Service wire size is the physical cross-sectional area (measured in AWG or kcmil) of the main conductors delivering power from the utility transformer to your home's main breaker panel, dictating the maximum continuous current the house can safely draw. In a real installation, this single specification dictates three physical realities: the maximum trip rating of your main breaker, the minimum trade size of the conduit or service mast required to route the cables, and the physical lug dimensions on both the meter socket and the panelboard. If you undersize it, the insulation degrades and the lugs overheat under load; if you oversize it, you waste hundreds of dollars on copper and struggle to bend the stiff wire into the panel's tight confines.
The Core Definition and Physical Path
When we talk about service conductors, we are strictly referring to the wires on the line side of the main disconnect. This is the segment that runs from the utility's point of demarcation (usually the output lugs of the meter socket) directly into the main breaker lugs of your interior or exterior panelboard. Once the wire passes through the main breaker and hits the bus bars, it ceases to be 'service wire' and becomes 'feeder' or 'branch circuit' wire, which are governed by entirely different ampacity derating rules.
The physical size of the wire determines its ampacity—its ability to dissipate the heat generated by electrical resistance. Because service wires carry the aggregate load of the entire house, they are the thickest conductors in residential wiring, typically ranging from #2 AWG for older 100-amp services up to 350 kcmil for massive 400-amp luxury builds.
The Copper vs. Aluminum Ampacity Reality
To understand how sizing works in the real world, let's look at a worked numeric example for a standard modern 200-amp residential service. You have two material choices: Copper or Aluminum. Both will safely carry the load, but the physics and the economics are vastly different.
According to the NFPA 70 National Electrical Code (NEC), specifically Table 310.12(A) for single-phase dwelling services, a 200A service requires a minimum of 2/0 AWG Copper or 4/0 AWG Aluminum.
Here is where amateur electricians get tripped up: the temperature column. Modern THHN/XHHW-2 wire insulation is rated for 90°C. If you look at the 90°C column in NEC Table 310.16, 4/0 Aluminum shows an ampacity of 205A. However, NEC 110.14(C) mandates that unless the equipment (your breaker lugs) is explicitly rated and tested for 90°C, you must use the 75°C column. In the 75°C column, 4/0 Aluminum is only rated for 180A.
So why does the code allow 180A-rated wire on a 200A breaker? NEC 310.12(A) provides a specific exception for whole-dwelling services, acknowledging that a 200A main breaker protects the wire because residential loads are highly diversified (your oven, AC, and EV charger rarely peak simultaneously). Therefore, 4/0 Aluminum is perfectly legal and safe for a 200A main breaker.
Where You Meet Service Wire Sizing in Practice
You typically only interact with service wire sizing during three specific project phases:
- The Panel Upgrade: Moving from a legacy 100A Federal Pacific or Zinsco panel to a modern 200A Eaton or Square D panel. The old #2 AWG aluminum service wire is now undersized for the new 200A main breaker, requiring the utility to pull new 4/0 AWG conductors through the existing mast or conduit.
- The Heavy Load Addition: You want to install a 60A Level 2 EV charger and a 50A hot tub. An Article 220 Load Calculation reveals your existing 150A service wire (1/0 AWG Aluminum) is maxed out. You must upgrade the service wire to 4/0 AWG and swap the main breaker to 200A before adding the new loads.
- Underground Lateral Replacements: If your home is fed via an underground trench rather than an overhead drop, the service wire (often a URD - Underground Residential Distribution cable) runs from the utility's green transformer box in your yard directly to your meter. If this wire degrades or you need more capacity, trenching and pulling new 4/0 direct-burial aluminum is required.
The Service Wire Size Decision Tree
Stop guessing and use this decision matrix to select your conductors. This table assumes standard 120/240V single-phase residential power, 75°C rated terminations, and a maximum 3% voltage drop for runs under 100 feet. For runs exceeding 100 feet, you must upsize by one AWG step to mitigate voltage drop.
| Main Breaker Size | Copper Wire Size (AWG) | Aluminum Wire Size (AWG) | Min. Conduit Trade Size (PVC Sch 80) |
|---|---|---|---|
| 100 Amps | #3 AWG | #1 AWG | 1.25 inch |
| 125 Amps | #2 AWG | #1/0 AWG | 1.5 inch |
| 150 Amps | #1 AWG | #2/0 AWG | 1.5 inch |
| 200 Amps | #2/0 AWG | #4/0 AWG | 2.0 inch |
| 320 Amps (Class 320) | #350 kcmil | #350 kcmil | 2.5 inch |
The Concrete Pick: For 95% of modern residential new builds and panel upgrades targeting 200 amps, buy 4/0 AWG Aluminum XHHW-2. It provides the exact ampacity required by NEC 310.12, fits cleanly into standard 2-inch PVC service conduit, and terminates securely into the dual-rated (Cu/Al) lugs of any modern 200A main breaker without requiring exotic anti-oxidant pastes beyond standard Noalox application.
Common Confusions and Code Caveats
Is service wire the same as feeder wire?
No. Service wire runs from the utility source to the main disconnect. Feeder wire runs from the main panel's bus bars to a subpanel. Feeders do not get the special dwelling-service ampacity allowances found in NEC 310.12; a 200A feeder to a detached garage subpanel strictly requires wire rated for 200A in the 75°C column (which means 250 kcmil Aluminum, not 4/0).
Can I use solid core wire for a 100A service?
No. Service entrance conductors are universally stranded. Solid wire above #8 AWG is virtually impossible to bend into the tight radius of a meter socket or panel lug without risking a snapped conductor or cracked insulation. Always purchase stranded THHN, XHHW-2, or pre-assembled SEU (Service Entrance Underground/Overhead) cable.
Do I need to apply the Noalox anti-oxidant compound?
If you are terminating aluminum service wire into mechanical lugs, yes. Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that generates heat. Brush the exposed aluminum strands with a wire brush and coat them with an oxide inhibitor like Noalox or Penetrox-E before torquing the lug to the manufacturer's specified inch-pound rating. (Note: Many modern breakers feature 'Al/Cu' rated lugs with pre-applied factory compound, but adding a fresh layer on the wire is best bench practice).
What if my utility drop wire is smaller than my panel wire?
The utility owns the wire up to the meter. It is incredibly common for a utility to supply a 100A overhead drop (using #2 Al triplex) to a house that has a 200A meter socket and a 200A main panel. This is legal and safe because the utility's transformer fuses protect their drop wire. However, your service wire (from meter to panel) must still be sized for the panel's main breaker (4/0 Al for 200A), regardless of what the utility hangs on the weatherhead.
When planning your upgrade, verify the exact ampacity and conduit fill ratios with your wire manufacturer's calculator, pull your permit, and let the utility handle the final termination at the meter. Stick to 4/0 Aluminum for 200A services, torque your lugs to spec, and your main panel will run cool and safe for decades.
