A wire size derating calculator helps you check a conductor after real job conditions are considered.
Before using the calculator, gather the information about the wire you plan to use and the conditions where it will be installed.
Select the Wire You Want to Check
Start with the conductor you are considering for the job.
You need to know:
- Wire size
- Conductor material, such as copper or aluminum
- Insulation type, such as THHN or THWN-2
For example, you may be checking whether a selected #6 copper THHN conductor will still work once the installation conditions are applied.
Know the Ambient Temperature
The ambient temperature is the temperature around the conductor where it will be installed.
A conductor in a normal indoor area may have different conditions than a conductor installed in:
- A hot attic
- A mechanical room
- A rooftop raceway
- An area exposed to higher temperatures
Higher temperatures can reduce the conductor’s allowable ampacity.
Count the Current-Carrying Conductors
You also need to know how many current-carrying conductors are installed together in the same raceway or cable.
More conductors grouped together can create more heat. That can require an adjustment to the conductor ampacity.
Do not confuse this with conduit fill.
Conduit fill checks whether the conductors physically fit inside the conduit. Current-carrying conductor count is used to check whether heat affects the conductor ampacity.
Check the Terminal Temperature Rating
The terminal temperature rating can limit the final allowable ampacity of the selected conductor.
Even when a conductor has 90°C insulation, the equipment terminals may be rated for a lower temperature.
Check the markings on the breaker, disconnect, panel, or equipment before entering the terminal rating.
Quick Example
Say you want to check #6 copper THHN.
Before using the calculator, gather:
- Selected wire: #6 AWG
- Material: copper
- Insulation type: THHN
- Ambient temperature: 40°C
- Current-carrying conductors: 7
- Terminal rating: 75°C
Enter those details into the calculator to see how the selected conductor performs after the job conditions are applied.
How Sparky Toolbox Helps
The Sparky Toolbox Wire Size, Derating, and Breaker Calculator puts the information in one place.
Select the conductor and enter the job conditions. The calculator shows:
- Insulation base ampacity
- Derated ampacity
- Terminal-limited ampacity
- OCPD result
That makes it easier to check the wire you are considering before ordering material or planning the circuit.
Important Reminder
Calculator results are planning tools. Always confirm conductor markings, equipment ratings, project requirements, approved plans, and local inspection requirements before installation.