How do you size an electrical wire using the NEC?

To size an electrical wire, determine the load current, apply the 125 percent factor for continuous loads where required, select the correct terminal temperature column, choose a conductor with sufficient ampacity, apply adjustment and correction factors, and then verify voltage drop for the installation.

Why does terminal temperature matter when sizing wire?

Terminal temperature matters because the allowable ampacity column is limited by the lowest temperature rating of the equipment terminals, conductor insulation, and connected devices. The conductor must be selected using a temperature column that matches those limitations.

Wire Sizing

Select the correct conductor size for ampacity and conditions.

Load Current (Amps)Conductor MaterialContinuous Load? (3+ hours) — NEC 215.2Terminal Temperature Rating — NEC 110.14(C)Ambient Temperature — Table 310.16Conductors in Raceway — Table 310.15(B)(1)Start Free Trial

How to Size a Wire — The 5-Step NEC Workflow

Calculating wire sizing in the 2023 NEC is a multi-step process. You cannot simply pick a wire size — you must satisfy three main criteria: Ampacity, Voltage Drop, and Terminal Temperature Ratings. [Source: NEC 110.14(C), 210.19(A), 215.2, 310.15, and 310.16]

Step 1 — Determine the Load (NEC 215.2)

Before looking at wire tables, define the circuit load. Continuous loads, such as lighting or EV charging, must be multiplied by 125%. Non-continuous loads use 100%. [Source: NEC 215.2]

Step 2 — Check Terminal Temperature Ratings (NEC 110.14(C))

This is a common point of failure. Most equipment is limited by its terminal rating, so the selected ampacity column must match the lowest temperature rating of the equipment, conductor, and termination. [Source: NEC 110.14(C)]

Step 3 — Select Wire Size (NEC 310.16)

Using the total load and the correct temperature column from Table 310.16, select a conductor size with ampacity equal to or greater than the calculated load. [Source: NEC 310.16]

Step 4 — Apply Adjustment and Correction Factors (NEC 310.15)

De-rate the wire when ambient temperature exceeds standard conditions or when more than three current-carrying conductors share a raceway or cable. [Source: NEC 310.15]

Step 5 — Verify Voltage Drop (NEC 210.19(A) FPN)

Use voltage drop as a design check after ampacity is satisfied. If voltage drop is too high, upsize the conductor even when the ampacity table appears acceptable. [Source: NEC 210.19(A) Informational Note]

Summary Checklist

StepActionNEC Reference

1Calculate total load and apply 125% for continuous loadsNEC 215.2

2Identify terminal temperature limitationsNEC 110.14(C)

3Find wire size in the correct ampacity columnNEC 310.16

4Adjust for ambient heat and conduit fill conditionsNEC 310.15

5Calculate voltage drop as a design-quality checkNEC 210.19

The answers we give you are for educational purposes only. Please verify with your code book, your journeyman or master electrician, and your local Authority Having Jurisdiction (AHJ).