Service wire sizing is the process of selecting the correct gauge, material, and insulation type for the main conductors bringing utility power to a building's main disconnect, ensuring they can handle the total calculated load without overheating or dropping excessive voltage. It is the foundational calculation that dictates the physical conduit diameter, the main breaker rating, the meter base lug capacity, and the overall safety margin of your home's power supply. People commonly confuse service wire sizing with feeder wire sizing. Feeders carry power from your main panel to a subpanel, whereas service conductors—governed strictly by NEC Article 230—run from the utility transformer or meter directly to your main service disconnect.
The Core Concept: What Service Wire Sizing Actually Dictates
When you size a branch circuit, you are matching a single load to a breaker. When you size service wire, you are matching the entire building's diversified load to the utility supply. This calculation changes the physical reality of your installation in three major ways:
- Conduit and Fitting Sizing: Stepping up from 100A to 200A service often means jumping from 1.5-inch to 2.5-inch or 3-inch conduit to accommodate the physical bend radius of thicker cables, drastically altering trenching and routing plans.
- Terminal Temperature Limits: Most main breaker lugs and meter sockets are rated for 75°C. Even if you buy 90°C rated wire, NEC 110.14(C) forces you to use the 75°C ampacity column for sizing, a trap that catches many DIYers who undersize their wire by reading the wrong table column.
- Voltage Drop Margins: The NEC minimum ampacity keeps the wire from melting, but it does not guarantee your lights won't dim when the HVAC compressor kicks on. Service wire sizing must account for the physical distance from the transformer to the panel.
Furthermore, while the NEC provides the baseline safety rules, your local utility company has the final say on the service drop or lateral. Utilities often publish a "Service Requirements" manual (sometimes called the Green Book or Blue Book) that mandates specific wire types, like requiring XHHW-2 in wet underground conduits rather than THHN.
The Math: A Worked Numeric Example for a 200A Upgrade
Let's look at the most common residential project: upgrading a 1970s home from a 100A service to a 200A service to support modern appliances and an EV charger. For single-phase dwelling services, we rely on NEC Table 310.12, which simplifies the sizing process and includes built-in diversity factors (meaning you don't have to manually calculate every single receptacle).
According to Table 310.12, a 200A residential service requires conductors with an ampacity of at least 83% of the service rating.
The Calculation:
200A × 0.83 = 166 Amps minimum required ampacity.
Now we look at the 75°C column of NEC Table 310.16 to find our wire gauge:
| Service Rating | Copper Wire Size (75°C) | Aluminum Wire Size (75°C) | Typical Cost per Foot (2026 Est.) |
|---|---|---|---|
| 100A | #4 AWG | #2 AWG | $1.50 - $4.00 |
| 150A | #1 AWG | #1/0 AWG | $3.00 - $7.50 |
| 200A | #2/0 AWG | #4/0 AWG | $6.50 - $14.00 |
| 300A/400A | #350 kcmil | #600 kcmil | $18.00 - $35.00+ |
For our 200A upgrade, we need a wire rated for at least 166A in the 75°C column. #2/0 AWG Copper is rated at 175A, and #4/0 AWG Aluminum is rated at 180A. Both meet the requirement. Because aluminum is significantly cheaper and lighter, #4/0 AWG Aluminum (usually XHHW-2 or SER cable) is the industry standard for 200A residential service entrances today.
Where You Meet This in Practice
You will directly apply service wire sizing principles in three specific scenarios on the jobsite or in your own home:
- Heavy Load Additions (EV Chargers & Heat Pumps): Adding a 60A continuous EV charger and a 40A electric heat pump to an older 100A panel will often push the calculated load past the panel's physical busbar rating. This triggers a service upgrade, requiring you to pull new #4/0 aluminum service wire from the meter to a new 200A main breaker panel.
- Meter Main Combo Installations: Modern code (NEC 2023/2026) increasingly requires outside emergency disconnects. This means moving the main breaker outside to a Meter Main combo unit. The wire from the utility meter to that outside disconnect is service wire, but the wire from that outside disconnect to your indoor subpanel is now technically a feeder, requiring a different grounding and bonding strategy (separating neutrals and grounds).
- Overhead vs. Underground Transitions: If you are running service wire from an overhead weatherhead down to an underground meter, the wire transitions from an aerial environment to a wet conduit environment. You must use wire rated for wet locations (like XHHW-2) for the entire run, as THHN is only rated for dry locations and will degrade if water enters the conduit.
Real-World Scenario Walkthrough: The 150-Foot Underground Mistake
To understand why ampacity isn't the only factor in service wire sizing, let's look at a real-world failure mode involving voltage drop.
The Setup: A homeowner is building a detached workshop with a 200A service. The utility transformer is at the street, and the meter/main panel is in the workshop, exactly 150 feet away. The trench is dug, and they install 2-inch PVC conduit.
The Numbers: Following NEC Table 310.12, they pull #4/0 AWG XHHW-2 Aluminum. It's rated for 180A, easily satisfying the 166A minimum requirement for a 200A service. The local inspector checks the ampacity, verifies the conduit fill, and passes the rough-in.
The Outcome: The system is energized. The homeowner turns on the mini-split HVAC, the air compressor, and plugs in their truck to charge. Immediately, the LED lights in the shop flicker violently, and the air compressor struggles to start, tripping its internal thermal overload.
What Went Wrong: They sized for ampacity (heat) but ignored voltage drop. The NEC recommends a maximum 3% voltage drop on service conductors for reasonable efficiency. Using the Southwire Voltage Drop Calculator, pushing 160A (80% of 200A for continuous loads) through 150 feet of #4/0 Aluminum results in a 4.2% voltage drop. The 240V nominal supply was arriving at the panel at roughly 230V. When the compressor tried to start, the inrush current caused the voltage to sag further below 220V, stalling the motor.
Service Wire Sizing FAQ
Can I use bare copper ground wire for my service entrance?
For a service entrance, the grounded conductor (neutral) and the grounding electrode conductor (GEC) have specific rules. You can use a bare copper GEC to connect your panel to ground rods or a ufer ground, but the neutral wire carrying return current must be fully insulated (typically white or gray). Never use bare wire for a current-carrying neutral.
Does the 83% rule apply to commercial buildings?
No. NEC Table 310.12 and the 83% diversity allowance apply strictly to single-phase dwelling services and feeders (houses, individual apartments). Commercial buildings, multi-family main services, and detached non-residential shops require a full Article 220 load calculation, and the service wire must be sized to 100% of the calculated non-continuous load plus 125% of the continuous load.
What is the difference between SER cable and individual THHN wires?
SER (Service Entrance Round) cable is a pre-assembled, jacketed cable containing the hot legs, neutral, and often a bare ground. It is commonly used for above-ground, indoor runs from the meter socket to the panel. Individual THHN/XHHW wires are pulled through conduit and are required for underground runs or long exterior exposures where physical protection and wet-location ratings are necessary.
Do I need to size the neutral wire the same as the hot wires?
For a standard single-phase 120/240V residential service, yes. While older codes allowed reduced neutrals in specific feeder calculations, modern practice and utility requirements almost universally mandate a full-size neutral for the service entrance to handle unbalanced 120V loads and ensure a robust fault-current path. Always pull a #4/0 neutral to match your #4/0 hot legs on a 200A service.
