Electricity Explained

Power Basics


It is helpful for growers who are using HID (High Intensity Discharge - i.e. Metal Halide and High Pressure Sodium) lighting to understand the basics of power. In this article we will also describe how a standard HID ballast (magnetic, or core & coil) works and what each component in the ballast does. First, we will describe some basic terminology:


  • Volts (V)
    In laymen’s terms a volt the difference in the electrostatic charge that exists between two points. It is this imbalance in the electrostatic charge that causes electrons to flow from one point to the next. Most households in the US operate on 110/120v while European countries typically use 220/240v.
  • Amps (A)
    An ampere or Amp is a unit of measure of the rate of current in an electrical conductor moving past a specific point in one second. Most circuit breakers in US households are rated for 15 - 20 amps.
  • Watts (W)
    A watt is a unit of power, equal to one joule per second. A watt is easily calculated by multiplying Volts (V) times Amps (A). For example, a lighting system that operates on 120v draws 5.5 amps of current; the total wattage of the lighting system is 120 x 5.5 = 660 watts.
  • Hertz
    Hertz is a unit of frequency - a cycle in alternating current of one cycle per second. In the United States, the common household electrical supply is at 60 hertz; this means that the current has 60 cycles per second; in Europe, line frequency is at 50 hertz.

120v vs 240v

Many people are misinformed about the method in which power companies charge for electricity usage. Many growers believe that if a ballast is rewired from 120v to 240v they will save up to 50% on the power bills. The reason for this is belief that households are charged by the amperage. This may be due to the fact that, on average, most households only have up to 200amps available and that each circuit breaker is rated in amps. The truth is that very little savings will be noted if growers changed their ballasts from 120v to 240v. Lighting systems that run on 240v will only require half the amperage but not half the wattage. The method for calculating this is: Amps x Volts = Watts.

For example: A lighting system that runs on 120v and draws 6 amps of current will consume 120v x 6 amps = 660w; this would typically be a 600w lighting system. If the same ballast was wired to 240v then the amperage would be 660w/240v = 2.75amps, but the wattage would still be the same.

Power companies do not charge by amperage or volts; they charge by kilowatt hours. For example one 1000w HID system being run for 1 hour will incur a cost of 1 kilowatt hour.

So then, why have 240v? There are many reasons for using 240v; however, the main reason for indoor gardeners has to do with amperage. If a grower is running 5 x 1000w lights at 120v which draws 9.5amps of current per lamp, then 5 separate 15 or 20amp circuit breakers would be required to run the 5 lighting systems. If the same grower was running their lights on 240v instead of 120v then three lights could be run on one 20 amp circuit.

There is another factor which makes 240v “better” than 120v. This has to do with voltage drop or the voltage lost due to resistance when power travels down a cable. The lower the resistance on the wire, the less the voltage drop. The shorter the distance between the power socket and the ballast, the lower the voltage drop. A thicker power cable will also reduce the voltage drop due to less resistance.

How do HID ballasts work in general?

HID lamps provide light from an electric discharge or arc and have a negative resistance characteristic that would cause them to draw excessive current, leading to instant lamp destruction if operated directly from line voltage. The ballast is a power supply for arc discharge lamps. Its purpose in HID lighting is to provide the proper starting voltage to initiate and maintain the lamp arc and to sustain and control lamp current once the arc is established.

HID lamps come in various type and wattage selections. Each lamp type and wattage requires specific starting and operating conditions to develop rated light output and operate the lamp within allowable limits. Ballasts and lamps are designed to meet standards for interchangeability between lamps and ballasts of the same type and wattage. A lamp must be operated by the ballast designed for that lamp, as improper matching of lamp and ballast may cause damage to the lamp or ballast, or both.

The magnetic ballast is an inductor consisting of copper coils assembled on a core - commonly referred to as a core & coil ballast. This assembly converts electrical power into a form appropriate to start and operate HID lamps. Ballasts for high pressure sodium lamps also include an igniter to start the lamp. The third major component is the capacitor, which improves the power factor, subsequently reducing line current draw, and also sets the lamp operating wattage.

Capacitors are a vital part of all core and coil type HID ballasts. The capacitor is responsible for maintaining power to the bulb once it has fired. It is also needed to regulate the power to the bulb; if there was no capacitor in the ballast the bulb would keep drawing more and more power until it finally burned out. Thus, the correct capacitors need to be selected for specific bulb wattages. There are two main types of capacitors on the market, wet caps and dry caps.

  • Wet Caps or Oil-Filled Capacitors
    Oil-Filled capacitors supplied today contain non-PCB oil and are a UL recognized component. Oil-filled capacitors are only supplied with ballasts where the capacitor operating voltage cannot be satisfied by Dry Film Capacitors. When required, the capacitor discharge resistor is connected across the capacitor terminals. The maximum case temperature for oil-filled capacitors is 90°C which makes it ideal for 150w – 400w lighting systems. These ballasts run cooler than that of the 600w and 1000w systems
  • Dry Caps or Dry Metalized Capacitors
    Dry Metalized Capacitors are available to fill almost all needs in HID ballast applications. Capacitor discharge resistors (when required) are installed within the capacitor case. Dry capacitors are UL recognized and contain no PCB material. The maximum allowed dry capacitor case temperature is 105°C which suits 600w and 1000w systems. These capacitors cost more but are well worth the extra expense. 
Igniters (or Starters)
An igniter is an electronic component that must be included in the circuitry of all high pressure sodium lighting systems. The igniter provides a pulse of at least 2500 volts or more to initiate the lamp arc. When the lighting system is energized, the igniter provides the required pulse until the lamp arc is established and automatically stops pulsing once the lamp has started. It also furnishes the pulse continuously when the lamp has failed or the socket is empty. Igniters are needed for all HPS lighting systems, but not for MH systems. The starting pulse of the igniter in an HPS can damage the internal components of an MH bulb.

Switchable Ballasts
Switchable ballasts save customers money, especially those customers who only need 1 or 2 lights. Instead of purchasing two lighting systems, MH and HPS, one switchable ballast can be used instead. The grower would normally start with an MH bulb for the vegetative stage of the crop. Once the crop is ready for flowering all the grower has to do is flip a switch on the ballast and change the MH bulb to an HPS bulb. The switch on the ballast isolates the igniter from the transformer and capacitor.

Important Safety Tips

  • If the exterior of the lamp cracks, punctures or breaks, turn off the unit immediately. The bulb may still work, but do not use it.
  • Allow bulbs to cool for 20 minutes before attempting to touch. Hot lamps may shatter or crack upon contact with water. Use caution when removing the lamps around water and when applying foliar sprays to plants in the area.
  • Do not remove or insert lamps while power is on.
  • Use a grounded timer and three-prong plugs when using timers with all HID fixtures.
  • Do not hang the fixture by its power cords. Use the supplied hanging hardware to hang the fixture.
  • Do not attempt to modify or rewire any lighting system. It may void the ballast warranty.
  • Do not open the light fixture housing. Any modifications should be done by an authorized service center or the manufacturer.
  • Do not touch the socket while the power is on - with or without a lamp inserted.
  • Do not use these light fixtures in wet areas. Water and electricity don’t go well together!
  • Do not energize lighting systems with voltage other than what the unit is made for. This is specified on the ballast or the packaging.
  • If using a remote ballast, make sure the lamp cord is plugged all the way into the female receptacle on the ballast.
  • Do not leave the lamp cord coiled while in operation.
  • Replace all power cords and lamps cords if the cable housing has been breached.
  • Keep ballasts elevated off the floor.
  • Do not attempt to use lamps other than specified lamps for the ballasts - higher or lower wattage, MH or HPS - this can lead to premature failure or damage to the lamps.
  • Use caution when disposing of used lamps, there are traces of Mercury and other harmful substances found inside the arc tube of lamps that need to be disposed of properly.
  • Lamps produce up to 4 BTU's per watt so use caution while handling them.
  • Capacitors can store a charge even while the transformers are off.