A 100 amp sub panel needs #4 AWG copper or #2 AWG aluminum wire. Choosing the wrong gauge creates dangerous heat buildup and violates NEC code. This guide covers wire gauge selection, voltage drop, conduit sizing, and installation best practices for a safe 100 amp sub panel feeder.
Key Takeaways:
Selecting the correct wire for a 100 amp sub panel is a straightforward NEC-driven decision. However, the quality of the installation determines long-term safety and reliability. In short, getting the gauge right matters as much as getting it done. When in doubt, consult a licensed electrician and always pull the required permits.
- Use #4 AWG copper or #2 AWG aluminum — the NEC minimum for 100A feeders.
- Upsize by one gauge for runs over 100 feet to control voltage drop.
- Always specify THHN/THWN-2 or XHHW-2 insulation rated at 75°C.
- Apply anti-oxidant compound on all aluminum terminations.
- Run four conductors: two hots, one neutral, one separate ground.
- Separate the neutral and ground bars inside every sub panel.
- Obtain permits and schedule inspection before energizing the panel.
Why Wire Size Matters for a 100 Amp Sub Panel
Every conductor has a maximum ampacity — the current it can safely carry without overheating. However, when current exceeds this limit, resistance converts electricity into heat. That heat degrades insulation and creates a fire risk over time. Choosing the right wire size for a 100 amp sub panel prevents this chain reaction entirely.
A 100 amp sub panel feeder carries the full load of all its branch circuits simultaneously. Therefore, even one size too small means the wire runs hot during peak demand. In addition, undersized wire causes voltage drop that hurts appliance performance and trips sensitive electronics.
The NEC recommends limiting voltage drop to 3% for feeders. For example, a 120V circuit should lose no more than 3.6 volts over the run. Finally, inspectors check wire gauge at rough-in. Getting it right the first time avoids costly rework.
NEC Code Requirements for a 100 Amp Sub Panel Feeder
The National Electrical Code (NEC), published by the NFPA, governs all sub panel wiring. For 100 amp feeders, NEC Table 310.16 is the primary reference. It lists allowable ampacity by conductor material, temperature rating, and installation method.
Minimum Wire Gauge per NEC Table 310.16
For copper conductors with 75°C insulation, #4 AWG is the minimum for 100 amps. For aluminum or copper-clad aluminum with 75°C insulation, the NEC requires #2 AWG. Aluminum carries only about 61% the conductivity of copper. As a result, it needs a larger cross-sectional area to carry the same current safely in a 100 amp sub panel feeder.
NEC Article 215 governs interior feeders. NEC Article 225 governs feeders to detached structures such as garages and workshops. Both require feeder conductors rated at least equal to the overcurrent device — in this case, 100 amps.
Derating Rules Under NEC 310.15(C)
When multiple current-carrying conductors share a conduit, the NEC requires a derating of ampacity. However, a standard 100 amp sub panel feeder includes only three current-carrying conductors: two hots and a neutral. The equipment grounding conductor does not count. Therefore, no derating is necessary for a typical 100A feeder. Always verify local NEC amendments before finalizing your design.
100 Amp Sub Panel Wire Gauge: Copper vs Aluminum
For a 100 amp sub panel, both #4 AWG copper and #2 AWG aluminum meet NEC requirements. However, each has distinct advantages depending on run length and budget. Choosing the right material upfront saves money and ensures your 100 amp sub panel feeder performs reliably for decades.
#4 AWG Copper: Best for Short Runs
Copper at #4 AWG is ideal for runs under 100 feet. It offers superior conductivity and excellent corrosion resistance. Also, copper does not require anti-oxidant compound at terminations. Its smaller diameter fits more easily into conduit and requires less fill space. However, copper costs significantly more per foot than aluminum.
#2 AWG Aluminum: Best for Long Runs
Aluminum at #2 AWG is the preferred choice for runs over 100 feet. Modern AA-8000 series alloy, required by NEC 310.14, addresses the brittleness issues of older aluminum wiring. For example, it handles thermal expansion cycling far better than the AA-1350 alloy used in the 1960s and 1970s.
When using aluminum, always apply anti-oxidant compound such as Noalox at all termination points. In addition, use terminals marked AL/CU or AL only. Finally, torque all connections to manufacturer specifications to prevent loosening.
Insulation Rating: Use THHN/THWN-2 or XHHW-2
Always specify conductors with THHN/THWN-2 or XHHW-2 insulation for any 100 amp sub panel installation. These carry a 75°C rating in wet locations and 90°C dry. However, use the 75°C column for ampacity calculations. Most breaker and panel terminals max out at 75°C. Therefore, the terminal rating limits your final ampacity, not the wire insulation rating.
How to Calculate Voltage Drop for a 100 Amp Sub Panel Feeder
Voltage drop increases as feeder run length increases. The NEC recommends no more than 3% voltage drop on feeder conductors. For a 240V circuit, that means no more than 7.2 volts lost over the run.
Voltage Drop Formula
VD = (2 × K × I × L) ÷ CM K = 12.9 (copper) or 21.2 (aluminum) | I = load amps | L = one-way run length in feet | CM = conductor area in circular mils
For example, a #4 AWG copper feeder (41,740 CM) at 100 amps over 75 feet gives: VD = (2 × 12.9 × 100 × 75) ÷ 41,740 = 4.6 volts. That equals 3.8% on 120V. Therefore, upgrading to #3 AWG or #2 AWG copper for that run is advisable.
Recommended Wire Gauge by Run Distance
| Run Distance | Copper Wire | Aluminum Wire | Notes |
| 0–100 ft | #4 AWG | #2 AWG | NEC minimum; voltage drop within 3% |
| 100–150 ft | #2 AWG | #1/0 AWG | Upsize to control voltage drop |
| 150–200 ft | #1 AWG | #2/0 AWG | Upsize again; calculate for actual load |
| 200+ ft | #1/0 AWG | #3/0 AWG | Consult voltage drop calculator; consider larger panel |
Online voltage drop calculators from Southwire and Cerrowire simplify these calculations for specific load levels and run lengths.
100 Amp Sub Panel Conduit and Breaker Sizing
Conduit Types for 100 Amp Sub Panel Feeders
For interior and protected exterior runs, use electrical metallic tubing (EMT). It is lightweight, easy to bend, and acts as the equipment grounding path. For underground runs, use rigid PVC conduit (Schedule 40 or 80). In areas subject to physical damage, use rigid metal conduit (RMC) or intermediate metal conduit (IMC).
Conduit Fill Calculations per NEC Chapter 9
For four #4 AWG THHN copper conductors (two hots, one neutral, one ground), a 100 amp sub panel feeder requires a minimum 1-inch EMT conduit. This follows the 40% fill limit for three or more conductors. For #2 AWG aluminum, however, the minimum conduit size increases to 1.25-inch EMT due to the larger conductor diameter. In addition, using 1.25-inch or 1.5-inch conduit for copper is common practice because it eases wire pulling on long runs.
Breaker Selection for the Main Panel
Install a 100-amp double-pole breaker in the main panel to protect the 100 amp sub panel feeder. Most residential panels support standard 100A breakers, including Square D QO, Siemens Q2100, and Eaton BR2100. However, verify compatibility with your specific panel brand and series. Not all breakers are interchangeable, even when they physically fit.
At the sub panel, NEC 225.32 requires a disconnect means within sight. The main breaker inside the sub panel satisfies this requirement. For detached structures, NEC 225.36 also requires the disconnect to be rated as service equipment. Therefore, a sub panel with a built-in main breaker is the most code-compliant solution.
Step-by-Step: How to Wire a 100 Amp Sub Panel
- Plan the installation
Measure the feeder run and calculate voltage drop. First, select the correct wire gauge and conduit type. Obtain all required permits before starting work. Most jurisdictions require an electrical permit and inspection for sub panel installations.
- Install the conduit
Run conduit from the main panel to the sub panel. Support EMT every 10 feet per NEC 358.30 and within 3 feet of each box. For underground runs, bury rigid PVC at 24 inches minimum depth per NEC Table 300.5.
- Pull the conductors
Apply wire-pulling lubricant to reduce friction. Pull all conductors simultaneously to avoid jamming. Label each conductor at both ends: black and red for hots, white for neutral, green or bare for the ground.
- Connect at the main panel
Turn off the main breaker before working inside the panel. Install the 100A double-pole breaker and connect the two hot conductors. Connect neutral to the main neutral bar and ground to the grounding bar. Torque all connections to manufacturer specifications.
- Connect at the sub panel
Connect the two hot conductors to the 100 amp sub panel main lugs or main breaker. Connect the neutral to the neutral bar and ground to the grounding bar. Remove the bonding screw or strap to separate neutral from ground. NEC 250.142(B) requires this separation in every sub panel.
- Verify and schedule inspection
Before energizing, confirm all connections are tight. Verify insulation is undamaged, neutral-ground bars are separated, and all covers are installed. Schedule inspection before turning on power.
Common 100 Amp Sub Panel Wire Sizing Mistakes to Avoid
Using Undersized Wire
The most dangerous mistake when wiring a 100 amp sub panel is using #6 AWG copper or #4 AWG aluminum. This error often occurs when installers confuse 60A and 100A sub panel requirements. Undersized wire runs hot under load, silently degrades insulation, and creates a fire hazard that may not appear for months or years.
Bonding Neutral to Ground in the Sub Panel
Bonding the neutral bar to the grounding bar inside a 100 amp sub panel violates NEC code. In the main service panel, neutral and ground bond at one point only. In all downstream panels, you must isolate these bars. Therefore, always remove the bonding screw from the sub panel neutral bar. Incorrect bonding creates parallel return paths that can energize equipment enclosures and create shock hazards.
Skipping Anti-Oxidant Compound on Aluminum
Aluminum oxidizes when exposed to air, forming a resistive layer on the conductor surface. As a result, connections without anti-oxidant compound overheat and eventually fail. Apply Noalox or Burndy Penetrox at all aluminum terminations in your 100 amp sub panel. This includes breaker lugs and panel terminal bars. It costs little and takes seconds to apply.
Ignoring Voltage Drop on Long Runs
Failing to calculate voltage drop before purchasing wire is a costly mistake. For example, upgrading from #4 AWG to #2 AWG copper after the fact means rewiring the entire run. Calculate voltage drop for your specific load and distance before buying materials. In addition, always secure conduit at proper intervals to prevent sagging and moisture accumulation inside the raceway.
Wire Gauge Comparison: Copper vs Aluminum
| Conductor | Gauge | Ampacity | Min. Conduit | Notes |
| Copper | #4 AWG | 100A @ 75°C | 1-inch EMT | Best for runs under 100 ft; smaller diameter |
| Aluminum (AA-8000) | #2 AWG | 100A @ 75°C | 1.25-inch EMT | Cost-effective for longer runs; requires anti-oxidant compound and AL-rated terminals |
| Copper-Clad Aluminum | #2 AWG | 100A @ 75°C | 1.25-inch EMT | Compromise option; verify local code acceptance first |
Frequently Asked Questions
What wire size do I need for a 100 amp sub panel?
Use a minimum of #4 AWG copper or #2 AWG aluminum with 75°C-rated insulation. This is based on NEC Table 310.16. However, for runs over 100 feet, upsize to #3 AWG or #2 AWG copper. For aluminum, use #1/0 AWG on runs of 100 to 150 feet.
Can I use aluminum wire for a 100 amp sub panel?
Yes, aluminum wire is acceptable and widely used. Use #2 AWG aluminum with AA-8000 alloy and THHN/THWN-2 insulation. Always apply anti-oxidant compound at terminations and use AL-rated or AL/CU-rated terminals. Modern AA-8000 aluminum is reliable when properly installed.
How far can a 100 amp sub panel be from the main panel?
There is no NEC maximum distance limit. However, voltage drop must stay within the 3% guideline. For example, #4 AWG copper at 75 feet calculates to 3.8% voltage drop at full load. Therefore, upsize to #2 AWG copper for runs of 100 to 150 feet. Always calculate voltage drop before purchasing wire.
Do I need a 4-wire or 3-wire feed for a sub panel?
Modern NEC (2008 and later) requires a 4-wire feeder: two hots, one neutral, and one equipment grounding conductor. The neutral and ground must be separated at the sub panel. The older 3-wire method is not permitted for new installations.
What breaker size do I need for a 100 amp sub panel?
Install a 100-amp double-pole breaker in the main panel. Verify it is compatible with your specific panel brand and series. The sub panel should also have a 100A main breaker. That main breaker serves as the disconnect required by NEC 225.32 and 225.36.
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