Electrical Calculators

Every calculator on WireRef works the same way the NEC does: start with the load, apply the code rules in order, and show what you need. All results cite the specific NEC section so you can verify the answer yourself or hand it to an inspector.

Voltage Drop CalculatorNEC 210.19(A)
Enter conductor size, material, length, and load current. Returns voltage drop percentage per NEC 210.19(A) Informational Note 4 and flags circuits exceeding 3% branch / 5% total recommendations.
Ampacity CalculatorNEC 310.15
Start with base ampacity from Table 310.16, apply temperature correction per 310.15(B)(1), bundling adjustment per 310.15(C)(1), and terminal limits per 110.14(C). Shows each derating step.
Conduit Fill CalculatorNEC Ch. 9
Select a conduit type and size, add conductors by gauge and insulation. Validates against Chapter 9 Table 1 fill percentages: 53% for one wire, 31% for two, 40% for three or more.
Panel Load CalculatorNEC 220
Dwelling unit load calculation per Article 220. Add general lighting, small appliance circuits, laundry, and fixed appliances. Applies demand factors from Tables 220.42 and 220.54 automatically.
Inspection Checklist55 items
Interactive pre-inspection prep tool. Select your project type (rough-in, final, service upgrade, EV charger) and get a complete checklist with NEC references and common fail reasons.
License Reciprocity Explorer50 states
Find which states accept your electrician license. Select your home state to see reciprocity agreements, CE requirements, and transfer details for all 50 states.

How NEC calculations work

The National Electrical Code uses a layered approach to conductor sizing. Base ampacity from Table 310.16 (or 310.17 for free air) is the starting point. From there, three independent adjustments apply in any order: ambient temperature correction reduces capacity when installations exceed 30°C, bundling adjustment accounts for heat buildup when more than three current-carrying conductors share a raceway, and terminal temperature limits per 110.14(C) cap the usable ampacity to what the equipment terminations can handle.

Voltage drop is not a code requirement but a recommendation. NEC 210.19(A) Informational Note 4 suggests limiting branch circuit drop to 3% and total circuit drop (feeder plus branch) to 5%. Many inspectors treat these as de facto requirements, and undersized conductors on long runs cause real performance problems: dimming lights, motor overheating, and tripped breakers.

Conduit fill limits exist to prevent conductor damage during pulling and to allow heat dissipation. Chapter 9, Table 1 sets the maximum cross-sectional fill area: 53% for one conductor, 31% for two, and 40% for three or more. These percentages apply to the conduit's internal area minus any fittings or splices.

When to use each calculator

Use the ampacity calculator when you know the wire size and need to verify it handles the load after derating. Use the voltage drop calculator when the run length is over 50 feet or the load is sensitive to voltage variation. Use the conduit fill calculator when planning a raceway with multiple circuits. Use the panel load calculator when sizing a service entrance or sub-panel for a dwelling unit.

Frequently asked questions

Are these calculators NEC compliant?

Every calculation cites the specific NEC 2023 section and table it references. Results are derived from the same tables and methods used in the codebook. However, your jurisdiction may have adopted a different NEC edition or local amendments that change the requirements. Always verify against your locally adopted code.

Do I still need an electrician?

Yes. These calculators help you understand what the code requires and verify that a proposed installation meets minimum standards. They do not replace the judgment of a licensed electrician who can evaluate your specific conditions: ambient temperature, conductor routing, equipment ratings, and local code amendments.

Why do the results show temperature derating?

NEC Table 310.16 ampacity values assume a 30°C (86°F) ambient temperature. Installations in attics, rooftops, or near heat sources operate above this baseline. Temperature correction factors from Table 310.15(B)(1) reduce the allowable ampacity to prevent conductor insulation from exceeding its rated temperature.

What is the 110.14(C) terminal limit?

Section 110.14(C) requires that conductors be sized based on the temperature rating of the equipment terminations, not just the conductor insulation. Most residential equipment is rated for 60°C or 75°C terminations. A 90°C THHN conductor connected to 75°C-rated equipment must use the 75°C column from Table 310.16, even though the insulation can handle more.
Requirements vary by state. NEC edition, licensing, permits, and GFCI rules differ by jurisdiction. Check your state or pick it below for personalized info across WireRef.
Important: For reference only. Not a substitute for a licensed electrician. Electrical work can cause serious injury, death, fire, or property damage if performed incorrectly. Always hire a licensed electrician for electrical work, especially for service upgrades, panel work, and 240V circuits. Values are derived from NFPA 70 (NEC) for educational purposes. Always verify against your locally adopted NEC edition and amendments - local jurisdictions may enforce stricter requirements. WireRef provides reference information only and is not responsible for work performed based on this content. NEC® is a registered trademark of the National Fire Protection Association. Free NEC access via NFPA · OSHA Electrical Safety · Terms of use.

NEC 2023 references verified April 2026