The ampacity of 3/0 AWG copper wire is 200 amps in the 75°C column and 225 amps in the 90°C column, per NEC Table 310.16. For nearly all residential and commercial terminations, the 75°C rating of 200A is the legal maximum allowable ampacity. While the wire's insulation may be rated for 90°C, the equipment lugs you connect it to are almost universally rated for 75°C, making 200 amps your functional limit.
How to Read the NEC 310.16 Ampacity Table for 3/0 Copper
Before pulling wire, you need to understand how the National Electrical Code (NEC) structures ampacity tables. Table 310.16 organizes wire sizes by material (copper vs. aluminum) and then splits them into three temperature columns: 60°C, 75°C, and 90°C. These columns correspond to the thermal rating of the wire's insulation and the equipment it connects to.
Per NEC 110.14(C), the temperature rating of the circuit is determined by the lowest rated component in the chain. For circuits rated over 100 amps, you must use the 75°C column unless the equipment manufacturer explicitly states the terminations are rated for 90°C (which is exceedingly rare for standard panels and breakers). Therefore, even if you buy 90°C-rated THHN wire, your 3/0 copper is legally capped at the 75°C ampacity of 200 amps at the termination points.
Below is the reference data for 3/0 AWG and its immediate neighboring sizes. This data is sourced directly from NFPA 70 (NEC) Table 310.16, assuming an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway.
| AWG Size | 60°C Column (TW, UF) | 75°C Column (THHW, THWN, RHW) | 90°C Column (THHN, XHHW-2) |
|---|---|---|---|
| 1/0 AWG | 125 A | 150 A | 170 A |
| 2/0 AWG | 145 A | 175 A | 195 A |
| 3/0 AWG | 165 A | 200 A | 225 A |
| 4/0 AWG | 195 A | 230 A | 260 A |
Derating Factors: When the Base Ampacity Drops
The 200-amp and 225-amp figures above assume ideal conditions: an ambient temperature of 30°C (86°F) and a maximum of three current-carrying conductors (CCCs) bundled in a conduit. Real-world jobsite conditions frequently violate these assumptions, triggering NEC derating requirements.
When you must derate wire, you always start with the 90°C column value (225A for 3/0 copper THHN), apply the derating multiplier, and then compare the result to the 75°C termination limit (200A). The final allowable ampacity is the lower of the two numbers.
1. Ambient Temperature Derating
If your conduit runs across a hot roof or through a high-temperature boiler room, the wire cannot dissipate heat as effectively. Per NEC Table 310.15(B)(1), if the ambient temperature is 46°C to 50°C (115°F to 122°F), you must multiply the 90°C base ampacity by 0.82.
- Calculation: 225A (90°C base) × 0.82 = 184.5A.
- Result: Because 184.5A is lower than the 200A termination limit, your 3/0 copper is now legally restricted to 184 amps (rounded down to the nearest standard breaker size, typically 175A).
2. Conduit Fill (Bundling) Derating
When you pull more than three CCCs through a single raceway, the wires heat each other up. If you pull four to six CCCs, NEC Table 310.15(C)(1) mandates an 80% adjustment factor.
- Calculation: 225A (90°C base) × 0.80 = 180A.
- Result: Your 3/0 wire is now capped at 180 amps. If you need a full 200 amps in a conduit with four CCCs, you must upsize to 4/0 AWG copper (which derates to 208A, safely above the 200A requirement).
What the Ampacity Table Cannot Tell You
Ampacity tables only address thermal limits—how much current the wire can carry before its insulation melts or degrades. They do not account for voltage drop or physical fit.
Voltage Drop: The NEC recommends keeping voltage drop under 3% for branch circuits and feeders. 3/0 copper has an alternating-current resistance of approximately 0.078 ohms per 1,000 feet at 75°C. If you are running a 240V, 200-amp feeder to a detached garage 200 feet away, the voltage drop calculation looks like this:
VD = (2 × Length × Current × Resistance) / 1000
VD = (2 × 200 × 200 × 0.078) / 1000 = 6.24 Volts
Percentage = (6.24 / 240) × 100 = 2.6%
At 200 feet, 3/0 copper passes the 3% rule. However, if that same run is 250 feet, the drop increases to 3.25%, violating the recommendation. In that scenario, you must upsize to 4/0 AWG or 250 kcmil copper to maintain power quality, even though 3/0 is thermally sufficient.
Physical Fit: 3/0 AWG is a thick, stiff cable (roughly 0.464 inches in bare diameter). It will not physically fit into the lugs of smaller 100A or 125A subpanels. Always verify the manufacturer's lug sizing data sheet before purchasing the wire.
3/0 Copper Wire Ampacity FAQ
Can I use 3/0 copper for a 200-amp residential service?
Yes. 3/0 AWG copper is the standard, code-compliant wire size for a 200-amp residential service entrance or feeder. Its 75°C ampacity is exactly 200 amps. The alternative, if you want to save money on material costs and are willing to work with thicker, lighter wire, is 4/0 AWG aluminum (which also has a 75°C ampacity of 180A, wait—4/0 AL is 180A at 60C and 205A at 75C, making it the correct aluminum equivalent for 200A services). Always apply an antioxidant compound (like Noalox) when terminating aluminum, though this is unnecessary for copper.
What size breaker do I use with 3/0 copper wire?
You should use a 200-amp breaker. Per NEC 240.4(B), you can round up to the next standard breaker size if the wire ampacity doesn't match a standard breaker exactly, but since 3/0 copper at 75°C is exactly 200 amps, and 200A is a standard breaker size (NEC 240.6), a 200A breaker is the precise and required match. Do not use a 225A breaker, as that would exceed the 75°C termination rating of the wire.
How does 3/0 copper ampacity compare to 3/0 aluminum?
Copper is a vastly superior conductor to aluminum. While 3/0 copper carries 200 amps at 75°C, 3/0 aluminum only carries 155 amps at 75°C. If you are trying to build a 200-amp feeder using aluminum wire, 3/0 is too small; you must step up to 4/0 AWG aluminum (rated for 205A at 75°C). Never substitute aluminum for copper at the same AWG size without checking the specific aluminum ampacity column.
Does the insulation type (THHN vs XHHW) change the 3/0 ampacity?
It changes the derating baseline, but not the final termination ampacity. Both THHN and XHHW-2 are rated for 90°C in dry locations, giving you the same 225A starting point for conduit fill and ambient temperature derating calculations. However, XHHW-2 is generally preferred by commercial electricians because it has a thinner insulation profile (allowing more wires in the same conduit) and better moisture resistance in wet locations compared to standard THHN/THWN-2. Regardless of which you choose, the termination limit remains 200 amps.
