The 4/0 wire amp rating is the maximum continuous electrical current a 4/0 AWG (read as "four-aught") conductor can safely carry without exceeding its insulation temperature limit, which the NEC sets at 230 amps for copper and 180 amps for aluminum in standard 75°C terminations.
In a real circuit, this single rating dictates whether your 200-amp residential service entrance will pass inspection, determines the physical size of your conduit, and sets the maximum overcurrent protection device (breaker) you can legally install at the panel. Pushing more current than the rated ampacity causes the insulation to degrade, off-gas, and eventually melt, leading to arc faults or structural fires.
The most common point of confusion among DIYers and junior electricians is mixing up the 90°C insulation rating printed on the wire jacket (like THHN/THWN-2) with the actual allowable circuit ampacity. Because of NEC 110.14(C), your circuit is almost always restricted to the 75°C column because standard panel lugs and breakers are only rated for 75°C, regardless of how heat-resistant the wire insulation itself might be.
The Core Ampacity Data for 4/0 AWG
To size a breaker or calculate a load, you must reference the correct temperature column. The table below pulls the exact allowable ampacities for 4/0 AWG conductors directly from NEC Table 310.16 (formerly 310.15(B)(16)), assuming an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.
| Conductor Material | 60°C (140°F) Column | 75°C (167°F) Column | 90°C (194°F) Column |
|---|---|---|---|
| Copper (THHN/THWN-2, XHHW) | 195 Amps | 230 Amps | 260 Amps |
| Aluminum (XHHW, THWN-2) | 150 Amps | 180 Amps | 210 Amps |
| Copper-Clad Aluminum | 150 Amps | 180 Amps | 210 Amps |
For comprehensive data on how ambient temperature correction factors apply to these baseline numbers, the Copper Development Association's ampacity tables provide excellent field-reference charts that combine NEC baseline data with temperature multipliers.
Worked Example: Sizing a 200-Amp Subpanel Feeder
Let’s apply these numbers to a common jobsite scenario: running a feeder from a 200A main panel to a detached garage subpanel 150 feet away to power a workshop, a welder, and a Level 2 EV charger.
Step 1: Base Ampacity Check
We want to protect the feeder with a 200A breaker. Looking at the 75°C column, 4/0 Copper is rated for 230A. Since 230A > 200A, the wire can safely handle the breaker's maximum trip threshold.
Step 2: Voltage Drop Calculation
While the NEC doesn't strictly mandate a voltage drop limit for feeders (it's an Informational Note recommending a max 3% drop for feeders), keeping it under 3% ensures your welder won't stall and your EV charger won't throw low-voltage errors. We use the standard single-phase voltage drop formula:
VD = (2 × K × I × L) / CM
- K (Copper resistivity) = 12.9
- I (Current) = 200A (using full breaker capacity for worst-case sizing)
- L (One-way length) = 150 feet
- CM (Circular mils for 4/0 AWG) = 211,600
Percentage: 3.65V / 240V = 1.52% Voltage Drop
At 1.52%, 4/0 copper easily passes the 3% recommendation.
Step 3: The Aluminum Feeder Trap
Because copper is expensive, many builders opt for aluminum. But look closely at the table: 4/0 Aluminum is only rated for 180 Amps at 75°C. You cannot put 4/0 aluminum on a 200A breaker for a subpanel feeder. The 180A wire would be over-fused. To use aluminum for a 200A subpanel feeder, you must step up to 250 kcmil Aluminum (rated 205A at 75°C), or drop the subpanel main breaker down to 175A.
Where You Meet 4/0 Wire in Practice
You won't find 4/0 AWG in standard 15A or 20A branch circuits. This is heavy-duty infrastructure wire. Here is where it actually shows up on the jobsite:
- 200A Residential Service Entrances: In many municipalities, 4/0 Aluminum SER (Service Entrance Cable) is the standard drop from the utility meter to the main 200A panel. This is permitted under a special NEC exception (NEC 310.12) that allows 4/0 Al to be rated at 200A specifically for whole-house service entrance conductors, bypassing the standard 180A feeder limit.
- Heavy-Duty Workshop Subpanels: When routing power to a detached building with high-draw equipment like CNC plasma cutters or large air compressors, 4/0 copper THHN pulled through conduit is the standard for maintaining low voltage drop over long distances.
- Solar and Battery Banks: In 48V off-grid solar systems, the DC current between a 5,000W inverter and the battery bank can exceed 120A continuously. Because DC arcs are harder to extinguish and voltage drop is critical at low voltages, 4/0 copper is frequently used for these short, high-amperage runs.
Common Confusions to Avoid:
Never confuse 4/0 AWG with 4 AWG. The "/0" denotes the number of zeros. 4 AWG is a relatively thin wire rated for only 85A at 75°C. Ordering "4 gauge" when you need "4-aught" will result in a catastrophic fire hazard if connected to a 200A breaker. Additionally, 4/0 wire is always stranded; solid 4/0 does not exist in standard building wire because it would be impossible to bend into a panel.
Frequently Asked Questions
What size conduit do I need for three 4/0 THHN wires?
Most DIYers assume a 2-inch pipe is plenty for 4/0 wire. It isn't. NEC Chapter 9 limits conduit fill to 40% when pulling three or more wires. A single 4/0 THHN wire has a cross-sectional area of roughly 0.3237 square inches. Three wires equal 0.9711 square inches. To keep this under the 40% fill limit, your conduit must have an internal area of at least 2.42 square inches. This requires a minimum of 3-inch PVC Schedule 40 (which offers 2.907 sq in of total area). If you try to jam three 4/0s into a 2-inch or 2.5-inch pipe, you will violate code and likely damage the wire insulation during the pull.
Can I use fine-strand welding cable for my inverter-to-battery connections?
Welding cable is incredibly flexible and makes terminating tight battery lug connections much easier than stiff THHN. However, standard welding cable is not NEC-rated for in-wall or in-conduit use because its jacket isn't rated for the same voltage/temperature profiles as THWN-2 or XHHW. For internal battery boxes or exposed inverter runs in a dedicated equipment room, it is widely accepted in the solar industry, but you cannot use it to wire a subpanel inside your home's walls.
Do I need to torque the lugs on 4/0 wire?
Absolutely. NEC 110.14(D) requires that terminations be torqued to the manufacturer's specified values using a calibrated torque tool. A 4/0 wire carrying 200A will generate significant heat at the termination point if the lug is loose. A loose lug increases resistance, which creates a thermal runaway loop: heat causes expansion, expansion loosens the connection further, and the lug eventually melts or catches fire. Always check the breaker or lug datasheet for the exact inch-pound rating (typically around 375 in-lbs for large 200A main lugs, but verify with the manufacturer).
