A 300 amp service wire size refers to the minimum physical cross-sectional area and insulation rating of the conductors required to safely carry a 300-ampere continuous load from the utility meter to the main service panel without exceeding thermal limits or code-mandated voltage drop thresholds. If you are pulling permits for a 300A main breaker, the direct answer is that you must use 350 kcmil Copper or 400 kcmil Aluminum conductors, assuming standard 75°C equipment terminations.
The Direct Answer: Sizing for 300 Amps
When you cross the 4/0 AWG threshold, the National Electrical Code (NEC) switches from the American Wire Gauge (AWG) numbering system to thousands of circular mils (kcmil). Sizing a 300 amp service wire size correctly means looking strictly at the 75°C column of NEC Table 310.16, because nearly all service panel lugs and main breakers rated for 100A or higher are tested and listed for 75°C terminations.
| Conductor Material | Minimum Size | 75°C Ampacity (Code Limit) | 90°C Ampacity (Wire Rating) | Avg. Cost per Foot (2026) |
|---|---|---|---|---|
| Copper (THHN/XHHW) | 350 kcmil | 310 Amps | 350 Amps | $48.00 - $58.00 |
| Aluminum (XHHW-2 / SER) | 400 kcmil | 305 Amps | 340 Amps | $19.00 - $25.00 |
Most residential and light commercial installations use Aluminum SER (Service Entrance Cable) due to the massive cost savings, but copper is preferred in highly corrosive environments or where physical space for conduit is severely restricted.
The Physics and Code: What This Changes in Your Installation
Choosing a 300 amp service wire size changes the physical reality of your rough-in. You are no longer dealing with floppy Romex; you are wrestling rigid, heavy cables that dictate your entire infrastructure.
A common and dangerous mistake is looking at the 90°C column of NEC 310.16 and assuming 300 kcmil Copper (rated 320A at 90°C) is sufficient. NEC 110.14(C) mandates that the ampacity of the wire is limited by the lowest temperature rating of any connected termination. Since your 300A breaker lugs are rated 75°C, your wire is legally limited to its 75°C ampacity. 300 kcmil Cu is only rated 285A at 75°C, which will fail inspection and risk melting the breaker lug under sustained load.
What people commonly confuse this with: Many DIYers confuse a "300A calculated load" with a "300A service breaker." If your actual calculated load is only 260A, NEC 240.4(B) allows you to use a wire rated for 260A and protect it with the "next size up" standard breaker (which would be 300A). However, if your utility drop and main panel are explicitly rated and fused as a 300-Amp Service, the conductors must have an ampacity of at least 300A before any next-size-up exceptions apply.
What this changes in the physical installation:
- Conduit Fill and Sizing: A single 400 kcmil Al conductor has an outside diameter of roughly 0.73 inches. A full 4-wire service (two hots, one neutral, one ground) requires a minimum 2.5-inch or 3-inch Schedule 80 PVC conduit to meet the NEC 40% fill rule.
- Bending Radius: Large kcmil cables fight you on every bend. The minimum bending radius for 400 kcmil is typically 5 to 7 times the cable diameter, meaning you need massive, sweeping elbows, not tight 90-degree sweeps.
- Lug Torque: A 300A breaker requires massive mechanical force to terminate properly. You will need a calibrated torque wrench, often set between 35 and 50 ft-lbs (check the breaker manufacturer's datasheet), to prevent thermal expansion from loosening the connection over time.
Worked Numeric Example: Voltage Drop on a 150-Foot Run
Ampacity keeps the wire from melting, but voltage drop ensures your appliances actually work. The NEC recommends a maximum 3% voltage drop for feeders. Let's calculate the real-world drop for a 300A load using 400 kcmil Aluminum over a 150-foot run from the meter to the panel.
Using the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM
- K (Specific resistance for Aluminum at 75°C AC) ≈ 21.2
- I (Current) = 300 Amps
- D (One-way distance) = 150 feet
- CM (Circular mils for 400 kcmil) = 400,000
VD = (2 × 21.2 × 300 × 150) / 400,000
VD = 1,908,000 / 400,000 = 4.77 Volts
Percentage Drop: (4.77V / 240V) × 100 = 1.98%.
For precise routing, always verify your specific cable's AC resistance using a tool like the Southwire Voltage Drop Calculator, as conduit material (PVC vs. Steel) slightly alters the reactance.
Where You Meet This in Practice
You will rarely see a 300 amp service wire size on a standard suburban tract home. This size enters the picture in three specific scenarios:
- Large Custom Homes (4,000+ sq ft): Homes featuring dual 5-ton HVAC systems, electric tankless water heaters (which alone can draw 120A+), and Level 2 EV charging stations. A 200A service will nuisance-trip under simultaneous peak loads here.
- Small Commercial and Maker Spaces: Welding shops, CNC machining garages, or boutique breweries running 3-phase motors, large air compressors, and resistive heating elements.
- Multi-Family Duplexes: Where a single 300A meter main is used to feed two separate 150A subpanels for individual units, requiring heavy feeder cables between the meter stack and the interior disconnects.
Frequently Asked Questions
Can I parallel smaller wires instead of using one massive 400 kcmil cable?
Yes. NEC 310.10(G) allows you to parallel conductors for services rated 1/0 AWG and larger, which is a common trick to make pulling wire easier. To achieve a 300A capacity, you can run two parallel sets of 1/0 AWG Copper (150A × 2 = 300A) or two parallel sets of 4/0 AWG Aluminum (180A × 2 = 360A). When paralleling, both sets must be the exact same length, material, and insulation type, and they must be grouped together in the same conduit to prevent inductive imbalance.
What size ground wire do I need for a 300 amp service?
According to NEC Table 250.66, the size of your Grounding Electrode Conductor (GEC) is based on the size of your largest ungrounded (hot) service conductor. If you are using 350 kcmil Copper or 400 kcmil Aluminum for your hots, you must use a minimum 2 AWG Copper or 1/0 AWG Aluminum grounding conductor. Do not downsize the ground wire just because it rarely carries current; it must be able to handle fault currents long enough to trip the utility's transformer fuses.
Does a 300 amp service require a specific conduit size?
There is no single "300A conduit size," as it depends on your wire material and insulation type. However, as a baseline, if you are pulling four individual THHN/XHHW-2 conductors (two hots, one neutral, one ground) sized at 400 kcmil Al, Chapter 9 Table 5 dictates that you need a minimum 2.5-inch trade size conduit (PVC or EMT) to stay under the 40% maximum fill capacity. If you are using pre-assembled SER cable, conduit fill rules do not apply to the cable itself, but the physical knockout on your panel must be large enough to accept a 2-inch or 2.5-inch hub fitting.
