Wire size can change when a circuit has a continuous load, high ambient temperature, or several current-carrying conductors in the same raceway.
This example shows how those factors affect wire size and breaker selection.
Quick Wire Size and Derating Example
Example: a 40-amp continuous load using #6 copper THHN conductors in a 40°C ambient temperature, with seven current-carrying conductors in the raceway.
Step 1: Calculate the Required Load
For a continuous load, calculate 125% of the load.
40 amps × 125% = 50 amps
The circuit needs a 50A overcurrent device for this example.
Step 2: Start With the 90°C THHN Ampacity
Because the conductor is THHN, use the 90°C ampacity as the starting point for correction and adjustment.
From NEC Table 310.16:
#6 copper THHN at 90°C = 75 amps
Step 3: Apply the Temperature Correction Factor
At 40°C, use the 90°C correction factor from NEC Table 310.15(B)(1)(1).
75 amps × 0.91 = 68.25 amps
Step 4: Apply the Current-Carrying Conductor Adjustment
There are seven current-carrying conductors in the raceway.
From NEC Table 310.15(C)(1), seven to nine current-carrying conductors use a 70% adjustment factor.
68.25 amps × 0.70 = 47.8 amps
After temperature correction and conductor-count adjustment, #6 THHN has an adjusted ampacity of 47.8 amps.
Step 5: Check the Terminal Limitation
THHN is 90°C insulation, but the terminals may have a lower temperature rating.
For this example, the adjusted ampacity is already 47.8A. That is below the #6 copper 75°C ampacity of 65A, so a 75°C terminal limitation would not reduce the result further.
Final adjusted ampacity = 47.8 amps
Step 6: Determine the Breaker Size
The continuous load calculation calls for a 50A breaker.
40 amps × 125% = 50 amps
The adjusted conductor ampacity is 47.8A. Since 47.8A is not a standard breaker size, the next standard size up is 50A.
Under NEC 240.4(B), the next standard size up may be permitted when all of that section’s conditions are met and no other rule limits the circuit.
For this example:
Breaker size = 50 amps
Final Results for This Example
- Load: 40 amps continuous
- Required breaker size: 50 amps
- Conductor: #6 AWG copper THHN
- 90°C base ampacity: 75 amps
- Ambient temperature: 40°C
- Current-carrying conductors: 7
- Adjusted ampacity: 47.8 amps
- Breaker size: 50 amps
How Sparky Toolbox Helps
Instead of looking up conductor ampacity, applying the temperature factor, applying the conductor-count adjustment, checking terminal limitations, and comparing breaker options by hand, enter the circuit details into Sparky Toolbox.
The Wire Size Calculator shows:
- Insulation base ampacity
- Derated ampacity
- Terminal-limited ampacity
- Wire size result
- OCPD and breaker results
That gives you a quick starting point before ordering wire or planning the circuit.
Important Reminder
This is one simple example. Wire size and breaker selection can change based on load type, conductor material, insulation rating, ambient temperature, current-carrying conductor count, terminal ratings, equipment instructions, and other job details.
Always verify the current Code, equipment markings, project requirements, approved plans, and local inspection requirements before installation.