100 Amp Wire Size Calculator
Enter your circuit length, supply voltage, wire material, and ambient temperature to find the minimum AWG gauge that meets the NEC 80% safety rule, then verify the voltage drop for your run length. The calculator covers single-phase and three-phase systems, copper and aluminum conductors, and 60 C, 75 C, and 90 C insulation ratings.
What wire size do you need for a 100 amp circuit?
The correct answer depends on conductor material, insulation temperature rating, run length, and ambient temperature - not just the breaker size. At a minimum, NEC Table 310.16 requires copper conductors rated at least 125 A to carry a continuous 100 A load (the 80% rule), which means 1 AWG copper at the 75 C column or 1/0 AWG aluminum. For runs beyond about 100 feet, voltage drop becomes the controlling factor and you typically need to step up to 1/0 AWG copper or 2/0 AWG aluminum to keep drop under 3%. For a 100 A residential service or subpanel at 240 V with a 100 ft run, 1/0 AWG copper in conduit is the most common choice. For longer runs, or aluminum feeders (common for main service entries), 2/0 AWG aluminum is standard. Always confirm with your local authority having jurisdiction, as some jurisdictions are more restrictive than the NEC baseline.
The NEC 80% rule and why it matters
The National Electrical Code requires that any circuit expected to operate for 3 hours or more - called a continuous load - must not exceed 80% of the circuit's rated ampacity. A 100 A breaker therefore needs wire rated for at least 125 A (100 / 0.80). This rule prevents the wire from running hot enough to degrade insulation, increase resistance, and eventually create a fire hazard. The same logic applies to the breaker itself: a 100 A breaker should not carry more than 80 A of continuous load. This calculator applies the 80% rule automatically before looking up the NEC table.
Voltage drop: why run length changes the answer
Resistance in a wire causes a voltage drop proportional to current and length. The NEC recommends keeping total voltage drop under 5%, with branch circuits ideally staying under 3% (FPN to NEC 210.19 and 215.2). The standard voltage-drop formula is VD = 2 x K x I x L / CM, where K is the conductor resistivity in ohm-circular mils per foot (12.9 for copper, 21.2 for aluminum at 75 C), I is the load current in amps, L is the one-way length in feet, and CM is the conductor cross-section in circular mils. Because voltage drop grows with distance, a wire that is adequately sized for ampacity at 50 feet may produce unacceptable drop at 200 feet, requiring a larger gauge. For example, 1 AWG copper at 100 A over a 200 ft run at 240 V produces roughly 3.1% drop - borderline. Stepping up to 1/0 AWG copper cuts that to about 2.5%, well within the recommended limit.
Copper vs aluminum: which should you choose?
Copper has lower resistivity and higher ampacity per gauge, making it the preference for branch circuits and shorter runs where the cost difference is modest. Aluminum is significantly lighter and less expensive per foot, which is why it dominates utility service entrances and long commercial feeders where large conductor sizes are needed. Aluminum requires anti-oxidant compound on connections and must use connectors rated for aluminum (marked AL or AL/CU). Never connect aluminum wire to a device rated only for copper - this is a common cause of connection failures. Aluminum is not listed by NEC for gauges smaller than 12 AWG. For 100 A service entrance conductors, aluminum 1/0 AWG (or larger for long runs) is standard practice in the industry.
Temperature derating and hot installations
All ampacity values in NEC Table 310.16 assume a 30 C (86 F) ambient temperature. Installations in attics, crawl spaces, or outdoor conduit in hot climates can easily reach 50 to 60 C, which reduces the wire's ability to shed heat. NEC Table 310.15(B)(1) (formerly 310.15(B)(2)(a)) provides the correction factors. For 75 C wire the factor is 0.94 at 31-35 C, 0.88 at 36-40 C, 0.82 at 41-45 C, 0.75 at 46-50 C, 0.67 at 51-55 C, and 0.58 at 56-60 C. The columns for 60 C and 90 C conductors differ, and the correction is above 1.00 when the ambient is below 26 C. This calculator applies the factor for the insulation rating you select. If your conduit runs through an attic, enter 50 to 55 C as a conservative ambient.
NEC 310.16 Ampacity - Copper and Aluminum (75 C rating)
| AWG / kcmil | Copper 60 C | Copper 75 C | Copper 90 C | Aluminum 60 C | Aluminum 75 C | Aluminum 90 C |
|---|---|---|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A | - | - | - |
| 12 AWG | 20 A | 25 A | 30 A | 15 A | 20 A | 25 A |
| 10 AWG | 30 A | 35 A | 40 A | 25 A | 30 A | 35 A |
| 8 AWG | 40 A | 50 A | 55 A | 30 A | 40 A | 45 A |
| 6 AWG | 55 A | 65 A | 75 A | 40 A | 50 A | 60 A |
| 4 AWG | 70 A | 85 A | 95 A | 55 A | 65 A | 75 A |
| 3 AWG | 85 A | 100 A | 115 A | 65 A | 75 A | 85 A |
| 2 AWG | 95 A | 115 A | 130 A | 75 A | 90 A | 100 A |
| 1 AWG | 110 A | 130 A | 145 A | 85 A | 100 A | 115 A |
| 1/0 AWG | 125 A | 150 A | 170 A | 100 A | 120 A | 135 A |
| 2/0 AWG | 145 A | 175 A | 195 A | 115 A | 135 A | 150 A |
| 3/0 AWG | 165 A | 200 A | 225 A | 130 A | 155 A | 175 A |
| 4/0 AWG | 195 A | 230 A | 260 A | 150 A | 180 A | 205 A |
Standard ampacity for conductors in conduit, cable, or earth, not more than three current-carrying conductors, ambient temperature 30 C (86 F). Values from NEC Table 310.16.
Frequently asked questions
What size wire do I need for a 100 amp service?
For most residential 100 A service at 240 V with a short run, you need a minimum of 1 AWG copper or 1/0 AWG aluminum, based on NEC Table 310.16 at the 75 C rating and the 80% continuous-load rule. For runs of 100 feet or more, voltage drop typically requires upsizing to 1/0 AWG copper or 2/0 AWG aluminum to stay within the recommended 3% drop limit.
Can I use 2 AWG wire for 100 amps?
At 75 C, 2 AWG copper is rated for 115 A, which does not meet the 125 A minimum required by the NEC 80% continuous-load rule for a 100 A circuit. You would need at least 1 AWG copper (130 A) at 75 C. At 90 C, 2 AWG copper is rated 130 A, which technically meets the threshold - but NEC 110.14(C) typically limits terminations to 75 C unless all equipment is 90 C rated, which is rarely the case in practice.
What is the difference between wire size for a 100 amp breaker and a 100 amp subpanel?
Functionally, the same wire sizing rules apply. The key difference is labeling: wires feeding a subpanel are called feeder conductors (governed by NEC Article 215) rather than branch-circuit conductors. Both must meet the 80% continuous-load rule. Subpanel feeders also require a separate equipment grounding conductor or grounded neutral, depending on whether the subpanel is a separate structure. The minimum conductor size remains 1 AWG copper or 1/0 AWG aluminum for a 100 A feeder at 75 C.
How do I reduce voltage drop on a long run?
Use a larger wire gauge - each standard AWG step up approximately increases circular mils by about 26%, reducing resistance and drop by a similar proportion. Other options include: raising the supply voltage (e.g., using 240 V instead of 120 V halves the current for the same power, cutting drop by half), running conduit in a straighter path to reduce actual length, or installing a sub-feeder panel closer to the loads to shorten the branch circuits.
Do I need a permit to install 100 amp service wiring?
In almost all jurisdictions, yes. Service entrance work and subpanel installations require an electrical permit and inspection. Rules vary by locality, but the NEC is adopted in most US states with local amendments. Always contact your local authority having jurisdiction before starting this type of work, and hire a licensed electrician for service entrance connections, which must be made at the utility meter base.
What conduit size is needed for 100 amp service conductors?
For three 1/0 AWG copper conductors plus a 6 AWG ground, a 1-1/4 inch EMT or Schedule 40 PVC conduit is typically required. For 2/0 AWG conductors (aluminum service), 1-1/2 inch conduit is often needed. Exact conduit fill is governed by NEC Chapter 9 Tables and depends on the number and type of conductors. Use a conduit fill calculator or NEC Annex C tables to confirm.
Is aluminum wire safe for 100 amp service?
Yes, when installed correctly. Aluminum wiring for service entrance feeders (1/0 AWG and larger) has a decades-long safety record. The key requirements are: use connectors and lugs rated for aluminum (marked AL or AL/CU), apply anti-oxidant compound to all connection points, and never mix aluminum and copper in the same terminal unless the connector is rated for both. Problems with aluminum wiring are almost always related to improper terminations, not the wire itself.