Wire Size Calculator
This tool is for informational and educational purposes only and does not constitute professional, legal, financial, or code-compliance advice. Figures, rates, codes, and requirements change over time and vary by jurisdiction, and may not reflect the latest local regulations. Results are estimates — always verify with an official source or a qualified professional before making decisions.

How to Size Electrical Wire (NEC Ampacity Walkthrough)

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Sizing electrical wire correctly protects against overheating, nuisance trips, and fire. The National Electrical Code (NEC, NFPA 70) lays out the framework, but the actual steps can feel scattered across multiple tables and articles. This page walks through the conceptual workflow in plain language so you understand what the calculator is doing — and what it is not doing. For any real installation, consult a licensed electrician and verify compliance with your local authority having jurisdiction (AHJ).

The Four-Step Sizing Workflow

Sizing a conductor generally follows this sequence. Each step feeds into the next, and skipping any one can produce an undersized — or unnecessarily oversized — wire.

  1. Determine the design load in amps.
  2. Apply the continuous-load rule (125%) if needed.
  3. Look up the minimum conductor size in NEC Table 310.16.
  4. Apply derating factors, then check voltage drop.

Step 1 — Determine the Load

Start with the actual current draw of the device or circuit, in amperes. For fixed appliances, use the nameplate full-load current (FLC). For lighting and receptacle circuits, add up the connected load or use the NEC's unit-load methods described in Article 220.

Do not use the breaker size as a proxy for load current. A 20 A breaker on a circuit drawing 12 A is not a "12-amp" circuit for sizing purposes — you size the conductor to handle what the breaker permits, not just what's plugged in today.

Step 2 — Apply the Continuous-Load Rule

NEC Article 100 defines a continuous load as one expected to run for three or more consecutive hours. Per NEC 210.20(A) and 215.3, the overcurrent device and conductor must be rated for at least 125% of the continuous load current. EV charger circuits are always treated as continuous per NEC Article 625.

In practice: if your load is 40 A and it runs continuously, your design current for sizing purposes becomes 50 A (40 × 1.25). Use that adjusted figure when looking up the conductor in Table 310.16.

Step 3 — NEC Table 310.16 and the 75°C Column

NEC 2023 Table 310.16 lists the allowable ampacity for conductors in a raceway or cable with no more than three current-carrying conductors at a 30°C (86°F) ambient temperature. The table has three temperature columns: 60°C, 75°C, and 90°C.

Most practical work uses the 75°C column. NEC 110.14(C) limits terminations at breakers and connectors to 75°C in most cases, regardless of the conductor's insulation rating. Only when every termination on a circuit is rated for 90°C can you apply the higher ampacity column — which is uncommon in residential and light-commercial installations.

Conceptual sizing workflow at a glance
Step What you determine NEC reference
1. Load Actual amps drawn by the circuit Article 220; nameplate FLC
2. Continuous rule Multiply by 1.25 if load runs ≥ 3 hours 210.20(A), 215.3, 625.42
3. Table 310.16 Minimum AWG from 75°C ampacity column Table 310.16; 110.14(C)
4a. Derate Reduce ampacity for high ambient or bundled conductors 310.15(B)(1); 310.15(C)(1)
4b. Voltage drop Check run length; upsize if drop exceeds target 210.19(A) informational note

Step 4a — Temperature and Conduit Derating

Table 310.16 assumes a 30°C (86°F) ambient temperature and no more than three current-carrying conductors in a raceway. When either condition changes, you must reduce the conductor's usable ampacity:

  • High-ambient correction: In attics, boiler rooms, or outdoor conduit exposed to summer sun, ambient temperatures can far exceed 30°C. NEC Table 310.15(B)(1) provides correction factors that reduce usable ampacity. The hotter the environment, the more you must derate — sometimes significantly. Use the calculator's derating guide or consult the table directly for your ambient conditions.
  • Conduit fill (bundling) adjustment: When four or more current-carrying conductors share a raceway, heat builds up and the allowable ampacity drops. NEC 310.15(C)(1) specifies adjustment factors for different conductor counts. Equipment grounding conductors generally do not count as current-carrying conductors for this purpose.

After derating, re-check whether the conductor you selected from Table 310.16 still has enough adjusted ampacity. If not, move up one or more AWG sizes until it does.

Step 4b — Voltage Drop Check

A wire sized correctly for ampacity can still cause problems if the circuit is long. Resistive losses along the conductor create a voltage drop from the panel to the load. The NEC's informational note at 210.19(A) recommends keeping branch-circuit voltage drop at or below 3%, with a combined feeder-plus-branch target of 5%. These are recommendations, not hard code requirements — but loads like motors and sensitive electronics often require tighter limits.

If your voltage-drop calculation shows a percentage above your target, upsize the conductor until the drop falls within range. The required size for voltage drop can exceed the size required for ampacity, particularly on long runs at modest currents. The final recommended size is always the larger of the two results.

Use the Wire Size Calculator to run both constraints simultaneously — enter your load, run length, conductor material, voltage, and phase, and the tool returns the minimum AWG that satisfies both ampacity and voltage-drop requirements.

Copper vs. Aluminum

Copper and aluminum have different resistivities and ampacity ratings at the same gauge. Aluminum conductors generally require a larger AWG to carry the same current as copper, and NEC 310.3(B) limits aluminum to 6 AWG or larger for most applications. For service entrances and feeders, aluminum is common and cost-effective; for branch circuits and smaller runs, copper is more practical. See the Copper vs. Aluminum Wire guide for a full comparison.

When to Call a Licensed Electrician

  • Any work in your main service panel or subpanel
  • New circuits requiring a permit in your jurisdiction
  • High-current loads — EV chargers, ranges, dryers, HVAC equipment
  • Circuits that involve derating, parallel conductors, or motor loads
  • Situations where the AHJ requires an inspection

This site and its calculator provide educational estimates. They are not a code compliance determination, an engineered design, or professional electrical advice. Always verify your local NEC adoption year and any AHJ amendments before any installation.

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