I once watched a guy try to feed a 200 amp subpanel with 6 AWG copper. Honestly? I had to bite my tongue so hard I nearly drew blood. The wire was barely rated for 65 amps in that particular run, and he was planning to pull double that. Look—confusing the differences in wire size for 100 vs 200 amp panels is one of the most common (and dangerous) mistakes I see from folks who are handy but haven't spent a decade inside electrical panels. If you get this wrong, you don't just trip a breaker. You get a fire.

So let’s cut the nonsense. You need to know exactly what conductor gauge works for each service, why the rules change based on distance, and when you can (and can’t) get away with aluminum. This isn’t theory. This is the difference between a safe install and a call to the fire department.

The Basic Rule That Everyone Forgets

The fundamental truth about wire size is that it’s not just about the breaker rating. It’s about ampacity—the maximum current a conductor can carry without overheating. For a 100 amp panel, you need a wire that can handle at least 100 amps continuously. For 200 amps, same deal. Simple, right? Not quite.

There’s a catch called the 80% rule. The National Electrical Code (NEC) says that for continuous loads (anything on for three hours or more), you shouldn’t load a circuit to more than 80% of its breaker rating. That means your wire size needs to handle 125% of the expected load in many cases. Seriously. It’s a big deal.

Ampacity and the 80% Rule

Let’s get specific. For a 100 amp panel supplying a continuous load (like a central AC or a shop full of tools), the wire needs to be rated for 125 amps. That pushes you from a typical 3 AWG copper to a 1 AWG copper in some situations. Most people don’t realize this until they’re halfway through a pull and the wire jams in the conduit.

For a 200 amp panel, the 80% rule means you’re looking at a wire rated for 250 amps under continuous load. Standard practice? 2/0 copper or 4/0 aluminum. But wait—that’s only if you’re running a short distance with good ambient conditions. Every time you toss in a hot attic or a long underground run, the wire size creeps up.

Why You Can't Just Guess the Gauge

Here’s where the differences in wire size for 100 vs 200 amp panels get interesting. It’s not a linear jump. You don’t just double the amperage and double the wire diameter. The math is logarithmic. A 100 amp service might use 4 AWG copper (good for 85 amps at 60°C) or 2 AWG aluminum. But a 200 amp service? That’s 2/0 copper or 4/0 aluminum. That’s a massive jump in physical size and cost.

Think of it like a garden hose. For 100 amps, you’re using a 5/8-inch hose. For 200 amps, you’re stepping up to a 3/4-inch fire hose. But the flow difference isn’t just double—it’s more like triple the volume capacity. That’s why running a 200 amp subpanel with old 100 amp wiring is a recipe for melted insulation and a bad day.

Wire Size for a 100 Amp Panel (The Sweet Spot)

A 100 amp panel is the workhorse of residential electrical work. It powers an average house without a central air or a big workshop. The standard recommendation is 4 AWG copper or 2 AWG aluminum for the main feeder. But honestly? I lean toward copper if you can swing the budget.

Why? Copper has higher ampacity for the same gauge, it doesn’t oxidize as aggressively, and it’s easier to terminate in small breakers. Aluminum is cheaper, but it requires anti-oxidant compound and careful torque specs. If you’re doing a 100 amp subpanel in a detached garage 50 feet away, aluminum is fine. But for a main panel? I’d go copper.

Copper vs Aluminum for 100 Amps

Here’s a quick breakdown of the wire sizes you’ll actually see in the field:

• 100 amp copper: 4 AWG is standard for a short run (under 100 feet). For longer runs or higher ambient temps, bump to 3 AWG or even 2 AWG.
• 100 amp aluminum: 2 AWG is the usual choice. But if you’re going 150 feet, you might need 1/0 aluminum to compensate for voltage drop.
• Temperature derating: If your conduit runs through an attic that hits 130°F, you’ll need to upsize the wire by one full gauge. Seriously. Check the NEC 310.16 table.

Most DIYers grab 4 AWG copper and call it a day. That works for a direct burial cable or a short conduit run. But if you have a 100 amp panel feeding a barn 200 feet away? You’ll need 2 AWG copper or 1/0 aluminum. The voltage drop at that distance will kill your efficiency.

Distance and Voltage Drop for 100 Amp Runs

Voltage drop is the silent killer of good intentions. For a 100 amp panel running 200 feet, you’re looking at a 3-5% voltage drop with standard wire. That means your 240V tool is getting 228V. Motors run hotter, lights dim, and electronics get cranky.

The rule of thumb I use: for every 100 feet, upsize by one gauge. So if you’re at 200 feet, go from 4 AWG to 2 AWG. At 300 feet? You’re in 1/0 territory. It’s not sexy, but it’s necessary. And it’s a huge part of the differences in wire size for 100 vs 200 amp panels because the distances are often longer for subpanels.

Wire Size for a 200 Amp Panel (Stepping Up the Game)

A 200 amp panel is the standard for modern homes. It handles electric car chargers, heat pumps, and a full kitchen without breaking a sweat. But the wire size required is no joke. You’re looking at 2/0 copper or 4/0 aluminum as the minimum. And that’s for a short run under ideal conditions.

I’ve seen people try to use 1/0 copper for a 200 amp service. Big mistake. That wire is only rated for 150 amps in most insulation types. It’ll run hot at 200 amps, and the breaker might not trip fast enough to save it. Don’t do it.

The Standard 200 Amp Service Cable

For a main service entrance (the cable from the meter to the panel), the utility often sets the rules. But for a subpanel? Here’s what I recommend:

• 200 amp copper: 2/0 AWG is the minimum. If you want a safety buffer or have a long run, go to 3/0 AWG.
• 200 amp aluminum: 4/0 AWG is standard. It’s bulky but cost-effective. Use it for long runs where copper would bankrupt you.
• Conduit fill: 2/0 copper in EMT requires a 1.5-inch conduit minimum. 4/0 aluminum needs 2-inch conduit. Plan accordingly.

The physical size difference is wild. A 4/0 aluminum cable is nearly the diameter of your thumb. Pulling it through conduit with two 90-degree bends is a workout. Seriously. Use cable lube and a buddy.

When You Need to Go Bigger (Temperature and Conduit)

The differences in wire size for 100 vs 200 amp panels really shine when you account for environmental factors. For 200 amps, the NEC tables get strict. At 86°F ambient (typical for a basement), you’re fine. But at 122°F (common in an unconditioned attic during summer), you have to derate by nearly 30%.

That means your 2/0 copper (rated 195 amps at 75°C) drops to about 136 amps at 122°F. So you need 3/0 or even 4/0 copper just to get back to 200 amps. Same for aluminum—4/0 drops to about 150 amps. So you’re looking at 250 kcmil or larger. It’s expensive. But it’s the code.

I once did a 200 amp panel for a house in Phoenix. The service mast was on the south side in direct sun. The ambient temp inside the mast hit 140°F. That 2/0 copper? Nope. Went with 350 kcmil aluminum to be safe. The client thought I was upselling. The inspector thanked me.

Copper vs Aluminum: The Tug of War

Every electrician has a preference. I’ll admit it—I prefer copper for anything under 200 amps. It’s forgiving, it doesn’t creep (loosen) under thermal cycling, and the terminations are easier. But for a 200 amp service running 300 feet? Aluminum saves you a fortune. The price difference can be 40-50%.

Here’s the kicker: aluminum requires special care. You must use anti-oxidant paste on all connections. You must torque lugs to spec (not just “tight”). And you can’t terminate aluminum directly into some breakers—they need CO/ALR ratings. Ignore that, and you’ll get a loose connection that arcs and burns. I’ve seen it happen.

Cost vs Conductivity

Let’s talk numbers. For a 100 amp panel running 100 feet:

• Copper (4 AWG): about $2.50 per foot. Total ~$250.
• Aluminum (2 AWG): about $1.20 per foot. Total ~$120.

For a 200 amp panel running 100 feet:

• Copper (2/0 AWG): about $5.00 per foot. Total ~$500.
• Aluminum (4/0 AWG): about $2.50 per foot. Total ~$250.

That’s a real savings. But remember: aluminum is thicker and harder to pull. If you have a long conduit with multiple bends, the labor cost might eat your savings. Evaluate your specific run before committing.

Terminal Compatibility and Oxidation

Here’s a detail that trips up beginners: aluminum expands more than copper when heated. Over years of heating and cooling cycles, the connections can loosen. That’s why you need to torque them properly and use a torque wrench. Don’t guess.

Also, aluminum forms a non-conductive oxide layer when exposed to air. That’s why you use the anti-oxidant goop. It seals the connection and prevents the oxide from blocking current flow. If you skip it, you’ll get high resistance, heat, and eventually a fire. I’ve seen a panel melt because a homeowner skipped the goop on a 200 amp aluminum feeder. Not pretty.