How to Run Electrical Wire Under Outdoor Decks

When you build a deck or a patio on your lawn, you will, of course, have to wire the thing itself or you may even have to run other electrical wires underneath it (except the main, of course). Regardless of what you’re wiring under, the wiring will eventually be exposed to water and other external elements.

There are several electrical wiring regulations that you need to follow when running electrical wire under outdoor decks. Each looks to minimize exposure to the environment and ensure safety for you and your family – as well as those who will have to conduct repair work on the same in the future.

The best way to run electrical wire under outdoor decks is to do it in a conduit and use an exterior-grade wire and box as the code requires. Furthermore, it is important that if the wiring is above-ground, it should be high enough that animals can’t nibble on it. In this article, we will take a close look at the intricacies involved in running electrical wire under outdoor decks.

Running Electrical Wires Under Outdoor Decks

Electrical wire spool

If we are to look at the code requirements for running electrical wires under outdoor decks, they offer specific guidelines for above and underground wiring. Under both circumstances, it is essential that you use external grade wiring and run it through conduits.

If the wiring is above ground, conduits will have to be fastened to the underside of your deck at least 1 foot high and must not be sagging at any point. It is essential that you fasten your wiring parallel to the joists and not your deck’s floor to eliminate any chance of a mishap.

Let’s look at these instructions in a bit more detail.

The Wire

We have discussed Romex wires quite a bit in our blog, but it is important to note here that you can’t just run Romex wire when running electrical wire under outdoor decks. Yes, it’s economical and is a nonmetallic wire, but Romex isn’t suitable for outside use.

When wiring, you will run a single wire per conduit, which is code requirement for all commercial buildings. Although you’re not building a commercial building, since the wire will be relatively exposed to the elements and animals, for your own safety, run just one wire per conduit.

When choosing the best wire for the task, make a point of choosing wires with a wet rating. When searching, you will find that wires will be rated as THHN or THWN. THHN means Thermoplastic High Heat-Resistant Nylon coated wire, while THWN means Thermoplastic Heat and Water-Resistant Nylon Coated. You need THWN wires for the job. There isn’t much of a cost difference, but there is a huge difference in utility.

The choice between solid or stranded wire is yours to make. Just remember that while stranded wire is easier to work with, it isn’t as easy to terminate (prepare the ends for connection). Non-metallic wires (NM) will work here and are actually used in many third world countries, but in the UK and US, using these is against code. Apart from code, if you are running NM wires, running them through a conduit is going to be a nightmare for you – not to mention how it will throw your calculations off.

Another problem you are likely to face is that NM wires aren’t labeled on the inside, thus introducing uncertainties. You might be able to install them correctly but the next person might not be so lucky.

We recommend investing in 3 rolls of wire for this. For an economical and lasting solution, we recommend that you invest in the 500 feet rolls. At the end, you will have some to spare as well; which a DIY-er such as yourself can always find use for later. The three rolls should be:

  1. Green/bare wires. This will be used for earthing.
  2. White/gray wire. Use this as your neutral. In Europe, Asia, and most of South America, white wire is used for neutral while in the US, gray wire is also used.
  3. Any other color you fancy. Again, Europe, Asia, and most of South America uses red while in the US there have been cases where we have seen a myriad of colors as the negative. Just remember to stick with the same color throughout your wiring.  

When running a wire for earthing, do so within a plastic conduit. Many recommend metal, but in our experience, it tends to rust pretty quickly in an outdoor environment – even if galvanized (which will present problems of its own).

The Conduit

A various electrical conduit installed outside
A various electrical conduit installed outside

Under no circumstances should you forget about the conduit when running an electrical wire under outdoor decks. Conduits are strictly required by code (and common sense, actually). Reasonable effort should be made to protect the conduit from damage. This means that if there is a damp/wet spot in your lawn or under the deck, try to work the conduit around it.

Furthermore, it should be high enough that animals or pests can’t nibble on it. We recommend going with a UF rated conduit and if the conduit is going to be exposed to sunlight, look for a UV stable one.

PVC conduits are known for their longevity and ability to offer safety to the wires inside and are therefore one of the best materials for conduits you can go with. Just make sure you don’t accidentally pour boiling water over the area. If you find any holes in the conduit, close them up, else the conduct can carry water along with the wire.

That is why you must use a wet-rate wire and box.

You can use PVC conduits for above- and below-ground conduits since they don’t rust, degrade, or deform. When running electrical wires above ground under outdoor decks, make a point of running and fastening it parallel to the joists.

Many will also recommend EMT since it is a much cheaper alternative compared to steel or PVC conduits but it isn’t as reliable as PVC, either.

Junction Boxes

When choosing the best junction box for running electrical wire under outdoor decks, you will have a lot of options to consider. There are PVC, metal, and even ceramic junction boxes to choose from.

One of the most commonly used metal junction boxes is made of aluminum. They will have a hole drilled into them for a ground screw and another to attach it. Don’t forget to attach a pigtail to the ground screw.

Ceramic junction boxes are the most cost-effective solution out there, but they are (in every sense of the word) cheap. We wouldn’t recommend using ceramic junction boxes under outdoor decks unless you are absolutely sure about what you’re doing.

PVC boxes present the most viable solution, but when buying, we recommend you go to an authorized dealer for the same – especially in Asia, South America, and the Middle East. This is because hardware stores don’t usually have all the parts necessary for proper installation. They are branded boxes which might not sit together properly.

You will need the right set of screws and wire fasteners (within the box) to ensure a tight connection. When installing, make sure you fasten the electrical wire after every 3 feet so that it doesn’t sway and put unnecessary pressure on the junction box.

Fastening

We mentioned in the previous section that you need to fasten the wire after every 3 feet so that the wire doesn’t wiggle around, sag, or allow animals to reach it. If you are using metal conduit or EMT, you can space your fasteners a bit more (no more than 5 feet, though).

If you are using heavier wire(s), you will need to invest in stronger fasteners – also called clips.

Regardless of whether you are using PVC, EMT, or full metal conduits, the best fasteners you can go with are made of metal. Just make sure the conduit isn’t wiggling too much, otherwise the metal fasteners can eat away the PVC.

Having said that, when it comes to fastening wiring, your building or municipality’s requirements will trump code requirements. You are supposed to follow whichever is stricter. For example, code doesn’t say anything about the connection between conduits and junction boxes, but many municipalities suggest that the entry and exit point of conduits through junction boxes must be adjacent to the structure’s walls.

On the other hand, there are some municipalities that actually allow you to bury electrical cable below deck without a conduit. However, it is important to note that not every type of wire can be buried and requirements vary from municipality to municipality. They suggest that UF cables or direct-bury cables are the only ones that can be buried.

Furthermore, when burying, these allowances dictate that the cable must be at least 18 inches below ground level – which is a hassle on its own.

And that’s all you need to know when running electrical wire under an outdoor deck; be it above- or underground. Keep these requirements in mind, be careful and have fun!

About Charlie D Paige

Charlie is a massive DIY fan, with dozens of DIY projects under his belt - ranging from tiling to electrics, and concrete pads to walls. Charlie loves tinkering, seeing how things works, the outdoors and playing with power tools... so is it any wonder that he's completed so many DIY jobs over the years?

Charlie loves spreading his hard-won DIY experience with the world via this blog.