Understanding voltage calculations for low voltage outdoor lighting projects.

I have read your 12 volt information pages on sizing low voltage wire and find them very helpful and informative. However, I need a layman's explanation to really understand the concept of amperage, wattage and voltage. Help!

You are not alone in your quest to understand AMPERAGE CALCULATIONS and CALCULATING WIRE SIZES for low voltage outdoor lighting systems. Most people and some electricians think just because it is low voltage that means you can use phone wire or speaker wire to hook up the lights. To the contrary, low voltage means higher amperage which translates into larger wire. Simply put, for low voltage lighting systems, the higher the voltage the smaller the wire and the lower the voltage the bigger wire you will need. Ultimately, the wire size is determined by the total wattage of the low voltage lighting system and the length of the wire to from the transformer to the lights! Thats why people get dim lights!

Here's are some simple examples of the math behind the concept:

100 watts / 12 volts = 8.33 amps
100 watts / 24 volts = 4.17 amps
100 watts /120 volts = .83 amps

12/2 cable is rated for 20 amps.
10/2 cable is rated for 30 amps.
8/2 cable is rated for 40 amps.

Now here's some Ohm's Law information to digest:

The electrical terms that go hand in hand with low voltage lighting systems are volt (for pressure), ampere (for volume), ohm (for resistance), and watt (for total power, volts times amperes). A current flow of one ampere at one volt is equal to one watt. A one hundred watt lamp for a 120 volt power source draws 0.83 amperes. For Ohm's Law, if the resistance is one ohm and there is one volt, then exactly one ampere will flow. If the same one volt power supply is used and there are two ohms of resistance, then one half ampere will flow. If per chance the power supply was too weak to provide one half ampere, the voltage will drop until some combination that obeys Ohm's Law is reached, for example you might end up measuring one quarter volt being put out by that "one volt" source providing one eighth of an ampere flowing through that two ohm resistance. The thinner the wire, the more resistance it has.

The cable lengths in your design will determine many further aspects of your lighting system. You want a cable wire that will deliver the best voltage to each individual fixture, and the best method for selecting this cable begins with a layout of your project. Be sure to include all landscape features, buildings, individual light fixtures, and power supplies in your drawing. Group your light fixtures into clusters of 2 to 6; do not form a group whose total wattages exceed 240 Watts. One cluster at a time, select a fixture - or a point between two fixtures - that is the most centralized and draw a line from the power supply to this point (your center point). From the center point, draw a line to the nearest fixture on the right, and do the same for the nearest fixture to the left. Repeat this process from the two newly connected fixtures until the entire cluster is connected. Click here to view a sample layout.

I am sorry this is not is not an easy subject to understand without considering the math. If you have any more questions regarding 12 volt lighting systems or calculating low voltage wire sizes, please give us a call at 800-239-2939. We have a great selection of LOW VOLTAGE LIGHTING PRODUCTS and we would be happy to help you design a system. Hope this helps!

Labels: Low Voltage Lighting, Voltage Calculations, Wire Sizing

posted by xinger @ 3:31 PM