Electrical

Wire Gauge Calculator

Find the recommended AWG wire gauge from load current and NEC ampacity, with an optional voltage-drop check.

Formula Required Ampacity = I × k, then select smallest gauge with Ampacity ≥ Required AmpacityReviewed Aug 22, 2026

Sizing a wire gauge starts with ampacity - how much current the conductor can carry safely - not voltage drop. This wire gauge calculator finds the smallest standard AWG wire size rated to carry your load current per NEC Table 310.16, then, if you supply a run length and supply voltage, checks whether that gauge also keeps voltage drop within an allowed percentage - upsizing further if a long run demands it. It is the same order electricians actually size wire in: safety (ampacity) first, efficiency (voltage drop) second.

Calculation Bench

Conductor Material

01Load Current

I · Circuit load current

02Continuous Load Factor

k · 1.25 for continuous (3+ hr) loads, 1 otherwise

03Run Length

L · Optional - enables the voltage-drop check

04Supply Voltage

V · Optional - pair with Run Length

05Max Voltage Drop

% · Applies only if Run Length/Supply Voltage are set

Solution

Enter load current and material to get a recommended wire gauge.

Required Ampacity = I × k, then select smallest gauge with Ampacity ≥ Required Ampacity

Formula Sheet

A_{req} = I \times k

V_{drop\%} = \dfrac{2 I \rho L}{A \, V} \times 100

AWGRecommended Wire Gauge
ILoad Current
kContinuous Load Factor
LRun Length
VSupply Voltage
ρResistivity

Variables & Units

Symbol	Variable	Description	Common Units
AWG	Recommended Wire Gauge	The smallest standard AWG wire size satisfying both ampacity and (if checked) voltage-drop requirements.
I	Load Current	The circuit's load current.	A
k	Continuous Load Factor	1.25 for a continuous load, 1 for non-continuous, per NEC 210.19(A).
L	Run Length	One-way conductor run length, from source to load.	m, ft
V	Supply Voltage	The circuit's nominal supply voltage.	V
ρ	Resistivity	Conductor material resistivity - fixed by the Material selection.

How to Use This Calculator

01Enter the load current in amps.
02Choose the conductor material - Copper or Aluminum.
03If the load runs continuously (3 hours or more), set the continuous load factor to 1.25, per NEC 210.19(A) - otherwise leave it at 1.
04Optionally enter the one-way run length and supply voltage to also check voltage drop, with a maximum allowed percentage (3% is the default, matching the NEC's usual guideline).
05The calculator returns the recommended AWG wire gauge, its rated ampacity, and - if a voltage-drop check ran - whether the gauge had to be increased beyond the ampacity minimum.

How the Formula Works

Ampacity is the maximum current a conductor can carry continuously without exceeding its insulation's temperature rating - it is a fixed, tabulated property per wire gauge and material, not something derived from a simple formula, which is why this calculator works from NEC Table 310.16's published values (75°C column) rather than computing it.

The calculator multiplies your load current by the continuous-load factor (1.25 for continuous loads, 1 otherwise) to get the required ampacity, then finds the smallest gauge in the table whose rated ampacity meets or exceeds it.

If a run length and supply voltage are given, the calculator checks that gauge's actual voltage drop - using the same Vdrop = 2 × I × ρ × L / A relationship as this site's Voltage Drop Calculator - against your allowed percentage. A short run at moderate current usually passes at the ampacity-minimum gauge; a long run can force a substantially larger gauge even though the smaller one is perfectly safe from an ampacity standpoint.

This is why ampacity and voltage drop are two separate checks a conductor has to pass, not one: a wire sized only for ampacity can still drop more voltage than acceptable over a long run, and a wire sized only for voltage drop on a short run can end up needlessly oversized.

Worked Example 01

40A continuous load, copper, ampacity only

Known

Load Current (I): 40 A
Material: Copper
Continuous Load Factor (k): 1.25

Formula

Required Ampacity = I × k, then select smallest gauge with Ampacity ≥ Required Ampacity

Substitution

Required Ampacity = 40 × 1.25 = 50 A → smallest copper gauge with ampacity ≥ 50 A

Result

8 AWG (rated 50 A)

A 40 A continuous load needs 50 A of ampacity once the 1.25 continuous-load factor is applied. 8 AWG copper is rated for exactly 50 A per NEC Table 310.16, the smallest standard gauge that qualifies.

Worked Example 02

20A load over a 30m run - voltage drop forces a larger gauge

Known

Load Current (I): 20 A
Material: Copper
Run Length (L): 30 m
Supply Voltage (V): 120 V
Max Voltage Drop: 3%

Formula

Vdrop% = (2 × I × ρ × L / A) / V × 100 - upsize gauge until Vdrop% ≤ limit

Substitution

Ampacity minimum = 14 AWG (20 A), but Vdrop% at 14 AWG ≈ 8.3% - check 12, 10 AWG, both still over 3% - 8 AWG ≈ 2.06%

Result

8 AWG (upsized from the 14 AWG ampacity minimum)

20 A alone only requires 14 AWG for ampacity, but over a 30 meter run, 14 AWG would drop about 8.3% of the supply voltage - far over the 3% limit. Checking each larger gauge in turn, only 8 AWG brings the drop down to about 2.06%, three sizes larger than ampacity alone would suggest.

Applications

01Getting a starting-point AWG recommendation for a branch circuit or feeder before consulting full code tables
02Checking whether a long run forces a larger gauge than ampacity alone would suggest
03Comparing copper vs aluminum gauge requirements for the same load
04Using wire gauge calculator and wire size calculator terminology for the same AWG sizing workflow

NEC Table 310.16 Ampacity by Gauge (75°C Column)

Gauge	Copper	Aluminum
14 AWG	20 A	—
12 AWG	25 A	20 A
10 AWG	35 A	30 A
8 AWG	50 A	40 A
6 AWG	65 A	50 A
4 AWG	85 A	65 A
3 AWG	100 A	75 A
2 AWG	115 A	90 A
1 AWG	130 A	100 A
1/0 AWG	150 A	120 A
2/0 AWG	175 A	135 A
3/0 AWG	200 A	155 A
4/0 AWG	230 A	180 A
250 kcmil	255 A	205 A
500 kcmil	380 A	310 A

Assumptions

01Ampacity is from NEC Table 310.16's 75°C column, for up to 3 current-carrying conductors at 30°C (86°F) ambient - the standard baseline condition, not a project-specific derated value.
02The voltage-drop check assumes a single-phase or DC circuit (k = 2) - a three-phase run should use this site's Voltage Drop Calculator directly, which supports the √3 case.
03Aluminum conductors are not offered below 12 AWG, matching standard practice - aluminum building wire smaller than 12 AWG isn't a standard product.

Where This Model Stops

01Does not apply ambient-temperature correction factors (NEC Table 310.15(B)(1)) for conductors in a hot attic, rooftop conduit, or other elevated-temperature location.
02Does not apply conductor-bundling adjustment factors (NEC 310.15(C)(1)) for more than 3 current-carrying conductors sharing a raceway or cable.
03This is a reference sizing estimate only - actual conductor selection must follow the applicable electrical code in full and, beyond a simple branch circuit, a licensed electrician or electrical engineer.

References

[1]
NEC Table 310.16 - Conductor Ampacities
National Electrical Code (NFPA 70)
75°C column ampacity values for copper and aluminum conductors, up to 3 current-carrying conductors at 30°C ambient - the table this calculator's gauge selection is based on.
[2]
American Wire Gauge (AWG) standard
Standard electrical engineering reference
Cross-sectional area per gauge from the standard AWG geometric progression, d(n) = 0.127mm × 92^((36-n)/39), cross-checked against the published 4/0 AWG = 211.6 kcmil reference point.

Frequently Asked Questions

Why does ampacity matter more than voltage drop for wire sizing?

Ampacity is a safety limit - exceeding it overheats the conductor and its insulation, a fire risk. Voltage drop is an efficiency and equipment-performance issue, not a direct safety hazard by itself. That's why ampacity is always checked first, with voltage drop as a secondary check that can call for a larger gauge on long runs, never a smaller one.

What's the 1.25 continuous load factor for?

NEC 210.19(A) requires branch-circuit conductors supplying a continuous load (one running 3 hours or more) to be sized for 125% of that load's current, to keep the conductor comfortably within its rating during sustained operation rather than sized to its bare minimum.

Does this calculator replace a licensed electrician?

No - it gives a reference starting point using the ampacity table and voltage-drop formula, but doesn't apply every NEC derating factor (ambient temperature, conductor bundling) or account for load diversity, panel schedules, or local code amendments. Always have final conductor sizing reviewed by a licensed electrician or electrical engineer.

Why does the calculator sometimes skip several gauge sizes at once?

Because voltage drop scales with run length, a sufficiently long run can push several consecutive gauges over the allowed percentage before finding one large enough - the worked example above skips from 14 AWG straight past 12 and 10 AWG to 8 AWG for exactly this reason.

Is wire gauge the same thing as wire size?

In common AWG-based sizing, yes: people often say wire gauge, wire size, and AWG size for the same conductor selection. This calculator uses AWG gauge labels such as 14 AWG, 8 AWG, and 1/0 AWG, then checks ampacity and optional voltage drop.

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