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what size wire for 100 amp sub panel

For a typical 100 amp sub panel, the commonly accepted starting point is:

  • #3 AWG copper conductors for 100A feeder at 75°C insulation rating.
  • #1 AWG aluminum conductors for 100A feeder at 75°C insulation rating.

However, that’s only the beginning of the story. Let’s walk through the details like you’d see in a solid forum “Quick Scoop.”

Quick Scoop: Short Answer

For most modern residential 100 amp sub panels:

  • Use #3 copper THHN/THWN-2 in conduit, or equivalent cable, for a 100A subpanel feeder when terminations are rated 75°C.
  • Use #1 aluminum XHHW-2 (or sometimes #2 Al where allowed by local code/voltage-drop limits) for a 100A subpanel feeder.
  • Ground wire is typically #8 copper or #6 aluminum for a 100A feeder, per common guidance for 100A services and subpanels.

You must still check:

  • Local electrical code (NEC plus local amendments).
  • Breaker and panel temperature rating (60°C vs 75°C).
  • Distance (voltage drop for long runs).
  • Whether copper or aluminum is allowed/preferred in your jurisdiction.

Why #3 Cu or #1 Al Is the “Go‑To”

Most technical guides that walk through NEC ampacity tables end up with the same baseline:

  • For a 100A subpanel feeder , they pick the 75°C column of NEC Table 310.16 and land on #3 AWG copper or #1 AWG aluminum as the matching ampacities for 100A.
  • Retail and trade sites that sell sub‑panel feeder cable highlight exactly those sizes as “for 100 amp sub‑panel” options (#3 Cu THHN, #1 or #2 Al XHHW-2).

These sizes are considered baseline because:

  • They’re large enough for 100A at 75°C before any derating for bundling or high ambient temperature.
  • They align with how most 100A breakers and subpanels are listed and labeled in real-world residential installs.

Think of #3 Cu / #1 Al as the default “forum answer” if nothing unusual is going on.

Distance: When You Need Bigger Wire

Distance can push you up a wire size for a 100 amp sub panel. One detailed guide specifically looks at a 100A subpanel 80–100 feet away and calls #4 copper or #2 aluminum suitable when voltage drop is factored in for that length. That shows how different authors and calculators sometimes upsize compared to the bare-table #3 Cu / #1 Al answer.

Common distance rules of thumb you see discussed:

  • Up to roughly 50–75 ft : baseline sizes (#3 Cu or #1–#2 Al) are often fine, assuming code tables and terminations are 75°C rated.
  • Around 80–100 ft : some pros will bump one size larger (e.g., #4 Cu or #2 Al) to keep voltage drop reasonable, depending on load profile and local practice.
  • For long runs (200+ ft), specialty guides focus almost entirely on voltage drop and may recommend substantially larger aluminum conductors, especially in buried conduit to outbuildings.

So when you ask “what size wire for 100 amp sub panel,” the true answer is:

It depends on distance, allowed voltage drop, copper vs aluminum, and how conservative your local code/inspector is.

Copper vs Aluminum for 100A Sub Panel

Both copper and aluminum are widely used, but they behave differently.

Copper (THHN/THWN-2, SE cable, etc.)

  • Typical go‑to: #3 AWG copper in conduit for 100A.
  • Some guides dealing with 100 ft runs suggest #4 Cu for 100A when using their particular NEC interpretation and voltage-drop assumptions.
  • Pros: Smaller physical size, easier pulls, more tolerant of slightly loose terminations.
  • Cons: Higher cost compared with aluminum; can be significant on long feeders.

Aluminum (XHHW-2, URD, etc.)

  • Code‑oriented technical articles call #1 Al the matching 100A ampacity size at 75°C.
  • Some how‑to and retailer content also list #2 Al XHHW-2 as a popular choice for 100A sub-panels, especially for moderate distances.
  • Pros: Much cheaper per foot, especially for runs to garages, barns, and sheds.
  • Cons: Larger diameter, needs correct anti-oxidant, torque, and lugs rated for aluminum; more sensitive to poor terminations.

On forums and calculator-based discussions, you’ll see DIYers and electricians going back and forth about #3 Cu vs #1 Al vs #2 Al for 100A feeders, often landing on upsizing slightly if the run is long or if they want extra headroom.

Code, Terminations, and Safety Notes

A key nuance in almost every professional article:

  • Those wire sizes come from NEC ampacity tables , usually Table 310.16, and assume 75°C terminations.
  • You must check whether your breaker and panel lugs are 60°C or 75°C rated; older or smaller equipment may effectively force you into 60°C ampacity rules, which can change the required gauge.
  • The 83% dwelling service rule in NEC Table 310.12 (often used to downsize main service conductors) does not automatically apply to subpanel feeders , a point specifically warned about in technical writeups.

Grounding and neutrals:

  • One common guide for 100A services suggests a #8 copper or #6 aluminum equipment grounding conductor for a 100A feeder.
  • In a subpanel , neutrals must be isolated from ground (separate neutral bar and ground bar, no bonding screw), which is a regular item in installation how‑tos and videos.

Because of these code subtleties and safety stakes, pros and authoritative resources constantly repeat:

  • Verify all sizing against the current NEC edition and local amendments.
  • Make load calculations and voltage-drop checks before finalizing conductor sizes.

Typical Real‑World Setups (Example Scenarios)

Here are a few example “forum-style” scenarios, pulling together what’s commonly recommended:

  1. Basement or attached garage subpanel, 20–40 ft away
    • Often: #3 Cu THHN/THWN-2 in conduit or SER cable with equivalent copper conductors from a 100A breaker to the subpanel.
 * Or: #1 Al XHHW-2 in conduit when cost matters and there’s space for pulling.
  1. Detached garage, 80–100 ft away
    • Some guides: #3 Cu / #1 Al is code-minimum; others upsize to #4 Cu / #2 Al for better voltage drop performance.
  1. Outbuilding 200+ ft away
    • Specialized advice focuses on voltage drop ; it may call for significantly larger aluminum conductors than the bare-nec 100A match (for example, bigger than #1 Al).

In real “ask an electrician” threads, you’ll see people post their distance, load, and conduit, then others respond with calculations that often end up recommending a slightly larger size than the bare minimum tables.

SEO Bits: Focus Keyword Usage

Since you explicitly mentioned content rules and SEO, here are the core points aligned with the keyword “what size wire for 100 amp sub panel” :

  • The most cited short answer to “what size wire for 100 amp sub panel” is #3 copper or #1 aluminum at 75°C, subject to local code and distance.
  • For longer runs, “what size wire for 100 amp sub panel” turns into a voltage-drop question, leading some experts to recommend #4 Cu or #2 Al at around 80–100 ft, and even larger for 200 ft lines.
  • Many “latest” how‑to articles published in 2023–2026 still frame “what size wire for 100 amp sub panel” around NEC Table 310.16 and emphasize not to misuse dwelling-service rules from Table 310.12 for subpanels.

Information gathered from public forums or data available on the internet and portrayed here.