How to Wire a Subpanel: NEC Compliance Guide

Understanding the Role of a Subpanel in an Electrical System

A subpanel, also commonly referred to as a panel sub, sub panel, or an electrical sub panel, is a smaller electrical panel that serves as a distribution point for a specific area or a set of high-demand loads. It receives power from the main service panel—often a 200 amp panel or a 200 amp main breaker panel—via a feeder circuit. The primary purpose of these subpanels is to add circuits when the main breaker panel 200 amp is full or to provide a convenient power distribution and disconnection point for a separate area, such as a detached garage, workshop, or a new addition. Installing an outdoor sub panel is also a common solution for supplying power to outdoor equipment like pool pumps or HVAC units.

Pre-Installation: Planning for NEC Compliance

Before a single wire is pulled, thorough planning is essential to ensure the installation complies with the National Electrical Code (NEC). This phase prevents costly mistakes and ensures the system’s safety and functionality.

Panelboard Load Calculation and Sizing

The first step is a detailed panelboard load calculation to determine the required capacity. You must calculate the anticipated load for the new circuits to correctly size the subpanel itself and the feeder. Common installations include a 50 amp sub panel for a range or welder, a 60 amp subpanel for a small workshop, or a 100 amp sub panel for a larger outbuilding or addition. The panel must have adequate physical space for all required breakers, including any future needs. While you might be working off a 200 amp electrical panel, the subpanel’s capacity is determined by its specific loads, not the size of the main service. A 100 amp breaker panel is a very common choice for residential additions. The panel itself will either be a main lug only panel or may come with a main breaker installed.

Feeder Conductor Sizing and Selection

Once the load is known, you must perform an ampacity calculation for feeders. This determines the correct gauge for the electrical feeder conductors. The calculation must account for conductor material (copper or aluminum), insulation type, ambient temperature, and the number of current-carrying conductors in the raceway. It’s also critical to calculate for voltage drop on feeder, ensuring it doesn’t exceed NEC recommendations (typically 3% for the feeder) to ensure equipment operates correctly. For nearly all modern installations, a 4-wire feeder installation (two hots, one neutral, one equipment ground) is required.

Step-by-Step Sub Panel Wiring Guide

With planning complete, you can proceed with the physical installation. Adhering to these steps ensures a compliant and safe setup. For worker safety, understanding new requirements is key, especially how 2023 NEC panelboard barriers improve worker safety when accessing energized parts.

1. Safety First: De-energize the Main Panel. Turn off the main breaker in the 200 amp breaker box (or other main panel). Use a voltmeter and a reliable circuit tracer or breaker finder to verify that power is off at the location where you will be installing the feeder breaker.
2. Mount the Subpanel. Securely mount the new electrical panel box. For installations in different building envelopes, be aware of how 2023 NEC panelboard enclosure requirements are different to ensure proper environmental protection. You might be installing a standard 100 amp breaker box or a specific Square D 200 amp panel as a large subpanel.
3. Run the Feeder Cable. Route the 4-wire feeder cable from the main panel to the subpanel location. Ensure it is properly supported and protected from physical damage per NEC Chapter 3.
4. Connect the Feeder at the Main Panel. In the main panel, connect the two ungrounded (hot) conductors to a new 240v breaker, such as a 60 amp breaker or 100 amp breaker. The size must match the feeder wire ampacity, providing correct overcurrent protection requirements. Connect the neutral conductor to the neutral bus bar and the equipment grounding conductor to the equipment grounding bus bar. In a main panel, these bars are typically bonded.
5. Connect the Feeder at the Subpanel. At the subpanel, connect the two hot conductors to the main lugs. This is where the term main lug only panel originates. Connect the neutral feeder conductor to the neutral bus bar.
6. CRITICAL: Isolate Neutral and Ground. This is the most crucial step in sub panel wiring. The neutral bus bar must be isolated from the panel enclosure. This is often called a floating neutral. Remove the factory-installed bonding jumper (usually a green screw or copper strap) that connects the neutral bar to the panel can. Install a separate equipment grounding bar, bonded directly to the metal enclosure, and connect the feeder’s equipment grounding conductor to this bar. This practice of separating neutral and ground is non-negotiable.
7. Install Branch Circuit Breakers. Install the new breakers for the branch circuits. This could be a 15 amp circuit breaker for lighting, a 20 amp breaker for outlets, or a 30 amp breaker for a dryer. You might install a 30 amp double pole breaker for a 240V load or a specific 40 amp breaker for an appliance. Even a 70 amp breaker might be used for a large load. Ensure you are using the correct type, whether a standard breaker, a dual function breaker, or special-purpose breakers like a shunt trip breaker.
8. Torque, Label, and Cover. Use a torque wrench to tighten all terminal lugs to the manufacturer’s specifications. This is one of the most overlooked but critical steps. Create an accurate panel schedule, clearly identifying each circuit. Knowing how to properly label switchgear, switchboards, and panelboards per NEC 2023 is a professional standard. Finally, install the electrical panel cover or electrical panel box covers.

The Critical Importance of Grounding and Bonding

The rules for grounding and bonding are what separate a safe subpanel installation from a hazardous one. The goal is to create a single point of bonding between the neutral and ground for the entire electrical system, which occurs only at the main service disconnect.

Subpanels in Detached Buildings: NEC 250.32 Requirements

When installing a subpanel in a detached building, NEC 250.32 requirements must be followed. This typically requires the installation of a local grounding electrode system (e.g., ground rods) at the detached building. The equipment grounding conductor in the feeder cable is still required and is used to bond the two buildings’ grounding systems together. A local disconnect box, such as a 200 amp disconnect or 100 amp disconnect, is often required at the outbuilding, which may be a simple ac disconnect box or a more robust fused disconnect.

Common Breaker and Panel Considerations

Professionals must be familiar with a wide array of products and troubleshooting scenarios. Navigating breaker types, brands, and legacy systems is part of the job.

• Breaker Compatibility: Always use circuit breakers specifically listed for the panel. You cannot install Siemens breakers in a Square D panel. Common brands include Square D breakers (with popular lines like Square D QO breakers), Eaton breakers (sometimes labeled ETN breaker), GE 20 amp breaker, and Bryant circuit breakers. Using the wrong breaker, like a Challenger type c120 2 pole in a non-listed panel, is a code violation. Products like the Eaton Corporation BD1520 duplex 15/20A circuit breaker or the Square D Homeline 30 amps surge 2-pole circuit breaker offer specific solutions but must be compatible. A lockable circuit breaker might be needed for safety disconnects.
• Legacy Panels: When working in older homes, you may encounter obsolete or dangerous panels. A Federal Pacific electrical panel with Federal Pacific breakers, or a Zinsco panel with Zinsco breakers, often requires a complete electrical panel replacement. The same can be true for older Westinghouse breakers, Crouse Hinds breaker models, ITE breakers (or ITE circuit breakers), Murray breakers, and some Bryant breakers. A challenger breaker replacement may be possible, but often a full panel swap-out by a qualified electrician for electrical panel replacement is the safest bet. The cost to change electrical panel is an investment in safety, especially when dealing with panels from Bryant Electric or Federal Pacific Panel.
• Breaker Types & Sizes: Beyond amperage, know your types. A hacr type breaker (or hacr breaker) is rated for air conditioning loads. A multi wire branch circuit requires a 2-pole common-trip breaker or two single-pole breakers connected with a listed handle tie to ensure simultaneous disconnection. Tandem breakers save space but are not allowed in all panels. You’ll encounter common breaker sizes and standard breaker sizes like a 15 amp breaker, 20amp breaker, square d 20 amp breaker, and up to a 200 amperes breaker or 200 amp circuit breaker.
• Troubleshooting: A common complaint is “breaker keeps tripping” or “why does my circuit breaker keep tripping?” This can be due to an overload, short circuit, or ground fault. Sometimes it’s a specific appliance, like a report of a “hisense 8000 tripping breaker,” or an “ac circuit breaker keeps tripping.” The first diagnostic step after a trip is knowing how to reset breaker. If it persists, use an electrical circuit tracer to identify the fault. The solution could be as simple as redistributing loads or as complex as finding and repairing a fault in the branch circuit wiring. Learning how to test a circuit breaker or how to test a breaker with a multimeter can confirm if the breaker itself is faulty, requiring a circuit breaker replacement or learning how to change a breaker.

A properly installed subpanel provides tremendous flexibility and is a hallmark of professional electrical work. By adhering strictly to NEC guidelines, especially regarding feeder sizing, overcurrent protection, and the critical separation of neutral and ground, you ensure a safe and durable expansion of any electrical system. When you need to fix electrical panel issues or perform an upgrade, these principles are your foundation. Whether you are dealing with a 400 amp meter base, a 200 amp meter base with disconnect, or a simple electrical box 200 amp setup, the rules for subpanels remain consistent.

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Primary Sources

• NFPA 70, National Electrical Code (NEC), 2023 Edition

Frequently Asked Questions (FAQ)

What is the most important rule for sub panel wiring regarding grounding?

The single most important rule for sub panel wiring is separating neutral and ground. The neutral bus bar must be isolated (floating) from the subpanel’s metal enclosure, and the green bonding screw or bonding jumper must be removed. A separate ground bar must be installed and bonded to the enclosure for all equipment grounding conductors.

My main panel is a 200 amp breaker panel; do I need a 200 amp subpanel?

Not necessarily. The size of the subpanel (e.g., 50 amp sub panel, 100 amp sub panel) is determined by the panelboard load calculation for the specific circuits it will serve, not the size of the main 200 amp main breaker panel. The feeder breaker in the main panel, which protects the subpanel and its feeder wires, will be sized according to this load, for example, a 100 amp breaker for a 100A subpanel.

Why does my circuit breaker keep tripping in my new subpanel?

A breaker keeps tripping for three main reasons: overload (too much current draw), a short circuit (hot wire touches neutral or ground), or a ground fault. First, investigate the loads on that specific branch circuit. If the problem persists, you may need a qualified electrician to diagnose the issue. Learning how to test a breaker can determine if the device itself is faulty, but persistent tripping usually indicates a wiring problem.

Can I use any brand of breaker, like Eaton circuit breakers, in my Square D electrical panel?

No. You must only use circuit breakers that are listed and classified for use in your specific electrical panel board. Using unapproved breakers, such as putting Crouse Hinds breakers or Eaton 20 amp breaker models in a panel not designed for them, is a serious code violation and fire hazard. It can also void the panel’s UL listing.