What wire size do I need for 100-amp service?
What wire size for 100-amp service?
For 100-amp service, you need a minimum of #4 AWG copper or #2 AWG aluminum wire. However, best practice calls for #2 AWG copper or #1/0 AWG aluminum to provide a safety margin, reduce voltage drop, and meet NEC (National Electrical Code) requirements at the standard 75°C terminal rating. The right choice depends on wire material, distance, installation method, and local code requirements.
| Wire material | Minimum wire size | Recommended wire size |
|---|---|---|
| Copper | #4 AWG | #2 AWG or #1 AWG |
| Aluminum / copper-clad | #2 AWG | #1 AWG or #1/0 AWG |
A 100-amp circuit at 240 volts can deliver up to 24,000 watts of electricity. Undersized wire creates dangerous heat buildup, voltage drop, and potential fire hazards. Choosing the correct wire gauge is one of the most critical decisions in any electrical installation.
Working with electrical panels, feeders, and service entrance wiring is hazardous and typically requires a licensed electrician and permits. Always consult your local building authority and follow the current edition of the NEC before beginning any electrical work.
Copper vs. aluminum wire for 100-amp service
Both copper and aluminum wire are acceptable for 100-amp service, but they differ in size requirements, cost, and performance. Copper is a better conductor, so it uses a smaller gauge for the same ampacity. Aluminum is significantly cheaper and lighter, making it the preferred choice for long runs and utility service entrances.
| Factor | Copper | Aluminum |
|---|---|---|
| Minimum gauge for 100 amps | #4 AWG (60°C) / #3 AWG (75°C) | #2 AWG (60°C) / #1 AWG (75°C) |
| Recommended gauge | #2 AWG | #1/0 AWG |
| Conductivity | Higher | About 61% of copper |
| Weight | Heavier | About 50% lighter |
| Cost | More expensive | Significantly cheaper |
| Flexibility | More flexible at smaller gauges | Less flexible, can be brittle |
| Thermal expansion | Lower | Higher (connections can loosen) |
When to choose copper
Copper wire is ideal for shorter runs, tight spaces where a smaller cable diameter matters, and applications where long-term reliability is the top priority. Copper connections are less prone to loosening over time because the metal expands and contracts less with temperature changes.
For residential 100-amp service panels, #2 AWG copper is the most commonly recommended size. It is rated for 115 amps at 75°C, providing a comfortable margin above the 100-amp breaker rating.
When to choose aluminum
Aluminum wire is the go-to choice for long feeder runs, utility service entrances, and budget-conscious projects. Its lower cost per foot can save hundreds of dollars on a long run to a detached garage or workshop. Modern aluminum alloy wire (AA-8000 series) has improved flexibility and resistance to fatigue compared to older aluminum wire.
For 100-amp service, #1 AWG aluminum meets the minimum NEC requirement at 75°C. Many electricians recommend #1/0 AWG aluminum as best practice to minimize voltage drop and future-proof the installation.
You may encounter references to the "83% rule" (NEC 310.12), which allows certain dwelling unit service conductors to be sized at 83% of the service rating. This rule applies only to main service entrance conductors for dwelling units, not to feeder circuits or sub-panels. Do not apply the 83% rule to a sub-panel feeder without verifying it meets your specific code scenario.
Types of wire used for 100-amp service
Several cable and conductor types are approved for 100-amp installations. The correct type depends on whether the wire runs through conduit, underground, or through a building's interior. Each type has specific insulation ratings and approved uses.
| Wire/cable type | Common use | Installation method |
|---|---|---|
| THHN / THWN-2 | Indoor and outdoor in conduit | Conduit only |
| XHHW / XHHW-2 | Indoor and outdoor in conduit, wet locations | Conduit only |
| SER (service entrance cable) | Service panels, sub-panels, interior runs | Surface mount or in walls |
| USE-2 / RHH / RHW-2 | Underground service entrance | Direct burial or conduit |
| UF-B | Underground feeder | Direct burial |
| Type MC | Indoor, protected runs | No conduit needed (armored) |
| URD (quadruplex) | Underground utility service | Direct burial |
Understanding each cable type helps you select the right product for your specific installation environment.
SER cable
SER (Service Entrance, Round or Rectangular) cable is one of the most popular choices for 100-amp panels and sub-panels. It contains two hot conductors, a neutral, and a ground wire in a single jacketed cable. Common configurations for 100-amp service include:
- 2-2-2-4 aluminum SER (two #2 hots, one #2 neutral, one #4 ground)
- 4-4-4-6 copper SER (two #4 hots, one #4 neutral, one #6 ground)
Individual conductors in conduit
Running individual THHN or XHHW conductors through conduit gives you more flexibility in sizing and routing. This method is common for underground runs in PVC conduit and for longer distances where voltage drop is a concern. You pull three current-carrying conductors (two hots and a neutral) plus a separate equipment grounding conductor.
URD quadruplex cable
The 2-2-2-4 Dyke quadruplex aluminum URD cable is a budget-friendly option for underground 100-amp service. It is rated for wet and dry locations, approved for direct burial, and widely available. This cable is a common choice for feeding detached garages and outbuildings.
Voltage drop and long wire runs
Voltage drop becomes a significant factor when wire runs exceed 100 feet. The NEC recommends keeping voltage drop below 3% for branch circuits and 5% total for the combined feeder and branch circuit. Exceeding these thresholds causes appliances to underperform, lights to dim, and motors to overheat.
| Distance (one way) | Copper wire size recommended | Aluminum wire size recommended |
|---|---|---|
| Up to 50 feet | #3 AWG or #2 AWG | #1 AWG |
| 50 to 100 feet | #2 AWG | #1/0 AWG |
| 100 to 150 feet | #1 AWG or #1/0 AWG | #2/0 AWG or #3/0 AWG |
| 150 to 200 feet | #1/0 AWG or #2/0 AWG | #3/0 AWG or #4/0 AWG |
The longer the wire run, the larger the conductor you need to compensate for voltage drop. At 150 feet with a full 100-amp load, even #2 copper can exceed the 3% recommended voltage drop. Always use a voltage drop calculator with your projected actual load, not just the breaker rating, to determine the optimal wire size.
How to calculate voltage drop
The formula for single-phase voltage drop is:
Voltage Drop = (2 × Length × Current × Resistance per foot) ÷ 1,000
Many free online calculators simplify this process. Enter the wire material (copper or aluminum), wire gauge, one-way distance, voltage (typically 240V for residential service), and the expected amperage load. The calculator returns both the voltage drop in volts and as a percentage.
When calculating voltage drop for a sub-panel feeder, use your projected load rather than the full 100-amp breaker rating. Most 100-amp sub-panels in garages and workshops draw far less than 100 amps continuously. This approach can prevent you from oversizing wire and overspending.
Ground wire size for 100-amp service
The equipment grounding conductor (EGC) for a 100-amp circuit must meet the minimums specified in NEC Table 250.122. The required ground wire size depends on the overcurrent protective device (breaker) rating, not the wire gauge of the circuit conductors.
| Ground wire material | Minimum size for 100-amp breaker |
|---|---|
| Copper | #8 AWG |
| Aluminum | #6 AWG |
These are the NEC minimums. If you upsize your circuit conductors to compensate for voltage drop, you may also need to upsize the grounding conductor proportionally. Your local inspector or a licensed electrician can confirm the correct ground wire size for your specific installation.
Grounding electrode system
For a detached building with a separate sub-panel, you typically need to install a grounding electrode system at the remote building. This usually consists of two ground rods driven at least 8 feet into the earth and spaced at least 6 feet apart, connected with a #6 AWG copper or #4 AWG aluminum grounding electrode conductor. This is in addition to the equipment grounding conductor that runs back to the main panel.
Wiring a 100-amp sub-panel
Feeding a 100-amp sub-panel in a detached garage or workshop is one of the most common reasons homeowners and electricians research 100-amp wire sizing. The installation requires four wires: two hot conductors, one neutral, and one equipment grounding conductor.
| Component | Copper size | Aluminum size |
|---|---|---|
| Hot conductors (2) | #3 AWG minimum / #2 AWG recommended | #1 AWG minimum / #1/0 AWG recommended |
| Neutral conductor | #3 AWG minimum (can be reduced) | #1 AWG minimum (can be reduced) |
| Equipment ground | #8 AWG minimum | #6 AWG minimum |
| Breaker in main panel | 100-amp double-pole | |
Key sub-panel wiring rules
- Four-wire connection: Sub-panels in detached buildings require a separate neutral and ground. Never bond the neutral and ground bars together in a sub-panel.
- Disconnect required: A detached building sub-panel needs a main breaker or disconnect switch at the building it serves.
- Conduit sizing: For individual conductors, a 1-1/4 inch PVC conduit is typically the minimum size for four #1 AWG aluminum conductors.
- Neutral reduction: The NEC allows reducing the neutral conductor size in certain feeder applications, though many electricians run full-size neutrals for simplicity.
Common cable configurations for 100-amp sub-panels
| Cable configuration | Material | Rated ampacity |
|---|---|---|
| 2-2-2-4 SER or URD | Aluminum | 100 amps |
| 1-1-1-3 SER | Aluminum | 110 amps |
| 4-4-4-6 SER | Copper | 85 to 100 amps (temperature dependent) |
| 2-2-2-4 SER | Copper | 115 amps |
For aluminum, 2-2-2-4 cable is the most widely available and cost-effective option. Some electricians prefer 1-1-1-3 aluminum for extra headroom, especially on runs approaching 100 feet. For copper, 2-2-2-4 copper SER provides the most robust connection but costs significantly more.
Where 100-amp service is used
A 100-amp electrical service is common in many residential and light commercial settings. Understanding whether 100 amps meets your needs helps you plan the correct wire size and panel configuration.
| Application | 100-amp service suitability |
|---|---|
| Small to mid-size homes (under 2,000 sq ft) | Typically sufficient |
| Older homes with gas heat and limited appliances | Typically sufficient |
| Detached garage or workshop sub-panel | Common and adequate |
| Homes over 3,000 sq ft with central A/C | Usually insufficient |
| Homes with electric heat | Usually insufficient |
| Agricultural buildings and barns | Often adequate |
| Construction site temporary power | Common choice |
A 100-amp panel can handle several 240-volt appliances such as a dryer, range, water heater, and basic air conditioning. If you plan to add an EV charger (typically 40 to 60 amps), make sure your total load calculation does not exceed the panel's capacity.
Frequently asked questions
Can I use #4 copper wire for 100-amp service?
#4 AWG copper is rated for 85 amps at 75°C according to NEC Table 310.16, which means it falls short of 100 amps at the standard terminal temperature rating. While some older references or 60°C column ratings list #4 copper at 95 amps, most modern installations require conductors rated at the 75°C column. Use #3 AWG copper as the minimum or #2 AWG copper for best practice.
Is #2 aluminum wire good enough for 100 amps?
#2 AWG aluminum is rated for 90 amps at 75°C, so it does not meet the minimum for a 100-amp breaker. You would need to use a 90-amp breaker with #2 aluminum. For full 100-amp service, use #1 AWG aluminum at minimum.
What size wire do I need for a 100-amp sub-panel 200 feet away?
At 200 feet, voltage drop is a major concern. You will likely need #1/0 or #2/0 copper, or #3/0 or #4/0 aluminum, depending on your projected load. Use a voltage drop calculator with your actual expected amperage to determine the exact size. A licensed electrician should verify the calculation.
Do I need a ground wire in my 100-amp feeder?
Yes. A four-wire feeder (two hots, one neutral, one equipment ground) is required for sub-panels, especially those in detached buildings. The minimum ground wire size is #8 AWG copper or #6 AWG aluminum for a 100-amp circuit. Never rely on the conduit alone as the grounding path.
What conduit size do I need for 100-amp wire?
For four individual #1 AWG aluminum THHN conductors plus a #6 AWG ground, you need at minimum a 1-1/4 inch PVC conduit. If you upsize the conductors for voltage drop, you may need 1-1/2 inch or 2-inch conduit. Always check NEC Chapter 9 fill tables for your specific conductor combination.
What is the difference between service entrance wire and feeder wire?
Service entrance wire runs from the utility meter to the main breaker panel. Feeder wire runs from the main panel to a sub-panel. Both can use similar cable types like SER, but the NEC has different sizing rules for each. The 83% dwelling unit service conductor sizing allowance applies only to service entrance conductors, not feeders.
How much does it cost to install 100-amp service?
The cost depends on wire material, run length, and labor. For reference, the cost to install a subpanel ranges from $500 to $1,500 for a 100-amp unit, including labor and materials. Electrical wiring costs average $4 to $9 per square foot for new installations, while electrician hourly rates range from $50 to $130. You may also need to replace your electrical panel if the existing one cannot support a 100-amp feeder breaker.
How do I identify the correct wire gauge?
Wire gauge is printed on the cable jacket or insulation of individual conductors. You can also reference a wire gauge amp chart for ampacity ratings across different wire sizes and insulation types. When identifying positive and negative wires, color coding on the insulation indicates the wire's function—black or red for hot, white for neutral, and green or bare for ground.