Wire Sizing

Selecting the correct wire size is one of the most critical tasks an electrician performs. Undersized wire causes overheating, voltage drop, and fire hazards. The NEC provides ampacity tables, temperature correction factors, and conduit fill derating factors that determine the minimum conductor size for any installation.

The Wire Sizing Process

Every wire sizing decision follows this sequence:

1. Determine the load current (from the equipment nameplate or calculated load)
2. Apply the 80% continuous load rule if applicable (multiply continuous load by 1.25)
3. Look up ampacity in NEC Table 310.16
4. Apply temperature correction if ambient temperature exceeds 30 C (86 F)
5. Apply conduit fill derating if more than 3 current-carrying conductors are in a conduit
6. Check voltage drop for long runs
7. Select the larger of ampacity-based size or voltage-drop-based size

NEC Table 310.16 — Ampacity (Abbreviated)

Allowable ampacities for insulated copper conductors in raceway, cable, or directly buried (based on ambient temperature of 30 C):

Important NEC rule: For circuits rated 100A or less, you must use the 60 C column for ampacity unless both the conductor and all terminations are rated 75 C. Most residential breakers and devices are rated 60 C.

Temperature Correction Factors

When the ambient temperature exceeds 30 C, conductor ampacity must be reduced:

$\text{Adjusted ampacity} = \text{Table ampacity} \times \text{Correction factor}$

Conduit Fill Derating (NEC 310.15(C))

When more than 3 current-carrying conductors share a conduit, heat buildup reduces ampacity:

Note: Neutral conductors carrying only unbalanced current and equipment grounding conductors do not count as current-carrying conductors.

$\text{Derated ampacity} = \text{Table ampacity} \times \text{Derating factor}$

Worked Examples

Example 1: Basic Residential Circuit

Scenario: A 120V, 20A general-purpose branch circuit in a home. Standard conditions (30 C ambient, 3 or fewer conductors in conduit). What wire size?

Step 1 — Load current: 20A breaker (non-continuous loads on this circuit).

Step 2 — Look up ampacity: From the 60 C column (residential terminations), #12 AWG is rated for 20A.

Step 3 — No correction needed (standard temperature, 3 or fewer conductors).

Answer: Use #12 AWG copper with 60 C insulation (or higher). This is the standard for 20A residential branch circuits.

Example 2: Continuous Load with Temperature Derating

Scenario: A commercial kitchen has a continuous 36A load on a 208V circuit. The conduit runs through a ceiling space at 40 C ambient. Three current-carrying conductors in the conduit. What wire size?

Step 1 — Apply 80% rule: $36 \times 1.25 = 45$ A minimum breaker and wire ampacity.

Step 2 — Apply temperature correction. Using 75 C wire in 40 C ambient, the correction factor is 0.88.

Step 3 — Find required table ampacity:

$\text{Required table ampacity} = \frac{45}{0.88} = 51.1 \text{ A}$

Step 4 — Select wire: From the 75 C column, #8 AWG is rated at 50A (not enough), #6 AWG is rated at 65A.

Answer: Use #6 AWG THWN copper. The 50A breaker provides the next standard size above 45A.

Example 3: Conduit Fill Derating

Scenario: Six current-carrying #10 AWG THHN conductors are pulled through a conduit in standard 30 C conditions. What is the adjusted ampacity of each conductor?

Step 1 — Base ampacity: #10 THHN (90 C column) = 40A.

Step 2 — Apply derating: 6 conductors = 0.80 factor.

$\text{Derated ampacity} = 40 \times 0.80 = 32 \text{ A}$

Step 3 — Terminal limitation: If terminations are 75 C rated, the 75 C column ampacity for #10 is 35A. The derated 90 C value (32A) is less than 35A, so 32A governs.

Answer: Each conductor is rated for 32 amps after derating.

Practice Problems

Test your understanding with these problems. Click to reveal each answer.

Common Mistakes to Avoid

1. Using the 90 C column for final sizing. The 90 C ampacity is only a starting point for derating. If the terminals are rated 60 C or 75 C, you must also check those column values.
2. Forgetting to apply the 1.25 multiplier for continuous loads. A 40A continuous load requires 50A of conductor capacity, not 40A.
3. Not counting current-carrying conductors correctly. Neutrals on 3-phase, 4-wire circuits with non-linear loads count as current-carrying. Equipment grounds never count.
4. Stacking derating and correction factors incorrectly. When both apply, multiply the table ampacity by both: $\text{Adjusted} = \text{Table} \times \text{Temp factor} \times \text{Fill factor}$.
5. Ignoring voltage drop. Ampacity may be satisfied by #10 AWG, but voltage drop on a long run may require #8 or #6.

Key Takeaways

• Always start with the load current and apply the 1.25 multiplier for continuous loads
• NEC Table 310.16 gives base ampacity — adjust for temperature and conduit fill
• For circuits 100A or less with standard equipment, use the 60 C column
• The 90 C column is used as the starting point when applying derating factors, but the final ampacity cannot exceed the 75 C or 60 C column value for the termination rating
• Wire sizing must satisfy both ampacity and voltage drop — always check both

Math for Electricians