300 MCM Wire: Ampacity, Uses & Specs
NEC Table 310.16 ampacity for 300 MCM copper and aluminum service entrance and large feeder runs.
300 MCM wire is used for 300 A commercial service entrance, large solar inverter AC output feeders, EV DC fast-charger service conductors, utility-scale battery inverter interconnects, and industrial motor branch feeders. It carries 285 A at 75°C copper and 230 A at 75°C aluminum under NEC Table 310.16, making it a workhorse conductor for large residential and small commercial services and feeders.
300 MCM wire has a cross-sectional area of 300.0 kcmil (152.0 mm²) with a conductor diameter of 0.63 inches (16.0 mm). The copper weight is approximately 908.5 lbs per 1,000 feet, with a DC resistance of 0.0431 Ω/1,000 ft for copper and 0.0709 Ω/1,000 ft for aluminum at 75°C.
The physical dimensions of 300 MCM are defined by NEC Chapter 9, Table 8. These values are used for conduit fill calculations (NEC Chapter 9, Table 4), voltage-drop math, and mechanical connector sizing.
| Specification | Copper (Cu) | Aluminum (Al) |
|---|---|---|
| MCM Size | 300 | 300 |
| Area (kcmil) | 300.0 | 300.0 |
| Area (mm²) | 152.0 | 152.0 |
| Diameter (in) | 0.63 | 0.63 |
| Diameter (mm) | 16.0 | 16.0 |
| DC Resistance @ 75°C (Ω/1000ft) | 0.0431 | 0.0709 |
| Weight (lbs/1000 ft) | 908.5 | 275.28 |
| 60°C ampacity (A) | 240 | 190 |
| 75°C ampacity (A) | 285 | 230 |
| 90°C ampacity (A) | 320 | 260 |
NEC Table 310.16 ampacity for 300 MCM copper conductors in raceway, cable, or earth (not more than three current-carrying conductors), based on 30°C (86°F) ambient. Termination temperature ratings (NEC 110.14(C)) apply — most 100 A and smaller equipment is rated 60°C; most 100 A+ equipment is rated 75°C.
| Insulation Type | Temp Rating | Ampacity | Common Use |
|---|---|---|---|
| TW, UF | 60°C | 240 A | Direct-burial UF cable, wet locations, older wiring |
| RHW, THW, THWN, USE, XHHW (wet) | 75°C | 285 A | Panel feeders, service conductors, wet-rated raceway |
| THHN, XHHW-2, RHW-2, USE-2, RHH | 90°C | 320 A | Dry raceway, derating base ampacity, high-temp motors |
300 MCM Aluminum Ampacity
NEC Table 310.16 ampacity for 300 MCM aluminum and copper-clad aluminum conductors. Aluminum ampacity is roughly 78% of copper for the same size. Do not use aluminum smaller than 6 AWG on general-purpose branch circuits.
| Insulation Type | Temp Rating | Ampacity |
|---|---|---|
| TW, UF | 60°C | 190 A |
| RHW, THW, THWN, USE, XHHW (wet) | 75°C | 230 A |
| THHN, XHHW-2, RHW-2, USE-2, RHH | 90°C | 260 A |
Standard for 200-300 kW commercial solar inverter AC output feeders (typically 300-500 A), utility-scale battery storage DC feeders between combiners and inverters, and EV fleet DC fast-charge service laterals.
For a code-compliant solar PV installation using 300 MCM, apply the 125% continuous-load factor from NEC 690.8(B): the maximum continuous inverter output current must not exceed 228 A at 75°C copper terminations. Battery DC circuits under NEC 690.31(A) require the same 125% continuous-load derating.
Typical applications for 300 MCM wire
| Application | Typical Load | Notes |
|---|---|---|
| 300 A commercial service | 300 A continuous | Copper THWN/THHN service entrance |
| Utility-scale solar AC output | 285-320 A | 500 kW inverter output feeder |
| EV fleet DCFC service lateral | 300 A | 150-350 kW DC fast-charger service |
| Battery storage DC feeder | 300 A DC | Between combiners and inverter |
| Industrial motor feeder | 250-285 A | 480 V motor with soft-starter |
| Data center service | 285 A per phase | PDU distribution feeder at 480 V |
Installing 300 MCM wire correctly requires attention to termination temperature ratings, conduit fill, bending radius, and NEC 110.14 lug torque specifications. Miswiring at these points is the leading cause of thermal failures and permit rejections.
Verify termination temperature rating
at both ends (NEC 110.14(C)) — most 100 A+ gear is 75°C; you must use the 75°C column ampacity for 300 MCM.
Select insulation for the environment
THHN/THWN-2 covers most raceway installs; USE-2 or RHW-2 for direct-buried; PV wire for exposed solar DC runs.
Verify conduit fill
for three 300 MCM THHN conductors, use NEC Chapter 9 Table 4 fill percentage (40% for over-2-conductor).
Apply derating factors
NEC 310.15(B) ambient temperature correction if above 30°C, and 310.15(C) adjustment for more than 3 current-carrying conductors in the raceway.
Torque lugs to manufacturer spec
typically 275-500 in-lb for 300 MCM compression lugs; always use a calibrated torque wrench per NEC 110.14(D).
Support in vertical raceway
NEC 300.19 requires cable supports at maximum intervals; 300 MCM vertical support interval per Table 300.19.
Label the conductors
phase colors black/red/blue for 120/208 V, brown/orange/yellow for 277/480 V (NEC 200.6, 210.5).
NEC 210.19(A) FPN No. 4 recommends limiting branch-circuit voltage drop to 3% and total feeder + branch drop to 5%. Use the formula:
VD = (2 × K × I × L) / CM
- K = 12.9 for copper, 21.2 for aluminum (Ω-cmil/ft at 75°C)
- I = load current in amps
- L = one-way distance in feet
- CM = 300000 for 300 MCM
Voltage drop reference — 300 MCM copper at various loads/distances
| Load (A) | 50 ft | 100 ft | 200 ft | 300 ft |
|---|---|---|---|---|
| 71 A | 0.13% | 0.25% | 0.51% | 0.76% |
| 142 A | 0.25% | 0.51% | 1.02% | 1.53% |
| 213 A | 0.38% | 0.76% | 1.53% | 2.29% |
| 285 A | 0.51% | 1.02% | 2.04% | 3.06% |
Values shown for 240 V single-phase copper. Green = under 3% (branch circuit target). Yellow = between 3% and 5% (feeder acceptable but tight). Red = exceeds 5% total drop limit — upsize the conductor.
| NEC Article | What it Covers |
|---|---|
| NEC 110.14(C) | Termination temperature rating limitations |
| NEC 240.4 | Overcurrent protection of conductors |
| NEC 310.12 | Single-phase dwelling services (aluminum 3/0 for 200 A) |
| NEC 310.15(B) | Ambient temperature correction factors |
| NEC 310.15(C) | Adjustment for more than 3 current-carrying conductors |
| NEC 310.16 | Ampacities for insulated conductors — 300 MCM in this table |
| NEC Ch 9, Table 4 | Conduit fill percentages |
| NEC Ch 9, Table 5 | Cross-sectional area of 300 MCM conductor with insulation |
| NEC Ch 9, Table 8 | Bare-conductor properties |
| NEC Ch 9, Table 9 | AC resistance and reactance for cables in conduit |
PES Supply stocks 300 MCM in THHN, THWN-2, XHHW-2, and USE-2 insulations in both copper and aluminum. All conductors are UL Listed and manufactured to NEC and ASTM specifications. Explore our full wire and cable collection or view the master NEC ampacity chart for all wire gauges.
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What is 300 MCM wire?
300 MCM (also written 300 kcmil) is a large-conductor wire with 300,000 circular mils of cross-sectional area. MCM stands for 'thousand circular mils' — 300 MCM equals 300 kcmil. It has a diameter of 0.63 inches (16.0 mm) and is used for 285 A commercial services and feeders at 75°C copper terminations.
What is the ampacity of 300 MCM copper?
Per NEC Table 310.16, 300 MCM copper carries 240 A at 60°C, 285 A at 75°C, and 320 A at 90°C. Most switchgear rated 100 A and larger uses 75°C terminations, so the effective ampacity is 285 A for standard commercial installations.
What is the ampacity of 300 MCM aluminum?
Per NEC Table 310.16, 300 MCM aluminum carries 190 A at 60°C, 230 A at 75°C, and 260 A at 90°C. Aluminum is about 78% the ampacity of copper for the same size, but is much less expensive for large feeders.
Is 300 MCM the same as 300 kcmil?
Yes, 300 MCM and 300 kcmil are identical measurements — both mean 300,000 circular mils of conductor cross-section. MCM is an older abbreviation (thousand circular mils, using the Roman numeral M) and kcmil is the SI-compatible modern form used in current NEC editions.
What size breaker does 300 MCM copper use?
At 75°C copper terminations, 300 MCM copper is protected by up to a 285 A breaker per NEC 240.4. For continuous loads, apply the 80% rule per NEC 210.19(A) — a 300 MCM copper feeder can serve a continuous load of 228 A.
What conduit size do I need for 3 x 300 MCM THHN?
Three 300 MCM THHN copper conductors require a minimum 2-1/2 inch trade-size EMT or rigid conduit per NEC Chapter 9 Table 4 (40% fill). If you add an equipment grounding conductor, verify total fill with Table 5 cross-sectional areas.
Can I use 300 MCM aluminum for a 300 A service?
Aluminum 300 MCM at 75°C is rated 230 A per Table 310.16, which is not enough for a full 300 A service. For a 300 A commercial service in aluminum, use 350 MCM (250 A at 75°C) at a minimum, or step up to 500 MCM (310 A) for a full 300 A rating.
Is 300 MCM stranded?
Yes. All conductors 8 AWG and larger are stranded per NEC 310.6 (in raceway). 300 MCM is typically Class B stranding with 37 strands for building wire (THHN, XHHW) or Class C stranding with 61 strands for flexible cable.
How much does 300 MCM copper weigh?
300 MCM bare copper weighs approximately 908.5 lbs per 1,000 feet. Add roughly 5-10% for THHN or XHHW insulation. A single 500 ft reel of 300 MCM THHN copper weighs approximately 475-500 lbs — plan lifting equipment accordingly.
- Complete NEC Wire Ampacity Chart — full table for all AWG and MCM sizes
- Wire & Cable Collection — shop THHN, THWN, XHHW, USE, and PV wire
- 2/0 AWG Wire Specs
- 4/0 AWG Wire Specs
- 6 AWG Wire Specs
- 8 AWG Wire Specs
- 10 AWG Wire Specs
- 12 AWG Wire Specs
- 14 AWG Wire Specs
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