Should I Use a Metal or Plastic Junction Box?

It is tricky to choose a metal or plastic electrical box for your electrical connections without knowing your demands because selection depends on your kind of application. But don’t get confused! This article will clear all of your doubts related to plastic and metal junction boxes and you’ll be able to choose the best one for you. So keep learning!

Figure no 1 Metal and plastic junction box

1) Basic overview of metal and plastic junction boxes

“A junction box is an electrical box used to keep multiple wires in a manner that could prevent fatal electric shocks.”

• It has the following main two types with different features:

i) Metal junction box

ii) Plastic junction box

i) Metal junction box

“A metal junction box is a sturdy enclosure meant for the safety of wire connections and electrical systems against damage in residential, commercial or industrial electrical systems.”

It is made of metals like aluminium and steel that protect it against extreme weather conditions. This box is fire, water and dust resistant and is widely used in commercial and residential buildings. In the market, it is available in multiple sizes and shapes to meet the wiring needs. Moreover, it has several opening points from where wires and cables can pass and it is also easily accessible for maintenance.

Figure no 2 Metal junction box

ii)Plastic junction box

“A junction box which is made of plastic is a strong and thick, non-metallic box used to contain and insulate wiring and their connections for safety and thermal insulation in homes and businesses.”

It stays moisture and corrosion-free because of the plastic material so, it is advisable to use it in warm places such as the outdoors and bathrooms. Some of the light and rigid cost-effective materials used include polycarbonate, polypropylene, and acrylonitrile butadiene styrene among others. Consequently, it is useful for the simple self-service of replacement, modifications and repairs of electric wires and cables as well.

Figure no 3 Plastic junction box

2) Durability and Longevity: Metal junction box vs plastic

• Metal Junction Box: Metal junction boxes are more long-lasting as they have high melting points ( from 660°C to 1,538°C ) and are stronger than plastic boxes so, they can be used long-term.

• Plastic Junction Box: Plastic junction boxes are not so sturdy, so not suitable for outside areas. They are UV rays sensitive and can even melt ( having melting points ranging from 100°C to 350°C) in some situations, therefore less durable than metal junction boxes.

• Outcome:

So if you are going to install a junction box outside for some heavy-duty, then a metal junction box could prove best for you.

3) Corrosion-resistant: Metal vs Plastic electrical boxes

• Metal Junction Box: Metal boxes such as those made with galvanized steel or aluminium depend on protective coatings or rust-resistant materials and have some measure of resistance to corrosion. But with time, and in very dusty conditions metal boxes are corroded and this may be worse when the coating is missing.

Figure no 4 Rust on metal junction box

• Plastic Junction Box: Plastic junction boxes are resistant to corrosion, naturally. Because such materials as PVC or polycarbonate are used, they are also non-corrosive making them suitable for moisture, chemical or saline exposure.

• Outcome:

If you are going to install junction boxes in damp places, then go for plastic junction boxes or use aluminium junction boxes.

4) Fire-resistance: Plastic electrical boxes vs metal

• Metal Junction Box: For instance, metallic junction boxes, especially, made of steel or aluminium are impervious to fire. They remain intact and do not burn, even in a fire, thus giving an additional layer of safety in instances of electrical fire and heating.

• Plastic Junction Box: Plastic junction boxes are usually made flameproof to some extent. However, plastic is more likely to catch fire than metal. However, if plastic is subjected to extreme heat, it will melt.

• Outcome:

So, plastic junction boxes are best for places less prone to fire. And, metal junction boxes are best suited for high-risk areas.

5) Cost-effective: Plastic vs metal electrical boxes

• Metal Junction Box: metal junction boxes, such as aluminium and steel junction boxes, pose higher costs because of the raw material costs, manufacturing costs, and their overall heavier weight, which raises their overall shipping and installation costs.

• Plastic Junction Box: Plastic junction boxes can be constructed, transported, and installed at a lower cost than other junction boxes. They are easy to handle thus reducing labour and transportation costs thus making them economical for domestic and light commercial applications.

• Outcome:

It is more economical to use plastic junction boxes for ordinary, low-risk Interior installations. There are however circumstances where the metal junction boxes can be preferable for example extreme environments that need more physical strength, fire protection and or grounding in that case paying extra for metal junction boxes may be justified.

6) Conductivity: Metal outlet box vs Plastic 

• Metal Junction Box: Metals ( such as steel and aluminium) contain free electrons which can roam within the structure and are easily able to transfer current. In general, metals are good electrical conductors, because their conductivity lies between 106 to 108 S/m.

Figure no 5 Conductivity of metal junction box

• Plastic Junction Box: On the other hand, plastics consist of polymers that do not have ‘free electrons’, hence no current is conducted. The value of plastics electrical conductance is very small, less than 10-16 S/m. This characteristic of plastics is beneficial towards safety against electricity as insulation.

• Outcome:

Now its easy for you to choose a metal or plastic junction box, as metal junction boxes are recommended in settings where grounding attention is needed due to their high ability of electricity conduction but can cause shorting if care is not taken. Plastic junction boxes, on the other hand, being non-conductive materials provide better insulation and are more secure making them suitable for domestic and non-metallic wiring systems.

7) Applications of metal junction boxes

Due to many prominent features of metal junction boxes, they have a wide range of applications, like;

• Outdoor Use: As metal junction boxes are strong and sturdy, so, they can be used to safeguard electric wiring from harsh conditions outside like rain, storm, snow or even sunlight.

• Suitable for heavy Traffic Areas: You can use these boxes in heavy traffic areas, where there are high risks of electrical shocks for people. Therefore, it makes sure accidental safety to vehicles and passengers as well.

• Industrial Safety: You have to use metal junction boxes for the strong safety of electric-powered systems within factories. As they have to compete with extreme conditions like high temperatures and chemicals.

Figure no 6 Industrial safety

• High-Impact Areas: Machinery and construction sites often need a perfect system of wiring, so it could be the best option.

• High voltage installation: Where systems are carrying high currents, metal boxes can easily manage wires to minimize wire tangling.

8) Applications of plastic junction boxes

You can use plastic junction boxes in the following specific situations:

• Indoor Use: Plastic junction boxes are suitable for indoor use as they are weightless hence easily portable and are not too costly.

• Low Voltage Applications: Home lightning or low-power electronics don’t require heavy-duty protection. So, plastic boxes are suitable for them.

• Chemical Resistance: More so, for places where there may be little contact with chemicals, then these boxes would be sufficient.

9) Conclusion: Which one is better?

The selection of a plastic or metal electrical box totally depends upon the requirements. As we have discussed, plastic boxes are ideal for basic protection, low-voltage applications and DIY projects. While, metal boxes are preferred for tough environment, outdoor application, and high-voltage applications because of their high mechanical strength, moisture and impact resistance.

After considering all aspects, to get the strength and protection, metal is the way to go, whereas, if you want for cheap and easy solutions then you can opt for plastic junction boxes.