To wire a standard 200-amp main breaker panel box, you will land two 2/0 AWG aluminum (or 2 AWG copper) hot conductors on the main breaker lugs, the 2/0 AWG neutral on the neutral bus bar, and the #4 AWG copper grounding electrode conductor on the ground bus bar. In a main service panel, the neutral and ground bus bars must be bonded together. This guide assumes a standard 120/240V single-phase, 3-wire residential service using the 75°C ampacity column per NEC Article 310.12.
The Decision Path: Sizing Your Panel Box and Feeders
Before stripping wire, you must match the panel rating to the utility transformer and your calculated load. Use the decision table below to select your service entrance conductors and panel rating based on your NEC Article 220 load calculation.
| Calculated Load (Amps) | Panel Rating | Hot/Neutral Conductor Size (Aluminum) | Hot/Neutral Conductor Size (Copper) | Grounding Electrode Conductor (GEC) |
|---|---|---|---|---|
| Up to 100A | 100-Amp | #2 AWG XHHW-2 | #4 AWG THHN | #8 AWG Copper |
| 101A to 150A | 150-Amp | 1/0 AWG XHHW-2 | #1 AWG THHN | #6 AWG Copper |
| 151A to 200A | 200-Amp | 2/0 AWG XHHW-2 | 2 AWG THHN | #4 AWG Copper |
| 201A to 320A | 400-Amp (Class 320) | 400 kcmil XHHW-2 | 350 kcmil THHN | #3/0 AWG Copper |
Tools, Materials, and Safety Protocols
Required Tools and Materials
- Panel: Square D HOM200M200PC (200A, 40-space, main breaker)
- Feeders: 2/0 AWG Aluminum XHHW-2 (Black, Red, White/Gray) — rated 75°C
- GEC: #4 AWG Copper THHN (Green) for Grounding Electrode Conductor
- Grounding Rods: Two 5/8-inch x 8-foot copper-bonded ground rods with acorn clamps
- Wire Prep: Klein Tools 11057 wire strippers, utility knife, wire brush
- Anti-Oxidant: Ideal Noalox or equivalent aluminum-to-copper anti-oxidant paste
- Torque Tool: Calibrated inch-pound torque wrench or digital torque screwdriver (e.g., Klein 60718)
- Testing: Fluke 117 Digital Multimeter, Fluke 2AC Non-Contact Voltage Tester
Step-by-Step: How to Wire a Panel Box
Follow these numbered steps to terminate the service entrance conductors. Note that wire colors are explicitly called out for every termination to prevent cross-wiring.
- Mount and Secure the Enclosure: Mount the panel box plumb and level to the studs or masonry using 1/4-inch lag screws or concrete anchors. Ensure the center of the panel is roughly 60 inches from the finished floor (NEC 240.24 maximum reach requirement).
- Route and Strip the Service Entrance Conductors: Pull the Black, Red, and White 2/0 AWG aluminum feeders through the top hub or knockout. Strip exactly 1 inch of insulation from the ends of all three 2/0 AWG wires. Do not nick the aluminum strands.
- Apply Anti-Oxidant Paste: Coat the stripped ends of the Black, Red, and White 2/0 AWG aluminum wires with a thin layer of Noalox anti-oxidant paste. This prevents galvanic corrosion and high-resistance heating at the lug connections.
- Land the Grounding Electrode Conductor (GEC): Take the Green #4 AWG copper wire. Route it from the panel's ground bus bar out to the exterior grounding rods. Terminate the interior end into an available hole on the ground bus bar and torque to the manufacturer's specification (typically 35 in-lbs for #4 AWG).
- Install the Main Bonding Jumper: In a main service panel, the neutral and ground must be bonded. Install the green bonding screw or bonding strap provided by Square D, connecting the neutral bus bar directly to the panel enclosure/ground bus. Torque the bonding screw tightly (usually 50 in-lbs).
- Terminate the Neutral Conductor: Take the White 2/0 AWG aluminum wire. Land it in an available, appropriately sized lug on the neutral bus bar. Do not place it on the ground bar. Torque to 250 in-lbs (verify the label inside the panel door for exact Square D specs).
- Terminate the Hot Conductors: Take the Black 2/0 AWG aluminum wire and land it on the left lug of the 200-amp main breaker. Take the Red 2/0 AWG aluminum wire and land it on the right lug of the main breaker. Ensure no bare wire is exposed outside the lug barrel and no insulation is pinned inside the lug. Torque both main breaker lugs to 250 in-lbs.
- Dress the Wires: Route the Black and Red wires neatly along the side gutters, keeping them clear of the breaker stab openings. Use nylon cable ties to bundle the feeders to the panel's internal wire anchors.
Verification and Testing: Expected Meter Readings
Never assume a panel is wired correctly just because the breakers snap into place. You must verify the electrical characteristics before applying branch circuit loads.
- Visual & Mechanical Check: Tug gently on every landed wire to ensure it is seated. Verify the bonding screw is installed. Ensure no stray wire strands are touching the enclosure.
- Dead Continuity Test: With power OFF, set your multimeter to continuity/ohms. Measure between the Neutral bus and the Ground bus. You should read less than 1 ohm (close to 0.0Ω). If it reads open (OL), your bonding jumper is missing or loose.
- Energize and Measure Voltage: Have the utility energize the meter. Turn ON the 200A main breaker. Set your Fluke 117 to AC Voltage.
- Black (L1) to Red (L2): Expect 240V (acceptable range: 228V - 252V).
- Black (L1) to White (Neutral): Expect 120V (acceptable range: 114V - 126V).
- Red (L2) to White (Neutral): Expect 120V (acceptable range: 114V - 126V).
- White (Neutral) to Green/Ground: Expect less than 2V (ideally 0.0V to 0.5V). If you read higher than 2V, you have a high-resistance neutral connection at the utility transformer or your main neutral lug is loose.
The Most Common Botch: Neutral and Ground Bonding Errors
When learning how to wire a panel box, the most frequent and dangerous mistake involves the neutral-to-ground bond. The NEC (NFPA 70, Article 250.24) strictly dictates where and how this bond occurs.
The Botch: Missing Bond in Main, or Extra Bond in Subpanel
In a main service panel, the neutral bus and ground bus MUST be bonded together. If you forget the bonding screw, a ground fault will not trip the main breaker, leaving the panel enclosure energized at 120V during a fault—a lethal shock hazard.
Conversely, if you are wiring a subpanel (fed from this main panel), you MUST remove the bonding screw or strap. If you leave the bond in a subpanel, normal neutral return current will split and travel back to the main panel on both the neutral wire AND the bare ground wire. This creates 'objectionable neutral current' on the grounding system, which can cause random tripping of AFCI/GFCI breakers, electromagnetic interference on AV equipment, and shock hazards on grounded appliances.
The Secondary Botch: Double-Tapping Neutrals
Another frequent failure is landing two neutral wires under a single screw on the neutral bus bar. According to NEC 408.41, unless the bus bar lug is specifically listed and identified for two conductors (which some Square D ground bars are, but almost no neutral bars are), you must use one hole per wire.
Symptom of the Botch: Because aluminum and copper expand at different rates, and because two round wires cannot sit perfectly flat under a flat screw head, one wire will inevitably loosen over time. This causes arcing, high heat, melted wire insulation, and flickering lights on that circuit. The Fix: Buy a box of 12 AWG or 10 AWG pigtail wires and wire nuts to splice two circuits to a single pigtail, or simply upgrade to a panel with more neutral bus holes (like the Square D HOM3040M200PC which features 40 spaces and 80 circuits with ample neutral terminations).
For further reading on torque specifications and preventing loose connections, consult the Schneider Electric / Square D technical FAQs to ensure your specific panel model's lugs are tightened to the exact inch-pound rating printed on the device label.
