"100 amp Romex" is a common but technically incorrect term for a 100-amp rated non-metallic sheathed cable, as standard NM-B (the technical name for Romex) is neither manufactured nor legally rated for 100 amps under the National Electrical Code (NEC). When builders and DIYers search for this cable, they are actually looking for a 100-amp feeder wire to supply a subpanel, EV charger, or heavy appliance. Realizing that NM-B tops out at 95 amps changes your installation from a simple staple-and-run cable job to pulling individual THHN/THWN-2 conductors through PVC conduit or running Service Entrance (SER) cable.
The NEC Rules That Cap Romex Below 100 Amps
To understand why you cannot buy 100 amp Romex, you have to look at how the NEC treats non-metallic sheathed cables. Romex is technically classified as NM-B cable. The internal conductors are made of THHN wire, which has a 90°C temperature rating. However, the outer PVC jacket that binds the wires together cannot withstand that much heat.
Because of this jacket limitation, NEC Article 334.80 strictly mandates that the ampacity of NM-B cable must be determined using the 60°C column of NEC Table 310.16, regardless of the 90°C rating of the inner wires.
Here is what the 60°C column dictates for copper wire:
- 3 AWG Copper: 85 Amps
- 2 AWG Copper: 95 Amps
- 1 AWG Copper: 110 Amps
Major manufacturers like Southwire do not standardly produce 1 AWG NM-B cable for residential use because it is incredibly stiff, difficult to bend in standard junction boxes, and economically unviable. Therefore, the largest Romex you can practically buy is 2 AWG, which is legally capped at 95 amps.
Many DIYers assume they can use 2 AWG NM-B (95A) on a 100A breaker by citing the NEC 240.4(B) "next standard size up" rule. While 95A is not a standard breaker size, rounding up to 100A is only permitted if your actual calculated continuous and non-continuous load is strictly under 95A. If you are feeding a subpanel with a 100A main disconnect, the feeder must be rated for the full 100A. Most local inspectors (AHJs) will fail a 2 AWG NM-B feed on a 100A panel breaker.
Worked Example: Sizing a 100-Amp Subpanel Feeder
Since 100 amp Romex is off the table, what should you use? Let us run the numbers for a real-world scenario: running a 100-amp feeder from a 200A main panel to a detached garage subpanel, 60 feet away.
You have two primary code-compliant options that allow you to use the 75°C column (because SER and THHN in conduit terminations at modern breakers are rated for 75°C).
| Parameter | Option A: Copper THHN in Conduit | Option B: Aluminum SER Cable |
|---|---|---|
| Wire Size | 3 AWG Copper | 1 AWG Aluminum |
| Ampacity (75°C Col) | 100 Amps | 100 Amps |
| Resistance (per 1000ft) | ~0.245 Ω | ~0.201 Ω |
| Voltage Drop (60ft @ 100A) | 2.94V (1.22%) | 2.41V (1.00%) |
| Estimated Material Cost | ~$350 - $450 | ~$180 - $240 |
| Installation Method | PVC Schedule 80 Conduit | Stapled to framing / Surface mount |
Note: Voltage drop is calculated using the standard formula: VD = (2 × Length × Current × Resistance) / 1000. The NEC recommends keeping branch circuit and feeder voltage drop under 3% for efficiency, making both options perfectly viable. Data cross-referenced with Cerrowire's official ampacity charts.
Where You Meet This in Practice
You will typically need to solve the "100 amp wire" problem in three specific residential scenarios:
- Detached Garage or Workshop Subpanels: A 100A subpanel is the sweet spot for a garage that needs to run a table saw, air compressor, and lighting simultaneously without tripping a 60A feeder.
- Hardwired Level 2 EV Chargers: While most home EV chargers run on 50A or 60A circuits, some high-end commercial or dual-vehicle hardwired units (like the ChargePoint Home Flex configured for max output or Tesla Wall Connector paired setups) can require an 80A continuous load, which legally mandates a 100A breaker and 100A wire.
- Large Tankless Electric Water Heaters: Whole-home electric tankless units often require massive power. While they usually use multiple smaller breakers (e.g., three 40A breakers), some localized setups require a single 100A feed to a local disconnect switch before splitting the load.
What People Commonly Confuse With 100 Amp Romex
When people claim they have "100 amp Romex," they are usually misidentifying one of three other cable types:
1. Service Entrance (SER) Cable
SER cable looks very similar to NM-B. It has a gray or black non-metallic jacket and contains multiple insulated wires plus a bare neutral/ground. However, SER is rated for 75°C and is manufactured in 1 AWG and 1/0 AWG aluminum sizes specifically for 100A and 200A feeds. It is legal to use for subpanel feeders in many jurisdictions, though some local codes require it to be run in conduit if buried or run outside.
2. Mobile Home Feeder (MHF) Cable
MHF cable is another gray-jacketed cable often found in big box stores. It is typically sized as 4-4-4-6 (rated for 50A-100A depending on the exact code cycle and termination) or 2-2-2-4 (rated for 200A). It is designed specifically for feeding mobile homes and is often direct-burial rated, unlike standard Romex.
3. The 240.4(B) Rounding Misconception
As mentioned in the warning box above, many electricians and DIYers mistakenly believe that because 2 AWG NM-B is rated for 95A, and 95A is not a standard breaker size, they can automatically round up to a 100A breaker. While this is true for a specific load calculated at 92A, it is false for a 100A rated panel busbar. The feeder must match the panel rating.
Frequently Asked Questions
Can I use 2 AWG Romex on a 100 amp breaker?
No, you should not. 2 AWG NM-B Romex is rated for a maximum of 95 amps under the 60°C column. While NEC 240.4(B) allows rounding up to the next standard breaker size (100A) if your actual calculated load is under 95A, you cannot use it to feed a subpanel with a 100A main disconnect. For a true 100A feeder, use 3 AWG copper THHN in conduit or 1 AWG aluminum SER cable.
What is the cheapest wire for a 100 amp subpanel?
1 AWG or 1/0 AWG Aluminum SER (Service Entrance) cable is almost always the most cost-effective choice. Aluminum is significantly cheaper than copper, and 1/0 AWG aluminum is frequently used for 100A feeds to provide a buffer for voltage drop on longer runs, even though 1 AWG is technically sufficient for 100A at 75°C. Always verify that your local AHJ permits SER cable for interior subpanel feeders.
Does a 100 amp feeder require a separate ground wire?
Yes. NEC 250.122 requires an Equipment Grounding Conductor (EGC) for all feeders. For a 100A circuit, the EGC must be a minimum of 8 AWG copper or 6 AWG aluminum. If you use 4-wire SER cable, the bare wire bundled inside the jacket serves as your ground/neutral bond at the main, but remember that at the subpanel, the neutral and ground must be isolated on separate busbars.
Can I run 100 amp wire underground without conduit?
You cannot bury standard NM-B (Romex) or THHN wire directly in the dirt; they are not rated for direct burial or wet locations. To run a 100A feed underground, you must either use direct-burial rated cable like USE-2 or MHF (Mobile Home Feeder), or pull THWN-2 rated conductors through continuous, sealed PVC Schedule 80 conduit. The conduit method is the industry standard as it protects the wire from shifting soil and allows for future upgrades.
