ToolwickHow to Use This Wire Gauge & Ampacity Calculator
Pick a tab. Find Gauge takes your load in amps, the system voltage and phase, the one-way run length, wire material, insulation rating, breaker/terminal rating, installation method, ambient temperature and number of bundled conductors. It returns the smallest AWG that passes every check, the standard breaker size, the usable ampacity and the voltage drop. Check Wire tests wire you already own. Tell it whether the number you entered is a load or an installed breaker rating, because the 125% continuous factor only applies to loads. Voltage Drop uses sliders so you can see the effect of length and current on any gauge.
Why This Matters
Undersized wire is a quiet fire hazard. When a conductor carries more than its rating, it heats up, insulation hardens and cracks, and connections loosen, often long before a breaker trips. A 40 A continuous EV charger needs a 50 A breaker (40 × 1.25) and 6 AWG copper, not the 8 AWG that looks big enough. A 20 A table saw on 100 feet of 12 AWG at 120 V loses 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.9 V, or 6.6%. The breaker never trips, but the motor runs hot and starts poorly.
Two rules catch people out. NEC 240.4(D) caps 14, 12 and 10 AWG copper at 15, 20 and 30 A breakers, so 14 AWG on a 20 A breaker is never right, even though 14 AWG at 90°C looks rated for 25 A. And NEC 110.14(C) limits you to the rating of the breaker and lug terminals, so premium 90°C wire does not buy extra capacity at the terminals. Electricians, DIYers, solar and RV installers and workshop builders all need ampacity and voltage drop right together.
How It's Calculated
The calculator works in five steps:
- Design load: amps × 1.25 for continuous loads, otherwise amps.
- Breaker size: round the design load up to the next standard size in NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A and so on.
- Usable ampacity is the lowest of three limits: the table ampacity at your insulation rating multiplied by the ambient and bundling factors, the table ampacity at the terminal rating, and the 240.4(D) cap.
- Pick the smallest AWG whose usable ampacity is at least the breaker size. This is deliberately conservative, because NEC 240.4(B) would sometimes allow the next breaker size above the wire's rating.
- Voltage drop check: if drop exceeds 3%, step up to the first gauge that meets 3%. If none does, the tool reports its largest size and says the drop is still too high.
Three-phase: VD = (1.732 × K × I × L) ÷ CM
VD% = VD ÷ system voltage × 100
K is 12.9 for copper and 21.2 for aluminum (ohm·cmil/ft). I is the load current, L is the one-way length in feet, and CM is the conductor's circular-mil area. Voltage drop uses the actual load, not the 125% figure. The "auto" terminal setting follows NEC 110.14(C): the 60°C column for circuits of 100 A or less, 75°C above that. The code allows 75°C for some larger conductors on smaller circuits, but the tool stays on the safer 60°C side. Choose 75°C only if your breaker and equipment are marked for it.
Worked Example
A permanently wired 20 A continuous heater, 120 V single-phase, 60 ft from the panel. Copper 75°C (THWN) wire in conduit, auto terminals, 30°C ambient, three or fewer conductors. These are the tool's default Find Gauge inputs.
- Design load: 20 × 1.25 = 25 A. The standard breaker is 25 A.
- Terminals: the breaker is 100 A or less, so the 60°C column limits the terminals.
- 14 AWG: 20 A (75°C) vs. terminal 15 A vs. cap 15 A gives 15 A usable, which is too small.
- 12 AWG: 25 A vs. terminal 20 A vs. cap 20 A gives 20 A usable, which is still too small.
- 10 AWG: 35 A vs. terminal 30 A vs. cap 30 A gives 30 A usable, which is at least 25 A, so it passes.
- Voltage drop: 2 × 12.9 × 20 × 60 ÷ 10,380 = 2.98 V, or 2.49% of 120 V. That is under 3%.
Result: 10 AWG copper with a 25 A breaker.
Tips & Common Mistakes
- Never round down. If the load falls between standard breaker sizes, the breaker goes up and the wire must support that size.
- Read the breaker label. Unless the breaker and lugs are marked 75°C, treat them as 60°C when the circuit is 100 A or less.
- NM-B cable is a 60°C product. Choose 60°C insulation for Romex, even though the conductors inside are rated 90°C.
- Enter one-way length. The formula already doubles it for the return path.
- Don't mix up load and breaker in Check Wire. A 20 A breaker entered as a "load" with the continuous box ticked would be treated as 25 A. Choose the right entry type first.
- Count bundled conductors. Four or more current-carrying conductors in one raceway reduce ampacity (80% for 4–6, 70% for 7–9). Hot attics and garages reduce it further. This tool doesn't cover kcmil sizes above 4/0, parallel runs or motor circuits.
Frequently Asked Questions
What AWG wire for a 20-amp circuit?
For a non-continuous 20 A load, 12 AWG copper is standard, because 240.4(D) caps 12 AWG at a 20 A breaker. If the load is continuous, it must be sized at 125%, which means a 25 A breaker and normally 10 AWG copper.
What's the difference between AWG and wire diameter?
AWG is an inverse scale, so a lower number means thicker wire. Cross-sectional area roughly doubles every 3 gauge steps, and diameter doubles about every 6. Ampacity grows more slowly than area: going from 10 AWG (30 A at 60°C) to 4 AWG (70 A) takes six steps and quadruples the area but only a bit more than doubles the ampacity.
How much voltage drop is acceptable?
The NEC informational notes recommend no more than 3% on a branch circuit or feeder and 5% overall. These are recommendations, not enforceable limits. Sensitive electronics and long motor runs do better under 2%.
Can I use aluminum wire instead of copper?
Yes, select Aluminum. The calculator uses aluminum's higher resistivity (K = 21.2) and its own ampacity column in Table 310.16, which usually means going up about two sizes. Free-air aluminum values aren't included, so conduit values are used instead. Aluminum building wire below 12 AWG isn't recognized.
What is the NEC 240.4(D) small-conductor rule?
It limits overcurrent protection for 14, 12 and 10 AWG copper to 15, 20 and 30 A. For aluminum, 12 AWG is capped at 15 A and 10 AWG at 25 A. The tool applies these caps automatically, so it never pairs a small wire with an oversized breaker.
Why doesn't 90°C wire let me use a smaller gauge?
The lowest of the wire, terminal and 240.4(D) limits always governs, and most breakers and lugs are only rated for 60°C or 75°C. 90°C wire mainly helps by giving more headroom before ambient heat or bundling derating drops you below the terminal limit.