The correct wire size for a 100 amp service located 200 feet from the source is the specific gauge that satisfies both the thermal ampacity required to prevent conductor overheating and the maximum allowable voltage drop to ensure equipment operates correctly at the far end of the run. For a standard 240V single-phase residential feeder spanning 200 feet, you must upsize from the minimum code-allowed gauge: use 2/0 AWG Aluminum (XHHW-2) to safely handle the full 100A breaker capacity under a 3% voltage drop limit, or 1 AWG Copper (THHN) if you prefer copper. If your actual calculated continuous load will never exceed 80 amps, 1/0 AWG Aluminum is mathematically sufficient and highly cost-effective.
What Voltage Drop Changes in a Real Circuit
To understand why distance forces you to buy thicker wire, you have to separate ampacity from voltage drop. Ampacity is the thermal limit of the wire—its ability to shed heat without melting the insulation or starting a fire. Voltage drop is the performance limit—the loss of electrical pressure over distance due to the inherent resistance of the metal.
Think of it like water flowing through a 200-foot garden hose. A narrow hose (smaller wire) might be strong enough to handle the pressure without bursting (ampacity), but by the time the water reaches the nozzle 200 feet away, the flow is a weak trickle (voltage drop).
- Motors and Compressors: Air compressors, well pumps, and table saws in a detached garage will draw higher amperage to compensate for low voltage, leading to overheated windings and tripped thermal overloads.
- Electronics and Lighting: Sensitive electronics may experience brownouts or fail to boot, while incandescent and halogen lights will visibly dim.
- EV Chargers: Level 2 EV chargers will throttle their charging speed or throw fault codes if the voltage at the terminal drops below 208V.
What people commonly confuse it with: DIYers and junior apprentices frequently confuse ampacity with voltage drop. They look at NEC Table 310.16, see that #3 AWG Copper or #1 AWG Aluminum is rated for 100 amps at 75°C, and assume the job is done. They ignore the 200-foot distance, resulting in a circuit that is perfectly safe from fire but completely inadequate for running heavy machinery at the far end.
The Worked Numeric Example: Calculating the Drop
The NEC (Informational Note to 310.15(B)) recommends a maximum voltage drop of 3% for feeders. On a 240V system, 3% equals 7.2 volts. Let us run the exact math for a 200-foot run using the standard single-phase voltage drop formula:
VD = (2 × K × I × L) / CM
- K = Conductor resistivity (12.9 for Copper, 21.2 for Aluminum)
- I = Current in Amps (100A)
- L = One-way length in feet (200)
- CM = Circular Mils of the wire (found in NEC Chapter 9, Table 8)
Testing 1/0 AWG Aluminum at Full 100A Load
1/0 AWG Aluminum has a CM of 105,600. Plugging in the numbers for a full 100A draw:
VD = (2 × 21.2 × 100 × 200) / 105,600 = 848,000 / 105,600 = 8.03V
8.03V is 3.34% of 240V. This fails the strict 3% recommendation for a full 100A load.
Testing 2/0 AWG Aluminum at Full 100A Load
2/0 AWG Aluminum has a CM of 133,100.
VD = (2 × 21.2 × 100 × 200) / 133,100 = 848,000 / 133,100 = 6.37V
6.37V is 2.65% of 240V. This passes comfortably, making 2/0 AWG the correct pick if you plan to load the panel to its absolute maximum.
The 80A Reality Check
In reality, a 100A breaker should only be loaded to 80% continuously (80A). If we recalculate the 1/0 AWG Aluminum at 80A, the drop is 6.42V (2.67%). This is why 1/0 AWG Aluminum is widely used in the field for 100A subpanels—the actual continuous load rarely hits 100A.
Where You Meet This in Practice
You will encounter the 100-amp, 200-foot scenario primarily in rural or large suburban properties where the main service panel is at the street or the front of the house, and the load center is far away. Common real-world applications include:
- Detached Garages and Workshops: Running a 100A subpanel to power a 240V welder, air compressor, and heavy lighting.
- Agricultural Outbuildings: Feeding a barn or pole building for livestock heaters, water pumps, and shop tools.
- Long-Driveway EV Charging: Installing a dedicated 100A panel at the end of a long driveway to support dual Level 2 EV chargers (which require 60A continuous capacity each).
- Secondary Dwelling Units (ADUs): Feeding a detached guest house or backyard studio located at the rear of a deep lot.
Decision Tree: Picking Your Exact Wire and Conduit
Use this decision matrix to finalize your materials list. Do not guess; match your specific load profile to the row below.
| Installation Scenario | Max Continuous Load | Recommended Conductor | Minimum Conduit Size (PVC Sch 40) |
|---|---|---|---|
| Heavy Workshop / Full Panel Utilization (Welders, multiple 240V tools running simultaneously) | Up to 100A | 2/0 AWG Aluminum XHHW-2 (or 1 AWG Copper THHN) |
2-inch (to allow for pulling radius and heat dissipation) |
| Standard Detached Garage / ADU (Lighting, standard outlets, occasional 240V tool use) | Under 80A | 1/0 AWG Aluminum XHHW-2 (or 3 AWG Copper THHN - though Cu VD will be ~4% at 100A) |
1.5-inch (minimum), 2-inch preferred for easier pulling |
| Dedicated EV Charger Run (No subpanel, direct feed to a hardwired 48A/60A charger) | 48A Continuous (60A breaker) | 4 AWG Copper THHN (or 2 AWG Aluminum XHHW-2) |
1-inch |
Default Recommendation: If you are unsure of your future load, buy 2/0 AWG Aluminum XHHW-2 and pull it through 2-inch PVC conduit. The upfront cost difference in wire and conduit is negligible compared to the labor cost of digging a 200-foot trench a second time.
Installation Realities: Lugs, Torque, and Terminations
Sizing the wire correctly is only half the battle. Terminating large-gauge aluminum wire 200 feet away from your main panel introduces specific jobsite hazards that you must manage.
- Anti-Oxidant Compound is Mandatory: Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that generates intense heat at the lug. You must apply a listed anti-oxidant paste (like Noalox or Ideal Noalox) to the stripped aluminum conductor before inserting it into the breaker or subpanel lug.
- Respect the 75°C Column: Even though XHHW-2 and THHN wire are rated for 90°C in the conduit, the lugs on almost all residential breakers and subpanels are only rated for 75°C (per NEC 110.14(C)). You must use the 75°C column in NEC Table 310.16 for your final ampacity check.
- Use a Calibrated Torque Screwdriver: NEC 110.14(D) strictly requires that terminations be tightened to the manufacturer's specified torque using a calibrated tool. Guessing the tightness on a 2/0 AWG lug will result in either a loose connection (arcing and fire) or a stripped lug (ruined breaker). Check the breaker label for the exact Newton-meter or lb-in spec.
- Grounding the Detached Structure: Per NEC 250.32, a detached building fed by a 100A subpanel requires both an Equipment Grounding Conductor (EGC) pulled with your feeder wires (sized per Table 250.122, typically #8 AWG Copper for a 100A feeder) AND a local grounding electrode system (like two 8-foot ground rods) at the detached building. Do not bond the neutral to the ground bar in the subpanel; keep them strictly isolated.
Common Questions on Long-Run 100A Feeders
Can I use direct burial cable instead of pulling wire in conduit?
Yes, you can use Underground Service Entrance (USE-2) or Mobile Home Feeder (MHF) cable buried directly in a trench (typically 24 inches deep per NEC Table 300.5). However, MHF cable is often limited to 90°C ratings and specific ampacities. For a 200-foot run, finding direct-burial 2/0 AWG aluminum MHF cable can be difficult and expensive compared to buying standard 2/0 XHHW-2 and pulling it through Schedule 40 PVC conduit, which only requires an 18-inch trench depth and offers superior physical protection against future digging.
Do I need to pull a separate neutral wire if I only have 240V loads?
If your 100A subpanel will exclusively feed 240V loads (like a dedicated well pump or EV charger) and absolutely zero 120V lighting or receptacles, the NEC technically allows you to omit the neutral. However, in 99% of subpanel installations, you will eventually need 120V for a lightbulb, a receptacle, or a smart panel sensor. Always pull four wires (Hot, Hot, Neutral, Ground) to a subpanel to avoid expensive retrofits later. Size the neutral to match the ungrounded (hot) conductors unless a specific calculated load reduction applies.
Where can I verify these NEC voltage drop rules?
Voltage drop in the National Electrical Code is primarily covered under informational notes rather than strict enforceable mandates for most branch circuits, but it becomes a mandatory enforceable rule for specific applications (like sensitive electronic equipment or certain fire pumps). For standard feeder design, the 3% recommendation is the industry standard for performance. You can review the official code language and sizing tables via the National Fire Protection Association (NFPA) or consult detailed field applications via Electrical Contractor Magazine's code columns.
