The 10/2 Subpanel Reality Check: 120V vs. 120/240V
The most common trap when searching for how to wire a sub panel with 10/2 wire is misunderstanding what 10/2 actually provides. Standard 10/2 NM-B (Romex) contains three physical wires: one black (hot), one white (neutral), and one bare copper (equipment ground).
However, if your project is strictly a 120V-only subpanel—such as a small backyard shed powering only LED lights and a single 120V receptacle circuit—10/2 NM-B is perfectly acceptable. In this configuration, the black wire feeds the single hot bus, the white wire feeds the isolated neutral bus, and the bare wire feeds the ground bus. You are limited to 120V loads, but you gain the full 30A capacity of the 10 AWG copper.
Ampacity, Voltage Drop, and Sizing Data
Before pulling any wire, we must establish the baseline assumptions for our sizing calculations. Wire ampacity is not a single fixed number; it changes based on insulation type, termination ratings, and installation method.
- Conductor Material: Copper (Aluminum requires different sizing, addressed below).
- Temperature Column: 60°C for NM-B (per NEC 334.80, even though the internal THHN insulation is rated 90°C, the ampacity is strictly capped at the 60°C column). 75°C for THHN in conduit, limited by standard breaker termination ratings.
- Ambient Temperature: 30°C (86°F). Higher temperatures require derating.
- Installation Method: Single cable in free air or standard conduit fill (no more than 3 current-carrying conductors).
Below is the spec-sheet data comparing 10 AWG configurations for subpanel feeds. According to the Cerrowire ampacity charts and NEC Table 310.16, 10 AWG copper at 60°C is rated for exactly 30 amps.
| Parameter | 10/2 NM-B (120V Only) | 10/3 NM-B (120/240V) | 4x #10 THHN (Conduit) |
|---|---|---|---|
| Max Breaker Size | 30A (Single-Pole) | 30A (Double-Pole) | 30A (Double-Pole) |
| Max Continuous Load (80%) | 24A | 24A per leg | 24A per leg |
| Max Power Capacity | 2,880W (at 120V) | 5,760W (at 240V) | 5,760W (at 240V) |
| Ampacity Column Used | 60°C (NEC 334.80) | 60°C (NEC 334.80) | 75°C (Terminations) |
| Insulation Rating | 90°C (THHN inside) | 90°C (THHN inside) | 90°C (THHN/THWN-2) |
Why 10 AWG and Not One Size Smaller?
You might wonder why you cannot just use 12/2 NM-B on a 20A breaker. NEC 240.4(D) strictly caps 12 AWG copper overcurrent protection at 20A. Applying the 80% continuous load rule, a 12 AWG circuit can only safely handle 16A continuously (1,920W). If your shed requires a 1,500W space heater (12.5A) and a 1,200W miter saw (10A) simultaneously, a 12 AWG wire will overheat and the 20A breaker will trip. 10 AWG gives you 24A continuous headroom (2,880W), preventing nuisance trips and voltage sag during motor startups.
Voltage Drop Check at 50 Feet
Wire size isn't just about ampacity; it's also about voltage drop. The NEC recommends a maximum 3% voltage drop on branch feeders. Let's calculate the drop for a 10 AWG copper 120V circuit carrying a full 30A load over a 50-foot one-way distance, using the standard Southwire voltage drop formula (VD = 2 x K x I x D / CM):
- K (Copper resistance) = 12.9 ohms
- I (Current) = 30A
- D (Distance) = 50 ft
- CM (Circular mils for 10 AWG) = 10,380
VD = (2 x 12.9 x 30 x 50) / 10,380 = 3.72 Volts.
3.72V / 120V = 3.1% drop.
This is right on the edge of the 3% recommendation. If your shed is 80 feet away, the drop exceeds 5%, and you must upsize to 8 AWG wire to maintain proper voltage for sensitive electronics and motors.
Step-by-Step: Wiring a 120V-Only Subpanel With 10/2
If you have confirmed your load is strictly 120V and your distance is under 60 feet, follow this procedure to wire the subpanel safely.
- Prepare the Main Panel: Install a 30A single-pole breaker in an available slot. Strip the 10/2 NM-B jacket back to where it enters the panel clamp, leaving about 1/4 inch of jacket inside the clamp for grip. Strip 3/4 inch of insulation from the black and white conductors.
- Land the Main Panel Connections: Connect the black wire to the 30A breaker terminal. Torque the screw to the manufacturer's specification (typically 20 in-lbs for Square D Homeline/QO 10 AWG terminals). Land the white wire on the main neutral bar and the bare wire on the main ground bar. Note: In the main panel, neutral and ground are bonded together.
- Route and Secure the Cable: Run the 10/2 to the subpanel, securing it with NM-B staples within 8 inches of the panel and every 4.5 feet thereafter. Keep the cable at least 1.25 inches from the face of framing studs to prevent drywall screw punctures.
- Isolate the Subpanel Buses: This is the most critical step. In a subpanel, the neutral and ground must never touch. Remove the green main bonding screw or bonding strap from the subpanel's neutral bar. Verify the neutral bar is floating (isolated) from the metal panel enclosure, while the ground bar remains bonded to the enclosure.
- Terminate at the Subpanel: Land the black wire on the subpanel's hot bus bar (or a single-pole main breaker if the subpanel has one). Land the white wire on the isolated neutral bar. Land the bare wire on the ground bar. Torque all terminal screws to 20 in-lbs.
- Verify and Test: Double-check that no bare ground wires are touching the neutral bar. Turn on the 30A breaker at the main panel. Use a multimeter at the subpanel to verify you have 120V between the hot bus and neutral bus, and 0V between neutral and ground.
What Changes the Answer: Derating, Aluminum, and AHJ Rules
The 30A limit for 10/2 NM-B is not absolute. Several environmental and material factors force you to change your wire size or installation method.
Bundling and Conduit Fill Derating
If you run multiple cables through the same bored holes in framing or pull multiple THHN wires into a single conduit, the wires heat each other up. Per NEC 310.15(C)(1), if you have 4 to 6 current-carrying conductors in a single raceway, you must derate the ampacity to 80%. For 10 AWG THHN (rated 40A at 90°C for derating purposes), 80% yields 32A, which still safely clears the 30A breaker. However, if you bundle 7 to 9 conductors, the derating drops to 70% (28A), forcing you to upsize to 8 AWG wire to maintain a 30A capacity.
The Aluminum Variable
Never treat aluminum and copper interchangeably. Aluminum expands and contracts more than copper under thermal load, which historically caused loose connections and fires. If you are using aluminum wire (such as SER cable for longer outdoor runs), 10 AWG aluminum is not rated for 30A. You must upsize to at least 8 AWG aluminum to achieve a 30A rating, and you must use an anti-oxidant compound (like Noalox) on the terminations and torque them to the specific aluminum specifications listed on the breaker lug.
When an Engineer or AHJ Must Confirm
While standard residential sheds and detached garages fall under standard NEC grounding and bonding rules, you must pull a permit and have your local Authority Having Jurisdiction (AHJ) inspect the work in the following scenarios:
- Runs exceeding 100 feet: Voltage drop becomes severe, and the AHJ may require an engineered voltage drop calculation or an upsized feeder.
- Extreme ambient temperatures: If the 10/2 NM-B is routed through an attic space in a region like Phoenix, AZ, where attic temperatures exceed 110°F (43°C), NEC Table 310.15(B)(1) requires a temperature correction factor that drastically reduces the wire's ampacity.
- Agricultural or commercial outbuildings: Buildings housing livestock or commercial equipment fall under NEC Article 547 or Article 225, which have strict equipotential bonding and GFCI requirements that override standard residential subpanel rules.
By understanding the strict 120V limitation of 10/2 wire, respecting the 60°C ampacity column for NM-B, and isolating your subpanel buses, you can safely power a small detached structure without risking overheated conductors or code violations.
