Any discussion about water and electrical lighting really must include the all-important topic of safety. Just as devices such as electric motors and in-pool lights must be grounded and steel structures bonded, so too lighting around bodies of water be installed following basic safety guidelines.

Ensuring that lighting systems used near water are safe starts with the equipment, which means using 12-volt lighting systems. You can use 120-volt systems, which must be installed by a licensed electrician, but I believe and argue that for the vast majority of applications, 12-volt systems are the best choice because the lower voltage is inherently less hazardous. (LED systems have come on strong in a big way in recent years and can provide high levels of safety and performance, but only if you use the right equipment. That's a separate discussion we can cover in a future issue.)

The quality of the equipment is equally important. For years, I've also argued that the inexpensive plastic fixtures designed for the do-it-yourself market, sold mainly at a "box stores," are not suitable for landscape lighting of any kind and especially applications near water. These products are extremely susceptible to damage and in general have inferior service lives. Instead, you should always think in terms of using quality cast brass or other non-ferrous (non rusting) metal alloy fixtures such as copper or stainless steel that are designed to endure the rigors of outdoor applications.

This is also why wiring next to water should always be installed in conduit. In my work, all cable runs are in conduits and all connections are made with weathertight heavy-duty splice kits. Some installers will argue that direct burial cable is acceptable in some applications and certainly more affordable. When you factor in the added safety and durability of conduit systems, however, as well as the ease of replacing wiring or expanding systems, using conduit is the only way to go.

The bottom line is that you do not want any part of the system, the fixtures, transformers or wiring, to become submerged in water at any time. To be clear, that means that low-voltage garden lighting equipment should never be used in water that comes in contact with people. And you want the entire systems to be as resistant to damage as humanly possible.

The next layer of protection involves proper grounding of transformers. Individual low voltage fixtures do not need to be grounded, but all transformers must be grounded in accordance with both manufacturer recommendations and the National Electric Code. Also, every component of a lighting system that can potentially come in contact with water must be protected with ground-fault circuit interrupters (GFI).

In some instances pool-and-spa rated transformers must be used. Always make it a point to know the NEC and local safety codes while working around pools and spas for the safety of your client and for your own potential exposure to liability.

Finally, and equally important to all of the above, fixtures installed around water must be set back from the water's edge. Municipalities vary in their requirements, but as a matter of general practice, I observe a six-foot set back. Essentially the idea is that you don't want someone being able to touch a light fixture while part of his or her body is still in the water. It would be a highly improbably scenario, but in the event the system loses ground for some reason and there's 120 volt current flowing through the fixture itself, a person partially submerged who touches such a fixture could be seriously injured.

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Just to clarify, don't mix up the circuit of the self contained bulb with the circuit created when the applied voltage leaks into the pool water creating another circuit which of course you dont want to be swimming in.
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Just an FYI...12v Pool and Spa lights are NOT DC! They are 12VAC.
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There are 12VDC Pool and Spa lights, please check the specs on the lights that you are using.
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Scott is correct its the amperage thats dangerous,static electricity can have thousands of volts but almost
no amperage, it will not kill you.I bet you would not the leads of both terminals of your car battery at the same time. (please do not!!!!)
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12V is much safer than 120V. The current flowing through your body is depending on the voltage and the resistance of your body, so it will be 10 times less at 12V. DC is much safer than AC. Our body also acts as a capacitor. Capacitors block DC (up to a certain voltage) but let AC flow through. This means our body has a much higher resistance to DC than it has to AC. So to add both up: 12V DC is much much much safer than 120V AC.
Did anybody consider the hazard of splitting water into hydrogen and oxygen? If you have a defective system in which a current can flow through the water, above 1.25V water can be split into hydrogen and oxygen. Just put a 9V battery in a glas of water and you can see both gasses form in the form of bubbles floating to the surface. The mixture of hydrogen and oxygen is highly explosive. This can become dangerous especially if you cover your swimming pool.
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I have spent over 20 years in the field, and can recall on (2) different occasions when low voltage transformers/drivers went bad and put out line voltage.... (1) of those occasions, the transformer was supplying power to landscape lights located inside a pool enclosure. Fortunately my lights were installed with metal spikes and they grounded out tripping the circuit.
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I'm considering adding some 12v strip led lighting under the lip of the edge of my pool above the water line (About 20m of lights), I have purchased waterproof (silicon sealed) RGB strip lights which are 5M in length each and ill be joining them and silicon sealing the joints. A shielded extension cable will transfer the 12v current to the strips from a brick utility shed where the transformer (230v -> 12v) and the lighting control box will be mounted, it is also where the pump, SWG and filter reside. The whole power system is wired into the my house mains which has a residual-current circuit breaker. I just want to be sure that this is going to be safe as I have just DIY level knowledge of electrics. Any other advice you can offer. I have the control box but still need to purchase a good transformer that will be suitable for this type of installation. Can Anyone recommend any? Thanks
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NEC 411 & 680 PROHIBIT the installation of low voltage landscape lighting within 10 feet of the waters edge, regardless of how it is powered.

In 2015, we will see that changed. But it will require non-metallic components, bonding if metal, powered by underwater swimming pool lighting rated transformers (with isolated primary & secondary windings) and other requirements.

Swimming pool contractors are bound by all of the NEC, not just section 680.
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Mike is correct. 12 volts is way safer. The voltage drives or pushes the current through the circuit. If a swimmer is part of the circuit, this swimmer is a resistor and is resisting the or limiting the current flowing through the circuit. The higher the voltage across the circuit, the less effective the resistance of the swimmer in the circuit. The current required by the lamp is 10 times higher with 12 volt lights but it is the current passing through the swimmer that is what we need to be concerned with. The formula is E=IxR. Where E is the voltage pushing on the swimmer. R is the resistance of the swimmer. The resistance is fixed regardless of the voltage. Solve for the current passing thru the swimmer. If the resistance of the swimmer is 600 ohms and is exposed to 12 volts then .02 amp will flow through the swimmer stopping the heart. The same swimmer would need to have a resistance of 6000 ohms to limit the current in a 120 volt circuit. Water lowers the swimmers resistance. I hope this helps.
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Scott, if I had to jump into a pool that unknowingly had voltage leaking into it, I'd take the 12 volt current over the 120 volt alternative. Enough said.