What Size Wire for 100 Amp Service? A Detailed Guide

Learn what size wire for 100 amp service you need with this detailed guide. Get tips on materials, distance, and building codes.

When I first started working on electrical projects, I remember staring at a giant spool of wire at the hardware store, completely clueless. I was helping a family friend upgrade their service to 100 amps, but the question kept nagging me: What size wire should I use for 100 amp service? If you’ve ever been in that boat, I can assure you, you’re not alone. Choosing the right wire size isn’t just about grabbing the thickest wire you can find. It’s a calculated decision, influenced by factors like material, distance, and even local building codes. Getting it wrong can lead to problems that quickly become a money maker for electronic repair shops.

In this article, I’m going to walk you through everything you need to know about selecting the correct wire size for a 100 amp service. Whether you’re a DIY enthusiast or an experienced electrician, this guide has something for you.

Why Wire Size Matters

Before we dive into the specifics, let’s talk about why wire size matters in the first place. Imagine trying to funnel water through a straw. A tiny straw can handle only so much water at once before it overflows or bursts. The same concept applies to electrical wiring.

If the wire is too small, it can overheat, causing energy losses or even fires. On the other hand, using a wire that’s unnecessarily large can waste money. Striking the right balance is key, not just for safety, but also for efficiency and cost-effectiveness.

I learned this lesson the hard way during an early project. Back then, I thought, “Bigger has to be better, right?” I ended up using a wire gauge that was overkill, spending more money than I needed and struggling to work with an unnecessarily bulky wire. Trust me, it pays to get it right the first time.

Factors That Determine Wire Size for 100 Amp Service

Choosing the right wire size isn’t as simple as looking at an ampacity chart. Several factors come into play:

1. Material (Copper vs. Aluminum)

Copper and aluminum are the two most common materials for electrical wiring. Copper is more conductive, meaning you can use smaller wires for the same current. Aluminum, while less expensive and lighter, requires a thicker gauge to handle the same load.

2. Distance (Voltage Drop)

The farther electricity has to travel, the more energy it loses along the way. This is called voltage drop. For long distances (over 100 feet), you may need to size up your wire to compensate.

3. Insulation Type

Different insulation types, like THHN, XHHW, or USE-2, have different temperature ratings and purposes. For example, underground installations often require water-resistant insulation like XHHW or USE-2.

4. Local Codes and Regulations

Electrical codes vary by region. While the National Electrical Code (NEC) provides general guidelines, always consult your local building codes to ensure compliance.

NEC Guidelines for Wire Sizing

The NEC Table 310.16 is the gold standard for determining wire size. For a 100 amp service

• Copper Wire: Use #2 AWG for most installations.
• Aluminum Wire: Use #1/0 AWG.

However, these sizes assume standard conditions. If you’re dealing with long distances, high temperatures, or other unique factors, adjustments may be required.

Copper vs. Aluminum Wire: Which is Better?

When I first started working with aluminum wire, I was skeptical. It’s lighter and cheaper, but I worried about its lower conductivity and reputation for overheating. After using it for several projects, I’ve learned that aluminum is perfectly safe when installed correctly, especially for long-distance runs.

Here’s a quick comparison:

Feature	Copper Wire (#2 AWG)	Aluminum Wire (#1/0 AWG)
Conductivity	High	Moderate
Weight	Heavy	Light
Cost	Expensive	Affordable
Corrosion	Resistant	Needs anti-oxidant paste
Durability	Longer lifespan	Shorter lifespan

If you’re working on an underground installation or a project with a tight budget, aluminum is a solid choice. Just remember to use anti-oxidant compound on the connections to prevent corrosion.

Voltage Drop Considerations

Voltage drop can be a sneaky problem. Imagine running a marathon but only getting 90% of the oxygen you need, it’s not ideal. Similarly, if your wire isn’t sized to account for voltage drop, your devices may not perform as expected.

Example:

If you’re installing a 100 amp service panel 150 feet from the power source:

• For Copper Wire, use #1 AWG.
• For Aluminum Wire, use #2/0 AWG.

The general rule of thumb is to keep the voltage drop under 3%. You can calculate this using online tools or a formula (more on that later).

Wire Insulation Types and Their Applications

Not all wires are created equal. Insulation plays a big role in where and how a wire can be used.

Insulation Type	Features	Best For
THHN/THWN	Heat-resistant, thin, versatile	Indoor conduit
XHHW	Water-resistant, thicker insulation	Underground or outdoor use
USE-2	Durable, direct burial	Underground service entrances

For most 100 amp underground services, XHHW or USE-2 aluminum wire is a popular choice.

DIY Installation: A Step-by-Step Guide

If you’re anything like me, you enjoy rolling up your sleeves and tackling projects yourself. Here’s a simplified roadmap:

1. Turn Off Power: Always start by cutting power to the area you’re working on. Safety first.
2. Choose Your Wire: Select the right size and type based on this guide.
3. Route the Wire: Whether through conduit or underground, make sure the wire is securely routed.
4. Connect the Panel: Attach the wires to the breaker panel, following the manufacturer’s instructions.
5. Inspect Your Work: Double-check connections, especially for aluminum wires (use anti-oxidant compound).
6. Call for Inspection: Most areas require a licensed inspector to approve your work.

Pro Tip: If you’re new to electrical work, consider hiring a professional for the final connections.

Tools and Calculators to Simplify Your Project

I can’t tell you how many times a good calculator or tool has saved my bacon. Here are a few you should consider:

• Voltage Drop Calculator: Plug in your distance, wire size, and material to get precise recommendations.
• NEC Reference Tables: Keep a copy of the NEC ampacity chart handy for quick checks.
• Wire Strippers and Multimeters: These are essential for any electrical project.

Cost Breakdown for 100 Amp Service Wiring

Here’s a rough idea of what you can expect to spend:

Item	Estimated Cost
Copper Wire (#2 AWG)	$2.50–$3.50 per foot
Aluminum Wire (#1/0 AWG)	$1.00–$1.50 per foot
Conduit (PVC)	$0.75–$1.50 per foot
Breaker Panel	$50–$200 depending on features

If you’re working with a tight budget, aluminum wire and PVC conduit are great ways to save money without sacrificing quality.

Key Takeaways

1. Use #2 AWG copper or #1/0 AWG aluminum for most 100 amp installations.
2. For distances over 100 feet, size up to account for voltage drop.
3. Always follow local codes and consult a licensed electrician for inspections.
4. Choose the right insulation for your environment, XHHW or USE-2 for underground, THHN for indoor.

Additional Resources

1. National Electrical Code | CPSC.gov: Overview of the NEC, a widely adopted model code for safe electrical installations.
2. Electrical Codes and Standards | Minnesota Department of Labor and Industry: Details on the 2023 NEC adoption and its effective implementation.
3. National Electric Code (NEC) | FEMA: Insights into the NEC as the benchmark for safe electrical design and installation. Let me know if you need further adjustments.