For a 100A main breaker in sub panel, use 3 AWG copper THHN wire. For a 60A main breaker in sub panel, use 6 AWG copper. The breaker rating must never exceed the wire's adjusted ampacity. This assumes copper conductors, 75°C terminations, 30°C ambient temperature, and EMT conduit.
Sizing the Feeder Wire and Main Breaker in Sub Panel
- Material: Copper (Aluminum requires different sizing, detailed below)
- Termination Rating: 75°C (Standard for modern Square D, Eaton, and Siemens panels)
- Ambient Temperature: 30°C (86°F)
- Raceway: EMT conduit with no more than three current-carrying conductors
- Insulation: THHN/THWN-2
When sizing a main breaker in sub panel, the fundamental rule is that the overcurrent protective device (OCPD) must protect the weakest link in the circuit. If you are installing a 100A subpanel, you might wonder why you cannot use 4 AWG copper wire, which is rated for 85A in the 75°C column of NEC Table 310.16.
NEC 240.4(B) allows you to round up to the next standard breaker size if the wire ampacity does not match a standard breaker. The standard breaker sizes are 80A, 90A, 100A, 110A, etc. Since 4 AWG is rated 85A, the next standard size up is 90A. You cannot legally protect 85A wire with a 100A breaker because 100A is not the next standard size above 85A. Therefore, to use a 100A main breaker in sub panel, you must step up to 3 AWG copper, which is rated exactly 100A at 75°C. For a 60A breaker, 6 AWG copper (rated 65A) is acceptable because 60A is a standard size and is lower than the wire's ampacity.
Ampacity, Voltage Drop, and Derating Factors
Wire sizing is not just about matching the breaker; it is about managing heat and voltage loss over distance. Below is the reference data for copper THHN/THWN-2 conductors in the 75°C column.
| AWG Size | 75°C Ampacity (Copper) | Standard Breaker Match | Circular Mils (CM) |
|---|---|---|---|
| 6 AWG | 65A | 60A | 26,240 |
| 4 AWG | 85A | 80A or 90A | 41,740 |
| 3 AWG | 100A | 100A | 52,620 |
| 2 AWG | 115A | 110A | 66,360 |
Voltage Drop Check at 100 Feet
Ampacity tables assume a short run. If your subpanel is located 100 feet from the main panel, you must check voltage drop. The NEC recommends a maximum 3% voltage drop on feeders. Let us calculate the drop for a 100A load on 3 AWG copper at 240V over 100 feet.
Formula: VD = (2 × K × I × L) / CM
Variables: K = 12.9 (copper), I = 100A, L = 100 ft, CM = 52,620 (for 3 AWG).
Calculation: (2 × 12.9 × 100 × 100) / 52,620 = 4.9 Volts.
Percentage: 4.9V / 240V = 2.04%.
Because 2.04% is well under the 3% threshold, 3 AWG copper is perfectly adequate for a 100-foot run. If the run were 160 feet, the drop would hit 3.26%, requiring an upsizing to 2 AWG copper to maintain code recommendations.
What Changes the Wire Size Answer?
The baseline answer changes immediately if you alter the installation environment. Never treat aluminum and copper interchangeably; aluminum has higher resistance and requires larger gauges (e.g., 1 AWG aluminum for 100A). Furthermore, bundling wires generates excess heat.
| Condition Change | Impact on 100A Subpanel Sizing | Required Action |
|---|---|---|
| Switching to Aluminum (XHHW-2) | 1 AWG Aluminum is rated 100A at 75°C | Use 1 AWG Al; ensure lugs are rated CO/ALR or use Noalox antioxidant. |
| 4 to 6 Current-Carrying Conductors in Conduit | NEC 310.15(C)(1) requires 80% derating | Upsize to 1 AWG Copper (130A × 0.8 = 104A). |
| Ambient Temp 110°F (43°C) in Attic | THHN 90°C column derating applies before termination limits | Apply 0.82 correction factor; 2 AWG copper recommended. |
| Run exceeds 140 feet at full 100A load | Voltage drop exceeds 3% (7.2V) | Upsize to 2 AWG or 1 AWG copper to mitigate drop. |
When to Call an Engineer or the AHJ
While the NEC provides the baseline math, local Authority Having Jurisdiction (AHJ) inspectors and professional engineers must confirm your design under specific conditions. You must pull a permit and consult an engineer if:
- Continuous Loads Dominate: If your subpanel primarily feeds continuous loads (defined by NEC Article 100 as loads expected to run for 3 hours or more, like commercial lighting or EV chargers), the OCPD must be rated at 125% of the continuous load. A 100A continuous load requires a 125A breaker and 1 AWG copper wire.
- Selective Coordination is Required: In critical facilities (hospitals, data centers), an engineer must perform a selective coordination study to ensure the subpanel breaker trips before the main service breaker. Standard residential breakers do not guarantee this overlap.
- High Available Fault Current: If your utility transformer supplies massive fault current (e.g., >22,000 Amps), standard 10kAIC residential breakers may violently fail. An engineer must calculate the fault current to specify 22kAIC or 42kAIC breakers.
Frequently Asked Questions
Does a main breaker in sub panel need to match the main panel?
No. The main breaker in a subpanel acts as a local disconnect switch and protects the specific branch circuits fed by that subpanel. It is entirely normal to have a 400A main service panel feeding a subpanel with a 100A main breaker. The only hard rule is that the subpanel breaker cannot exceed the ampacity of the feeder wire connecting the two panels, nor can the sum of all subpanel breakers exceed the capacity of the main service panel without a proper NEC Article 220 load calculation.
Can I use a 100A main breaker in sub panel with 100A feeder wire?
Yes, provided the wire is genuinely rated for 100A under your specific installation conditions. As established, 3 AWG copper THHN in the 75°C column is rated exactly 100A. However, if that same 3 AWG wire is routed through a hot attic (ambient temperature above 30°C) or bundled with other circuits, its derated ampacity drops below 100A. In those derated scenarios, you must either upsize the wire to 2 AWG or downsize the breaker to 90A to remain code-compliant.
Why install a main breaker in sub panel if the NEC doesn't require it?
The NEC does not strictly require a main breaker in a subpanel if the subpanel is in the same building as the main service disconnect. However, installing one is highly recommended for three reasons: it provides a local emergency shutoff (crucial for detached garages or workshops), it allows you to de-energize the entire subpanel for maintenance without walking back to the main house panel, and it provides an extra layer of overcurrent protection if the feeder wire is ever downgraded or damaged in the future.
What happens if my main breaker in sub panel trips before the main panel breaker?
This is called selective coordination, and in a standard residential setup, it is largely left to chance. Breakers operate on thermal-magnetic curves. If a massive short circuit occurs in the subpanel, both the 100A subpanel breaker and the 200A main panel breaker experience the exact same instantaneous magnetic surge. Often, the larger main breaker might trip first due to slight manufacturing variances in the trip coils. If this happens, you simply reset the main panel breaker and the subpanel breaker. If you require guaranteed downstream tripping, you must purchase specialized, time-delay breakers engineered for coordination, which are rarely used in residential construction.
