Romex Wire Size Chart: Complete Ampacity & Sizing Guide

A Romex wire size chart shows the relationship between wire gauge (14, 12, 10, 8, 6 AWG), maximum ampacity (current-carrying capacity), appropriate circuit breaker size, and typical applications—helping you select the correct wire size for safe, code-compliant electrical installations. Choosing the wrong wire size can lead to overheating, fire hazards, and code violations, while oversizing wastes money on unnecessarily expensive wire.

Understanding wire sizing is one of the most critical aspects of electrical work. Too small, and the wire overheats under load, creating a fire risk. Too large, and you've spent more than necessary. The right size ensures safe operation, meets electrical code requirements, and optimizes your budget.

This comprehensive guide provides complete Romex wire sizing charts, explains how to choose the correct size for any application, covers ampacity ratings, voltage drop calculations, and helps you avoid common sizing mistakes.

Quick Reference: Romex Wire Size Chart

Standard Residential Romex Sizes

Important: Always match circuit breaker size to wire gauge—never exceed the wire's ampacity rating.

Understanding Romex Wire Sizing

What Is AWG?

AWG = American Wire Gauge

The standard system for measuring wire diameter:

• Higher number = Thinner wire (less capacity)
• Lower number = Thicker wire (more capacity)

WIRE SIZE COMPARISON (Approximate Diameters)

14 AWG: ━━━  (2.1 mm / 0.083')
12 AWG: ━━━━  (2.7 mm / 0.106')
10 AWG: ━━━━━  (3.3 mm / 0.130')
8 AWG:  ━━━━━━  (4.3 mm / 0.170')
6 AWG:  ━━━━━━━━  (5.4 mm / 0.212')

Thicker = More capacity

What Is Ampacity?

Ampacity = Maximum current (amps) a wire can safely carry continuously

Factors affecting ampacity:

• Wire size (larger = more capacity)
• Insulation temperature rating (NM-B = 90°C)
• Ambient temperature (hotter environment = lower capacity)
• Bundling (multiple cables together = lower capacity)
• Length (longer runs = voltage drop considerations)

Complete Romex Ampacity Chart

Standard NM-B Cable Ratings (60°C Column)

While Romex is rated for 90°C, NEC requires using the 60°C ampacity column for standard residential circuits:

Source: NEC Table 310.16 (formerly 310.15(B)(16))

Why use 60°C column?

• Most terminations (breakers, outlets, switches) rated for 60°C or 75°C
• Safety margin
• Code requirement for most residential installations

Romex Wire Size by Application

Lighting Circuits

Recommended: 14/2 Romex

Typical applications:

• Ceiling lights
• Recessed lighting
• Wall sconces
• Ceiling fans (without heaters)
• Under-cabinet lighting

Note: Some electricians prefer 12/2 for lighting to allow future flexibility.

General Outlet Circuits (Bedrooms, Living Rooms)

Recommended: 12/2 Romex (preferred) or 14/2 Romex (acceptable)

Why 12/2 is preferred:

• More capacity for modern loads
• Allows power tools, vacuums
• Future-proofs installation
• Only slightly more expensive

Kitchen Counter Outlets (Small Appliance Circuits)

Required: 12/2 Romex (Code requirement)

Code requirements (NEC 210.11(C)(1)):

• At least two 20-amp circuits
• Serves countertop outlets only
• Cannot serve other loads

Appliances on these circuits:

• Toaster
• Coffee maker
• Blender
• Microwave (if not dedicated)
• Food processor

Bathroom Outlet Circuits

Required: 12/2 Romex

Code requirements:

• 20-amp circuit required (NEC 210.11(C)(3))
• GFCI protection mandatory
• Can serve multiple bathrooms (if no other loads)

Dedicated Appliance Circuits

Dishwasher

Garbage Disposal

Refrigerator

Microwave (Built-in)

Note: Some high-wattage microwaves may require 10/2 and 30-amp circuit. Check manufacturer specifications.

240-Volt Appliances

Electric Water Heater

Typical ratings:

• 30-50 gallon: 3,800-4,500 watts
• 240V × 20A = 4,800 watts capacity

Window Air Conditioner (Large)

Always check manufacturer specifications

Electric Clothes Dryer

Note: Some large dryers require 8/3 and 40-amp circuit.

Electric Range/Oven

Typical ratings:

• Small ranges: 6,000-8,000 watts
• Large ranges: 12,000-15,000 watts

Central Air Conditioner

Must check manufacturer nameplate for exact requirements

Romex Wire Size Selection Guide

Step-by-Step Sizing Process

Step 1: Calculate Total Load

Add up the wattage of all devices on the circuit:

Example: Kitchen counter outlets
- Coffee maker: 1,000W
- Toaster: 1,200W
- Blender: 400W
Total: 2,600W

Step 2: Calculate Required Amperage

Formula: Amps = Watts ÷ Volts

Example:
2,600W ÷ 120V = 21.7 amps

This exceeds 20 amps, so you need:
- Multiple circuits, OR
- Larger wire (not allowed for standard outlets)

Solution: Two 20-amp circuits (code requires this for kitchens anyway)

Step 3: Apply 80% Rule

Continuous loads (operate for 3+ hours) must not exceed 80% of circuit capacity:

20-amp circuit × 80% = 16 amps continuous maximum
15-amp circuit × 80% = 12 amps continuous maximum

Step 4: Select Wire Size

Based on calculated amperage:

Step 5: Check Voltage Drop

For runs over 50-100 feet, voltage drop becomes significant.

Voltage Drop Considerations

Why Voltage Drop Matters

Long wire runs cause voltage drop:

• Resistance in wire
• Less voltage at appliance
• Reduced performance
• Potential code violation

Acceptable Voltage Drop

NEC recommendations:

• 3% maximum for branch circuits
• 5% maximum combined (feeder + branch)

Example calculation:

120V circuit with 3% drop:
120V × 0.03 = 3.6V drop
116.4V delivered (acceptable)

120V × 0.05 = 6V drop
114V delivered (marginal)

Voltage Drop Chart for Romex

120V circuits at 80% capacity:

🔴 = Over 5% (upsize wire recommended)
🟡 = 3-5% (marginal, consider upsizing)
🟢 = Under 3% (acceptable)

When to Upsize for Voltage Drop

Upsize one gauge when:

• Run exceeds 100 feet
• Voltage drop calculation shows >3%
• Running to outbuilding/detached garage
• Powering sensitive electronics

Example:

150-foot run to garage for outlets
Standard: 12/2 for 20-amp circuit
Voltage drop: 5.1% (marginal)
Better: 10/2 for reduced drop to 3.2%

Derating Factors

Multiple Cables Bundled Together

When multiple cables run together, heat buildup reduces ampacity:

Example:

Five 12/2 Romex cables bundled together:
Normal ampacity: 20A
Derated: 20A × 80% = 16A maximum
Must use 15-amp breaker

Where bundling occurs:

• Multiple cables through same hole
• Tight bundle in conduit
• Bunched together in attic/crawl space

High Ambient Temperature

Standard ampacity assumes 86°F (30°C) ambient temperature:

Where this applies:

• Attics in summer
• Near heating equipment
• Uninsulated outdoor enclosures

Romex Wire Gauge Chart (Physical Dimensions)

Wire Diameter and Coverage

Common Romex Wire Sizing Mistakes

Mistake #1: Matching Wire to Outlet Rating (Not Load)

❌ Wrong thinking: 'I'm installing 20-amp outlets, so I need 12-gauge wire.'

✅ Correct approach: Calculate actual load and required circuit capacity. Outlet rating is maximum, not requirement.

Reality:

• 20-amp outlets can be on 20-amp circuit (12 AWG)
• 15-amp outlets can be on 15-amp circuit (14 AWG)
• 15-amp outlets can be on 20-amp circuit (12 AWG)
• 20-amp outlets CANNOT be on 15-amp circuit

Mistake #2: Undersizing Based on Single Appliance

❌ Wrong: 'My microwave is 1,000W = 8.3A, so 14-gauge is plenty.'

✅ Correct: Account for continuous operation and code requirements:

• Continuous loads: 125% (1.25×)
• Code minimums (kitchen = 20A minimum)

Mistake #3: Ignoring Voltage Drop on Long Runs

❌ Wrong: '100 feet to shed, 12-gauge for 20-amp circuit.'

✅ Correct: Calculate voltage drop, upsize if needed:

• 100 feet 12 AWG at 16A = 3.4% drop (marginal)
• Better: Use 10 AWG = 2.1% drop

Mistake #4: Oversizing Breaker to Wire

❌ Dangerous: 'Wire seems hot, I'll use a 30-amp breaker on this 14-gauge.'

✅ Safe: Wire determines maximum breaker size:

• 14 AWG → MAX 15-amp breaker
• 12 AWG → MAX 20-amp breaker
• Never exceed wire capacity!

Mistake #5: Not Accounting for Derating

❌ Wrong: 'Six cables bundled, all good for 20 amps each.'

✅ Correct: Apply derating factor:

• 6 cables = 80% derating
• 20A × 80% = 16A maximum
• Use 15-amp breakers

Quick Reference: Choose Your Wire Size

By Circuit Type

By Amperage Need

Frequently Asked Questions

Can I use 12-gauge wire on a 15-amp circuit?

Yes, absolutely. You can always use larger wire than the minimum required. It's safer and provides more capacity. The breaker size is determined by the smallest wire in the circuit.

Can I put a 20-amp breaker on 14-gauge wire?

NO. This is dangerous and violates code. 14-gauge wire has a maximum ampacity of 15 amps. Using a 20-amp breaker creates a fire hazard—the wire will overheat before the breaker trips.

How do I know what size Romex is in my walls?

Check the printing on the cable jacket:

• '14/2' or '14/3' = 14-gauge
• '12/2' or '12/3' = 12-gauge
• '10/2' or '10/3' = 10-gauge

Also check your breaker panel—breaker size indicates minimum wire size (15A = 14 AWG minimum, 20A = 12 AWG minimum).

Is 14-gauge wire still allowed?

Yes, for 15-amp circuits. However, many electricians prefer 12-gauge throughout the house for consistency and future flexibility.

What happens if I use wire that's too small?

Wire that's too small for the load will:

• Overheat under load
• Potentially melt insulation
• Create fire hazard
• Violate electrical code

The circuit breaker should trip before this happens (if properly sized).

What size wire for 100 amp sub-panel?

Depends on distance:

• Under 50 feet: 3 AWG copper
• 50-100 feet: 2 AWG copper
• Over 100 feet: 1 AWG or larger

Always calculate voltage drop for sub-panel feeders.

Conclusion

Selecting the correct Romex wire size ensures safe, code-compliant electrical installations that handle expected loads without overheating. The fundamental rule is simple: match wire size to circuit ampacity, and never exceed the wire's capacity with an oversized breaker.

Key takeaways:

• 14 AWG (14/2, 14/3): Maximum 15-amp circuits—lighting and bedroom outlets
• 12 AWG (12/2, 12/3): Maximum 20-amp circuits—kitchens, bathrooms, general outlets
• 10 AWG (10/2, 10/3): Maximum 30-amp circuits—water heaters, dryers, large appliances
• 8 AWG and larger: 40+ amp circuits—electric ranges, sub-panels

Always calculate loads, account for voltage drop on long runs, apply derating factors when cables are bundled, and follow NEC requirements. When in doubt, upsize—it's always safer and provides future flexibility.

Related Guides:

• What Is Romex Wiring? - Complete guide to NM-B cable
• Can You Run Romex in Conduit? - Installation methods
• What Does NM-B Stand For? - Technical specifications
• Electrical Conduit Guide - Alternative wiring methods

Planning electrical work? Always verify local code requirements, obtain necessary permits, and consider hiring a licensed electrician for complex installations. Improper wire sizing creates serious fire and safety hazards.