The wire size for service entrance refers to the specific American Wire Gauge (AWG) or kcmil cross-sectional area of the conductors that carry power from the utility's point of attachment to your home's main service disconnect breaker. In a real installation, this wire size dictates the maximum continuous ampacity your home can draw before the conductors overheat, and it directly controls the voltage drop under heavy load. Most commonly, DIYers and junior apprentices confuse service entrance conductors with standard subpanel feeder wires, mistakenly applying 100% continuous load sizing rules to the service entrance instead of utilizing the residential 83% sizing allowance.
What Service Entrance Wire Size Actually Means (and What It Changes)
The service entrance is the absolute bottleneck between the utility grid and your home's electrical system. When we talk about "wire size" in this context, we are strictly talking about the physical cross-sectional area of the metal conductor, which determines its electrical resistance and its ability to dissipate heat.
Think of the service entrance wire as the main highway on-ramp to your neighborhood; if the lanes (wire gauge) are too narrow for the rush hour traffic (amperage), the friction generates heat, which in electrical terms means insulation meltdown, voltage sag, and severe fire risk. Choosing the correct size changes two critical factors in your installation:
- Thermal Limits: It ensures the wire can carry the peak diverse load of your home without exceeding the temperature rating of the breaker lugs (typically 75°C).
- Voltage Drop: It keeps the voltage at your main panel within the acceptable 114V–126V range (for a 120V nominal system) when high-draw appliances like HVAC compressors and electric ranges kick on simultaneously.
The 83% Rule: Where Most DIYers Get the Math Wrong
If you look at standard ampacity tables, you might assume a 200A service requires a wire rated for a full 200A. This is where the National Electrical Code (NEC) provides a specific exception for residential dwellings.
Under NEC Article 310.12 (formerly 310.15(B)(7)), the code recognizes that residential loads are "diverse." You are almost never running your electric oven, dryer, HVAC, EV charger, and water heater at 100% capacity at the exact same millisecond. Because of this diversity factor, the NEC allows service entrance conductors for single-family dwellings to be sized at 83% of the service rating.
83% of a 200A service = 166A minimum required ampacity.
This is the exact point where people confuse service entrance wires with feeder wires. If you were running a feeder to a detached garage subpanel that had a 200A continuous industrial load, you would need a full 200A-rated wire. But for the main service entrance to a house, the 83% rule applies, saving you significant money and making the physical installation much easier due to the smaller, more flexible wire diameter.
Worked Example: Sizing for a Standard 200A Residential Upgrade
Let’s run the exact math for the most common residential project today: upgrading an older 100A service to a modern 200A service to support EV charging and heat pumps.
- Calculate Target Ampacity: 200A × 0.83 = 166A.
- Select Temperature Column: Most residential main breaker lugs are rated for 75°C. Even if you buy 90°C THHN wire, NEC 110.14(C) requires you to use the 75°C column for termination sizing.
- Consult the Table: Looking at standard manufacturer ampacity tables for the 75°C column, we evaluate Copper vs. Aluminum.
| Conductor Material | AWG Size | Ampacity (75°C Col) | Approx. Cost per Foot (2026) | Physical Handling |
|---|---|---|---|---|
| Copper | 2/0 AWG | 175A | $28.00 - $35.00 | Very stiff, heavy, hard to bend in tight panels |
| Aluminum | 4/0 AWG | 180A | $11.00 - $15.00 | Lighter, more flexible, requires anti-oxidant paste |
Both 2/0 Copper (175A) and 4/0 Aluminum (180A) exceed our 166A minimum requirement. However, because 4/0 Aluminum Service Entrance Round (SER) cable is less than half the cost of copper and significantly easier to pull through a mast or conduit, aluminum is the undisputed industry standard for residential service entrances.
Where You Meet This in Practice: Overhead vs. Underground
The physical environment dictates the exact jacket type of the wire you buy, even though the internal gauge remains the same.
Overhead Service Drops
For an overhead drop, the utility runs their own triplex/quadruplex cable to your weatherhead. From the weatherhead, down through the service mast, and into the meter/main panel, you will use SER (Service Entrance Round) cable. SER has a bare aluminum grounding conductor wrapped around the two insulated hot legs and the neutral. It is rated for wet locations and sunlight resistance, making it perfect for the exterior mast.
Underground Service Laterals
If your power comes from a pad-mounted transformer underground, you cannot use standard SER in direct burial. You must either use USE-2 (Underground Service Entrance) individual conductors pulled through a sweep of 2-inch or 2.5-inch PVC conduit, or direct-burial rated URD (Underground Residential Distribution) cable.
Decision Tree: Pick Your Exact Service Entrance Conductor
Stop guessing and use this decision matrix to select your wire. This assumes a standard single-family residential dwelling utilizing the NEC 83% rule and standard 75°C terminations.
| If Your Service Rating Is... | And You Prefer... | Then Your Required Wire Size Is... |
|---|---|---|
| 100 Amps | Aluminum (Cost-effective) | #2 AWG |
| 100 Amps | Copper (Premium/Compact) | #4 AWG |
| 200 Amps | Aluminum (Industry Standard) | 4/0 AWG |
| 200 Amps | Copper (Premium/Compact) | 2/0 AWG |
| 400 Amps | Aluminum (Required for cost) | 600 kcmil (or parallel 4/0s per 310.10(H)) |
The Default Recommendation: If you are doing a standard modern home upgrade or new build in 2026, your concrete pick is 4/0 AWG Aluminum SER cable for a 200A service. It perfectly satisfies the NEC 83% rule, keeps material costs under $15 a foot, and is what 99% of licensed electricians will pull for this exact job.
Frequently Asked Questions
Q: Can I use individual THHN wires in conduit instead of SER cable for an overhead mast?
A: Yes, you can pull individual THHN/THWN-2 conductors inside a rigid metal or Schedule 80 PVC service mast conduit. You will need two 4/0 AWG Aluminum hot legs, one 4/0 AWG Aluminum neutral, and one #4 AWG Copper (or #2 Aluminum) equipment grounding conductor. However, pulling four thick wires through a 2-inch mast is physically much harder than pulling a single pre-assembled SER cable, which is why SER is preferred for overhead masts.
Q: Do I need to apply voltage drop calculations for a 50-foot underground lateral?
A: The NEC does not mandate a strict voltage drop limit for service entrance conductors, only a recommendation (Informational Note in 310.12 suggests keeping it reasonable). However, for a standard 200A residential load, 4/0 aluminum will only experience roughly a 1.5% to 2% voltage drop over 50 feet, which is well within the acceptable 3% threshold for branch circuits and 5% for feeders. You only need to upsize to 250 kcmil if your underground run exceeds 150 feet.
Q: Why is the neutral wire the same size as the hot wires in SER cable?
A: In a standard 120/240V single-phase residential service, the neutral carries the unbalanced load between the two hot legs. Because the NEC requires the grounded (neutral) conductor to be capable of carrying the maximum unbalanced load—and because a fault on the line side relies on the neutral/ground bond to clear—the service entrance neutral must be a full-size conductor, identical in gauge to the ungrounded (hot) conductors.
