For a 200-amp subpanel feeder, use 3/0 AWG copper or 250 kcmil aluminum wire with a 200-amp double-pole breaker. If your calculated load is under 175A, the NEC next-size-up rule permits 2/0 AWG copper or 4/0 AWG aluminum. Assumes THHN in conduit, 75°C terminations, 30°C ambient.
- Conductor Material: Copper (primary) / Aluminum (alternative)
- Insulation: THHN/THWN-2 (90°C rated wire, but terminated at 75°C)
- Temperature Column: 75°C (per NEC 110.14(C) for equipment >100A)
- Ambient Temperature: 30°C (86°F)
- Installation Method: Raceway (PVC or EMT conduit), max 3 current-carrying conductors.
Note: Never interchange aluminum and copper sizing. Aluminum requires larger gauges and specific anti-oxidant paste (like Noalox) at terminations.
Feeder Sizing and Ampacity Data
The most common mistake DIYers make when planning a 200 amp sub panel wiring diagram is confusing service entrance sizing (NEC 310.12) with feeder sizing (NEC 310.16). While 2/0 copper is legally permitted for a 200A residential service drop, a feeder to a subpanel must be sized based on the actual calculated load and standard ampacity tables. Below is the data-dense matrix for 200A class feeders based on the 75°C column of NEC Table 310.16.
| Target Breaker | Calculated Load | Copper AWG (75°C) | Aluminum AWG (75°C) | Min EGC (Cu) | Approx 2026 Cost/ft (Cu) |
|---|---|---|---|---|---|
| 150A | Up to 150A | 1/0 AWG | 2/0 AWG | 8 AWG | $3.80 |
| 200A | 176A - 200A | 3/0 AWG | 250 kcmil | 6 AWG | $6.10 |
| 200A | Up to 175A | 2/0 AWG | 4/0 AWG | 6 AWG | $4.90 |
| 225A | Up to 225A | 4/0 AWG | 300 kcmil | 4 AWG | $7.80 |
The 75°C Termination Trap and Voltage Drop Adjustments
Why use 3/0 copper for a full 200A load and not 2 AWG or 1 AWG? The answer lies in the termination temperature ratings. THHN wire is manufactured with 90°C insulation. If you look at the 90°C column, 2/0 copper is rated for 195A, which looks close enough to 200A to trigger the NEC 240.4(B) 'next-size-up' breaker rule.
However, NEC 110.14(C) strictly mandates that you must use the 75°C ampacity column because the lugs inside standard residential load centers (like Eaton BR or Square D Homeline) are only rated for 75°C. At 75°C, 2/0 copper drops to 175A. Therefore, to carry a true 200A continuous load, you must step up to 3/0 copper (rated 200A at 75°C).
Voltage Drop: When Distance Forces an Upsize
Ampacity tables only tell half the story. If your subpanel is located far from the main breaker, voltage drop will force you to increase the wire gauge. The NEC recommends a maximum 3% voltage drop for feeders. Using the standard Southwire voltage drop formula (VD = 2 x K x I x D / CM) for a 240V circuit pulling a full 200A:
- At 150 feet (using 3/0 Cu): Voltage drop is 4.61V (1.92%). This is well within the 3% limit. 3/0 copper is perfectly fine.
- At 250 feet (using 3/0 Cu): Voltage drop jumps to 7.68V (3.2%). You have exceeded the 3% recommendation.
- The Fix: Upsize to 4/0 copper (CM = 211,600). At 250 feet, 4/0 copper drops the loss to 6.09V (2.53%), restoring compliance.
200 Amp Sub Panel Wiring Diagram and Grounding Rules
When executing your 200 amp sub panel wiring diagram, the physical routing and termination of the 4-wire feeder is where inspectors find the most violations. Since the 2008 NEC cycle, a 4-wire feeder (2 Hots, 1 Neutral, 1 Ground) is strictly required for all detached structures and subpanels.
| Conductor | Standard Color | Main Panel Termination | Subpanel Termination |
|---|---|---|---|
| Hot Phase A | Black | 200A Breaker Lug A | Subpanel Main Lug / Breaker A |
| Hot Phase B | Red | 200A Breaker Lug B | Subpanel Main Lug / Breaker B |
| Neutral | White | Main Neutral Bar (Bonded) | Isolated Neutral Bar (Floating) |
| Equipment Ground | Green / Bare | Main Ground Bar | Ground Bar (Bonded to Enclosure) |
Equipment Grounding Conductor (EGC) Sizing: Do not guess the ground wire size. Per NEC Table 250.122, a 200-amp breaker requires a minimum 6 AWG copper or 4 AWG aluminum equipment grounding conductor. If you upsize your hot conductors for voltage drop (e.g., moving from 3/0 to 4/0 copper), NEC 250.122(B) requires you to proportionally upsize the ground wire as well.
When to Pull the AHJ or an Engineer Into the Loop
The sizing rules above assume a standard, clean installation. You must consult your local Authority Having Jurisdiction (AHJ) or a licensed professional engineer if your installation hits any of the following edge cases:
- Ambient Temperature Derating: If your conduit runs through an attic where temperatures regularly exceed 113°F (45°C), the 75°C ampacity values must be derated using NEC Table 310.15(B)(1). A 3/0 copper wire in a 120°F attic loses roughly 15% of its ampacity, forcing an upsize to 4/0.
- Conductor Bundling: If you pull more than three current-carrying conductors through a single conduit (for example, routing two separate 240V circuits in one pipe), you must apply the derating factors in NEC 310.15(C)(1). Four to six conductors require an 80% derating multiplier.
- Parallel Conductors: If your distance is so extreme that 4/0 copper isn't enough, you might consider running two parallel sets of smaller wires. NEC 310.10(G) allows parallel conductors, but only for sizes 1/0 AWG and larger. Your subpanel lugs must be explicitly rated and listed for parallel wire termination; otherwise, the AHJ will fail the inspection.
- Local Utility Amendments: Some municipalities and utility companies have local amendments that override the NEC next-size-up rule, mandating 4/0 aluminum or 3/0 copper for all 200A feeders regardless of the calculated load. Always pull a permit and verify local codes before buying wire.
