A 100 amp cable is an electrical conductor sized to safely carry a continuous or non-continuous current of up to 100 amperes without exceeding its thermal limits or violating voltage drop thresholds. What it changes in a real installation is the physical capacity of your feeder or branch circuit to deliver power to high-draw subpanels, EV chargers, or workshop machinery without tripping breakers or melting insulation.
Choosing the right wire for a 100A circuit is rarely as simple as looking at a single number on a chart. The National Electrical Code (NEC) requires you to factor in insulation temperature ratings, termination limits, conductor material, and the physical length of the run. Below, we break down the exact AWG sizes, the math behind voltage drop, and the termination rules that catch most DIYers off guard.
Sizing a 100 Amp Cable: The Core Rules & AWG Table
The baseline for wire sizing in the US is NEC Article 310.16, which dictates ampacity based on conductor material, size, and insulation temperature rating. However, the most critical rule for a 100 amp cable is found in NEC 110.14(C): your wire's allowable ampacity is limited by the lowest temperature rating of any connected component (breaker, lug, or terminal).
Most modern breakers and lugs are rated for 75°C. Older equipment, or specific cable types like NM-B (Romex), are strictly limited to the 60°C column. You can use 90°C wire (like THHN) for derating purposes, but the final circuit ampacity cannot exceed the 75°C or 60°C limits of the terminations.
| Conductor Material | AWG / kcmil Size | 60°C Ampacity (NM-B / Older Lugs) | 75°C Ampacity (THWN / Modern Lugs) | 90°C Ampacity (THHN / Derating Base) |
|---|---|---|---|---|
| Copper | 4 AWG | 70A | 85A | 95A |
| Copper | 3 AWG | 85A | 100A | 110A |
| Copper | 2 AWG | 95A | 115A | 130A |
| Copper | 1 AWG | 110A | 130A | 145A |
| Aluminum | 2 AWG | 75A | 90A | 100A |
| Aluminum | 1 AWG | 75A | 100A | 120A |
| Aluminum | 1/0 AWG | 100A | 120A | 135A |
Source: Adapted from the Cerro Wire Ampacity Chart and NEC Table 310.16.
For a standard 100A feeder using THWN/THHN in conduit with modern 75°C lugs, 3 AWG Copper or 1 AWG Aluminum is the minimum legal size. If you are pulling NM-B cable, you must jump to 1 AWG Copper or 1/0 AWG Aluminum to satisfy the 60°C column requirement.
Where You Meet 100 Amp Cables in Practice
You will typically encounter the need for 100 amp cable in high-load residential and light commercial scenarios where standard 20A or 50A branch circuits fall short.
- Residential Subpanels: A 100A subpanel is the standard upgrade for a detached garage or workshop running multiple 240V tools simultaneously. It requires a 4-wire feeder (2 hots, 1 neutral, 1 ground) pulled through PVC or buried as direct-bury URD.
- High-Power EV Chargers: While most home Level 2 chargers run on 50A or 60A circuits, commercial dual-charger pedestals or high-end units like the ChargePoint Home Flex (when configured for 80A continuous output) require a 100A breaker and corresponding 100A wire to satisfy the NEC 125% continuous load rule.
- Welders and CNC Plasma: Heavy-duty stick/TIG welders (e.g., Lincoln Electric 250A+ output models) and large CNC plasma tables often require dedicated 100A branch circuits to handle the massive inrush and continuous draw without voltage sag.
- Solar Inverter AC Tie-ins: A large residential or light commercial solar array (e.g., 15kW to 20kW) feeding a string inverter will often output near 80A continuous, necessitating a 100A breaker and 100A rated conductors for the AC disconnect tie-in.
Worked Example: Voltage Drop and Derating on a 100A Feeder
Ampacity tables only tell you if the wire will melt. They do not tell you if your equipment will actually work at the other end. NEC 210.19 (Informational Note) recommends keeping voltage drop under 3% for feeders. Let's calculate a real-world scenario.
To find the voltage drop, we use the standard single-phase formula:
VD = (2 × K × I × D) / CM
- K (Specific resistance of copper) = 12.9
- I (Current) = 100 Amps
- D (One-way distance) = 150 Feet
- CM (Circular mils for 3 AWG copper) = 52,620
Calculation:
VD = (2 × 12.9 × 100 × 150) / 52,620
VD = 387,000 / 52,620 = 7.35 Volts
Percentage Drop:
(7.35V / 240V) × 100 = 3.06%
At 3.06%, you are technically over the 3% recommended threshold for feeders. While legally permissible (the NEC drop limits are generally advisory, not mandatory, unless specified by local AHJ), it is poor practice. To fix this, you bump up one size to 2 AWG Copper (CM = 66,360).
Recalculating with 2 AWG:
VD = 387,000 / 66,360 = 5.83 Volts (2.43%). This is well within the safe 3% margin.
Derating Gotcha: If you decide to pull two separate 100A circuits through the same conduit, you now have 6 current-carrying conductors. NEC Chapter 9, Table 310.15(C)(1) requires an 80% derating factor. Your 3 AWG THHN (110A at 90°C) drops to 88A. You would be forced to use 1 AWG THHN just to maintain your 100A baseline after derating.
Common Confusions: Breaker Ratings, Terminations, and Wire Types
When sizing a 100 amp cable, DIYers and even some licensed electricians frequently trip over three specific NEC rules. Understanding these will save you from failing an inspection or creating a fire hazard.
1. The 90°C Wire vs. 75°C Termination Trap
Almost all modern THHN/THWN wire you buy at the hardware store is rated for 90°C. However, the lugs inside standard 100A breakers and load centers are almost universally rated for 75°C. You cannot use the 90°C column to determine your base wire size. You can only use the 90°C column as a starting point for derating (like the conduit fill example above). The final, derated ampacity must still meet or exceed 100A in the 75°C column. For copper, this means 3 AWG is your absolute minimum for THHN in conduit.
2. NM-B (Romex) is Strictly 60°C
If you are wiring a 100A interior subpanel using NM-B cable (commonly known by the brand name Romex), NEC 334.80 strictly limits the ampacity to the 60°C column, regardless of the fact that the physical insulation might feel thick. Looking at the table above, 3 AWG copper at 60°C is only 85A. Therefore, you must use 1 AWG copper NM-B for a 100A circuit. Because 1 AWG NM-B is incredibly stiff, expensive, and hard to find, most electricians switch to THHN/THWN pulled through EMT or PVC conduit for 100A runs.
3. Aluminum Oxidation and Torque Specs
Aluminum is significantly cheaper than copper (often 50% less for 100A feeders), making 1 AWG or 1/0 AWG aluminum the go-to for long subpanel runs. However, aluminum oxidizes when exposed to air, creating a high-resistance layer that generates heat. You must apply an anti-oxidant compound (like Noalox) to the stripped wire before inserting it into the lug. Furthermore, NEC 110.14(D) requires you to use a calibrated torque screwdriver or torque wrench to tighten the lugs to the manufacturer's specified inch-pounds. Hand-tightening a 100A aluminum feeder is a guaranteed way to cause a thermal failure at the breaker lug within the first year.
