When to Use a Plastic or Metal Electrical Box

Choosing the right electrical box for your project is an important decision that requires careful consideration of several factors. Both plastic and metal boxes have advantages and disadvantages that make them better suited for certain applications. This comprehensive guide will examine when it is best to use a plastic electrical box versus a metal one.

Introduction

Electrical boxes, also known as junction boxes, are containers used to connect and house electrical wires and devices. They protect the connections from exposure, contain faults like shorts and overheating, and prevent shock and fire hazards.

Electrical boxes come in two main materials:

Plastic – Typically made of PVC or other polymers. Lightweight, corrossion-resistant, and inexpensive.
Metal – Usually made of galvanized steel. Sturdy, durable, and provides effective grounding.

There are several factors to weigh when deciding between plastic or metal for an electrical box in a project. Considerations include:

Environment – Is the box for indoor or outdoor use? Wet, damp, or dry conditions?
Electrical Load – How much electricity will pass through the box? Heavier loads require metal.
Physical Protection – Does the box need to be extra sturdy and withstand impacts?
Cost – Plastic is cheaper in most cases.
Grounding – Metal boxes provide an earth ground connection.

By evaluating these criteria against the specific demands of your electrical project, you can determine if a plastic or metal electrical box is more suitable.

This article will examine these factors in-depth and provide guidance on when a plastic or metal electrical box is the optimal choice.

Plastic Electrical Boxes

Plastic electrical boxes, including PVC and other polymeric boxes, have several inherent qualities that make them well-suited for many applications:

Non-Conductive

Plastic provides electrical insulation and isolation. This non-conductive property prevents arcs, shorts, and electrocution hazards. Plastic boxes are especially important for protecting sensitive electronics.

Resist Moisture

Plastic boxes resist water corrosion and moisture damage far better than metal. This makes them ideal for damp areas like basements or covered outdoor outlets.

Lightweight

Plastic boxes are much lighter than metallic boxes. This allows for easier transport and installation. Their light weight also makes them useful in mobile applications like RV electrical systems.

Cost-Effective

Plastic is inexpensive compared to metal. Plastic electrical boxes are an affordable option for many budgets.

Easy to Modify

It is simple to cut, drill, and reconfigure plastic boxes during or after installation. Metallic boxes are more challenging to alter once placed.

Chemical Resistance

Plastic boxes withstand exposure to salts, alkaline cleaners, oils, and other chemicals that can degrade or corrode metal. This suits them for industrial settings.

Limitations

While advantageous in many situations, plastic electrical boxes also have some drawbacks:

Not as sturdy or impact-resistant as steel boxes
Typically limited to lower voltage/amperage electrical loads
Does not provide an electrical ground connection
Can melt or deform when exposed to high heat
Prone to UV damage when used outside

When to Use Plastic

Given their strengths and limitations, some situations where plastic electrical boxes excel include:

Low voltage indoor wiring, like networking or speaker cables
Adding new outlets and switches in dry areas
Housing electronic controls and components
Temporary or portable power connections
Corrosive chemical environments
Areas that require electrical insulation
Outdoor fixtures and receptacles that are covered or shielded
Places where a lightweight box facilitates installation
Cost-driven projects where plastic saves substantially on material expense

Metal Electrical Boxes

Metal electrical boxes, typically made from galvanized or stainless steel, provide their own set of benefits for electrical systems:

Sturdy and Durable

Metal boxes are extremely strong, impact-resistant, and long-lasting. They stand up to knocks, drops, vibration, and abuse.

Fire Resistant

The non-combustible metallic construction is fireproof and contains arcs, sparks, and heat. This prevents electrical fires.

Heavy Duty Capacity

Metal safely accommodates higher voltage, amperage, and wattage loads. Plastic boxes are prone to melting under high wattage.

Provides Grounding

The conductive metal links the ground wire to the earth, giving a safe path for stray currents. Plastic cannot ground.

Secure Mounting

Metal boxes attach firmly to wall studs and surfaces. Installation is rigid and permanent. Plastic boxes require more fastener support.

Limitations

The disadvantages of metal electrical boxes include:

Corrodes when exposed to water or chemicals
Heavier and harder to install than plastic
Can be a shock hazard if shorted or improperly grounded
More expensive than plastic versions
Difficult to modify once installed
Conducts heat and cold readily

When to Use Metal

Here are some ideal applications for metal electrical boxes:

High wattage devices like large motors or commercial lighting
Connecting heavy gauge or high voltage wires
Wet, outdoor, or underground locations when properly sealed
Boxes supporting heavy fixtures like ceiling fans
Areas vulnerable to impacts like garages or workshops
Systems that require reliable grounding
Retrofitting old homes with ground wires
Situations wherepermanent, rigid installation is preferred
Highly vibrating environments like equipment or appliance controls

Indoor Electrical Boxes

When wiring a residential or commercial interior space, both plastic and metal boxes can be suitable options depending on the specific demands.

Plastic for Indoors

For most general indoor wiring, plastic boxes provide several advantages:

Lower Cost – Plastic is cheaper to manufacture and purchase, saving on construction budgets.
Lighter Weight – Easier to carry boxes up ladders and maneuver them into tight spaces during installation.
Easier Installation – Plastic boxes can be quickly cut, drilled, and positioned. Simple to attach outlets and switches.
Non-Conductive – Safely insulates basic household 120V circuits. Prevents inadvertent live wire contacts.
Chemical Resistance – Withstands exposure to cleaners, oils, mold, and other household chemicals that can degrade metal.

However, plastic does have limitations indoors:

Not as sturdy if mounting heavy fixtures
Avoid high amperage/wattage appliances or lighting
Provides no path to ground if accidentally energized
Easier to damage with impact

Metal for Indoors

Metal boxes provide extra durability, capacity, and safety for more demanding interior electrical systems:

Heavy Loads – Safely supports high wattage lighting, motors, heaters that exceed plastic ratings.
Secure Mounting – Firmly anchors and spreads weight of large fixtures across studs. Reduces sagging.
Fire Safety – Contains internal shorts, sparks, and heat.
Impact Resilience – Withstands crunches and collisions from objects.
Grounding – Provides critical ground path in the event of a short circuit or fault. Reduces shock risks.
Permanent Install – Rigid support for hardwired connections that won’t need changes.

The tradeoffs are that metal boxes are more expensive, heavier, and conduct electricity if energized accidentally.

Outdoor Electrical Boxes

Outdoor electrical systems face the additional challenges of moisture, ultraviolet exposure, temperature swings, and physical impacts.

Plastic for Outdoors

Plastic boxes can be successfully utilized outside provided proper precautions are taken:

Use Weatherproof Boxes – Must be rated for wet locations and sealed to prevent water intrusion.
Provide Shade – Avoid direct sun exposure which can degrade plastic over time. Install under eaves or shielded by fixtures.
Support Weight – Heavier fixtures require extra mounting support to avoid sagging and damage over time.
Limit Vibration – Plastic fatigue can lead to cracking. Use extra containment for vibrating equipment like pumps.
Prevent Impacts – Position boxes out of harm’s way. Use protective bollards for exposed locations.
Check Condition Periodically – Inspect plastic boxes for UV and water damage. Replace deteriorated boxes.

Plastic boxes work well for covered outlets, low-voltage landscape lighting, wiring in conduit, and protected soffit or eave lights.

Metal for Outdoors

For more extreme outdoor conditions, metal electrical boxes offer unbeatable durability:

Waterproof when Sealed – Avoid corrosion with gaskets and potting compounds. Stainless steel resists rust.
Withstand Sun Exposure – Avoid UV damage issues associated with plastic enclosures.
Supports Heavy Fixtures – Anchor weight of large exterior lights, equipment, and devices.
Vibration Resistant – Rigidly secures electrical connections despite movement from lawn equipment, pumps, and vehicles.
Heat Tolerance – Metal dissipates heat better than plastic which can droop or melt.
Impact Protection – Sturdy steel prevents crushed boxes from flying projectiles, vehicles, tree branches.
Permanent Installation – Boxes are long-lasting when correctly installed.

Metal boxes are ideal for pool equipment, pumps, outdoor kitchens, direct-burial applications, and other high-exposure areas.

Dry Locations

Inside buildings, electrical boxes in dry areas like upper floors, offices, and conditioned mechanical rooms are less prone to moisture exposure. Both plastic and metal boxes can work well in these locations.

Plastic in Dry Areas

Lower Cost – Savings on both material and installation labor.
Lighter Weight – Easier to carry and install multiple boxes.
Insulation Ability – Safely isolates wire connections.
Chemical Resistance – Withstands exposures like cleaning solutions or machinery lubricants.
Flexible Mounting – Adjustable, easy to position boxes to fit needs.

Metal in Dry Areas

Heavy Loads – Serves high wattage devices beyond plastic’s capacity.
Fire Safety – Contains internal shorts and sparks.
Secure Mounting – Anchors weight of large fixtures on ceilings and walls.
Grounding – Critical for fault and short circuit protection.
Permanent Install – Hardwired boxes that won’t need changes.

Both plastic and metal boxes can be successfully utilized in dry interior locations. Choose the right one based on the specific demands of the application.

Damp Locations

For areas prone to moisture like basements and bathrooms, the electrical box material requires careful choice.

Avoid Plastic in Wet Areas

While plastic boxes resist water well initially, over time moisture absorption can lead to:

Corrosion of wires, screws, and components
Deformation or cracking of the box
Mold growth
Loss of structural integrity
Electrical shorts

Plastic should only be used in damp locations for low-voltage wiring and with rigorous waterproofing measures.

Use Metal in Wet Areas

Metal boxes are inherently water-resistant as long as properly sealed and supplemented with appropriate precautions:

Conduit Seals – Use water-tight conduit fittings and sealant.
Gaskets – Rubber or foam gaskets around covers and openings keep moisture out.
Box Coatings – Apply waterproofing paints or potting compounds internally and externally.
Drain Holes – Provide weep holes for condensation drainage.
Elevate – Mount boxes on vertical walls or columns to avoid standing water.
Vapor Barriers – Install an external plastic membrane over the box to repel moisture.

With proper detailing for the conditions, metal electrical boxes reliably withstand damp environments.

High Electrical Loads

Heavy wattage lighting, large motors, industrial equipment, and similar high-draw electrical devices require an electrical box that can safely handle the demands.

Avoid Plastic for Big Loads

Plastic boxes have load limits based on material properties and melt temperatures. Typical capacity limits are:

1400 watts for receptacle outlets
800 watts for lighting fixtures
1 horsepower for motor loads

They cannot reliably support larger electrical loads over time. The box is prone to overheating, drooping, catching fire, and other modes of failure. Stick to lower voltage, lower amperage applications.

Use Metal for Big Loads

Metal boxes are designed to handle thousands of watts or multiple horsepower loads. For example:

Commercial lighting circuits can demand 20,000+ watts.
Large motors like elevators can draw 10+ horsepower.

With suitable gauge wiring and proper installation, metal boxes can operate safely at these elevated loads. The sturdy construction, noncombustible properties, and heat dissipation capabilities enable metal to withstand the significant electrical demands.

Always check manufacturers’ specifications and electrical codes for the confirmed ratings. But metal is inherently capable of managing higher wattage, current, and voltages than plastic alternatives.

Physical Protection

For electrical boxes installed in areas prone to physical impacts, crushed boxes, severed wires, and component damage can result. The box material choice factors into impact resilience.

Avoid Plastic in Impact Zones

Plastic boxes are easily broken by collisions with objects. Their brittle nature makes them prone to:

Cracking from surface impacts
Shattering if struck hard
Fragmenting when crushed
Permanent deformation from lower force blows

Plastic cannot withstand abuse like metal. One good smash can ruin the integrity of the box, requiring full replacement. Positioning plastic boxes safely away from harm is crucial.

Use Metal in Impact Zones

In contrast, sturdy metal electrical boxes resist damage from impacts:

Sheet metal construction withstands direct blows.
Metal boxes regain their shape after impacts without damage.
High-quality steel provides extreme impact toughness.
Edges and corners can be reinforced for additional strength.
Metal electrically bonds to ground, avoiding shocks if physically deformed.

Metal stands up to incidental contact as well as deliberate abuse. Boxes in garages, workshops, and industrial settings benefit from the ruggedness of metallic construction.

Grounding Ability

A key safety function electrical boxes provide is grounding. This gives stray electrical current a safe path to dissipate. Metal grounded boxes are critical for providing this protection.

Plastic Cannot Ground

Plastic’s electrical insulation properties prevent grounding.
No earth ground path exists through the non-conductive material.
Any short, fault current, or leakage remains “hot”.
If energized, the box can transmit dangerous voltage.

Metal Provides Grounding

The conductive metal directly connects to ground terminals.
Stray currents safely pass through the box to the earth.
Faults trip breakers instead of lingering as shock hazards.
A grounded box does not become energized if shorted.

In older homes, properly installed metal boxes can provide effective ground connections that plastic cannot. This significantly improves electrical safety.

Permanent Versus Temporary Use

Whether an electrical box needs to serve as a lasting rough-in or enable modifications down the road dictates optimal materials.

Plastic for Changes

Plastic boxes allow for alteration:

Simple to install initially as temporary power during construction.
Easy to relocate, add, or remove boxes as needs evolve.
Simple to cut, drill, and reconfigure.
Lightweight if accessing boxes in attics or walls.

Plastic suits electrical systems that anticipate adjustments, upgrades, and flexibility.

Metal for Permanence

Metal secures solid permanent power distribution:

Once installed correctly, metal boxes become part of the building structure.
Difficult to modify later without power tools.
Heavier weight deters tampering or theft.
Rigid support for hardwired connections.
Withstands long-term fixed installation.

Choose metal boxes when the electrical system design won’t need revisions after construction.

Cost Factors

Electrical box material costs can contribute significantly to overall electrical project budgets. In most cases, plastic boxes provide major savings over metal.

Plastic is Cheaper

Economical plastic resin materials, streamlined manufacturing, and commodity availability make plastic boxes highly affordable:

Blank PVC boxes under $1 when purchased in bulk.
Simple designs keep fabrication costs low.
Minimal metal components like screws and clamps.
Lightweight characteristics reduce shipping costs.
Common material avoids supply shortages.

Metal Carries Premium

The durability and performance of metal comes with extra production costs:

Galvanized steel sheet metal is more expensive.
Complex box designs increase manufacturing complexity.
Thicker metal uses more material.
Painting and coatings add cost.
Heavier weight increases shipping charges.
Can face material shortages and commodity volatility.

If keeping project budgets down is a necessity, plastic boxes can provide major savings. But for long-term value, metal excels.

Conclusion

For indoor low voltage wiring, plastic electrical boxes offer affordability, light weight, and safety.
Outdoor applications demand waterproof metal boxes that won’t degrade and can ground electrical systems.
Only metal provides the ruggedness to withstand heavy usage demands like high wattage and physical impacts.
Damp areas require meticulously sealed metal boxes to avoid moisture damage.
Consider costs, but don’t compromise safety – invest in metal boxes for demanding electrical loads.
Plan ahead whether the system will need changes. Plastic allows modifications, while metal secures permanent installations.

By considering the unique needs of each