Key takeaways
- Wire is sized by two limits — ampacity (the current it can safely carry) and voltage drop over the run.
- The NEC 240.4(D) rule caps #14 copper at 15 A, #12 at 20 A, and #10 at 30 A no matter what the larger tables say.
- Long runs need a bigger wire to hold voltage drop under ~3%.
- Always use the larger of the ampacity gauge and the voltage-drop gauge.
Two limits decide the size
Wire is sized by two limits — ampacity (the current it can safely carry) and voltage drop over the run. Ampacity protects the conductor from overheating; voltage drop protects the load from running on too little voltage. You size for both, then pick whichever forces the heavier conductor.
The NEC small-conductor rule
Ampacity tables list values that look generous for small wire, but the NEC overrides them with the 240.4(D) small-conductor rule. Regardless of any higher table ampacity, overcurrent protection limits #14 copper to 15 A, #12 copper to 20 A, and #10 copper to 30 A. The practical consequence: a 20 A circuit needs #12, not #14, even though a thermal table might suggest #14 could carry it. Start every job by reading the breaker size off this rule.
Then check the run length
Ampacity is only the floor. Over a long run the conductor's own resistance bleeds off voltage, and a load fed too low draws more current, runs hot, and works poorly. To keep voltage drop under about 3% on a branch circuit, a long run often needs a wire one or two gauges larger than ampacity alone would require. The longer the home run, the more likely voltage drop — not the breaker — sets your size.
The wire size calculator runs both checks at once and returns the controlling gauge, so you don't have to look up circular mils by hand.
Common circuits (copper, short runs)
For short runs where voltage drop isn't a concern, ampacity and the 240.4(D) rule give you these starting points:
| Breaker / load | Copper wire |
|---|---|
| 15 A | #14 |
| 20 A | #12 |
| 30 A | #10 |
| 40 A | #8 |
| 50 A | #6 |
| 100 A | #2 |
A worked example: a 50 A subpanel 100 ft away
Say you're feeding a 50 A subpanel 100 ft from the main at 240 V. By ampacity alone the table allows #8 copper. But run the voltage-drop check: circular mils = (2 × 12.9 × 50 × 100) ÷ (240 × 3) = 129,000 ÷ 720 = ~17,900 circular mils. That exceeds #8 (16,510 cmil) and lands you on #6 copper (26,240 cmil), which holds drop comfortably under 3%. Voltage drop, not ampacity, sets the final size here — exactly the case the formula is built to catch.
When the load is an air conditioner or heat pump, size the circuit from its actual draw first — the HVAC BTU calculator helps you nail the unit's load before you pick the wire.
Frequently asked questions
What wire size do I need for a 20 amp circuit?
Use #12 copper. The 240.4(D) small-conductor rule caps #14 copper at 15 A, so a 20 A circuit needs #12 even if a table lists a higher ampacity for #14.
Does the length of the run change the wire size?
Yes. On long runs voltage drop becomes the limit. To hold drop under ~3% you often step up one or two gauges beyond what ampacity alone requires.
How do I calculate wire size for voltage drop?
Circular mils needed = (2 × K × Amps × Distance) ÷ (Volts × VD%), with K = 12.9 copper or 21.2 aluminum. Use the larger of the ampacity gauge and the voltage-drop gauge.