Common picks electricians reach for first
- 15A branch circuit: #14 Cu
- 20A branch circuit: #12 Cu
- 30A branch circuit: #10 Cu
- 60A feeder: #6 Cu or #4 Al
- 100A feeder: #3 Cu or #1 Al
- 200A residential service: 2/0 Cu or 4/0 Al
Temperature-column check before you trust the answer
The conductor insulation rating is not automatically the final ampacity column. The lowest termination temperature in the circuit still controls the final usable ampacity.
Aluminum check before you order conductor
Aluminum often wins on price, but it usually needs the next larger size and deserves a fresh voltage-drop check. Use the copper vs aluminum comparison when the project is on the fence.
When to stop trusting the cheat sheet and rerun the math
- Hot ambient temperatures
- More than three current-carrying conductors
- Long runs where voltage drop matters
- Material changes from copper to aluminum
- Anything near a service or feeder limit
Frequently Asked Questions
Is this enough to size every conductor?
No. This is a quick reference. The live calculator is still the right tool when derating, voltage drop, or material changes matter.
What is the most common wire-sizing mistake?
Using the wrong temperature column or forgetting that small-conductor breaker caps still apply even if the raw ampacity looks higher.
When should I stop trusting the cheat sheet?
The moment the run is long, the ambient is hot, the conduit is crowded, or the job switches from copper to aluminum.
Should I use this instead of checking voltage drop?
No. Ampacity and voltage drop are separate questions. A conductor can pass one and still fail the other.
Use The Live Tool
Wire Size Calculator
Use the live conductor-sizing workflow when the quick reference is not enough to defend the final answer.