Looking for reliable wiring solutions for your next project? “#3 cu wire” is a top choice among homeowners and professionals alike, prized for its excellent conductivity and versatility in various electrical applications. Whether you’re upgrading your home’s wiring or tackling a new installation, understanding the benefits and uses of #3 copper wire will help you make the right purchasing decision.

#3 Copper Wire: Types, Uses, Safety, and Selection Guide

Comparison Table: Common #3 Copper Wire Types and Applications

Wire Type	Common Use Cases	Indoor/Outdoor	Stranded/Solid	Direct Burial	Rated for Wet Locations	Typical Colors	Voltage Rating	Key Certification
THHN	General building wiring, conduit	Both	Stranded	No	Yes (as THWN-2)	Black	600V	UL, THWN-2
THWN-2	Wet/damp/dry locations in conduit	Both	Stranded	No	Yes	Black	600V	UL
SER Cable (Service Entrance)	Panel feeders, subpanels	Both	Stranded	No	Yes	Gray	600V	UL
NM-B	Indoor residential branch circuits	Indoor	Stranded	No	No	Varies	600V	UL
XHHW-2	Industrial, feeder circuits	Both	Stranded	Yes	Yes	Black	600V	UL

Types of #3 Copper Wire Explained

Understanding the right type for your project is crucial for safety, compliance, and performance. Here’s a breakdown of the most common #3 CU (copper) wire types you’ll encounter:

THHN/#3 Stranded Copper Wire

Usage: Widely used for general building wiring in conduit or other raceways.
Features: Thermoplastic High Heat-resistant Nylon-coated (THHN) allows for use in dry and damp locations. When dual rated as THWN-2, it is also suitable for wet locations.
Stranded Construction: Provides greater flexibility than solid core.
Color: Most commonly black for phase/hot wires.

SER Cable (#3 Copper Service Entrance)

Usage: Feeds power from an electric meter to the main distribution panel, used in both residential and commercial settings.
Multiple Conductors: Typically features multiple #3 copper wires plus a smaller ground.
Jacket: Heavy-duty vinyl for both indoor and outdoor use, but not suitable for direct burial unless specified.

NM-B (#3 Nonmetallic Sheathed Cable)

Usage: Indoor residential branch circuit wiring.
Not for Wet/Damp Locations: NM-B is for dry, protected environments only.
Ease of Use: Jacketed but not armored; flexible for pulling through walls.

XHHW-2 (#3 Cross-linked Polyethylene High Heat-resistant Water-resistant Wire)

Usage: High-demand feeder circuits and industrial applications.
High Water and Heat Resistance: Suitable for both wet and dry locations, with improved insulation.
Direct Burial Capable: Some variations may be directly buried (check label).

Usage of #3 CU Wire in Homes and Offices

In the Home

Electrical Service Feeders: Commonly runs from service panels to subpanels, or for large appliance circuits (electric ovens, HVAC units).
Conduit Installations: THHN/THWN versions are routed through PVC or metal conduit for branch circuits.
Indoor Branch Wiring: NM-B is suitable (though less common at #3 size) for heavy-demand indoor circuits, like home workshops.

In Commercial & Office Environments

Panel and Subpanel Wiring: Feeds power to secondary panels for office suites or equipment rooms.
Conduit Distribution: Commercial settings use THHN/THWN in metal or PVC conduit for durability and code compliance.
High-demand Circuits: Ideal for copy rooms, server rooms, or equipment-heavy zones requiring substantial amperage.

Outdoor and Special Applications

Feeder Circuits to Detached Buildings: SER or XHHW-2 wires can bring power to garages or outbuildings, typically routed through conduit.
Not for Direct Burial: Unless specifically rated (some XHHW-2 and UF types), #3 CU wire is not designed for direct burial.

Safety Tips When Working with #3 CU Wire

Electrical work can be hazardous. Follow strict safety precautions to protect both property and personal well-being:

De-energize Circuits: Always turn off power at the breaker panel before starting any electrical work.
Verify Voltage: Use a voltage tester to ensure circuits are not live.
Correct Sizing: Using undersized wire can overheat and cause fires; always choose #3 when code or load calculations require.
Use Conduit Where Required: Most #3 CU wire (e.g., THHN) must be installed inside conduit unless it is SER or another jacketed, code-compliant cable.
Observe Ampacity Limits: Do not exceed 110 amps for #3 copper, per standard rating tables (check NEC for adjustments based on conditions).
Check for Wet/Dry Ratings: Only use wire rated THWN-2, XHHW-2, or similar in wet or outdoor locations.
Tighten Connections Properly: Loose terminations cause arcing and heat buildup.
Obey Local and National Codes: Always consult NEC and local ordinances for installation, as allowances vary by jurisdiction.
Personal Protective Equipment (PPE): Wear insulated gloves, protective eyewear, and avoid metal jewelry during installation.
Hire a Pro if Unsure: When in doubt, consult or hire a licensed electrician.

How to Choose the Right #3 CU Wire

Selecting the appropriate #3 copper wire ensures long-term safety, code compliance, and reliable power delivery. Here’s a step-by-step process:

1. Determine Your Application

Feeders vs. Branch Circuits: For main-to-subpanel feeds, use SER or THHN in conduit. For indoor branch circuits, NM-B may suffice.
Location: Does the wiring run indoors, outdoors, or in potentially wet environments?
Special Conditions: Is the wire likely to be exposed to sunlight, chemicals, or mechanical damage? Choose insulation accordingly.

2. Check Ampacity Requirements

Load Calculation: Sum the amperage draw of all devices in the circuit. #3 CU wire is typically rated for 100–110 amps (depending on temperature ratings and installation environment).

3. Select the Insulation Type

THHN/THWN-2: Versatile for most indoor/outdoor conduit runs (wet/dry).
SER: Offers all-in-one cable for panel feeding, with multiple conductors.
XHHW-2: Enhanced insulation for more demanding or industrial environments.

4. Confirm Code Compliance

Voltage Rating: Ensure the wire’s voltage rating (usually 600V) matches your system.
Direct Burial/Egress: If burial is necessary, choose a wire rated for this use.

5. Buy the Right Amount

Sold by Foot or Reel: Most suppliers offer #3 CU wire by-the-foot for small jobs, or by 500- or 1,000-ft reels for larger projects.
Leftover Considerations: Order a few extra feet to account for routing or cutting errors.

Practical Tips and Best Practices

Label Both Ends: Mark each wire for function (phase, neutral, ground) to avoid confusion during installation or inspection.
Measure Runs Accurately: Precise measurement prevents waste and shortfalls.
Use Pulling Lubricant: For long conduit runs, a small amount eases pulling and prevents insulation damage (especially for SIMpull THHN).
Anchor Wires Properly: Use clamp connectors and secure wires at intervals to prevent sagging or abrasion.
Avoid Overfilling Conduit: Adhere to conduit fill tables for safe, code-compliant installations.
Check for Local Rebates: Some utility companies offer energy-efficiency rebates for using properly sized wire, minimizing voltage drop.
Inspect for Damage: Never install wires with nicked or cut insulation—replace them immediately.

Technical Comparison Table: #3 Copper Wire Attributes

Attribute	THHN/THWN-2 Stranded	SER Cable (Service Entrance)	NM-B (Indoor Only)	XHHW-2
Conductor Material	Copper	Copper	Copper	Copper
Stranded or Solid	Stranded	Stranded	Stranded	Stranded
Typical Ampacity (60ºC/75ºC)	85A/100A	100–110A	85A/100A	100A+
Voltage Rating	600V	600V	600V	600V
Maximum Length per Reel	1,000 ft	500–1,000 ft	1,000 ft	1,000 ft
Suitable for Wet Locations	Yes (if marked THWN-2)	Yes	No	Yes
Suitable for Outdoor	Yes (in conduit)	Yes	No	Yes
Direct Burial	No	No	No	Yes (certain models)
Jacket Color	Black	Gray	Varies	Black
Typical Weight (per foot)	~0.19–0.25 lbs	Varies (multi-conductor)	~0.19 lbs	~0.19–0.21 lbs
Certification	UL Listed	UL Listed	UL Listed	UL Listed

Conclusion

3 copper (CU) wire is a robust, reliable choice for powering large residential and commercial circuits. Designed for heavy loads, it comes in several types like THHN/THWN-2, SER, NM-B, and XHHW-2, each with unique strengths and suitable applications. When choosing, pay close attention to your environment, load, local codes, and safety practices. Opt for quality, code-rated wire to ensure years of safe, efficient operation.

FAQ

What does “#3 CU wire” mean?
“#3 CU wire” refers to an American Wire Gauge (AWG) size 3 electrical conductor made from copper (CU). It is thicker than most household branch circuit wires and is intended for high-capacity applications, like feeder lines and large appliances.
Is #3 copper wire suitable for both indoor and outdoor use?
Yes, but only if it’s appropriately rated for the environment. THHN/THWN-2 and XHHW-2 types can be used outdoors when installed in conduit and are rated for wet locations. NM-B is for indoor, dry use only.
Can #3 copper wire be used for direct burial?
Generally, no. Standard THHN/THWN-2 and SER wires are not rated for direct burial. For burial, you must use wire specifically marked as direct burial, such as certain types of XHHW-2 or UF cable.
What is the ampacity (current rating) of #3 copper wire?
At standard temperatures, #3 copper wire is rated between 85–110 amps, depending on installation conditions, insulation type, and code requirements. Always check the NEC tables and apply local code adjustments.
What’s the difference between THHN and THWN-2?
THHN is rated for dry or damp environments, while THWN-2 includes extra protection for wet locations and is rated to 90°C in both wet and dry conditions. Many modern wires are dual-rated (THHN/THWN-2).
How do I know if #3 copper wire is stranded or solid?
Most #3 AWG wire used for feeders and larger circuits is stranded, making it more flexible for pulling through conduit. Product labeling and specifications will clarify the construction.
Can #3 copper wire be used as the main service entrance wire?
It can be used for subpanels or large branch circuits, but the main service entrance may require a larger gauge depending on total service amperage. Consult your electric utility or inspector.
What color is standard for #3 copper wire insulation?
Black is standard for phase/hot wires, especially THHN/THWN-2, but other colors may be available. Always verify insulation color aligns with wire function per local codes.
Do I need to run #3 copper wire in conduit?
THHN/THWN-2 wire must be run in conduit. SER cable has its own jacket and does not require conduit in most cases, except where required by code for protection.
Are there cost-saving alternatives to #3 copper wire?
Aluminum wire (of a different, larger gauge) can sometimes be used, but has different ampacity and installation requirements. Copper remains the preferred choice for safety, conductivity, and longevity, especially in residential settings.

Use this complete guide to make confident, code-compliant decisions about your next project involving #3 copper wire. Always prioritize safety and check with a licensed electrician or local inspector if in doubt.

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#3 CU Wire Guide: Types, Uses, and Buying Tips