Electrical Wire Sizing: NEC Table 310.16 + Free Calculator

What size wire do you need? Start with your load current in amps, apply the 125% continuous load rule if needed, look up the minimum conductor size in NEC Table 310.16 using the correct temperature column for your equipment terminations (usually 75°C), apply any derating factors for heat or bundled conductors, and finally verify voltage drop for the circuit length. The wire must meet all four requirements — ampacity, proper table column, derating, and voltage drop — to be code-compliant.

Quick tool: Use our free Wire Size Calculator to find the right conductor for any circuit — checks ampacity, derating, and voltage drop in seconds.

Wire sizing is one of the highest-value skills on the Texas journeyman electrician exam. Examiners expect you to move quickly from circuit current to the correct wire size using NEC Table 310.16, and they regularly test your understanding of derating and voltage drop. This guide walks you through the complete four-step process, with worked examples and common exam traps.

The Four-Step Wire Sizing Process

Wire sizing is a sequential process. Each step builds on the last, and skipping any step can lead to unsafe or code-violating installations.

1. Determine the load current — Start with the circuit amperage, and multiply continuous loads by 1.25 per NEC 210.19(A), 215.2, and 230.42(A).
2. Select the temperature column — Choose the correct column (60°C, 75°C, or 90°C) in NEC Table 310.16 based on your equipment terminal ratings. NEC 110.14(C) limits your selection.
3. Apply derating factors — Adjust ampacity downward if the ambient temperature is above 30°C or if you have more than three current-carrying conductors in the same conduit.
4. Check voltage drop — Verify that the voltage drop over the circuit length does not exceed limits set by the NEC and the local authority.

If any step fails, upsize the wire and re-check. The final wire size must pass all four tests.

Step 1: Determine the Load Current

The first step is to find the amperage that will flow through the circuit.

For non-continuous loads, use the actual expected current. For example, a motor that draws 30 amps during operation requires a wire rated for at least 30 amps.

For continuous loads (like lighting circuits that run 3+ hours without stopping, heating, refrigeration, or medical life-support equipment), you must apply a 125% multiplier per the NEC. This is one of the most frequently tested rules on the exam.

Adjusted Current = Load Current × 1.25

For example, a circuit with a continuous load of 80 amps becomes:

Adjusted Current = 80 × 1.25 = 100 amps

You then use 100 amps to select the wire size from the table, not 80 amps.

Why the 1.25 Rule?

The NEC applies the 125% multiplier because continuous loads generate heat. Wires operating continuously at their maximum ampacity degrade faster than intermittently loaded wires. By oversizing for continuous loads, you protect the wire's insulation and reduce the risk of failure.

Step 2: NEC Table 310.16 and the Terminal Temperature Rule

Once you have the adjusted load current, you look it up in NEC Table 310.16 — Allowable Ampacities of Insulated Conductors Rated Up to 2000V, 60°C Through 90°C, Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth, and in Free Air.

The table has three columns: 60°C, 75°C, and 90°C. These are the insulation temperature ratings of the conductor.

• 60°C — Lower rating, smaller ampacity. Used for older wiring and budget installations.
• 75°C — Standard for most commercial and residential circuits. Used by devices with 75°C terminals.
• 90°C — Higher rating, larger ampacity. Used for THHN and XHHW-2 wire, and in derating calculations.

The Terminal Temperature Rule (NEC 110.14(C))

You cannot use the 90°C ampacity directly for selection. NEC 110.14(C) states:

Conductors shall be of a size that their ampacity under the conditions of use permits the safe carrying of the loads without exceeding the lower of the temperature ratings of the connected equipment, the conductor insulation, or both.

In plain English: Your circuit ampacity is limited by whichever is lower — the conductor insulation rating or the equipment terminal rating.

• Most breakers are rated 60°C or 75°C.
• Most load centers and panelboards are rated 75°C.
• If you use a 90°C conductor (THHN) but connect it to a 75°C breaker, you must use the 75°C ampacity from the table.

For the Texas exam, the standard approach is to use the 75°C column for most circuits, unless the question explicitly states otherwise.

Step 3: Derating Factors

After finding the ampacity in Table 310.16, you must check whether derating factors apply. Derating reduces the allowable ampacity below the table value.

Ambient Temperature Derating (NEC 310.15(B)(2))

If the conductor is installed in an environment hotter than 30°C (86°F), the ampacity must be reduced. For example, conductors in an attic in summer or near a furnace may be derated.

The NEC provides a correction table (Table 310.15(B)(2)(a)):

| Temperature | 60°C | 75°C | 90°C | |:---|:---:|:---:|:---:| | 30°C and below | 1.00 | 1.00 | 1.00 | | 35°C | 0.91 | 0.94 | 0.96 | | 40°C | 0.82 | 0.88 | 0.91 | | 45°C | 0.71 | 0.82 | 0.87 | | 50°C | 0.58 | 0.75 | 0.82 |

Apply the correction:

Derated Ampacity = Table Ampacity × Correction Factor

Conductor Bundling Derating (NEC 310.15(C)(1))

If you have more than three current-carrying conductors in the same conduit or raceway, ampacity is reduced per the bundling table (Table 310.15(C)(1)):

| Number of Conductors | Derating Factor | |:---|:---:| | 4–6 | 0.80 | | 7–9 | 0.70 | | 10–20 | 0.60 | | 21–30 | 0.50 | | 31–40 | 0.45 | | 41 and above | 0.40 |

Important: Neutral conductors do not count toward this derating unless the circuit is a multiwire branch circuit or the neutral carries unbalanced current.

Combining Multiple Derating Factors

If both ambient temperature and bundling apply, multiply both factors together, then multiply the table ampacity:

Final Ampacity = Table Ampacity × Temperature Factor × Bundling Factor

For example:

• Table ampacity at 75°C: 30 amps
• Ambient temperature: 40°C (factor 0.88)
• Six conductors in raceway (factor 0.80)

Final Ampacity = 30 × 0.88 × 0.80 = 21.1 amps

You would need to select a wire size rated for at least 21.1 amps.

Step 4: Voltage Drop Check

After confirming ampacity, you must verify that voltage drop is acceptable. Voltage drop reduces the voltage at the load, which can cause motors to overheat, lights to dim, and controls to malfunction.

The NEC (Informative Annex D.2) recommends:

• Branch circuit: Voltage drop ≤ 3%
• Feeder: Voltage drop ≤ 2%
• Combined (feeder + branch): Voltage drop ≤ 5%

Voltage drop depends on:

• Wire size (smaller = more drop)
• Conductor length (longer = more drop)
• Load current (higher = more drop)
• Conductor material (copper is lower resistance than aluminum)

For most short residential circuits, voltage drop is negligible. For long runs or high-current circuits, you may need to upsize the wire to reduce voltage drop below the limit.

Use our Voltage Drop Calculator to check any circuit, or see the full calculation method in Voltage Drop Calculations: Formula + Electrician Exam Examples.

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Quick Reference: Wire Size Chart (75°C, No Derating)

This table shows the minimum wire size for common loads at 75°C. Always verify derating and voltage drop for your specific installation.

| Load (amps) | Copper Size | Aluminum Size | |:---|:---:|:---:| | 15 | #14 AWG | #12 AWG | | 20 | #12 AWG | #10 AWG | | 30 | #10 AWG | #8 AWG | | 40 | #8 AWG | #6 AWG | | 50 | #6 AWG | #4 AWG | | 60 | #4 AWG | #2 AWG | | 100 | #3 AWG | #1 AWG | | 200 | 2/0 AWG | 4/0 AWG |

Source: NEC Table 310.16 (Allowable Ampacities, 75°C column)

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Worked Examples

Example 1: Simple Residential 20A Branch Circuit (No Derating)

Scenario: You're installing a kitchen outlet circuit. The breaker is 20 amps, rated 75°C. The circuit is 30 feet long, with 6 outlets (not all continuously loaded). Ambient temperature is normal (30°C). Only three wires in the conduit.

Step 1: Load Current

• Non-continuous load: 20 amps
• No 1.25 adjustment needed

Step 2: Table 310.16

• Look up 20 amps in the 75°C column → #12 AWG copper (rated 20A)

Step 3: Derating

• Ambient temperature: 30°C → Factor = 1.00 (no derating)
• Conductors in raceway: 3 → No bundling derating
• Final ampacity: 20 × 1.00 × 1.00 = 20 amps ✓

Step 4: Voltage Drop

• Distance: 30 feet, current 20 amps, #12 copper
• Voltage drop ≈ 0.6% (well under 3% limit) ✓

Answer: #12 AWG copper wire. This is standard for kitchen outlets.

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Example 2: Commercial Circuit with Derating (Continuous Load + Bundling)

Scenario: An HVAC feeder carries a continuous load of 40 amps to a commercial unit. The feeder is 100 feet long. It's run in a conduit with 8 other current-carrying conductors. The ambient temperature in the mechanical room is 40°C. Equipment is rated 75°C.

Step 1: Load Current

• Continuous load: 40 amps
• Adjusted: 40 × 1.25 = 50 amps

Step 2: Table 310.16

• Look up 50 amps in the 75°C column → #6 AWG copper (rated 56A at 75°C)

Step 3: Derating

• Ambient temperature: 40°C → Factor = 0.88
• Bundling: 8 conductors total (7–9 range) → Factor = 0.70
• Final ampacity: 56 × 0.88 × 0.70 = 34.5 amps ✗

The #6 AWG is now only rated for 34.5 amps after derating, but we need 50 amps. Upsize.

• Try #2 AWG copper (rated 95A at 75°C)
• Final ampacity: 95 × 0.88 × 0.70 = 58.5 amps ✓

Step 4: Voltage Drop

• Distance: 100 feet, current 50 amps, #2 copper
• Voltage drop ≈ 1.2% (under 3% limit) ✓

Answer: #2 AWG copper wire.

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Example 3: Long-Run Circuit Requiring Voltage Drop Upsize (30A, 200 feet)

Scenario: You're running a 30-amp circuit from the panel to a remote garage 200 feet away. Load is non-continuous. Ambient temperature is normal. No bundling derating. Breaker and equipment are 75°C.

Step 1: Load Current

• Non-continuous load: 30 amps

Step 2: Table 310.16

• Look up 30 amps in the 75°C column → #10 AWG copper (rated 30A)

Step 3: Derating

• Ambient: 30°C → Factor = 1.00
• Bundling: 3 conductors → No derating
• Final ampacity: 30 amps ✓

Step 4: Voltage Drop

• Distance: 200 feet (one-way), current 30 amps, #10 copper
• Voltage drop ≈ 2.9% (very close to the 3% limit for a branch circuit) ⚠️

At 200 feet, we're just barely within the 3% limit with #10 AWG. However, if we upsize to #8 AWG:

• Voltage drop ≈ 1.8% ✓

Answer: #8 AWG copper is safer and meets best practice. The ampacity of #10 is sufficient (30A vs. 40A rating), but voltage drop requires the upsize.

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Common Exam Traps

Trap 1: Using the Wrong Temperature Column

Mistake: You see a problem that says "What size wire for 30 amps?" and immediately look up 30 amps in the 90°C column because it allows smaller wire.

Reality: NEC 110.14(C) limits you to the 75°C column for most equipment. The 90°C column is used only as a starting point for derating calculations (you apply derating to the 90°C value, then round up to the next standard size, and verify the final ampacity is acceptable at 75°C).

Exam response: Unless the problem explicitly states 90°C terminations, use the 75°C column.

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Trap 2: Forgetting the 1.25 Continuous Load Multiplier

Mistake: A problem states "30 amps continuous load." You look up 30 amps in the table and select #10 AWG.

Reality: The NEC requires 30 × 1.25 = 37.5 amps to be used. #10 AWG is only rated 35A at 75°C, which is insufficient. You need #8 AWG (40A).

Exam response: Always multiply continuous loads by 1.25 before consulting the table. Examiners specifically test this rule.

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Trap 3: Ignoring Derating Factors

Mistake: You select a wire size that meets the table ampacity but don't apply derating for 8 conductors in a conduit and 40°C ambient temperature.

Reality: After derating, the wire's real-world ampacity may be much lower than the table value. The final ampacity must still meet the load.

Exam response: Always ask: "Are there more than 3 conductors? Is the ambient temperature above 30°C?" If yes to either, apply derating.

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Trap 4: Not Checking Voltage Drop

Mistake: You select a wire size based on ampacity alone and don't verify voltage drop.

Reality: Voltage drop can require a larger wire than ampacity does, especially on long runs. You may have an "acceptable" ampacity but unacceptable voltage drop.

Exam response: For any circuit longer than 50 feet, estimate voltage drop or use the calculator. Problems often include the distance to test this knowledge.

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Related Study Guides

For deeper dives into wire sizing and related topics, explore these resources:

• Conductor Sizing: NEC Table 310.16 Explained — Detailed breakdown of Table 310.16 columns, ampacity values, and how to read the table.

• Voltage Drop Calculations: Formula + Electrician Exam Examples — The full voltage drop calculation method, with worked examples and the formula.

• Texas Electrician Exam Calculations: Complete Breakdown — Collection of high-value calculation problems from past exams.

• Wire Size Calculator — Free calculator that finds the correct wire size and checks ampacity, derating, and voltage drop in seconds.

• Ampacity Chart & Reference — Printable reference table of NEC Table 310.16 and common derating factors.

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Frequently Asked Questions

Question: What size wire do I need for 100 amps?

Answer: For a 100-amp circuit with 75°C-rated terminations, you need at minimum #3 AWG copper (rated 100A in the 75°C column of NEC Table 310.16) or #1 AWG aluminum (rated 100A at 75°C). If the circuit is a continuous load, multiply by 1.25 first — 125 amps requires #1 AWG copper or 1/0 AWG aluminum. Always verify derating factors and voltage drop for long runs.

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Question: How do I determine wire size for a circuit?

Answer: Follow four steps: (1) determine the load current, applying the 125% multiplier for continuous loads, (2) look up the minimum wire size in NEC Table 310.16 using the correct temperature column for your terminations (usually 75°C), (3) apply any derating for ambient temperature or conductor bundling, and (4) check voltage drop for long runs. The wire must satisfy all four requirements.

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Question: What is the difference between 60°C, 75°C, and 90°C wire ratings?

Answer: These are the insulation temperature ratings. The 90°C column (THHN, XHHW-2) allows the highest ampacity but most equipment terminals are rated 75°C, so NEC 110.14(C) limits your final selection to the 75°C column for most circuits. The 90°C column is used as a starting point when you need to apply derating factors for heat or bundled conductors.

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Question: Does wire size affect voltage drop?

Answer: Yes. Smaller wires have more resistance, causing more voltage to be lost over the length of the run. The NEC recommends keeping branch circuit voltage drop under 3% and total drop under 5%. For long runs, you may need to upsize the wire beyond what ampacity alone requires. Use a voltage drop calculation or our free wire size calculator to check.

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Summary

Electrical wire sizing requires four sequential checks:

1. Calculate the load current, applying the 1.25 multiplier for continuous loads.
2. Use NEC Table 310.16 to find the minimum wire size, always using the 75°C column unless otherwise specified (per NEC 110.14(C)).
3. Apply derating factors for ambient temperature and bundled conductors.
4. Verify voltage drop is within acceptable limits for the circuit length.

The final wire size must pass all four tests. Skipping any step or using the wrong table column are the most common exam mistakes. Use the worked examples above to practice, and bookmark our Wire Size Calculator for quick answers on the job or during study sessions.

Master wire sizing, and you've solved one of the highest-yield topics on the Texas journeyman electrician exam.