When it comes to electrical installations, the choice between a metal or plastic junction box depends on the unique requirements of your project. Plastic boxes are cost-effective and lightweight. They are widely used for standard residential and low-voltage applications. 

Metal boxes are durable, fire-resistant, and good for grounding. They are used in demanding environments and critical installations. In this article, we'll break down the characteristics, pros, and cons of each box so you can make an informed decision regarding metal vs plastic electrical boxes.

What are Electrical Junction Boxes?

An electrical box or a junction box is a box of electrical installations that houses and protects electrical connections. It  ensures electrical safety and keeps these connections safe from the environment. Electrical boxes prevent things like water and debris from causing problems.

The electrical box encloses electrical connections and components within a secure, insulated housing. This safeguards against accidental contact, prevents exposure to moisture, and mitigates fire hazards. Inside of an electrical box, wire connections are joined together, and switches, outlets, or fixtures are mounted. In this way, the electricity is distributed safely throughout a building.

Electrical boxes come in two primary materials: plastic and metal. Plastic junction boxes are lightweight, affordable, and resistant to corrosion. They are suitable for dry environments. Metal junction boxes  are more durable, they are fire resistant and are used for wet or high-temperature areas.

When To Use A Plastic Electric Box?

Let’s review situations when you should use a plastic electrical box:

• When working with non-metallic sheathed cables (NM or Romex) entering or exiting the box.
• In situations where grounding is handled separately from the box.
• For retrofit or remodel projects, particularly when attaching to drywall instead of wall studs.
• In low-stress installations where the box won't be subjected to excessive strain.
• When dealing with open wall constructions where the thickness of the wall covering is uncertain (especially useful with adjustable plastic boxes).
• When undertaking DIY electrical projects with simplified requirements.
• In applications involving non-conductive conduit systems, where the electrical box material won't impact conductivity or interfere with the system's insulation.
• For installations in areas with minimal exposure to moisture and environmental factors, where the corrosion resistance of metal boxes isn't necessary.

When To Use A Metal Electrical Box?

These are the cases when to use a metal electrical box:

• When working with metallic sheathed or BX wires.
• In cases where grounding relies on the box itself.
• For exposed locations where the box is not recessed into the wall.
• When attaching the box to wall studs instead of drywall.
• In gangable installations requiring box connectivity.
• When installations apply significant strain on the box.
• For experienced DIY electricians seeking added durability and reliability.
• In industrial or commercial settings where regulations and safety codes may mandate the use of metal boxes for added protection and durability.
• When the installation requires electromagnetic interference (EMI) shielding. Metal boxes can provide an effective barrier against interference in certain sensitive electronic or communication systems. 

When Can Both Metal and Plastic Boxes Be Used?

• Indoor residential wiring. Both metal and plastic boxes are suitable for typical indoor residential wiring projects where moisture and extreme conditions are not a concern.
• Low-voltage installations. In low-voltage applications such as data, telephone, or cable TV wiring, both types of boxes can be used.
• Non-conductive environments. When working in non-conductive environments like PVC conduit systems, both metal and plastic boxes are viable options.
• DIY electrical work. DIY electricians may choose either metal or plastic boxes for simpler projects that don't involve complex wiring configurations or specific code requirements.
• Interior wall installations. Both types are used in interior wall installations that don't require additional protection or grounding.
• Light fixture mounting. When mounting light fixtures or switches, both types of boxes can be used, depending on fixture weight and personal preference.
• Local building codes. Compliance with local building codes and regulations may dictate the use of either metal or plastic boxes in specific situations. 

Plastic vs. Metal Juction Boxes

Pros and Cons of a Metal Electrical Box

Pros of a Metal Electrical Box

• Durability. Metal electrical boxes are highly durable and can withstand physical impact and environmental stress better than plastic boxes. They provide long-lasting protection for electrical connections.
• Fire resistance. They are more fire-resistant because of their non-combustible nature. It reduces the risk of fires spreading through electrical systems.
• Grounding capability. Metal boxes can serve as effective grounding points for electrical circuits. It enhances safety by providing a path for fault currents.
• Security. Metal boxes provide a robust barrier against tampering and unauthorized access. They are suitable for secure or public installations.
• Versatility. They are compatible with various wiring methods and conduit systems, offering flexibility in wiring configurations.

Cons of a Metal Electrical Box

• Cost. Metal boxes are generally more expensive than plastic counterparts, which can impact project budgets.
• Weight. They are heavier than plastic boxes. It makes installation potentially more challenging, especially for ceiling-mounted fixtures.
• Corrosion. In humid or corrosive environments, metal boxes may rust over time.
• Conductive material. The metal construction makes them conductive, which can pose a risk if live wires come into contact with the box's interior. Proper grounding is essential to mitigate this risk.
• Heat transfer. Metal boxes can transfer heat from electrical components. It can affect sensitive devices or cause overheating in enclosed spaces.

Pros and Cons of a Plastic Electrical Box

Pros of a Plastic Electrical Box

• Cost-Effective. Plastic electrical boxes are generally more budget-friendly than metal ones. They are an economical choice for projects that are on budget. 
• Lightweight. They are easier to handle and install, especially in ceiling or wall-mounted applications.
• Non-conductive. Plastic is an insulating material, which means it does not conduct electricity. In this way, the risk of electrical shock is reduced. 
• Resistance to corrosion. Plastic boxes are immune to corrosion and suitable for use in damp or humid environments.
• Aesthetic options. They are available in various sizes and styles.

Cons of a Plastic Electrical Box

• Durability. Plastic boxes are less durable compared to metal ones and may not withstand physical impact or environmental stress as effectively.
• Limited fire resistance. Plastic is flammable and may not provide the same level of fire resistance as metal.
• Grounding challenges. Plastic boxes cannot serve as grounding points for electrical circuits. It requires additional grounding methods.
• Tampering risk. Plastic boxes may be more vulnerable to tampering or unauthorized access compared to metal ones.
• Limited versatility. They may not be suitable for all wiring methods or conduit systems. 

Sum Up

The choice between metal and plastic electrical boxes depends on the needs of the electrical installation. Metal boxes are durable, fire resistant, and offer grounding capability. They are suitable for demanding environments and critical applications. 

Plastic boxes offer cost-effectiveness, lightweight construction, and corrosion resistance. They are suitable for standard residential and low-voltage installations where durability and fire resistance are not primary concerns. Selecting the right box ensures electrical safety and functionality tailored to the project's unique requirements.