Definition

Opposition to current flow in ohms (Ω). Determined by material, cross-section, length, and temperature.

Regulatory Context

NEC regulatory requirements for resistance:

Article 334 (Nonmetallic-Sheathed Cable (NM/Romex)):

Not permitted in commercial buildings over 3 stories
Must be supported within 12 inches of box, every 4.5 feet
334.80: ampacity based on 60 C column of Table 310.16

Article 310 (Conductors for General Wiring):

Table 310.16: allowable ampacities of insulated conductors
310.15(C): derating for more than 3 current-carrying conductors
Temperature correction factors for ambient over 30 C

Troubleshooting

Troubleshooting issues related to resistance:

Wrong breaker size: Using 20A breaker on 14AWG circuit (max 15A per 240.4(D))
Missing nail plates: Cables through studs within 1.25 inches of edge require steel plates per 300.4
Overcrowded box: Box fill exceeds allowable volume per 314.16, common in renovation work
Missing GFCI: Bathroom, kitchen, garage, outdoor, and crawl space receptacles require GFCI per 210.8

Safety Considerations

Safety requirements relevant to resistance in electrical work:

Ground fault protection of equipment required on 480Y/277V services over 1000A per 230.95
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
PPE categories 1-4 based on incident energy level (cal/sq cm): Cat 1 = 4 cal/sq cm, Cat 4 = 40 cal/sq cm

Exam Focus Areas

On the Ibew Apprenticeship, Journeyman Electrician, Master Electrician exam(s), questions about resistance typically test:

Conductor sizing, overcurrent protection, and grounding requirements
NEC code references and their correct application
Safety procedures including LOTO, arc flash, and approach boundaries
Load calculations for residential and commercial installations

Step-by-Step Procedure

Opposition to current flow in ohms (Ω). Determined by material, cross-section, length, and temperature. Resistance calculations tested on IBEW, JE, and ME exams.

Relevant formula: Power (DC) is P = E x I = I squared x R = E squared / R. Variables: P (watts), E (volts), I (amps), R (ohms).

Why It Matters

Resistance calculations tested on IBEW, JE, and ME exams.

Practice This Topic

Practice ibew alg equations → IBEW APPRENTICESHIP Practice je th ohms power → JOURNEYMAN ELECTRICIAN

Ready to practice for the IBEW APPRENTICESHIP?

Adaptive practice powered by Item Response Theory targets your weak areas. Start with 3 free sessions.

Start free practice →

Frequently Asked Questions

What formulas are needed to calculate resistance?▼

For resistance: Conduit fill: 1 wire=53%, 2 wires=31%, 3+ wires=40% of conduit area. Variables: Wire area from Chapter 9 Table 5, conduit area from Table 4. Example: 4 x 12AWG THHN (0.0133 sq in) = 0.0532 sq in. 3/4 inch EMT allows 0.213 sq in (40%). OK..

What are common code violations related to resistance?▼

For resistance: Backstab connections: Push-in connections on receptacles fail more often than screw terminals, poor practice though not a code violation. No AFCI protection: Bedrooms and living areas in new construction require AFCI per 210.12. Overcrowded box: Box fill exceeds allowable volume per 314.16, common in renovation work.

What NEC articles cover resistance?▼

For resistance: Article 480 (Batteries): 480.5: battery location ventilation requirements. Article 240 (Overcurrent Protection): 240.4(D): small conductor protection (14AWG=15A, 12AWG=20A, 10AWG=30A).