The correct 100amp wire size is the minimum cross-sectional area of a conductor required to safely carry 100 amps of current without exceeding its thermal insulation limits or causing dangerous voltage drop. Getting this right changes everything about your physical installation: it dictates your conduit fill capacity, the minimum bend radius in your junction boxes, the torque specs on your terminal lugs, and whether your breaker will even accept the wire. The most common point of failure for DIYers and junior apprentices is confusing the 90°C insulation rating of modern THHN wire with the 75°C or 60°C termination limits of their breakers, leading to dangerously undersized installations that pass a visual check but fail under thermal load.
The Core Sizing Table: NEC Ampacities for 100 Amps
To size a wire for a 100-amp breaker or feeder, we look directly at NEC Article 310, specifically Table 310.16. This table is the absolute authority on conductor ampacity, but it is frequently misread because it contains three distinct temperature columns.
| Conductor Material | 60°C Column (NM-B / Romex) | 75°C Column (THHN in Conduit) | 90°C Column (Derating Only) | Minimum Size for 100A Breaker |
|---|---|---|---|---|
| Copper | #3 AWG (100A) | #3 AWG (100A) | #2 AWG (115A) | #3 AWG |
| Aluminum | #1 AWG (100A) | #1 AWG (100A) | #1 AWG (110A) | #1 AWG |
How to Read the Temperature Columns
- 60°C Column: Used for Non-Metallic Sheathed Cable (NM-B, commonly known as Romex) and any breaker or terminal explicitly marked for 60°C. If you are running #3 AWG copper NM-B, you are limited to 100A.
- 75°C Column: The standard for almost all modern breakers, lugs, and THHN/THWN-2 wire in conduit. Most 100A breakers are rated for 75°C terminations, making #3 Copper or #1 Aluminum your baseline.
- 90°C Column: Never use this column to size your breaker. The 90°C rating of THHN wire is only used as a starting point for ampacity derating (e.g., when you have more than three current-carrying conductors in a single conduit run). The termination point at the breaker remains limited to 75°C.
Where You Meet 100A Wire Sizing in Practice
You rarely pull 100-amp wire for standard branch circuits. This gauge of wire is reserved for heavy-duty feeder applications and dedicated high-draw appliances. Here is where you will actually use these sizes on the jobsite:
- Subpanel Feeders: Supplying a 100A detached garage, basement workshop, or barn subpanel. This is the most common residential application.
- Hardwired EV Chargers: Many Level 2 EV chargers are rated for 80A continuous charging. Under NEC Article 210.20, continuous loads require the branch circuit to be sized at 125% of the load (80A × 1.25 = 100A). Therefore, an 80A EV charger mandates a 100A breaker and 100A wire sizing.
- Large Hot Tubs and Spas: Luxury spas with multiple high-wattage heaters and multi-speed circulation pumps frequently require a 100A feed to an outdoor GFCI disconnect panel.
- Tankless Electric Water Heaters: While many use multiple 40A breakers, some high-flow commercial or large-residential models require a single 100A feed to a local fused disconnect.
Worked Example: Sizing a 100A Subpanel Feeder with Voltage Drop
Ampacity tables only tell half the story. If your run is long, a wire that is legally large enough to prevent a fire might still cause a severe voltage drop, starving your equipment and overheating motors. The NEC recommends keeping voltage drop under 3% for branch circuits and 5% for feeder/branch combinations.
The Scenario: You are installing a 100A subpanel in a detached garage. The conduit run is 150 feet from the main panel. You expect a maximum continuous load of 100A at 240V.
The Formula:
Voltage Drop (VD) = (2 × K × I × L) / CM
- K = Resistivity constant (12.9 for Copper, 21.2 for Aluminum)
- I = Current in Amps (100A)
- L = One-way length in feet (150 ft)
- CM = Circular Mils of the wire cross-section
Testing #3 AWG Copper
The Circular Mils (CM) for #3 AWG Copper is 52,620.
VD = (2 × 12.9 × 100 × 150) / 52,620
VD = 387,000 / 52,620 = 7.35 Volts
To find the percentage: (7.35V / 240V) × 100 = 3.06%.
Result: This is slightly over the recommended 3% threshold. While legally permissible for ampacity, it is poor practice for a full 100A load at this distance.
Testing #1 AWG Aluminum (The Cost-Effective Fix)
The CM for #1 AWG Aluminum is 105,600.
VD = (2 × 21.2 × 100 × 150) / 105,600
VD = 636,000 / 105,600 = 6.02 Volts
Percentage: (6.02V / 240V) × 100 = 2.50%.
Result: #1 AWG Aluminum keeps you well under the 3% drop limit, and because aluminum is significantly cheaper and lighter than copper, it is the superior choice for this specific 150-foot run. You can verify these calculations dynamically using tools like the Southwire Voltage Drop Calculator.
Common Sizing Mistakes and Code Caveats
Do I need to upsize the wire if the load is continuous?
Yes. If your 100A breaker is protecting a continuous load (defined by the NEC as a load expected to run for 3 hours or more, like an EV charger or commercial lighting), you must size the wire for 125% of the load. 100A × 1.25 = 125A. In this case, #3 AWG Copper (100A) is illegal. You must step up to #1 AWG Copper (130A at 75°C) or #1/0 AWG Aluminum.
Can I mix copper and aluminum on the same 100A run?
You can, but only if you use properly rated bimetallic lugs and an approved splicing method inside an accessible junction box. Never twist copper and aluminum together with a standard wire nut; the galvanic corrosion will create a high-resistance fault that will melt the connection and start a fire.
What happens if I put four 100A circuits in the same large conduit?
You trigger NEC Article 310.15(C)(1) for conduit fill derating. When you have 4 to 6 current-carrying conductors in a single raceway, you must derate the ampacity to 80% of the 90°C column value. If you are using #3 AWG THHN Copper, the 90°C rating is 115A. 115A × 0.80 = 92A. Your wire is now only legally good for 92 amps, meaning it will trip a 100A breaker under full load. You would need to upsize to #2 AWG or #1 AWG to compensate for the shared heat.
Does local code override these NEC tables?
Absolutely. The NEC is a model code. Your local Authority Having Jurisdiction (AHJ) or municipal inspector may have local amendments—such as mandating copper-only for residential feeders or requiring a strict 2% voltage drop limit instead of 3%. Always pull your local electrical code supplement before purchasing wire.
