Service entrance wire for a 100-amp system is the primary feeder cable that delivers up to 100 amps of current from the utility meter to the main breaker panel, universally sized at #1 AWG copper or #1/0 AWG aluminum under standard 75°C termination rules. This specific wire gauge dictates your home's absolute maximum power draw before the main utility breaker trips, determines the physical conduit diameter required for the run, and governs the mechanical lug compatibility at both the meter base and the panelboard. DIYers and junior apprentices commonly confuse service entrance wire with standard branch circuit wire (like 14 AWG or 12 AWG NM-B), and they frequently misuse the 90°C ampacity column on wire charts, mistakenly believing they can use smaller wire because its 90°C insulation rating exceeds 100 amps.
Core Sizing Rules and the 75°C Termination Limit
The most critical rule when sizing service entrance wire for a 100 amp panel is NEC 110.14(C), which governs termination temperature limits. Even if you purchase THHN wire with 90°C insulation, the lugs inside standard residential breaker panels and meter bases are almost universally rated for a maximum of 75°C. Therefore, you must size your wire based on the 75°C column of NEC Table 310.16, not the 90°C column.
Think of the 75°C rating as the speed limit of a highway on-ramp; even if the main highway (the wire insulation) is engineered to handle traffic moving at 90 MPH, the on-ramp (the breaker lug) caps your safe speed at 75 MPH. If you push 90°C thermal loads through a 75°C lug, the lug will overheat, oxidize, and eventually fail, regardless of how cool the wire itself remains.
| Wire Material | AWG Size | 75°C Ampacity (Termination Limit) | 90°C Ampacity (Derating Only) |
|---|---|---|---|
| Copper | #3 AWG | 100 Amps | 115 Amps |
| Copper | #2 AWG | 115 Amps | 130 Amps |
| Aluminum | #1 AWG | 100 Amps | 115 Amps |
| Aluminum | #1/0 AWG | 120 Amps | 135 Amps |
While #3 AWG Copper and #1 AWG Aluminum technically hit exactly 100 amps in the 75°C column, the industry standard practice is to use #2 AWG Copper or #1/0 AWG Aluminum. This is because NEC 230.42 requires service conductors to handle 125% of any continuous loads (loads expected to run for 3 hours or more). Bumping up one size provides a necessary buffer for continuous loads, mitigates voltage drop, and accounts for the fact that #3 Cu and #1 Al are often difficult to source at local electrical supply houses compared to the ubiquitous #2 Cu and #1/0 Al.
Worked Numeric Example: Voltage Drop on a 75-Foot Run
Ampacity tells you if the wire will melt; voltage drop tells you if your appliances will actually work. Let's calculate the voltage drop for a realistic installation where the meter mast is located 75 feet away from the main panel (a common scenario when the utility drop is at the street and the panel is deep inside the house).
The Scenario:
- System: 120/240V Single Phase, 100A Main Breaker
- Actual Continuous Load: 80 Amps (assuming an 80% continuous load factor for a fully loaded 100A panel)
- Distance (D): 75 feet (one-way)
- Wire Selected: #1/0 AWG Aluminum THHN in PVC conduit
The Math:
We use the standard single-phase voltage drop formula: VD = (2 x K x I x D) / CM
- K (Specific resistance for Aluminum at 75°C) = 21.2
- I (Current in Amps) = 80
- D (Distance in feet) = 75
- CM (Circular Mils for #1/0 AWG) = 105,600
VD = (2 x 21.2 x 80 x 75) / 105,600
VD = 254,400 / 105,600
VD = 2.41 Volts
To find the percentage drop on a 240V system: (2.41 / 240) x 100 = 1.00%. This is well below the NEC recommended maximum of 3% for feeders (NEC 210.19(A) Informational Note). If we had attempted to use #2 AWG Aluminum (CM = 66,360) to save money, the drop would be 3.83V (1.6%), which is still acceptable but leaves zero margin for future load additions or extreme temperature derating. This numeric proof is exactly why #1/0 AWG Aluminum is the undisputed champion for 100A service entrance runs.
Where You Meet This in Practice
You will typically encounter the need to size and pull 100-amp service entrance wire in three specific real-world scenarios:
- Upgrading Older Homes: Homes built in the 1950s and 60s often have 60-amp fuse boxes wired with #6 AWG copper. When upgrading to a modern 100-amp breaker panel to support central air conditioning or electric ranges, the existing wire is woefully undersized and must be replaced with #1/0 Al or #2 Cu from the new meter base to the new panel.
- Detached ADUs and Large Workshops: When wiring a detached Accessory Dwelling Unit (ADU) or a heavy-duty woodworking shop, a 100-amp subpanel is often installed. The feeder wire from the main house to the detached structure acts as the service entrance for that specific building, requiring the same rigorous sizing, burial depth (usually 24 inches for PVC), and grounding rod protocols as a primary utility service.
- Meter Loop Replacements: If a roof leak damages the meter base, or if the utility requires a meter relocation to the exterior property line, the 'meter loop' (the conductors from the weatherhead, through the meter, to the panel) must be pulled fresh. This requires sweeping bends in rigid metal conduit (RMC) to accommodate the stiffness of #1/0 AWG wire.
Frequently Asked Questions
What exact size wire do I need for a 100 amp service entrance?
The absolute minimum code-allowed size is #3 AWG Copper or #1 AWG Aluminum, based on the 75°C column of NEC Table 310.16. However, professional electricians almost exclusively use #2 AWG Copper or #1/0 AWG Aluminum. This one-size bump accommodates the 125% continuous load multiplier required by NEC 230.42, reduces voltage drop on longer runs, and ensures the wire physically fits securely into standard 100A breaker lugs without needing to be trimmed.
Can I use aluminum service entrance wire for a 100 amp panel?
Yes, aluminum is the industry standard for service entrance feeders due to its significantly lower cost and lighter weight compared to copper. The critical requirement is that you must use anti-oxidant paste (like Noalox) on the stripped aluminum conductors before terminating them, and you must torque the lugs to the exact inch-pound specification printed on the breaker or panel label. Failure to torque aluminum properly leads to thermal expansion/contraction cycles that loosen the connection over time, causing arcing and fires.
Does 100 amp service entrance wire need to be installed in conduit?
It depends on the cable type and the location. If you are using individual THHN/THWN conductors, they must be installed in a raceway (such as Schedule 80 PVC, EMT, or Rigid Metal Conduit) to protect them from physical damage. However, if you are using Type SER (Service Entrance Round) or SEU (Service Entrance Underground) cable, the NEC allows it to be run exposed along the surface of interior walls or fished through wall cavities without conduit, provided it is secured and supported per NEC 334. You can verify specific installation rules via resources like the Southwire installation guides.
What is the difference between SER and SEU cable for a 100 amp service?
Both are armored service entrance cables, but their grounding configurations differ. SEU (Service Entrance Unarmored/Underground) has a concentric neutral/ground wire that wraps entirely around the hot conductors, and it only contains two insulated hot wires and the bare concentric neutral. SEU is typically used between the meter and the panel where the neutral and ground are bonded. SER (Service Entrance Round) contains two insulated hot wires, a separate insulated neutral wire, and a separate bare ground wire. SER is required when running a feeder to a subpanel where the neutral and ground must remain isolated, or when local utility codes mandate a 4-wire configuration from the meter to the main disconnect.
