For most outdoor electrical installs, GRP (glass reinforced plastic, or fibreglass) is the safest all-round choice. It does not rust, it does not conduct electricity and there is no paint to look after. Stainless steel earns its place where a specification or design calls for a metal enclosure. Painted steel can work on sheltered sites where someone will keep the coating in good order. Thermoplastic is best kept for small, light-duty boxes. The rest of this article explains why, so you can pick the right material before you order.
What an outdoor enclosure has to cope with
Indoors, almost any enclosure will do the job. Outside, the same box has to deal with rain, standing water, dust, frost, sunlight and the occasional knock from a van or a strimmer. It may sit there for years with nobody looking at it. So the questions that matter are simple:
- Will the material corrode or break down where it is sited?
- Does the body conduct, and does that change how you bond and earth it?
- How much maintenance will it need over its life, and who will do it?
- Is it big enough for the kit, with room for cabling and future changes?
- Can it be fixed down securely?
The options side by side
This is a general comparison of the materials. Performance varies by product, grade and finish, so always check the individual enclosure against your site.
| Material | Corrosion | Conductive body | Upkeep | Typical use |
|---|---|---|---|---|
| GRP | Does not rust | No | Low: clean it and check seals and hinges | Outdoor cabinets, from small housings to walk-in enclosures |
| Painted steel | Rusts once the coating is damaged | Yes | Touch up or repaint damaged coating | Sheltered or indoor sites |
| Stainless steel | Resists corrosion; depends on grade and environment | Yes | Cleaning to keep the finish | Where a metal enclosure is specified |
| Thermoplastic | Does not rust | No | Low, but check for sun and impact damage | Small, wall-mounted boxes |
GRP
GRP is a composite of glass fibre and resin. It does not rust, so a scratch or chip does not turn into a corrosion problem, and there is no coating to repaint. The body is non-conductive, so the enclosure itself is not a conductive path if a fault occurs inside. Your earthing arrangements still follow your installation design.
Our GRP cabinets and enclosures are IP55-rated, which means protected against dust and low-pressure water jets. They are manufactured in Wales using self-extinguishing fire-retardant resin, with a UV-stabilised gel coat, stainless steel hinges and bolt-down flanges. Every model comes in Grey or Green.
Painted steel
Painted or powder-coated steel is strong and familiar. The weak point outdoors is the coating. Once it is chipped or scratched, the steel underneath can start to rust, and on a remote site that may go unnoticed until it is a real problem. If you choose painted steel for an outdoor job, budget for inspections and touch-up work over its life. A steel body is also conductive, so it needs bonding and earthing in line with your design.
Stainless steel
Stainless steel resists corrosion well and suits jobs where the client, the design or the standard you work to calls for a metal enclosure. How well it holds up depends on the grade and the environment, so check the grade against the site before you specify it, particularly near the coast or around chemicals. Like painted steel, it is conductive and needs bonding and earthing.
Thermoplastic
Thermoplastic enclosures (moulded plastic boxes) are light, cheap to buy in small sizes and do not rust. They are good for small junction boxes and wall-mounted controls. They are usually less suited to large, free-standing outdoor housings. Sunlight and impact can affect some plastics over time, so check the product data for UV and impact performance before using one in full sun or at ground level where it may get knocked.
Questions to ask before you choose
- Does the specification name a material? If it calls for metal, that settles it. If not, you have a free choice.
- How exposed is the site? Coastal, damp and roadside locations are hard on painted steel. GRP does not rust.
- Who will maintain it? If the answer is nobody, pick a material that does not rely on a coating.
- How big is the kit? Small controls may fit a plastic box. Distribution gear, meters and controls usually need a proper floor-standing cabinet.
- How will it be fixed? A cabinet with bolt-down flanges gives you a secure fixing to a base or plinth.
Two GRP cabinets that cover a lot of outdoor jobs
If GRP fits your job, these two models cover a wide range of outdoor installs.
FS3 single door cabinet
The FS3 is the widest cabinet in our single door range at 935 x 460 x 1140 mm (W x D x H) and 18 kg. It gives you a good amount of room without stepping up to a double door unit. From £955 + VAT.
FD3 double door cabinet
The FD3 is a double door cabinet measuring 1510 x 615 x 1470 mm (W x D x H). The double door opening is 1350 x 1240 mm (W x H), so getting kit in and working on it is straightforward. It has an 18 mm ply backboard (1220 x 1220 mm) for mounting equipment and an integral 100 mm GRP base flange for fixing down. It weighs 75 kg. From £1,800 + VAT.
| Model | Doors | Size (mm) | Weight | Price |
|---|---|---|---|---|
| FS3 | Single | 935 x 460 x 1140 | 18 kg | From £955 + VAT |
| FD3 | Double | 1510 x 615 x 1470 | 75 kg | From £1,800 + VAT |
The short version
Outdoors, with no metal requirement in the specification, GRP is usually the practical answer: no rust, no conductive body and very little upkeep. Choose stainless steel when metal is specified, painted steel only where the coating will be looked after, and thermoplastic for small boxes. If you would like a hand matching a cabinet to your kit, browse the full range or send us your dimensions for a quote. Typically 48hr delivery on items we hold in stock.