What Size Aluminum Wire is Best for a 100 Amp Sub-Panel?

Thanks for stopping by Wire America.  We’re a dedicated resource for high-quality residential, commercial, industrial, & utility wire & cable. Our goal is to provide the technical clarity and specialized products you need to get your project done right. Below, we’re answering a frequent question from our customers regarding wiring a 100 amp sub-panel:

What Size Aluminum Wire for a 100 Amp Sub-Panel?

Choosing the right aluminum wire for a 100-amp sub-panel is the smartest way to keep your project on budget. While copper is a great conductor, modern aluminum alloy provides the tremendous reliability at a fraction of the cost.

The "best" wire isn't just about the gauge—it’s about the environment. In this guide, we’ll help you choose between SER, THHN, XHHW, USE-2/RHH/RH-2, & URD so you can buy with confidence and pass your inspection the first time.

The Quick Answer: 100A Aluminum Sizing Table

Choosing Your Cable Type

1. Aluminum SER (The Indoor/Dry Choice)

If you are running power from your main panel to a sub-panel inside the house—such as an upstairs addition or a basement—SER (Service Entrance) is your best option.

• The Benefit: It is a bundled cable containing two hots, a neutral, and a ground all within one jacket. It is easy to staple to studs and typically does not require conduit.

• The Link: Shop our #2 AWG Aluminum SER Cable—perfect for interior sub-panel runs in residential garages or home additions.

2. XHHW-2 Aluminum (The Conduit Choice)

If you are running your wire through a pipe—common in garages, shops, or unfinished basements—you want individual conductors.

• The Benefit: XHHW-2 Building Wire is often preferred over THHN/THNW-2 Building Wire for sub-panels because its insulation is more resistant to moisture and heat, making it easier to pull through long conduit runs.

• The Link: Need to pull through conduit? Order your #2 AWG Aluminum XHHW-2 by the foot to get exactly what your job requires.

3. Aluminum URD (The Direct Burial Choice)

If your 100-amp panel is going into a detached shed, barn, or ADU (Accessory Dwelling Unit), you need URD (Underground Residential Distribution).

• The Benefit: It is ruggedly insulated to be buried directly in a trench without the need for a full conduit run.

• The Link: Running power to a detached garage or ADU? Shop our #2-2-4 Triplex Aluminum URD (Ramapo) for standard 3-wire runs, or our #2-2-2-4 Quadruplex Aluminum URD (Dyke) if your local code requires a dedicated 4-wire setup.

4. XLP-USE-2 / RHH / RHW-2 (The Multi-Rated Choice)

This is the "All-In-One" single conductor wire that contractors love for its versatility.

• The Benefit: It carries multiple ratings (USE-2 / RHH / RHW-2). This means it is tough enough for direct burial in the dirt, but also approved for use inside the house. You can run it from the trench, into the conduit, and all the way to the panel without having to splice wires.

• The Link: Looking for the most versatile outdoor-to-indoor wire? Shop our #2 AWG Aluminum USE-2 / RHH / RHW-2 to get exactly the footage your project requires.

• The Tech: The XLP (Cross-Linked Polyethylene) insulation is highly resistant to chemicals, moisture, and sunlight.

"Always calculate your total load and check local codes before selecting your gauge. For runs over 100ft, you may need to upsize to prevent voltage drop."

The 83% Rule Explained

One reason you see #2 Aluminum recommended so often for 100A panels is found in NEC 310.12.

• The Rule: For certain residential services and feeders, the code allows you to size the wire at 83% of the breaker rating (100A x 0.83 = 83A).

• The Result: Since #2 Aluminum is rated for 90 Amps at 75°C, it covers the 83-Amp requirement perfectly. This makes it the most cost-effective, code-compliant choice for a 100A residential service.

How Wire America Can Help

We offer #2 Aluminum in all configurations—SER, XHHW-2, XLP/USE-2/RHH/RHW-2 and URD—to ensure you have the right wire for your next sub-panel installation. 

Whether you need 100 feet for a garage or 500 feet for a barn, we offer cut-to-length service on all our building and utility wire. With our fast shipping and professional-grade aluminum conductors, we help you get your power up and running without the "copper tax."

Call us to discuss your project: 833-268-8644

Let's Wire America! 🇺🇸

At Wire America, we believe in genuine conversation. You'll always talk with real people who understand that every project requires careful product selection. We're here to ensure you feel educated, confident, and great about your final purchase. Feel free to give us a call to discuss your project! You can reach us at: 833-268-8644

⚠️ Important Disclaimer: Safety and Compliance

Wire America provides this information as a guide only. Electrical installations must strictly adhere to the National Electrical Code (NEC) and all state, local, and jurisdictional requirements, which vary widely. Always consult a licensed electrician, electrical engineer, or the local inspecting authority before purchasing material or starting any electrical project. Wire America assumes no liability for the installation, use, or interpretation of these guides.