Electrical Wire Gauge Calculator - AWG Size Tool

Electrical Wire Gauge Calculator

An electrical wire gauge calculator recommends a safe wire size, measured in AWG (American Wire Gauge), for a home electrical circuit. It uses three inputs — the load in amps or watts, the one-way wire run in feet, and the system voltage — to pick the smallest wire that can carry the current without overheating and without losing too much voltage along the way.

Wire that is too thin for its load can overheat, melt its insulation, or start a fire. Wire that is too thin for a long run can also cause a large voltage drop, which makes lights dim and motors run hot. This tool checks both problems at once and returns the smallest gauge that passes both tests.

How to Use This Wire Gauge Calculator

1. Pick a project type (optional). Presets for a garden pond pump, shed wiring, a tiny house circuit, or a workshop fill in typical values.
2. Enter the load. Type the amps directly, or switch to watts and let the calculator divide by the voltage to get amps.
3. Enter the one-way distance from the breaker panel to the device, in feet. The calculator doubles this internally to account for both the outgoing and return conductor.
4. Choose 120V or 240V, the two standard residential voltages.
5. Check "continuous load" if the circuit will run 3 hours or more without a break, such as an EV charger or a space heater. NEC rule 210.19(A)(1) requires the wire to be rated for 125% of that kind of load.
6. Read the result. It shows the recommended gauge, that gauge's rated current, the voltage drop percentage, and a safety label: safe, caution, or unsafe.

How to Calculate Wire Gauge

The calculator runs two checks for every gauge, starting at 14 AWG and moving up to thicker wire until both pass.

1. Convert watts to amps

If the load is given in watts, it is first converted to amps:

$I = \frac{P}{V}$

where I is current in amps, P is power in watts, and V is the system voltage. A 200-watt pump on a 120V circuit draws 200 ÷ 120 = 1.67 amps.

2. Add the continuous-load margin

For a continuous load, the required current is multiplied by 1.25 before the ampacity check (the voltage-drop check still uses the real current):

$I_{required} = I \times 1.25$

3. Check ampacity

Ampacity is the maximum current a gauge can carry. The calculator uses the following ratings, which are the NEC Table 310.16 60°C column for uncoated copper. This is the column that applies to NM-B ("Romex") and UF cable — the types used in the calculator's presets — even though the individual conductors inside that cable are rated for 90°C. A thicker cable in metal conduit with 75°C-rated terminations can legally carry more current on some of these gauges, but this calculator does not assume that setup.

A gauge passes this step if its rated current is at or above the required current from step 2.

4. Check voltage drop

Even a gauge with enough ampacity can lose too much voltage over a long run. The calculator uses the resistance values above:

$V_{drop} = \frac{2 \times R \times I \times L}{1000}$

$V_{drop\%} = \frac{V_{drop}}{V_{source}} \times 100$

Here R is the wire's resistance in ohms per 1000 feet, I is the actual current in amps (not the continuous-load-adjusted figure), L is the one-way distance in feet, and the 2 accounts for current traveling out on one conductor and back on the other.

The calculator labels the result:

• Safe — voltage drop under 3%
• Caution — voltage drop from 3% up to 5%
• Rejected — voltage drop over 5%; the calculator tries the next thicker gauge instead

If no gauge up to 2 AWG satisfies both checks, the calculator returns "2 AWG or Larger" with an unsafe label and recommends a licensed electrician.

Worked Example

Consider a 15-amp load on a 100-foot one-way run at 120V, not continuous (the calculator's shed-wiring preset).

Try 14 AWG. Its ampacity is 15A, which just meets the 15A load. Its voltage drop is:

$V_{drop} = \frac{2 \times 3.07 \times 15 \times 100}{1000} = 9.21\text{ V}$

$V_{drop\%} = \frac{9.21}{120} \times 100 = 7.68\%$

That is above the 5% cutoff, so 14 AWG is rejected even though it passed the ampacity check.

Try 12 AWG. Its ampacity is 20A, which covers the 15A load. Its voltage drop is:

$V_{drop} = \frac{2 \times 1.93 \times 15 \times 100}{1000} = 5.79\text{ V}$

$V_{drop\%} = \frac{5.79}{120} \times 100 = 4.83\%$

That is under the 5% cutoff, so 12 AWG is accepted. Because 4.83% is above 3%, the result is labeled "caution" rather than "safe." The calculator recommends 12 AWG wire with a caution flag on this run.

Common Project Types

• Garden pond pump: typically a few hundred watts, often 50–200 feet from the house, on a GFCI-protected 120V outdoor circuit.
• Shed wiring: 15–20 amp circuits, commonly 50–150 feet from the main panel.
• Tiny house circuits: a mix of 120V and 240V circuits over short indoor runs.
• Workshop power tools: 20 amp or larger circuits, often 240V for heavy equipment, over runs of 20–100 feet.

Safety Notes

A licensed electrician should handle service panel work, any job that needs a permit, and any installation the user is not confident doing safely. The wire gauge must always match the circuit breaker rating, and local electrical code should be checked before starting a project. This calculator is a planning aid, not a substitute for code compliance review.

Frequently Asked Questions

What wire gauge do I need for 20 amps?

12 AWG copper wire is the minimum for a 20-amp circuit; its ampacity is exactly 20A. On a 120V run, voltage drop stays under 3% (safe) up to roughly 47 feet and under 5% up to roughly 78 feet. Past about 78 feet, this calculator switches the recommendation to 10 AWG.

How far can I run 12 AWG wire at 20 amps?

At 20 amps and 120V, 12 AWG stays in the "safe" voltage-drop band (under 3%) for about 47 feet. Between roughly 47 and 78 feet it is still usable but flagged "caution" (3–5% drop). Beyond about 78 feet the drop exceeds 5%, and the calculator recommends 10 AWG instead.

What does AWG stand for?

AWG stands for American Wire Gauge, the standard system for wire diameter used in North America. A smaller AWG number means a thicker wire: 10 AWG is thicker than 14 AWG and can carry more current.

What is the difference between 12 AWG and 14 AWG wire?

12 AWG is thicker and rated for up to 20 amps. 14 AWG is thinner and rated for up to 15 amps. 12 AWG is standard for kitchen and bathroom outlet circuits and other 20-amp circuits; 14 AWG is standard for 15-amp lighting and outlet circuits.

What happens if I use wire that is too thin for the load?

Undersized wire can overheat, melt its insulation, and create a fire risk. It can also cause excessive voltage drop, which dims lights and can damage motors, and it may repeatedly trip the breaker. It also fails inspection under most electrical codes.

Is 10 AWG wire OK for 30 amps?

Yes. 10 AWG copper wire is rated for 30 amps on the 60°C column used by NM-B and UF cable, and it is the standard gauge for many 30-amp circuits, such as electric water heaters and window air conditioners. The breaker must still match the wire gauge, and voltage drop should be checked for long runs.

About These Numbers

This calculator uses the NEC Table 310.16 60°C ampacity column and NEC Chapter 9 Table 8 resistance values for uncoated copper conductors, with a 3% caution threshold and a 5% rejection threshold for voltage drop on branch circuits. Local code can differ from the National Electrical Code, and only a licensed electrician or local inspector can confirm what a specific installation requires.