Understanding Wire Gauge: How to Choose the Right Wire Size

The AWG System Explained

Wire gauge sizing in North America follows the American Wire Gauge (AWG) system, a standardized method dating back to the 1850s. Understanding AWG is fundamental to safe, code-compliant electrical installations.

How AWG Numbering Works

The AWG system operates counterintuitively: smaller numbers indicate larger wire diameters. This inverse relationship confuses many beginners but follows a mathematical progression based on wire drawing manufacturing processes.

Key AWG values:

14 AWG: 0.0641 inches diameter (1,620 circular mils)
12 AWG: 0.0808 inches diameter (2,580 circular mils)
10 AWG: 0.1019 inches diameter (4,107 circular mils)
8 AWG: 0.1285 inches diameter (6,530 circular mils)
6 AWG: 0.1620 inches diameter (10,380 circular mils)
4 AWG: 0.2043 inches diameter (16,510 circular mils)
2 AWG: 0.2576 inches diameter (26,240 circular mils)
1 AWG: 0.2893 inches diameter (33,100 circular mils)
1/0 (0) AWG: 0.3249 inches diameter (41,740 circular mils)
2/0 (00) AWG: 0.3648 inches diameter (52,620 circular mils)
3/0 (000) AWG: 0.4096 inches diameter (66,360 circular mils)
4/0 (0000) AWG: 0.4600 inches diameter (83,690 circular mils)

Circular mils measure wire cross-sectional area. One circular mil equals the area of a circle with 1 mil (0.001 inch) diameter. The formula relates diameter to circular mils:

Circular Mils = Diameter in mils²

Why the inverse numbering? AWG was developed around the wire drawing process, where wire is pulled through successively smaller dies. Each gauge represents a specific number of drawing operations — more draws = thinner wire = higher gauge number.

The mathematical relationship: Each decrease of 3 gauge sizes roughly doubles the cross-sectional area. Moving from 14 AWG to 11 AWG (not a standard size, but mathematically) doubles the area. Going from 12 AWG to 6 AWG (6 steps) quadruples the area.

NEC Table 310.16: The Ampacity Reference

The National Electrical Code Table 310.16 (previously 310.15(B)(16)) specifies the current-carrying capacity (ampacity) of conductors based on insulation temperature ratings and installation conditions.

Temperature Columns: 60°C, 75°C, and 90°C

Modern wire insulations are rated for different maximum operating temperatures:

60°C insulation (140°F):

Types: TW, UF
Older standard, less common in new construction
Lower ampacity for given wire size

75°C insulation (167°F):

Types: THWN, THW, THWN-2, USE, XHHW
Standard for most residential and commercial wiring
Balanced performance and cost

90°C insulation (194°F):

Types: THHN, THWN-2, XHHW-2, RHW-2
Highest ampacity for given wire size
Common in modern installations

Critical Rule: Terminal Temperature Ratings Govern

Here's the key principle that trips up many electricians: Even if your wire has 90°C insulation, you often cannot use the 90°C ampacity column.

Why? Because device terminals (circuit breakers, receptacles, switches) have their own temperature ratings, typically 60°C or 75°C. Using the 90°C column with 75°C-rated terminals would allow excess current that overheats the terminal connection, even though the wire itself could handle it.

NEC 110.14(C) specifies:

Circuits rated 100 amps or less (or 14 AWG through 1 AWG wire): Use 60°C column unless devices are listed for 75°C
Circuits rated over 100 amps: Use 75°C column if terminals are rated 75°C (most are)
The 90°C column is used primarily for derating calculations, not base ampacity

Exception: You can apply temperature derating to the 90°C ampacity, then compare the derated value to the 75°C or 60°C ampacity. If the derated 90°C value is higher than the appropriate terminal column, you may use the smaller wire size. This allows using thinner wire in hot attics or bundled conduit runs.

Ampacity Table for Copper Conductors (NEC 310.16)

| Wire Size | 60°C (Terminal Limited) | 75°C (Typical) | 90°C (Insulation Only) | |-----------|-------------------------|----------------|------------------------| | 14 AWG | 15 A | 20 A | 25 A | | 12 AWG | 20 A | 25 A | 30 A | | 10 AWG | 30 A | 35 A | 40 A | | 8 AWG | 40 A | 50 A | 55 A | | 6 AWG | 55 A | 65 A | 75 A | | 4 AWG | 70 A | 85 A | 95 A | | 3 AWG | 85 A | 100 A | 110 A | | 2 AWG | 95 A | 115 A | 130 A | | 1 AWG | 110 A | 130 A | 150 A | | 1/0 AWG | 125 A | 150 A | 170 A | | 2/0 AWG | 145 A | 175 A | 195 A | | 3/0 AWG | 165 A | 200 A | 225 A | | 4/0 AWG | 195 A | 230 A | 260 A |

Residential quick reference (using 75°C terminals for breakers 60A+):

15A circuit: 14 AWG minimum (but 12 AWG preferred)
20A circuit: 12 AWG minimum
30A circuit: 10 AWG
40A circuit: 8 AWG
50A circuit: 6 AWG (using 75°C column: 65A capacity)
60A circuit: 6 AWG (using 75°C column: 65A capacity)
70A circuit: 4 AWG (using 75°C column: 85A capacity)
100A circuit: 3 AWG (using 75°C column: 100A exactly)

Continuous Load Rule: 125% Multiplier

NEC 210.19(A)(1) and 210.20(A) require conductors and overcurrent protection to be sized for 125% of continuous loads.

A continuous load runs for 3 hours or more. Examples:

Lighting in commercial spaces
HVAC equipment
Appliances that run extended periods

Formula:

Minimum Conductor Ampacity = Continuous Load × 1.25

Example: A baseboard heater draws 20 amps continuously. You cannot put it on a 20A circuit with 12 AWG wire, even though 12 AWG is rated 25A in the 75°C column.

Required capacity: 20A × 1.25 = 25A

Using 60°C terminal column:

12 AWG = 20A (insufficient)
10 AWG = 30A ✓

You must use 10 AWG wire and a 30A breaker for this 20A continuous load when following the 60°C terminal limitation.

If terminals are rated 75°C (verify on the device):

12 AWG = 25A ✓ (exactly meets 25A requirement)
Could use 12 AWG wire with 25A breaker (if such a breaker existed; standard is 20A or 30A)

In practice, this heater goes on a 30A circuit with 10 AWG wire.

Copper vs Aluminum Conductors

Aluminum wiring was popular in the 1960s-70s during copper shortages but fell out of favor for branch circuits due to connection failures. It's still used for service entrance conductors and large feeders where cost savings justify careful installation practices.

Ampacity Comparison

Aluminum has approximately 61% the conductivity of copper, requiring larger wire for equivalent ampacity.

| Copper Size | 75°C Ampacity | Aluminum Equivalent | 75°C Ampacity | |-------------|---------------|---------------------|---------------| | 12 AWG | 25 A | 10 AWG | 25 A | | 10 AWG | 35 A | 8 AWG | 35 A | | 8 AWG | 50 A | 6 AWG | 50 A | | 6 AWG | 65 A | 4 AWG | 65 A | | 4 AWG | 85 A | 2 AWG | 90 A | | 2 AWG | 115 A | 1/0 AWG | 120 A | | 1/0 AWG | 150 A | 3/0 AWG | 155 A |

Cost-benefit analysis: Aluminum wire costs 50-70% less per pound than copper, but you need a larger size. For long runs or large services (200A+), aluminum can save 30-40% on material costs despite the size increase.

Aluminum Wire Installation Requirements

Aluminum requires special handling to prevent connection failures:

AL-CU rated devices: Use only devices marked "AL-CU" or "CO/ALR" (copper-aluminum revised) for aluminum wire terminations. Standard devices marked only for copper will fail over time.
Anti-oxidant compound: Apply oxide-inhibiting compound to aluminum conductors before terminating. Aluminum oxidizes rapidly when exposed to air, creating a resistive layer.
Torque specifications: Follow manufacturer torque requirements precisely. Aluminum is softer than copper and cold-flows under pressure, loosening connections over time if not properly tightened.
No mixing: Never directly splice copper and aluminum (different expansion rates and galvanic corrosion). Use approved copper-to-aluminum connectors or transition devices.
Larger landing spaces: Panel bus bars and lugs must be sized for the larger aluminum conductor.

Where aluminum makes sense:

Service entrance conductors (overhead or underground)
Feeders to sub-panels or detached buildings
Large commercial and industrial installations
Underground service laterals (aluminum is less prone to theft than copper)

Where to avoid aluminum:

Branch circuits (use copper for 15A, 20A, 30A circuits)
Flexible cords and portable equipment
Residential receptacle circuits (connection failures are safety hazards)

Common Insulation Types and Applications

Wire insulation protects against shorts, moisture, and physical damage while determining the wire's temperature rating and approved uses.

THHN / THWN / THWN-2

THHN: Thermoplastic High Heat-resistant Nylon-coated

90°C dry locations
Most common wire for conduit installations
Thin insulation, easy to pull through conduit
Available in all common sizes

THWN: Thermoplastic Heat and Water-resistant Nylon-coated

75°C wet locations, 90°C dry locations
Approved for outdoor conduit, underground in conduit
Moisture and oil resistant

THWN-2: Upgraded version

90°C wet and dry locations
Better moisture resistance than THWN
Replaces both THHN and THWN in most applications (dual-rated)

Common uses: Conduit runs, panels, commercial installations, any application requiring individual conductors

NM-B (Romex)

NM-B: Non-Metallic sheathed cable, Building wire

90°C insulation, but 60°C ampacity due to NEC 334.80 (terminals and overcurrent devices)
Two or three insulated conductors plus bare ground in a PVC outer jacket
Residential standard for concealed wiring in dry locations
Cannot be used in wet locations, embedded in concrete, or exposed to physical damage
Sizes: 14/2, 12/2, 10/2 (most common); also 14/3, 12/3 for 3-way switches, multi-wire branch circuits

Color coding:

White jacket: 14 AWG
Yellow jacket: 12 AWG
Orange jacket: 10 AWG
Black jacket: 8 AWG, 6 AWG

Ampacity: Use 60°C column for NM cable (even though insulation is 90°C-rated) per 334.80. Exception: you can apply 90°C derating for bundling, then compare to 60°C ampacity.

UF-B (Underground Feeder)

UF-B: Underground Feeder, Building wire

90°C insulation (but often limited to 60°C ampacity like NM-B)
Moisture and sunlight resistant
Conductors embedded in solid thermoplastic (not individual insulated wires like NM)
Approved for direct burial and wet locations
Common for outdoor circuits (yard lighting, detached garages, well pumps)

Direct burial depth requirements:

120V circuits: 12 inches minimum depth (18 inches recommended)
120V-240V under GFCI protection: 12 inches
Over 240V or without GFCI: 24 inches

XHHW / XHHW-2

XHHW: Cross-linked High Heat-resistant Water-resistant

90°C dry, 75°C wet
Excellent moisture and chemical resistance
Common in industrial and commercial installations

XHHW-2: Improved version

90°C dry and wet locations
Suitable for direct burial when marked "sunlight resistant"

Typical uses: Feeders, service conductors, underground in conduit, harsh environments

USE / RHW / RHH

USE: Underground Service Entrance

75°C wet and dry (some versions 90°C dry)
Designed for direct burial service laterals
Often combined rating: RHW-2/USE-2 for maximum versatility

RHW / RHW-2: Rubber or thermosetting, Heat and Water-resistant

RHW: 75°C wet and dry
RHW-2: 90°C wet and dry
Flexible, durable, suitable for wet locations and direct burial

RHH: Rubber or thermosetting, High Heat-resistant

90°C dry locations
Often combined with RHW-2 as RHH/RHW-2

Temperature Derating for Bundled Conductors

When multiple current-carrying conductors are bundled together (in conduit, cable, or raceway), heat dissipation is reduced, requiring ampacity derating.

NEC 310.15(C): Adjustment Factors

| Number of Current-Carrying Conductors | Derating Factor | |----------------------------------------|-----------------| | 1-3 | 100% (no derating) | | 4-6 | 80% | | 7-9 | 70% | | 10-20 | 50% | | 21-30 | 45% | | 31-40 | 40% | | 41+ | 35% |

What counts as current-carrying?

Hot (ungrounded) conductors: YES
Neutral on multi-wire branch circuit with balanced loads: NO (doesn't count)
Neutral on circuit with nonlinear loads (computers, LED lights): YES (counts due to harmonics)
Ground (equipment grounding conductor): NO
Travelers on 3-way switches: YES (when carrying current)

Example 1: Six 12 AWG THHN conductors (three 2-wire circuits) in 3/4" EMT conduit:

Count: 6 current-carrying conductors
Derating: 80%
12 AWG THHN: 30A (90°C column) × 0.80 = 24A derated
Compare to 60°C terminal limit: 20A
Derated ampacity (24A) exceeds terminal limit (20A), so 12 AWG is acceptable for 20A circuits ✓

Example 2: Ten 10 AWG THHN conductors in conduit (five 2-wire circuits for 30A loads):

Count: 10 current-carrying conductors
Derating: 50%
10 AWG THHN: 40A (90°C) × 0.50 = 20A derated
Compare to 60°C terminal limit: 30A (required for 30A circuit)
Derated ampacity (20A) is less than required (30A) — must upsize to 8 AWG
8 AWG THHN: 55A × 0.50 = 27.5A derated (still less than 30A required)
Must use 6 AWG: 75A × 0.50 = 37.5A ✓ exceeds 30A requirement

This demonstrates why heavily loaded conduits quickly require larger conductors or additional conduits to split circuits.

Ambient Temperature Correction Factors

Wire ampacity tables assume 30°C (86°F) ambient temperature. Higher ambient temperatures require additional derating.

NEC 310.15(B) Ambient Temperature Correction (for 90°C insulation):

| Ambient Temp (°C) | Ambient Temp (°F) | Correction Factor (90°C wire) | |-------------------|-------------------|-------------------------------| | 30°C or less | 86°F or less | 1.00 (no correction) | | 31-35°C | 87-95°F | 0.96 | | 36-40°C | 96-104°F | 0.91 | | 41-45°C | 105-113°F | 0.87 | | 46-50°C | 114-122°F | 0.82 | | 51-55°C | 123-131°F | 0.76 | | 56-60°C | 132-140°F | 0.71 |

Combined derating: When both bundling and ambient temperature derating apply, multiply both factors.

Example: Eight 10 AWG THHN conductors in an attic where ambient temperature reaches 50°C (122°F):

Base ampacity (90°C column): 40A
Bundling factor (7-9 conductors): 0.70
Ambient temperature factor (50°C): 0.82
Combined derated ampacity: 40A × 0.70 × 0.82 = 23A

If these circuits are 30A continuous loads:

Required capacity: 30A × 1.25 = 37.5A
10 AWG only provides 23A after derating
Try 8 AWG: 55A × 0.70 × 0.82 = 31.6A (still insufficient)
Try 6 AWG: 75A × 0.70 × 0.82 = 43A ✓ meets 37.5A requirement

The conductor size jumped from 10 AWG (typical for 30A) to 6 AWG due to combined derating.

Voltage Drop Considerations

Voltage drop is the reduction in voltage along a conductor due to wire resistance. Excessive voltage drop causes lights to dim, motors to overheat, and equipment to malfunction.

NEC Recommendations

The NEC doesn't mandate maximum voltage drop, but provides recommendations in Informational Note (Fine Print Note):

3% maximum on branch circuits
5% maximum total from service entrance to furthest outlet (2% feeder + 3% branch)

Many jurisdictions and engineering standards enforce these as requirements, not suggestions.

Voltage Drop Formula

For single-phase circuits:

Voltage Drop = (2 × L × I × R) / 1,000

Where:

L = One-way length in feet
I = Current in amperes
R = Resistance per 1,000 feet (from NEC Chapter 9, Table 8)
Factor of 2 accounts for current traveling out and back (both conductors)

Resistance values for copper conductors (at 75°C, from NEC Table 8):

| Wire Size | Resistance (Ω per 1,000 ft) | |-----------|-----------------------------| | 14 AWG | 3.14 Ω | | 12 AWG | 1.98 Ω | | 10 AWG | 1.24 Ω | | 8 AWG | 0.778 Ω | | 6 AWG | 0.491 Ω | | 4 AWG | 0.308 Ω | | 2 AWG | 0.194 Ω | | 1/0 AWG | 0.122 Ω |

Voltage Drop Example

A 240V well pump draws 20A continuously, located 300 feet from the panel. What wire size prevents excessive voltage drop?

Try 10 AWG (ampacity: 35A at 75°C, exceeds 20A × 1.25 = 25A requirement):

Voltage drop = (2 × 300 × 20 × 1.24) / 1,000 = 14.88V
Percentage: 14.88V / 240V = 6.2% (exceeds 3% recommendation)

Try 8 AWG:

Voltage drop = (2 × 300 × 20 × 0.778) / 1,000 = 9.34V
Percentage: 9.34V / 240V = 3.9% (still high, but acceptable for some applications)

Try 6 AWG:

Voltage drop = (2 × 300 × 20 × 0.491) / 1,000 = 5.89V
Percentage: 5.89V / 240V = 2.5% ✓ (within 3% recommendation)

Result: Use 6 AWG despite ampacity only requiring 10 AWG. Voltage drop, not ampacity, governs long runs.

When Voltage Drop Governs

Voltage drop becomes the limiting factor (more restrictive than ampacity) on:

Long runs: 100+ feet for branch circuits, 200+ feet for feeders
Low voltage circuits: 120V circuits drop voltage percentage faster than 240V
High current: Motors, heaters, large appliances
Sensitive equipment: Electronics, computers, medical devices (may require less than 2% drop)

Rule of thumb: For runs over 100 feet, always calculate voltage drop before finalizing wire size.

Service Entrance Conductor Sizing

Service entrance conductors run from the utility connection (meter or service point) to the main disconnect. These must handle the calculated service load per NEC Article 220.

Service Load Calculation (Simplified)

For residential single-family dwellings (NEC 220.82 Optional Calculation):

General lighting and receptacles: 3 VA per sq ft
Small appliance circuits: 1,500 VA each (minimum 2 required)
Laundry circuit: 1,500 VA
Appliances at nameplate rating: Range, dryer, water heater, HVAC, etc.
Apply demand factors from NEC 220.82 Table

Example calculation for a 2,000 sq ft home:

Lighting: 2,000 sq ft × 3 VA = 6,000 VA
Small appliance: 2 circuits × 1,500 VA = 3,000 VA
Laundry: 1,500 VA
Electric range: 12,000 VA
Electric dryer: 5,000 VA
Water heater: 4,500 VA
HVAC (air conditioner): 6,000 VA (largest load, A/C or heat)
Total: 38,000 VA

Apply NEC 220.82 demand:

First 10 kVA at 100%: 10,000 VA
Remainder (28,000 VA) at 40%: 11,200 VA
Total demand: 21,200 VA

Service ampacity:

Ampacity = 21,200 VA / 240V = 88.3A

This calculation supports a 100A service. Service conductors must handle 100A.

From NEC 310.16 (75°C column):

Copper: 3 AWG (100A exactly)
Aluminum: 1 AWG (110A)

In practice: Many jurisdictions and utilities require minimum 100A service for new construction, with 200A increasingly common to accommodate future loads (EV chargers, heat pumps, solar).

Feeder and Sub-Panel Conductor Sizing

Feeders supply power from the main panel to sub-panels or large equipment. Sizing follows similar principles as services but with additional considerations.

Feeder Sizing Steps

Calculate the total load on the feeder using NEC Article 220
Size conductors for the calculated load (125% for continuous loads)
Check voltage drop (often more critical due to longer distances)
Size based on the larger of ampacity or voltage drop requirement

Example: Feeder to a detached garage sub-panel, 100 feet away, with 40A calculated load (not continuous).

Ampacity requirement: 40A

From NEC 310.16 (75°C copper):

8 AWG = 50A ✓

Voltage drop check (assuming 240V feeder):

8 AWG: (2 × 100 × 40 × 0.778) / 1,000 = 6.22V drop
Percentage: 6.22 / 240 = 2.6% ✓ (acceptable for feeder within 2% budget)

Result: 8 AWG copper is acceptable.

If distance were 200 feet:

8 AWG: (2 × 200 × 40 × 0.778) / 1,000 = 12.45V = 5.2% (exceeds 2% feeder recommendation)
Try 6 AWG: (2 × 200 × 40 × 0.491) / 1,000 = 7.86V = 3.3% (better, but still high for feeder alone)
Try 4 AWG: (2 × 200 × 40 × 0.308) / 1,000 = 4.93V = 2.05% (marginal)
Try 3 AWG: (2 × 200 × 40 × 0.245) / 1,000 = 3.92V = 1.6% ✓

At 200 feet, you'd need 3 AWG to keep feeder drop below 2%, even though ampacity only requires 8 AWG.

Grounding Conductor Sizing

Equipment grounding conductors (EGC) protect against faults by providing a low-impedance path to ground. The EGC must be sized per NEC 250.122.

NEC Table 250.122: Minimum EGC Size

| Overcurrent Device Rating | Copper EGC | Aluminum EGC | |---------------------------|------------|--------------| | 15A | 14 AWG | 12 AWG | | 20A | 12 AWG | 10 AWG | | 30A | 10 AWG | 8 AWG | | 40A | 10 AWG | 8 AWG | | 60A | 10 AWG | 8 AWG | | 100A | 8 AWG | 6 AWG | | 200A | 6 AWG | 4 AWG | | 300A | 4 AWG | 2 AWG | | 400A | 3 AWG | 1 AWG | | 500A | 2 AWG | 1/0 AWG | | 600A | 1 AWG | 2/0 AWG |

Key principle: EGC size is based on the overcurrent device rating (breaker or fuse size), not the conductor ampacity.

Example: A circuit uses 6 AWG conductors (65A capacity at 75°C) protected by a 50A breaker.

EGC is sized for the 50A breaker, not the 6 AWG conductor
From Table 250.122: 50A requires 10 AWG copper EGC

Exception for upsized conductors: When you upsize conductors to compensate for voltage drop, you must proportionally upsize the EGC.

Example: 30A circuit, 300 feet to a subpanel, voltage drop requires 3 AWG conductors (instead of 10 AWG for ampacity).

Standard EGC for 30A: 10 AWG
Conductors upsized from 10 AWG to 3 AWG (increase in circular mils: 33,100 / 10,380 = 3.19×)
EGC must upsize proportionally: 10 AWG to larger size with 3.19× the area
10 AWG = 10,380 circular mils
Required EGC: 10,380 × 3.19 = 33,112 CM ≈ 1 AWG (33,100 CM)

Use 1 AWG EGC when upsizing 30A circuit conductors to 3 AWG for voltage drop.

Common Wire Sizing Mistakes

Mistake 1: Using 14 AWG on 20A Circuits

Many DIYers and inexperienced electricians incorrectly use 14 AWG wire on 20A circuits, seeing that 14 AWG is rated 20A in the 75°C column.

Why it's wrong: NEC 240.4(D) limits small conductor protection:

14 AWG: Maximum 15A overcurrent protection
12 AWG: Maximum 20A overcurrent protection
10 AWG: Maximum 30A overcurrent protection

Even though 14 AWG can technically handle 20A, the NEC prohibits 20A breakers on 14 AWG circuits for safety margin.

Correct: Always use 12 AWG (or larger) for 20A circuits.

Mistake 2: Ignoring Terminal Temperature Ratings

Installers often use the 90°C ampacity column for all calculations, forgetting that device terminals limit to 60°C or 75°C.

Example error: Using 10 AWG THHN (40A in 90°C column) on a 40A circuit with 60°C-rated terminals.

10 AWG at 60°C terminal limit: 30A (insufficient for 40A)
Correct: 8 AWG (40A at 60°C column)

Always check device terminal ratings and use the appropriate ampacity column.

Mistake 3: Not Accounting for Continuous Loads at 125%

Running a 20A continuous load (like baseboard heat) on a 20A circuit with 12 AWG wire seems logical but violates NEC.

Why it's wrong: Continuous loads require 125% capacity:

20A load × 1.25 = 25A required capacity
12 AWG at 60°C = 20A (insufficient)
Correct: 10 AWG and 30A breaker

Mistake 4: Mixing Copper and Aluminum Without Proper Connectors

Direct splicing copper to aluminum creates galvanic corrosion and resistive connections that overheat.

Correct methods:

Use listed copper-to-aluminum connectors (anti-oxidant compound required)
Use transition lugs rated for both metals
Maintain separate copper and aluminum circuits when possible

Mistake 5: Undersizing for Voltage Drop

Sizing wire solely for ampacity without checking voltage drop leads to poor performance on long runs.

Example: 100A subpanel feeder, 150 feet away, using 3 AWG (100A capacity).

Voltage drop: (2 × 150 × 100 × 0.245) / 1,000 = 7.35V at 240V = 3.06%
Exceeds 2% feeder recommendation
Should use: 2 AWG or larger to reduce voltage drop below 2%

Always calculate voltage drop for circuits over 75-100 feet.

Practical Wire Sizing Decision Tree

Follow this decision tree for selecting wire size:

Determine the load current (in amperes)
Is the load continuous (3+ hours)? If yes, multiply by 1.25
Check NEC Table 310.16 for minimum wire size based on load
Verify terminal temperature rating: Use 60°C column for most circuits ≤100A
Apply bundling derating if >3 current-carrying conductors in raceway
Apply ambient temperature correction if over 86°F
Calculate voltage drop if run exceeds 100 feet
Select the largest wire size required by any of the above factors
Size grounding conductor per NEC 250.122 based on breaker size (upsize if conductors were upsized for voltage drop)

Actionable Takeaways

To size wire correctly and safely:

Understand AWG inversely: Smaller numbers = larger wire. Memorize common sizes (14, 12, 10, 8, 6, 4, 2 AWG).
Use the correct ampacity column: 60°C for most residential circuits, 75°C for circuits >100A with listed terminals, 90°C only for derating calculations.
Apply the 125% rule for continuous loads: Baseboard heat, lighting, anything running 3+ hours needs 125% capacity.
Derate for bundling and temperature: More than 3 current-carrying conductors or ambient temps >86°F require derating.
Calculate voltage drop on long runs: For circuits over 100 feet, voltage drop often governs, not ampacity.
Never use 14 AWG on 20A circuits: Even though ampacity might allow it, NEC 240.4(D) prohibits it.
Size grounding conductors per Table 250.122: Based on breaker size, not wire size (except when upsizing for voltage drop).
Use AL-CU rated devices for aluminum: Standard devices will fail with aluminum wire. Apply anti-oxidant compound.
Check terminal temperature ratings: Your calculation is only as good as the weakest link — often the device terminal.
When in doubt, go one size larger: The cost difference between 12 AWG and 10 AWG is minimal, but the safety margin is substantial.

Wire sizing combines ampacity, derating, voltage drop, and code requirements. Master these principles, and you'll design electrical systems that perform safely and reliably for decades.