If you are researching how to hook up a breaker box for a garage, workshop, or addition, you are likely installing a subpanel fed from your home's main service panel. The direct answer for a standard 100A subpanel is that it requires a 4-wire feeder (two hot legs, one neutral, one equipment ground), strict isolation between the neutral and ground buses, and landing the hot conductors on the main lugs while the feeder breaker in the main panel provides the overcurrent protection.
Decoding the Breaker Box Diagram: Symbols and Terminal Map
Before pulling wire, you must understand the single-line diagram typically found on the panel schedule or manufacturer datasheet. In standard electrical schematics, a breaker is represented by a rectangle with a diagonal line and a toggle switch symbol. The hot bus bars are thick solid vertical lines branching off the main lugs. The neutral bus is often a dashed line or explicitly labeled 'N', while the ground bus uses the universal earth symbol (a circle with three descending horizontal lines).
Here is the exact terminal mapping for a standard 24-space 100A subpanel (like a Square D Homeline or Eaton BR):
| Terminal / Bus Name | Physical Location | Function & Connection Rule |
|---|---|---|
| Main Lugs (Phase A & B) | Top center, directly below the main disconnect (if present) | Accepts the two hot feeder wires. Torque to manufacturer spec (typically 250 in-lbs for 3 AWG). |
| Isolated Neutral Bar | Left or right side, mounted on plastic insulators | Accepts the feeder neutral and all branch neutrals. Must NOT be bonded to the enclosure in a subpanel. |
| Ground Bar (Equipment Ground) | Left or right side, bolted directly to the steel enclosure | Accepts the feeder ground and all branch grounds. Bonds the enclosure to earth. |
| Branch Breaker Terminals | Staggered down the center hot bus bars | Accepts branch circuit hot wires. Plug-on or bolt-in depending on panel brand. |
Node-by-Node Trace: Source to Load Path
To wire this safely, trace the current path from the utility source to your final load. We will trace the hot, neutral, and ground paths separately, as their routing rules differ drastically.
The Hot Path (Polarity and Phasing)
- Source: Utility transformer feeds your main panel's service entrance.
- Feeder Breaker: A 2-pole 100A breaker in the main panel connects to Phase A and Phase B bus bars.
- Feeder Cable: Two hot conductors (Black and Red) carry 120V each, 180 degrees out of phase, yielding 240V across them.
- Subpanel Main Lugs: Black lands on Lug A, Red lands on Lug B.
- Subpanel Bus Bars: Voltage distributes down the alternating center stabs.
- Load: Branch breakers clip onto the stabs. A single-pole breaker draws 120V from one phase; a double-pole draws 240V across both.
The Neutral Path (Current Return)
The neutral carries the unbalanced return current. It traces from the main panel's neutral bar, through the white feeder wire, to the subpanel's isolated neutral bar. From there, individual white branch wires carry return current back to the subpanel neutral bar. Crucial rule: The subpanel neutral bar must have its green bonding screw or strap removed. If neutral and ground are bonded at the subpanel, return current will flow on the ground wire, creating a shock hazard.
The Ground Path (Fault Clearing)
The Equipment Grounding Conductor (EGC) carries zero current under normal operation. It traces from the main panel's ground bar, through the bare/green feeder wire, to the subpanel's ground bar. Because the ground bar is bolted directly to the steel enclosure, it bonds all metal parts. If a hot wire touches the metal box, the EGC provides a low-impedance path back to the source, tripping the breaker instantly.
Decision Tree: Sizing Your Feeder and Breakers
Wire sizing is dictated by NEC Article 310 and the 75°C column of the ampacity tables, as most modern panel terminals are rated for 75°C. Use this decision path to select your exact materials.
| Condition / Scenario | Action Required | Resulting Material Pick |
|---|---|---|
| Run distance is under 50 feet | Size for 100A ampacity only. | 3 AWG Copper or 1 AWG Aluminum |
| Run distance is 50 to 120 feet | Upsize one step to limit voltage drop to <3%. | 2 AWG Copper or 1/0 AWG Aluminum |
| Budget is tight, but conduit space allows | Use Aluminum (XHHW-2) to save ~40% on wire cost. | 1 AWG Aluminum (under 50ft) |
| Terminating on small lugs or DIYer prefers ease | Use Copper (THHN) for smaller bend radius and easier termination. | 3 AWG Copper (under 50ft) |
Step-by-Step Physical Hookup and Torque Specs
With your materials selected and the main feeder breaker turned OFF and locked out, follow this sequence to land the wires.
- Verify Dead: Use a non-contact voltage tester and a multimeter to confirm the main panel's bus bars are de-energized where you will be working.
- Strip the Feeder Wires: Strip exactly 3/4 inch of insulation from the 3 AWG hot and neutral wires. Do not nick the copper strands; a nicked strand creates a hot spot under load.
- Land the Ground First: Route the 8 AWG green ground wire to the subpanel's ground bar. Tighten the set screw. This ensures the enclosure is bonded before you introduce any voltage.
- Land the Neutral: Route the 3 AWG white neutral to the subpanel's isolated neutral bar. Ensure the green bonding screw on this bar is removed or retracted.
- Land the Hots: Insert the Black and Red 3 AWG wires into the subpanel's main lugs. Ensure no insulation is caught under the lug plate, and no bare copper is exposed outside the lug.
- Torque to Spec: Use an inch-pound torque screwdriver. Check the label inside the Schneider Electric / Square D panel cover. For 3 AWG copper, the spec is typically 250 in-lbs. Do not guess; under-torqued lugs arc and melt, while over-torqued lugs snap the screw or deform the wire.
- Route and Land Branch Circuits: Install your branch breakers, land the branch hots, neutrals (on the isolated bar), and grounds (on the bonded bar). Keep neutrals and grounds paired by circuit number for easy troubleshooting.
Meter Verification: Proving the Connections
Before energizing any branch loads, you must verify the electrical characteristics of the subpanel with a digital multimeter (DMM) set to AC Volts (200V or 600V range).
- L1 to Neutral (Black to White): Read should be 120V (acceptable range 114V - 126V). This proves Phase A and the neutral return are intact.
- L2 to Neutral (Red to White): Read should be 120V. This proves Phase B and the neutral return are intact.
- L1 to L2 (Black to Red): Read should be 240V. This proves you have both legs of the split-phase supply and they are correctly out of phase.
- Ground to Neutral (Green to White): Read should be 0.0V to 0.5V. If you read 120V here, your subpanel neutral is floating (disconnected at the main) or you have a severe fault. If you read high voltage under load, your neutral and ground are illegally bonded at the subpanel, causing neutral current to backfeed onto the ground wire.
- Ground to Enclosure: Read should be 0.0V. This proves the ground bar is physically bonded to the steel box.
Final Safety and Code Caveats
Hooking up a breaker box is highly methodical, but local codes always supersede general guides. Under NEC 250.32, a separate equipment grounding conductor is mandatory for subpanels; you cannot rely on a metallic conduit alone as the sole ground path for a 100A feeder in most residential jurisdictions. Furthermore, if your subpanel is in a detached structure, you will also need to drive two 8-foot copper ground rods and bond them to the subpanel's ground bar, while still keeping the neutral isolated. Always pull a permit and have your local Authority Having Jurisdiction (AHJ) inspect the rough-in and final connections before closing the panel deadfront.
