Wire sizing is not a single table lookup.
A proper conductor calculation is a chain of steps: determine the load, calculate required ampacity, select a conductor, apply any temperature correction and conductor-count adjustment factors, verify the terminal temperature limitation, and then select an OCPD that is permitted by the NEC.
Skipping any one of those steps can mean a failed inspection, nuisance tripping, or an overheated conductor behind a finished wall.
This guide covers general branch-circuit and feeder conductor sizing under the 2026 NEC. Special rules can override this normal workflow for motors, transformers, HVAC equipment, welders, EV charging equipment, photovoltaic systems, fire pumps, dwelling services, and other equipment covered elsewhere in the NEC.
The Key Code Sections
- NEC Table 310.16 — allowable ampacities for insulated conductors rated 0–2000V, installed with not more than three current-carrying conductors in a raceway, cable, or earth, based on 30°C ambient temperature
- NEC Table 310.15(B)(1)(1) — ambient temperature correction factors for conductors rated 100°C or less
- NEC 310.15(B)(2) — rooftop temperature adder
- NEC Table 310.15(C)(1) — adjustment factors for more than three current-carrying conductors
- NEC 110.14(C) — terminal temperature limitations
- NEC 240.4 — conductor overcurrent protection
- NEC 240.4(B) — next-higher-standard-size OCPD rule
- NEC 240.4(D) — small-conductor overcurrent protection limits
- NEC 240.6(A) — standard OCPD ratings
Step 1 — Determine the Required Ampacity
Start with the load, not the conductor table.
For a continuous load, the conductor must be sized at 125% of the continuous load. A continuous load is one expected to operate for three hours or more.
Required ampacity = (Continuous load × 1.25) + Noncontinuous load
Example — 30A Continuous Load
30A × 1.25 = 37.5A minimum conductor ampacity
The conductor must have a final allowable ampacity of at least 37.5A after all applicable correction factors, adjustment factors, and terminal limitations have been applied.
Step 2 — Select a Starting Ampacity From Table 310.16
Table 310.16 assumes:
- No more than three current-carrying conductors
- A 30°C (86°F) ambient temperature
- Conductors installed in a raceway, cable, or earth
- No special equipment rule that changes the normal ampacity process
The table has three temperature columns:
| Temperature Column | Common Insulation Types | Typical Use |
|---|---|---|
| 60°C | TW, UF | 60°C-rated conductor insulation or equipment terminals |
| 75°C | THWN, XHHW, RHW | Common commercial and industrial equipment terminals |
| 90°C | THHN, THWN-2, XHHW-2 | Starting point for correction and adjustment calculations only |
Ampacities Used in the Worked Example
| Copper Conductor | 75°C Ampacity Used in Examples | 90°C Starting Ampacity Used in Examples |
|---|---|---|
| #8 AWG Cu | 50A | 55A |
| #6 AWG Cu | 65A | 75A |
| #4 AWG Cu | 85A | 95A |
Educational example only: This chart contains only the conductor sizes used in the worked example. Verify the complete NEC ampacity table, conductor insulation marking, terminal rating, and local requirements before installation.
Aluminum generally requires a larger conductor size than copper to achieve the same ampacity.
Always use the correct conductor material column.
Step 3 — Apply Ambient Temperature Correction
Table 310.16 is based on 30°C ambient.
If conductors run through an attic, boiler room, mechanical room, rooftop raceway, or any space hotter than 30°C, apply the correction factor from Table 310.15(B)(1)(1).
Selected Temperature Correction Factors Used in This Article
| Example Ambient Temperature | 75°C Insulation Factor | 90°C Insulation Factor |
|---|---|---|
| 40°C / 104°F | 0.88 | 0.91 |
| 45°C / 113°F | 0.82 | 0.87 |
Educational example only: These are the correction factors used in this article’s worked examples. Verify the complete current table and local requirements before sizing an installation.
Using the 90°C Column for Correction
When the conductor insulation is rated 90°C, such as THHN, THWN-2, or XHHW-2, start the correction calculation from the 90°C ampacity column.
However, the final allowable ampacity cannot exceed the lowest applicable terminal-temperature limitation.
- If both ends are rated 75°C, the final ampacity cannot exceed the 75°C column value.
- If the installation is limited to 60°C terminals, the final ampacity cannot exceed the 60°C column value.
- The conductor insulation rating helps with correction and adjustment calculations but does not override the terminal rating.
Example — #6 Copper, 90°C Insulation, 40°C Ambient, 75°C Terminals
90°C base ampacity: 75A
75A × 0.91 = 68.25A
75°C terminal cap for #6 Cu: 65A
Final allowable ampacity: 65A
Without using the 90°C column, the result would be:
65A × 0.88 = 57.2A
That does not automatically require an upsized conductor.
It only requires upsizing if the circuit needs more than 57.2A of allowable ampacity.
Wet-Location Warning
THHN by itself is dry-location insulation.
For wet locations, verify the conductor carries a wet-location rating such as THWN or THWN-2.
Do not assume a THHN marking alone is sufficient for a wet location.
Rooftop Temperature Warning
For a raceway or cable exposed to direct sunlight on or above a rooftop, NEC 310.15(B)(2) can require a 33°C (60°F) temperature adder.
Where the bottom of the raceway or cable is less than 3/4 inch above the roof, add 33°C to the outdoor ambient temperature before applying the temperature correction factor.
This can substantially increase the required conductor size.
Step 4 — Apply Adjustment Factors for More Than Three Current-Carrying Conductors
This is not a conduit-fill calculation.
Conduit fill determines whether conductors physically fit inside a raceway.
This step is an ampacity adjustment for more than three current-carrying conductors installed together for a continuous length exceeding 24 inches.
When too many current-carrying conductors are grouped together, heat builds up and their allowable ampacity must be reduced.
Selected Conductor-Count Adjustment Factors Used in This Article
| Current-Carrying Conductors | Adjustment Factor Used in Examples |
|---|---|
| 4–6 conductors | 80% |
| 7–9 conductors | 70% |
Educational example only: These are selected adjustment factors used to explain the worked examples. Verify the complete current table before applying an adjustment factor to an installation.
What Counts as a Current-Carrying Conductor
- Each ungrounded conductor carrying load current
- A neutral conductor in a 2-wire circuit
- A neutral conductor in a 3-wire circuit consisting of two phase conductors and a neutral from a 4-wire, three-phase wye system
- A neutral conductor in a 4-wire, three-phase wye circuit where the major portion of the load is nonlinear and harmonic current is present on the neutral
What Does Not Count as a Current-Carrying Conductor
- Equipment grounding conductors
- A neutral that carries only unbalanced current from the other conductors of the same circuit
- A typical 120/240V single-phase multiwire branch-circuit neutral carrying only imbalance
Step 5 — Apply Both Factors When Both Apply
When both ambient-temperature correction and current-carrying-conductor adjustment apply, multiply both factors together, then compare the result to the terminal-temperature limitation.
Final ampacity = (90°C base) × (temperature factor) × (conductor-count factor)
Final ampacity must not exceed the applicable terminal-temperature column.
Example — #6 Copper, 90°C Insulation, 40°C Ambient, Six Current-Carrying Conductors, 75°C Terminals
90°C base ampacity: 75A
Temperature factor at 40°C: 0.91
Conductor-count factor for six conductors: 0.80
75A × 0.91 × 0.80 = 54.6A
75°C terminal cap: 65A
Final allowable ampacity: 54.6A
The adjusted value governs because it is lower than the terminal-temperature limitation.
Step 6 — Verify the Terminal Temperature Limitation
This is one of the most commonly missed steps.
The final allowable ampacity cannot exceed the lowest applicable terminal-temperature rating.
- Equipment rated 100A or less, or marked for conductors #14 AWG through #1 AWG, must generally be sized from the 60°C column under 110.14(C)(1)(a), unless the equipment is identified for use with a higher conductor-temperature rating.
- Equipment rated over 100A, or marked for conductors larger than #1 AWG, is generally based on 75°C terminations unless marked otherwise.
- Conductors with 90°C insulation can use the 90°C column for correction and adjustment calculations, but the terminal rating determines the maximum final ampacity.
Do not assume every breaker, disconnect, panelboard, or piece of equipment is rated 75°C.
Read the equipment marking.
Step 7 — Apply the Small-Conductor Rule
NEC 240.4(D) limits the maximum OCPD for common small conductors.
These limits apply even if the conductor ampacity table value appears higher.
| Conductor | Maximum OCPD |
|---|---|
| #14 AWG copper | 15A |
| #12 AWG aluminum or copper-clad aluminum | 15A |
| #12 AWG copper | 20A |
| #10 AWG aluminum or copper-clad aluminum | 25A |
| #10 AWG copper | 30A |
Educational quick reference: Verify the current NEC and any special equipment rule before choosing an OCPD.
For example, #12 copper is listed at 25A in the 75°C column of Table 310.16.
That does not allow a normal #12 Cu branch circuit to be protected by a 25A breaker.
The small-conductor rule limits it to a 20A OCPD unless another specific NEC rule applies.
Apply any needed correction and adjustment factors first.
Then verify that the conductor still has enough allowable ampacity for the load and that the OCPD does not exceed the small-conductor limit.
If derating reduces a #10 copper conductor below 30A, you may need to upsize the conductor so it meets the required load ampacity after correction and adjustment.
Step 8 — Select the OCPD
Once the final allowable ampacity is confirmed, select an OCPD that:
- Meets the load requirement
- Does not exceed the conductor's final allowable ampacity
- Complies with the small-conductor rule where applicable
- Is a standard OCPD rating listed in NEC 240.6(A)
The Next-Higher-Standard-Size Rule
NEC 240.4(B) may permit the next higher standard OCPD size, but it is not automatic.
The rule can apply only when all of these conditions are met:
- The conductor ampacity does not match a standard OCPD rating
- The next higher standard OCPD rating is 800A or less
- The conductors are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads
- No other NEC section prohibits the increase
The next-higher-standard-size rule does not override the small-conductor limits in 240.4(D).
Common Standard OCPD Sizes
15A, 20A, 25A, 30A, 35A, 40A, 45A, 50A, 60A, 70A, 80A, 90A, 100A, 110A, 125A, 150A, 175A, 200A, 225A, 250A, 300A, 350A, 400A, 450A, 500A, and 600A.
Worked Example — Full Derating Chain
Scenario:
- 40A continuous load
- 75°C terminals
- Seven current-carrying conductors in one raceway
- 45°C ambient temperature
- Copper conductors marked THHN/THWN-2
Step 1 — Required Ampacity
40A × 125% = 50A minimum required ampacity
Test #8 Copper
90°C base ampacity: 55A
Temperature correction:
55A × 0.87 = 47.85A
Conductor-count adjustment:
47.85A × 0.70 = 33.5A
75°C terminal cap: 50A
Final allowable ampacity: 33.5A
#8 copper does not meet the required 50A ampacity.
Test #6 Copper
90°C base ampacity: 75A
Temperature correction:
75A × 0.87 = 65.25A
Conductor-count adjustment:
65.25A × 0.70 = 45.7A
75°C terminal cap: 65A
Final allowable ampacity: 45.7A
#6 copper does not meet the required 50A ampacity.
Test #4 Copper
90°C base ampacity: 95A
Temperature correction:
95A × 0.87 = 82.65A
Conductor-count adjustment:
82.65A × 0.70 = 57.9A
75°C terminal cap: 85A
Final allowable ampacity: 57.9A
#4 AWG copper meets the 50A required ampacity.
A 50A breaker is permitted because it does not exceed the conductor’s final allowable ampacity of 57.9A.
Common Mistakes
Calling Conductor Adjustment “Conduit Fill Derating”
Conduit fill determines whether conductors physically fit in the raceway.
Conductor adjustment reduces ampacity when too many current-carrying conductors are grouped together.
They are different calculations.
Reading the 90°C Column as the Final Ampacity
The 90°C column is the starting point for temperature correction and conductor-count adjustment.
The final allowable ampacity cannot exceed the lowest applicable terminal-temperature limitation.
Assuming Every Circuit Uses the 75°C Column
Many circuits rated 100A or less are limited to 60°C terminals unless the equipment is marked or listed for 75°C use.
Always verify the terminal rating.
Counting Every Neutral as a Current-Carrying Conductor
A neutral that carries only unbalanced current does not count.
A neutral carrying harmonic current or carrying full circuit current does count.
Applying the Small-Conductor Rule Before Derating
The small-conductor rule sets the maximum OCPD.
It does not create ampacity.
Apply correction and adjustment factors first, verify the conductor meets the load, then confirm the OCPD is permitted.
Assuming the Next Standard Breaker Size Is Always Allowed
The next-higher-standard-size rule in 240.4(B) has conditions.
It is not a blanket permission to round every conductor ampacity upward.
Forgetting the Rooftop Temperature Adder
A raceway close to a rooftop surface exposed to direct sunlight can require a 33°C temperature adder before selecting the correction factor.
This can change the required conductor size dramatically.
Using the Wrong Material Column
Copper and aluminum have different ampacities.
Do not size aluminum conductors from the copper column.
Final Takeaway
The general conductor-sizing process is:
1. Determine the load. 2. Apply the continuous-load rule where required. 3. Select a starting conductor ampacity from Table 310.16. 4. Apply ambient-temperature correction where required. 5. Apply conductor-count adjustment factors where required. 6. Verify the terminal-temperature limitation. 7. Verify the small-conductor rule where applicable. 8. Select an OCPD that is permitted by Article 240.
A conductor is only properly sized when it meets the required load ampacity after every applicable correction, adjustment, terminal limitation, and overcurrent-protection rule has been considered. ```