Everything that we do in lighting requires power, so you need to overcome any fear and understand power for the most wonderful gift it is. In order to help you understand what a wonderful gift power and electricity are I want to introduce you to the basics of electricity and those aspects that are important to lighting.
Electricity is the force that energizes all that we (lighting people) do. We will begin by defining a few terms related to electricity. The four terms we need to understand are Voltage, Amperage, Resistance, and Wattage.
We will use a water model to help define and understand these terms (See diagram). If we look at a water pipe as an example of an electrical wire, and in the example, the water pipe is hooked up to a water wheel (lamp), then we could call the friction that the wheel has while turning “the resistance to the flow of (be it water or electricity)”. All light bulbs present a certain resistance to the flow of electricity.
If we turn on or open a faucet, water starts to flow. This flow is just like bringing the level up on a dimmer or throwing a light switch. When you open the valve the water flows because of the pressure coming from the tank of water, the pressure of the water in the pipe is directly comparable to the voltage in an electrical circuit. As you bring up a dimmer you are increasing the voltage, just as when you open the valve and increase the flow the pressure increases to the maximum pressure in the tank. The pressure in an electrical circuit is voltage and the most common maximum voltages that you will deal with are 120, and 240. 120 is the common household outlet voltage in North America, 240 is found in all household breaker panels and used for large appliances. In Europe the standard household outlet is 240 volts.
The next term to consider is amperage. The size of the pipe carrying the water is the corollary for amperage. The larger the diameter of a pipe, the more water can flow. The larger the gauge, or diameter of a wire, the more current can flow. Fuses and circuit breakers are rated in amperage. They provide protection in an electrical circuit by sitting in line with the wire coming from the power company to the device being powered. When you try to draw more amperage through the circuit than the fuse or circuit breaker is rated for, the fuse or circuit breaker will “blow”. The larger the fuse or circuit breaker the more power you can draw, or for us, the more lighting fixtures we can turn on at one time.
In our water model we could measure the amount of water flowing past the wheel as it “spins” the wheel. The amount of water flowing as well as the pressure and rate of flow controls the speed of the spinning wheel. In the case of our lighting instrument, we control the power with our dimmer. The measurement of the amount of power flowing past the light bulb, our spinning wheel in the water model, is called wattage. In electrical terms we measure wattage, the amount of amperage or current flowing past a point at a certain voltage or pressure. We can define “Power” as the measurement of how much work is being done. In the case of the water model example, we would multiply the amount of water that flows by the pressure, or how much we opened the tap.
If the friction or resistance is a small number and we open the tap all the way then the amount of water will be large. If the friction is large then the amount of water flowing will drop down. However the resistance provided by the wheel is not a factor we are concerned about in lamps, this is a fixed value. What we are concerned with is how much amperage is being used by a lamp, not how much resistance it places in the circuit, this value changes as we add more lamps. In order to calculate the amperage we need to calculate this value based on information provided, to that end we need to go back in history.
In order to really understand electricity and power we need to discuss the work of Georg Simon Ohm who in 1827 published his theories about electricity which we now know as Ohm’s Law. Ohm worked out the mathematical relationship between the 4 properties of electricity.
Here are the 2 formulas that you will use most. See Ohms Law Equation Chart for all variations of the formula.
Wattage: The measure of the amount of power/work being done.
Amperage: The availability of power (controlled by the gauge of the wire and breaker or fuse rating.)
Voltage: The pressure of flow
Resistance: The friction of the light bulb or other device.
Wattage = W or P
Voltage = V
Amperage = A or I
Resistance = R
A = V / R Amperage = Volts divided by Resistance
W = V
A or P = I * E Wattage = Voltage times Amperage
The most important relationship for a lighting designer is; how much current is required to power a fixture. The formula is P (watts) = I (amps) x E (volts). In most situations we know what the wattage is and what the voltage is, in the United States the voltage is normally 120 volts. The wattage is found by looking at the lamp in the fixture. A little algebra work on the equation and the amperage is found by dividing the wattage by the voltage. Therefore a 1000 watt lamp divided by 120 volts draws 8.33 amps.
So now armed with this simple equation we can calculate exactly how many amps our lighting system will require in order to run. Now we need to talk about where we get the power to run this system. A good friend of mine loves to start discussions about dimming systems by talking about “the big wa-hrr” (that’s wire, as you might guess he’s from the south). All dimmers are rated in watts, meaning how many watts they can control. The big wire refers to how much power is available to feed the dimmer system. Dimmer systems come in a variety of sizes, the smallest for our purposes, is one 600 watt wall mount dimmer, the biggest single dimmer rack will handle 48 6000 watt dimmers or a total of 28800 watts, or 2400 amps at 120 volts, or 800 amps per phase in a 3 phase system.
No matter how big the wire you have, if you know how much power is available, the amperage of the circuit breaker or the fuses, you can design a lighting system that will fit within the limitations. One last important point, circuit breakers and their rating, if you have a small dimmer pack and are plugging it into a wall outlet look at the rating on the breaker that controls that outlet. The standards for these breakers are 15 and 20 amps. What you need to know is that the rating of 15 or 20; is for short time periods, over a long time the value is de-rated to 80% which means that you can only allow for 18 amps continuous draw, and continuous draw is the standard draw for most dimming applications. The same de-rating does not apply to the breakers on theatrical dimmer racks. So a small 4 dimmer pack with 4 500 watt lamps cannot be run at full on a 20 amp breaker.
So take the time to do the math and calculate how many lights you can run from the available power. Also remember you are in control of when they are drawing power, I once designed a show with 420 amps worth of lights in the air and only 300 amps of available power, we never did turn everything on at once.
If you have any questions or would like more clarification on your specific requirements, please send me an email,firstname.lastname@example.org, I am always looking for questions to include in this column, feel free to contact me.