The size of service entrance wire refers to the specific American Wire Gauge (AWG) or kcmil cross-sectional area of the conductors running from the utility's point of attachment or meter base to the main service disconnect, sized to safely carry the building's total calculated electrical load without exceeding temperature ratings or allowable voltage drop. Changing this wire size directly dictates the physical conduit diameter required, the bending radius inside the meter base, the terminal lug compatibility at the main breaker, and the exact voltage drop experienced at the panel under peak load. People commonly confuse service entrance conductors with feeder conductors (which run from the main panel to a subpanel) and mistakenly apply the same sizing rules, ignoring that service entrance neutrals and grounding requirements operate under entirely different National Electrical Code (NEC) articles.
The Core Physics: Ampacity, Voltage Drop, and Termination Limits
Sizing service entrance conductors is governed by NEC Article 230. The fundamental constraint is not just the wire's ability to carry current, but the temperature limit of the equipment it connects to. Under NEC 110.14(C), termination provisions for equipment rated 100 amps or more are generally evaluated using the 75°C column of Table 310.16, even if the wire insulation is rated for 90°C (like THHN or XHHW-2).
For single-family dwellings, the NEC provides a specific demand factor. Because residential loads are highly diversified (your oven, EV charger, and HVAC rarely peak simultaneously), NEC 310.15(B)(7) allows residential service entrance conductors to be sized at 83% of the service rating. This means a 200A residential service does not require a wire rated for a continuous 200A; it requires a wire rated for 166A (200 x 0.83).
Where You Meet This in Practice: Meter to Panel Routing
In the field, the size of service entrance wire dictates your physical routing strategy. You will typically encounter two scenarios:
- Overhead Service: Conductors run from a weatherhead down through a rigid metal or intermediate metal conduit (IMC) mast to the meter socket. The mast must withstand physical wind load and the mechanical tension of the utility's service drop.
- Underground Service: Conductors run from the utility's underground pedestal or transformer pad, through Schedule 40 or 80 PVC, into the bottom of the meter base.
When pulling 4/0 AWG aluminum or 2/0 AWG copper, the physical stiffness of the wire becomes your primary enemy. You cannot use tight 90-degree conduit elbows. You must use sweeping bends to respect the minimum bending radius (typically 4 to 8 times the cable diameter, depending on the insulation type). If you undersize the conduit, the wire will jam, and you risk damaging the insulation or snapping the conductor during the pull.
Worked Example: Sizing for a 200A Residential Upgrade
Let's calculate the exact requirements for a standard 200A, single-phase 120/240V residential service. Assumptions: Aluminum conductors, XHHW-2 insulation, 75°C termination column, and a 50-foot run from the meter base to the main panel.
Step 1: Apply the 83% Residential Rule
200A × 0.83 = 166A minimum required ampacity.
Step 2: Select the Conductor Size
Looking at the 75°C column for Aluminum, 4/0 AWG XHHW-2 has an ampacity of 180A. Since 180A > 166A, 4/0 AWG is code-compliant for the hot legs and the neutral.
Step 3: Verify Voltage Drop
While the NEC mandates ampacity, it only recommends keeping voltage drop under 3% for branch circuits and feeders combined. Using the standard voltage drop formula for single-phase: VD = (2 × K × I × L) / CM.
- K (DC resistance constant for Aluminum) ≈ 21.2
- I (Assumed peak continuous load) = 160A
- L (One-way length) = 50 feet
- CM (Circular mils for 4/0 AWG) = 211,600
VD = (2 × 21.2 × 160 × 50) / 211,600 = 1.60 Volts.
At 240V, a 1.60V drop is 0.66%. This is well within acceptable limits, meaning no upsizing is required for this distance. For a deeper look at measuring and mitigating drop on long runs, refer to this Fluke guide on voltage drop testing.
| Material | AWG Size | 75°C Ampacity | Approx. Cost per Foot (2026) | Voltage Drop (160A Load) |
|---|---|---|---|---|
| Aluminum (XHHW-2) | 4/0 AWG | 180A | $3.50 - $4.50 | 1.60V (0.66%) |
| Copper (THHN/THWN-2) | 2/0 AWG | 175A | $11.00 - $14.00 | 1.28V (0.53%) |
Common Confusions: Service Entrance vs. Feeder Conductors
The most frequent mistake DIYers and junior apprentices make is treating service entrance wire identically to feeder wire. They are governed by different code articles and have different grounding rules.
Service Entrance Conductors (Article 230): These have no overcurrent protection on the line side (utility side). The neutral (grounded conductor) must be sized to carry the maximum unbalanced load, but it also serves as the primary fault current path back to the utility transformer in the event of a line-to-ground fault before the main breaker. Therefore, the service neutral can never be smaller than the required Grounding Electrode Conductor (GEC).
Feeder Conductors (Article 215): These run from the main panel to a subpanel. They must include a separate Equipment Grounding Conductor (EGC). The neutral and ground must be kept strictly isolated at the subpanel. You cannot use the neutral as a fault path for the subpanel's equipment ground.
Frequently Asked Questions About Service Entrance Wire Sizing
What size of service entrance wire do I need for a 100 amp panel?
For a 100A residential service, applying the 83% rule yields a minimum required ampacity of 83A (100 x 0.83). According to the 75°C column, #2 AWG Aluminum (rated 90A) or #4 AWG Copper (rated 85A) are the minimum acceptable sizes. Always verify with your local utility, as some require a minimum of #1/0 AWG Aluminum for mechanical strength on overhead spans regardless of the electrical load.
Can I use aluminum wire for my service entrance instead of copper?
Yes, aluminum is the industry standard for modern residential service entrances due to its significant cost advantage and lighter weight. You must use AA-8000 series alloy aluminum (like XHHW-2 or THWN-2). When terminating aluminum, you must apply an anti-oxidant inhibitor paste (like Noalox) to prevent galvanic corrosion, and you must torque the lugs to the exact inch-pound specification printed on the breaker label using a calibrated torque screwdriver.
Does the neutral wire need to be the same size as the hot legs?
For a standard single-phase, 3-wire 120/240V residential service, yes. The neutral (grounded conductor) is typically sized identically to the ungrounded (hot) conductors. This is because the neutral must be capable of carrying the maximum unbalanced 120V load, which in a worst-case scenario could approach the full capacity of one hot leg. Furthermore, it must meet the minimum sizing requirements tied to the Grounding Electrode Conductor.
How does voltage drop affect my service entrance wire size?
If your home is set far back from the street, the total distance from the utility transformer to the meter, plus the meter to the main panel, can easily exceed 150 feet. While the utility owns the wire to the meter, the cumulative voltage drop matters. If the total calculated drop approaches 3% at peak load, you must upsize your service entrance conductors by one or two AWG sizes (e.g., moving from 4/0 Al to 250 kcmil Al) to ensure your panel receives a stable 235V-240V, preventing brownouts on sensitive electronics and HVAC compressors.
