Can you use metal electrical boxes outside?

Whether you can use metal electrical boxes outside depends on several factors. The main considerations are the type of metal, the weatherproof rating, and local electrical codes.

Types of Metal Electrical Boxes

The most common types of metal used for electrical boxes are:

• Steel – Most standard electrical boxes are made of steel. This is a durable and affordable option.
• Aluminum – More lightweight and corrosion-resistant than steel but more expensive.
• Stainless Steel – The most corrosion-resistant option but also the most expensive.
• Cast Iron – Very durable but very heavy and prone to rusting.

Steel and aluminum boxes are generally acceptable for outdoor use as long as they meet weatherproofing requirements. Stainless steel is ideal for coastal areas or other wet, salty environments. Cast iron should be avoided for outdoor applications because of its tendency to rust.

Weatherproof Ratings

For outdoor use, electrical boxes need to have weatherproof ratings to prevent moisture ingress. The main ratings are:

• Weatherproof (WP) – Protects against rain, snow, and sleet but not immersion or constant dampness.
• Rain Tight (RT) – Same protection as weatherproof but also seals when cover is open.
• Watertight (WT) – Prevents water intrusion even when immersed in water up to 1 meter for 30 minutes.

Weatherproof boxes are the minimum requirement for any outdoor installation. Rain tight offers extra protection for horizontal wall mounts subject to rain. Watertight boxes should be used in wet locations like irrigation boxes.

NEC Code Requirements

The National Electrical Code (NEC) has requirements for the use of metal boxes outdoors:

• Boxes must be corrosion-resistant or well-galvanized steel.
• Aluminum boxes require special caution markings.
• Boxes in direct soil contact must be listed for this use.
• Only raintight or wet location boxes within 10 feet of the ground.
• GFCI protection required for receptacles.

Always check with local inspectors to ensure compliance with electrical codes for your area. The use of non-metallic boxes may also be an option.

Conclusion

In summary, metal boxes can be safely used outside if the following guidelines are met:

• Made of suitable corrosion-resistant metals like stainless steel
• Have weatherproof, rain tight or watertight ratings
• Designed for in-ground use if buried
• Meet all applicable NEC requirements
• Approved for use by local inspectors

With the right metal and weatherproofing, metal electrical boxes can provide safe and durable outdoor electrical connections. Always follow electrical codes and use boxes listed for the environment to prevent hazards.

Frequently Asked Questions

What are the most common types of metal electrical boxes?

The most common types of metal electrical boxes are steel, aluminum, stainless steel, and cast iron. Steel and aluminum are the most widely used.

What weatherproof rating do you need for outdoor electrical boxes?

At minimum, outdoor electrical boxes should be weatherproof (WP) rated. Rain tight (RT) offers more protection. Watertight (WT) should be used in wet areas or where water immersion is possible.

Can you bury a metal electrical box directly in the ground?

Only electrical boxes listed for direct burial can be placed underground. Regular metal boxes will corrode over time when buried.

Do metal electrical boxes require special markings or labels?

Aluminum boxes require markings indicating they are suitable for wiring methods. Stainless steel boxes should be marked as SEALTITE to indicate watertight design.

What NEC code applies to outdoor electrical boxes?

Key NEC requirements include use of corrosion-resistant metals, raintight boxes within 10 feet of the ground, GFCI protection for receptacles, and boxes rated for in-ground use if buried.

Comparison of Metal Electrical Boxes for Outdoor Use

Box Material	Corrosion Resistance	Cost	Weight
Steel	Low	Low	High
Aluminum	Medium	Medium	Low
Stainless Steel	High	High	Medium
Cast Iron	Low	Low	Very High

This table compares different types of metal electrical boxes for outdoor use based on important factors like corrosion resistance, cost, and weight.

Outdoor Electrical Box Installation Tips

Follow these tips for properly installing outdoor electrical boxes:

• Use boxes listed for wet locations with rain tight connectors.
• Caulk around boxes to prevent water intrusion.
• Slope conduit downward to facilitate drainage.
• Use box covers listed for outdoor use.
• Keep outdoor boxes accessible – don’t seal or cover.
• Use extra caution when wiring aluminum boxes.
• Ground and bond boxes as required by code.

Paying attention to proper weatherproofing, drainage, and accessibility helps ensure an outdoor electrical box will hold up to the elements.

Outdoor Electrical Box Grounding Requirements

Metallic electrical boxes must be properly grounded to ensure safety. The NEC requires boxes to be grounded if:

• The box has any metal parts exposed to contact.
• Wiring methods include a ground, like NM cable.
• Grounded devices like receptacles are installed.
• The box is fed with a grounded conduit.

Common grounding methods include a grounding pigtail connected to the ground wire, attachment to grounded outlet boxes, or connecting to a ground rod if no other ground source is available.

Size of Ground Wires

The grounding wire should be sized based on the box’s largest overcurrent device, or:

• #14 AWG for 15A circuits
• #12 AWG for 20A circuits
• #10 AWG for 30A circuits
• #8 AWG for 40A circuits
• #6 AWG for 60A circuits

Corrosion Protection for Outdoor Metal Boxes

Outdoor metal boxes should include corrosion protection measures such as:

• Galvanizing – Zinc coating applied to steel for cathodic protection.
• Stainless steel – Alloys like 304 and 316 are resistant to rust.
• Anodized aluminum – Electrolytic coating prevents oxidation.
• Powder coating – Baked on paint/epoxy fusion coating.
• Watertight gaskets – Rubber gaskets seal out moisture.

Plastic boxes provide inherently better corrosion resistance than metal but may not offer the same strength or heat dissipation.

Choosing the Right Outdoor Electrical Box Location

Tips for selecting the optimal outdoor box location include:

• Mount vertically on exterior walls if possible.
• Avoid low points where water may collect and puddle.
• Select areas protected from severe weather like northeast corners.
• Place under eaves or overhangs for increased weather protection.
• Allow adequate clearance from windows, doors, and vents.
• Ensure convenient access for maintenance and repairs.

The ideal location will keep outdoor electrical boxes protected from rain and flooding while still allowing easy access when needed.

Outdoor Electrical Box Maintenance

Regular maintenance is required to keep outdoor metal boxes in good working order:

• Inspect for signs of corrosion, cracked seals, and moisture ingress.
• Test GFCIs and breakers for proper function.
• Check covers and gaskets for tight fit and pliability.
• Clear any dirt, leaves, or debris from box.
• Confirm connections are clean, dry, and secure.
• Touch up nicks and scratches with corrosion inhibiting paint.

Conducting periodic inspections and preventative maintenance extends the life of outdoor electrical boxes.

Preventing and Fixing Rust on Outdoor Metal Boxes

Strategies for preventing and dealing with rust on outdoor metal electrical boxes include:

• Use rust-inhibiting sprays and clear coats.
• Replace rusted screws with stainless steel versions.
• Remove rust with sandpaper, steel wool, or wire brushes.
• Touch up bare metal with cold galvanizing compound.
• Paint over rust spots with outdoor-rated enamel.
• Replace severely deteriorated boxes.

Addressing rust issues quickly prevents further corrosion damage and equipment failures down the line.

Safety Tips for Working on Outdoor Electrical Boxes

Follow these safety precautions when working on outdoor electrical boxes:

• Turn off power at the breaker before starting work.
• Verify power is off by testing with a non-contact voltage tester.
• Review electrical box labeling before making any connections.
• Use insulated tools and wear personal protective equipment.
• Never alter outdoor electrical boxes to compromise weatherproofing.
• Make sure all connectors and gaskets are in good condition.
• Replace any damaged electrical parts – don’t repair.

Exercising caution protects against electric shocks and other hazards when performing outdoor electrical box repairs and replacements.

Conclusion

Metal electrical boxes can definitely be used in outdoor environments as long as proper precautions are taken. Choosing suitable corrosion-resistant metals, maintaining weatherproof ratings, following electrical codes, practicing safe installations, and conducting preventative maintenance enables outdoor metal boxes to withstand the elements.

With informed material selection and careful installation, metal boxes provide a durable and cost-effective option for outdoor wiring. Just be sure to follow all guidelines related to required protection ratings, grounding, and moisture ingress prevention when utilizing metal electrical boxes on the exterior of buildings.