To install a standard 60-amp subpanel, you need a 2-pole 60A breaker at the main panel, 6 AWG copper THHN wire (or 4 AWG aluminum) in a 4-wire configuration (two hots, one neutral, one ground), and a main-lug subpanel. The black and red wires land on the brass breaker lugs, the white wire lands on the isolated silver neutral bus bar, and the bare copper wire lands on the green grounding screw bonded to the panel enclosure.
Sizing the Feeder and Subpanel Breaker
Before pulling wire, you must match the breaker size to the wire ampacity and the subpanel's main lug rating. The subpanel's main lugs are simply a disconnect and bus distributor; the overcurrent protection happens at the main panel's feeder breaker. Sizing is based on the 75°C column of NEC Table 310.16, as most modern breakers and lugs are rated for 75°C terminations.
| Feeder Breaker Size | Copper Wire (THHN/THWN-2) | Aluminum Wire (XHHW-2) | Minimum Conduit (EMT/PVC) | Common Use Case |
|---|---|---|---|---|
| 60 Amp (2-Pole) | 6 AWG | 4 AWG | 1 inch | Detached garage, small workshop, shed |
| 100 Amp (2-Pole) | 3 AWG | 1/0 AWG | 1.5 inch | Finished basement, large addition, EV charger prep |
| 125 Amp (2-Pole) | 1 AWG | 2/0 AWG | 2 inch | Heavy machinery shop, whole-house addition |
Tools, Materials, and Mains Safety Protocol
Working inside a live main panel is unforgiving. The bus bars remain energized even when the main breaker is off if the utility feed is still connected. For a DIYer, the safest approach is to have the utility pull the meter or install an upstream service disconnect. If you are proceeding with the main breaker off, strict verification is mandatory.
Required Tools & Materials:
- Breaker: 2-pole feeder breaker (e.g., Eaton BR260 or Square D HOM260) matching your panel brand.
- Wire: 4-conductor feeder (Black, Red, White, Bare) sized per the table above.
- Subpanel: Main-lug load center (e.g., Eaton BRP12L125) with an accessory ground bar kit (e.g., Eaton GBKP16).
- Tools: Calibrated torque screwdriver (NEC 110.14(D) requires calibrated torque), wire strippers, fish tape, NCVT, digital multimeter (e.g., Fluke 117), 1/2-inch or 3/4-inch drill bit for knockout punching.
Step-by-Step: How to Install a Subpanel and Route the Feeder
- Mount the Subpanel: Secure the subpanel enclosure to the studs using 2-inch lag screws. Ensure the center of the panel is roughly 48 to 52 inches from the finished floor to meet ADA and ergonomic reach requirements. Punch out the appropriate knockouts for your conduit entry.
- Run the Conduit and Pull Wire: Route your EMT or PVC conduit from the main panel to the subpanel. Pull the four THHN wires through the conduit. Leave at least 12 inches of slack inside both panel enclosures.
- Terminate at the Main Panel (Line Side):
- Black & Red (Hots): Strip 1/2 inch of insulation. Land these on the brass/gold lugs of the new 2-pole feeder breaker. Torque to the manufacturer's spec (typically 40-45 in-lbs for 6 AWG). Snap the breaker into the main panel's bus stab.
- White (Neutral): Strip 3/4 inch of insulation. Land this on an available silver screw on the main panel's neutral bus bar. Torque to spec.
- Bare (Ground): Land this on the main panel's grounding bus bar (or the combined neutral/ground bar if it's the service entrance panel). Torque to spec.
- Isolate the Subpanel Neutral: Before terminating any wires at the subpanel, locate the green bonding screw or bonding strap that connects the neutral bus bar to the panel enclosure. Remove it. In a subpanel, the neutral and ground must remain strictly separated. Install the accessory ground bar kit directly to the panel enclosure using the pre-tapped holes.
- Terminate at the Subpanel (Load Side):
- Black & Red (Hots): Land these on the two main silver lugs at the top of the subpanel. Torque to the lug manufacturer's specification (often 60 in-lbs for 6 AWG).
- White (Neutral): Land this on the isolated silver neutral bus bar. Ensure it does not touch the panel enclosure.
- Bare (Ground): Land this on the newly installed green accessory ground bar, which is physically bonded to the metal enclosure.
The Most Common Botch: Subpanel Neutral and Ground Bonding
The most frequent and dangerous mistake when learning how to install a subpanel is leaving the green bonding screw or jumper strap in place. This bonds the neutral to the ground at the subpanel, creating a parallel path for return current.
The Symptom: If you bond neutral and ground at the subpanel, normal 120V return current will split between the white neutral wire and the bare ground wire. Because the ground wire is connected to the panel enclosure, conduit, and any metal tool chassis plugged into the subpanel, the metal enclosure becomes energized with a few volts of potential. You will feel a 'tingling' shock when touching metal tools, and upstream GFCI or AFCI breakers may trip randomly due to the unbalanced current leaking onto the equipment grounding conductor. Per NEC Article 250, the neutral and ground are bonded only at the main service disconnect. Everywhere downstream, they must remain isolated.
Verify and Test: Expected Meter Readings
Before connecting any branch circuits or closing the subpanel deadfront, energize the main panel and turn on the new 2-pole feeder breaker. Set your digital multimeter to AC Voltage (V~) and take the following measurements at the subpanel's main lugs and bus bars:
- Black to Red (Hot to Hot): Expected 240V (Acceptable range: 228V - 252V)
- Black to White (Hot to Neutral): Expected 120V
- Red to White (Hot to Neutral): Expected 120V
- Black to Bare (Hot to Ground): Expected 120V
- White to Bare (Neutral to Ground): Expected < 2V. If you read 120V here, your neutral connection at the main panel is loose or missing. If you read >5V under load, your neutral wire is undersized or shared incorrectly.
Once readings are verified, turn the breaker off, install the branch circuit breakers, wire your loads, and secure the panel deadfront and cover.
Subpanel Installation FAQ
How to install a subpanel in a detached garage with a ground rod?
When running a feeder to a separate structure like a detached garage, NEC 250.32 requires a 4-wire feeder (two hots, neutral, ground) and a local grounding electrode system at the garage. You must drive two 8-foot copper-clad ground rods, spaced at least 6 feet apart, and connect them to the subpanel's ground bus bar using a continuous 6 AWG bare copper grounding electrode conductor (GEC). The neutral and ground remain isolated in the subpanel, but the ground bus is now tied to the earth via the rods to dissipate lightning and surge energy locally.
How to install a subpanel from a 200 amp main breaker?
The size of the main breaker (200A) does not dictate the subpanel feeder size; your calculated load does. You can install a 60A or 100A subpanel off a 200A main panel without issue, provided the main panel has physical space for the 2-pole breaker and the total calculated load of the house plus the new subpanel does not exceed the 200A service rating. Use the wire sizing table above based on the subpanel's specific feeder breaker, not the main breaker.
How to wire a subpanel with a 4 wire feeder if my old house only has 3 wire?
If you are retrofitting a subpanel in an existing structure that was previously fed with a 3-wire feeder (two hots and a neutral, with no separate ground), NEC 250.32 allows an exception for existing feeders installed before the 2008 code cycle. However, you must meet strict conditions: an equipment grounding conductor must not have been run previously, the neutral must be insulated, and the branch circuits must be protected by a GFCI. For any new installation in 2026, a 4-wire feeder is strictly mandatory. Do not pull a new 3-wire feeder.
