To wire up a standard 100-amp subpanel breaker box, you need a 100-amp double-pole feeder breaker at the main panel, 3 AWG copper conductors for the hots and neutral, an 8 AWG copper equipment grounding conductor, and strict adherence to neutral-to-ground isolation at the subpanel. This guide walks you through the exact decision matrix, termination points, and meter readings required for a safe, code-compliant installation.

The Decision Path: Sizing Your Feeder and Breaker

Before pulling any wire, you must match your calculated load to the correct breaker and conductor size. Undersizing causes nuisance tripping and voltage drop; oversizing wastes money and can violate terminal temperature ratings. Use the decision table below to find your exact configuration based on your continuous and non-continuous load calculations.

Calculated Load Feeder Breaker Copper Wire Gauge (75°C Column) Equipment Ground
Under 60 Amps 60A Double-Pole 6 AWG 10 AWG
61 to 100 Amps 100A Double-Pole 3 AWG 8 AWG
101 to 125 Amps 125A Double-Pole 1 AWG 6 AWG
The Concrete Pick: For a standard detached garage workshop running a 240V welder, EV charger, and general lighting, the default recommendation is a Siemens Q2100 100-Amp double-pole breaker at the main panel, feeding a Siemens P100L3CU 100-Amp subpanel. Run three individual 3 AWG THHN/THWN-2 copper wires (Black, Red, White) and one 8 AWG bare copper ground through 1.25-inch PVC conduit.

Tools, Materials, and Safety Callouts

⚠️ CRITICAL MAINS SAFETY WARNING: Working inside a breaker box involves lethal mains voltage. You must de-energize the main panel by shutting off the utility main breaker. Apply a lockout/tagout (LOTO) device to the main breaker handle to prevent accidental re-energization. Before touching any bus bar, verify the panel is dead using a properly functioning non-contact voltage tester and a CAT III or CAT IV digital multimeter. If you are not comfortable with this, hire a licensed electrician. Always follow OSHA Lockout/Tagout standards and local AHJ requirements.

Required Tools

  • Calibrated Torque Screwdriver: Required by NEC 110.14(D) for all breaker and lug terminations. A standard Wiha or Klein torque driver set to inch-pounds is mandatory.
  • Wire Strippers: Heavy-duty strippers capable of cleanly removing insulation from 3 AWG THHN without nicking the copper.
  • Digital Multimeter (DMM): CAT III/IV rated for verifying dead circuits and testing final voltages.
  • Insulated Hand Tools: VDE-rated screwdrivers and pliers for accidental contact protection.
  • Fish Tape & Wire Lubricant: Polywire fish tape and non-conductive pulling lube (e.g., Ideal Yellow-77) for conduit pulls.

Materials List

  • 1x Siemens Q2100 100-Amp Double-Pole Breaker (Main panel feeder)
  • 1x Siemens P100L3CU 100-Amp Main Lug Subpanel
  • 3x 3 AWG THHN/THWN-2 Copper Wire (Black, Red, White)
  • 1x 8 AWG Bare Copper Wire (Equipment Grounding Conductor)
  • 1.25-inch PVC Schedule 40 conduit and fittings (if running underground or surface-mounted)

Step-by-Step: How to Wire Up a Breaker Box

Follow these exact termination steps. Never leave a wire color unaccounted for, and never guess on torque values.

  1. Mount the Subpanel and Route Conduit: Secure the Siemens P100L3CU subpanel to the studs using the pre-punched mounting holes. Route your 1.25-inch PVC conduit from the main panel to the subpanel knockouts. Use a hub connector at the top of the subpanel for a watertight seal if the conduit enters from the outside.
  2. Pull the Feeder Conductors: Apply wire pulling lube to the 3 AWG THHN wires and the 8 AWG bare ground. Pull them through the conduit, leaving at least 6 inches of slack inside both the main panel and the subpanel. Strip exactly 3/4-inch of insulation from the ends of the Black, Red, and White wires.
  3. Terminate at the Main Panel (Source):
    • Black Wire (Hot 1): Lands on one of the brass terminal screws of the 100A Siemens Q2100 breaker.
    • Red Wire (Hot 2): Lands on the second brass terminal screw of the Q2100 breaker.
    • White Wire (Neutral): Lands on an available silver terminal screw on the main panel's neutral bus bar.
    • Bare Wire (Ground): Lands on an available terminal screw on the main panel's ground bus bar (or the combined neutral/ground bar if it's a main service panel).
  4. Isolate the Subpanel Neutral Bar: This is the most critical step. Locate the green main bonding screw or bonding strap inside the Siemens subpanel. Remove it completely. In a subpanel, the neutral bar must float (be electrically isolated) from the metal enclosure and the ground bar. Save the green screw in your pocket so it doesn't fall into the panel bus.
  5. Terminate at the Subpanel (Destination):
    • Black Wire (Hot 1): Lands on the top main lug (L1) of the subpanel.
    • Red Wire (Hot 2): Lands on the bottom main lug (L2) of the subpanel.
    • White Wire (Neutral): Lands on an available silver terminal screw on the isolated neutral bus bar. Do not mix grounds and neutrals on this bar.
    • Bare Wire (Ground): Lands on an available terminal screw on the ground bus bar, which is bolted directly to the metal panel enclosure.
  6. Apply Exact Torque: Using your calibrated torque screwdriver, tighten all breaker terminals and main lugs to the manufacturer's specified value. For the Siemens Q2100 breaker and P100L3CU main lugs accepting 3 AWG wire, the typical torque requirement is 40 inch-pounds. Always verify this against the printed label on the inside of the panel door or the breaker casing, as specs can vary slightly by manufacturing year.
  7. Install Branch Breakers: Snap in your branch circuit breakers (e.g., 20A AFCI/GFCI for receptacles, 30A double-pole for the welder) and terminate your branch wiring, ensuring all ground wires go to the ground bar and neutrals go to the isolated neutral bar.

The Most Common Botch (And How to Avoid It)

The Botch: Failing to remove the green bonding screw/strap in the subpanel, thereby bonding the neutral bar to the ground bar and the panel enclosure.

The Symptom: You will create a parallel neutral path. Because the neutral carries return current, bonding it to the ground bar at the subpanel forces a portion of that 120V return current to flow back to the main panel along the bare equipment grounding conductor. This energizes the metal panel enclosure, metal conduit, and the grounding pins on your outlets. You will experience mild shocks when touching appliances, stray voltage readings on metal workbenches, and upstream AFCI/GFCI breakers will nuisance-trip constantly because they detect an imbalance between the hot and neutral currents.

The National Electrical Code (NEC) Article 250.32 strictly prohibits bonding the neutral to the ground at any building or structure other than the main service disconnect. The ground wire is strictly for fault clearing; it must never carry normal operating current.

Verify and Test: Expected Meter Readings

Do not skip the testing phase. Once all terminations are torqued and the panel covers are temporarily set aside (keep faces clear of the bus bars), remove your LOTO device and energize the main panel feeder breaker.

Set your digital multimeter to AC Voltage (V~) and take the following measurements at the subpanel:

  1. Line 1 to Line 2 (Black to Red): Place one probe on the L1 main lug and the other on the L2 main lug. Expected Reading: 240V (Acceptable range: 230V - 250V).
  2. Line 1 to Neutral (Black to White): Place one probe on L1 and the other on the isolated neutral bar. Expected Reading: 120V (Acceptable range: 114V - 126V).
  3. Line 2 to Neutral (Red to White): Place one probe on L2 and the other on the neutral bar. Expected Reading: 120V.
  4. Neutral to Ground (White to Bare/Ground Bar): Place one probe on the isolated neutral bar and the other on the ground bar. Expected Reading: Less than 2.0V (Ideally 0.0V to 0.5V). If you read 120V here, your upstream main breaker is off, or you have an open neutral. If you read 5V-10V, you have severe voltage drop or a shared neutral botch upstream.
  5. Ground to Enclosure: Place one probe on the ground bar and the other on the bare metal of the subpanel box. Expected Reading: 0.0V. This confirms the ground bar is properly bonded to the enclosure.

If all readings match the expected values, de-energize the panel, install the dead-front cover and panel door, and apply the circuit directory label. Your 100-amp subpanel is now safely wired, correctly bonded, and ready to distribute power to your branch circuits.