AIEL Instructional
ALTERNATING CURRENT
ELECTRICAL POWER
Here is a short discussion of
alternating current (AC) power
and its cable as related to
stage lighting in Canada.
THE FOLLOWING MAY NOT BE REPRODUCED
WITHOUT PERMISSION FROM THE AUTHOR ©
With some exceptions(*), all non-transformered stage lights
in Canada use lamps that are rated and tested at 120 volts AC.
Always use this voltage in your calculations:
The Power Law: P = I x E
In electrical or physics terms this means: Power = Inductive Force times Electromotive Force. In more familiar language it means Power (Watts) = Current (Amperes) times Voltage. In very common terms, just remember: Watts = Amps x Volts. ( W = A x V )
So to determine what current is drawn by 2000 watts of lights, use simple algebra to rework the formula. Divide 2000 watts by 120 volts to get 16.67 amps. Thus, a 20 amp circuit is required if you intend to turn on this many lights at the same time. (A 20 amp circuit breaker is the next highest available above 16.67 amps.) Of course, you could have 10,000 watts of lights in your show, just don't turn on any more than 2400 watts, which is the maximum a 20 amp breaker can handle indefinitely. (20 amps x 120 volts = 2400 watts.)
Be sure the circuit selected is free from such things as a vending machine or an audio system. Otherwise, you will only be able to consume the available power left over after those items have consumed their power. So if your audio equipment draws 15 amps at maximum output, that leaves you with 5 amps for lights. 5 A x 120 V = 600 watts.
(*) Some stage lighting lamps are now rated at 115 volts.
These will draw more current and consume more power at
the higher 120 volts. As an example, the popular 575-watt,
115-volt lamps actually consume around 615 watts at 120
volts. Be sure to take this into consideration when calculating
total current draw if running these lamps at 120 volts.
Generally, extension cords must be used to plug in your lights. Select a smaller gauge cable for individual lights, while the larger gauges can power those small four-channel dimmer packs that mount next to the lights themselves.
Here is a chart to determine the size (wire gauge) required when using the common Cabtire type of cable to make up your cords:
Wire Gauge / Suggested
Number of Maximum Wattage
Conductors for up to a
30-Metre Run
18/3 750 W
16/3 1000 W
14/3 1500 W
12/3 2000 W
10/3 2500 W
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Please note that these ratings are for two current-carrying conductors within a common jacket. (The third is a ground lead.) The wattages have been derated from the maximum, as permitted by The Canadian Electrical Code, so as to allow for up to a 30-metre run. The chart shown below gives some main-power cable ratings: |
Wire Gauge / Suggested
Number of Maximum Wattage
Conductors for Each
Hot Conductor
8/4 4,000 W
6/4 6,000 W
#4 Single 12,000 W
#2 Single 24,000 W
For Continuous Usage Beyond a
15-Metre Run, Derate by 25%
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#8 and #6 cables in the table have three current-carrying conductors, with the fourth being a ground lead. They are contained within an outer covering. #4 and #2 above are rated as single conductor cables; however, loosely bundling 2, 3, or 4 current-carrying conductors together in open air for a main power feed does not change this as there is no heat-holding covering. Note that not all cable types of this gauge can handle the wattages listed. Insulation material must be considered. Consult manufacturers' specifications for their actual ratings. |
Cable Types: Use rubber covered cable such as Cabtire Type SJ for up to #12, from there up to #6, use Type S, and for large main power feed cables we suggest Tech cable or Coleflex/Ultraflex types. Do not use plastic covered cables such as lawn mower extension cords. The plastic covering splits under heavy use, the moulded ends become loose in short order (especially the ground prong at the male end), and orange or yellow cables running around the stage look tacky and are considered amateurish.
Be sure the Voltage Rating of your selected cable is above that of the voltage being used. Remember that in Canada, end-user panel feeds will have 208 or 240 volts between some conductors, so use 300 volt or higher ratings. Such ratings usually mean thicker insulation which also translates to more durability. Type SJ is rated at 300 volts while Type S is rated at 600 volts. Ultraflex is rated at 600 volts. Refer below for ideal voltages between conductors in Canada:
Conductor Voltage Voltage
Measurements Single Phase Three Phase
Hot to Hot: 240 V 208 V
Hot to Neutral: 120 V 120 V
Hot to Ground: 120 V 120 V
Neutral to Ground: 0 V 0 V
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As just mentioned, these are ideal voltages, but you are likely to see a higher or lower voltage in most places. It is best not to exceed any more than 5% higher or 10% lower for typical stage lighting equipment. Voltage between neutral and ground is not good. Because they are connected together at the service entrance, they should theoretically be at the same potential. Voltage present between the two is an indication of a poor connection of one or both, or of unequal, excessively long runs with a wire gauge that is too small for the intended purpose. In this situation, try to locate a better ground or neutral connection. |
If too small a gauge is used for the current being drawn, a drop of the voltage will be experienced along the cable run due to resistance in the conductors. This will result in dimmer lights, and possibly distorted audio if the latter is on the same cable. Also, heat will be generated, and over a period of time your cable insulation may break down, resulting in short circuits, melted connector housings, and a possible fire. So always determine your maximum current draw and use cable of an appropriate gauge.
For fast determination of typical cable gauges required to handle a given load, see the Electrical Considerations Tables, farther back. However, for those of you wishing to have more precise voltage drop numbers, the formula presented in this section will determine if the amount is excessive.
Generally, this desired amount should not exceed 5 to 10% lower than the specified consumer line voltage, which in Canada is 120 volts. (Some recommend not less than 3 to 5% lower when electric motors are involved. Most modern electronic equipment can compensate for a 10% loss, though.) Below that, oddities in operation will begin to occur, and farther below that point, the equipment will not function properly at all.
For example, cooling fans will begin to overheat as the voltage approaches, and then drops below, a certain threshold. Be aware that although electronic lighting boards and dimmers may be able to operate at 10% below rated voltage, stage lighting lamps will put out only 70% of their rated light at that voltage, so the stage will be dimmer than normal. It is best then, to keep the drop below 5%.
The following formula is for copper wire, Cabtire-style
cabling as used by lighting personnel. This will not
necessarily apply under all circumstances. The
factors shown also don't take AC reactance
into account, but do give an idea of what
to expect for a given wire gauge under
any load you might choose to attach.
VD = P ÷ V x CL x GF or where the current is known: VD = I x CL x GF
The first formula translates to: Voltage Drop = Power (Watts) divided by Volts times Cable Length (Metres) times the Gauge Factor. The second formula is used when the current draw (I) in Amps is already known. Regardless, think of the formula in common terms as Volts Dropped = Amps times Metres times Resistance.
The chart below gives the Gauge Factors to use for common cable sizes. They are rounded to 4 decimal places. Based only on resistance, these factors do not take AC line reactance into account. Examples using one of the factors follow the table.
Wire Gauge
Resistance Factors
18 .0418
16 .0264
14 .0166
12 .0104
10 .0066
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EXAMPLE #1:
A 1000-watt fixture is plugged into a 30-metre,
16-gauge extension cord at an initial 120 volts.
What is the voltage drop at the end of this cable?
* First, determine the current draw: Current = W ÷ V. So, 1000 ÷ 120 = 8.33 amps
* Next, retrieve the #16 Gauge Factor from the table -- .0264
* Now enter all into the formula: I x CL x GF = VD
8.33 x 30 x .0264 = ...
a drop of about 6.6 volts or 5.5%.
EXAMPLE #2:
A 2000-watt load is plugged into the same 30-metre,
16-gauge extension cord, again at 120 initial volts.
Now, what is the voltage drop at the end of this cable?
* As before, determine the current draw: Current = W ÷ V. So, 2000 ÷ 120 = 16.67 amps
* We already know the Gauge Factor of .0264 for #16 cable.
* Now enter all into the formula: I x CL x GF = VD
16.67 x 30 x .0264 = ...
a drop of 13.2 volts or 11%.
As you can see, 2000 watts of lights on a 16
gauge cable not only exceeds the recommendation
of the first table, but it is reenforced by the excessive
voltage drop just calculated. This cable will heat up fairly
quickly and will eventually cause problems. It is better to go
with at least a 14-gauge cable, but best to switch to 12 gauge,
which would work out to a voltage drop of only 5.2 volts, or 4.33%.
Please Realise: Actual results will vary depending on the purity of the copper in the wire, the condition of the conductors and connectors, the ambient temperature, and the wire temperature due to current flow and the heat-holding capacity of the cable covering. These calculations are meant only to give an idea to the user as to conditions that will be generated when using a specific cable.
Physical Testing: To best determine your particular situation and equipment, you may wish to set up a test bench with one accurate volt meter at each end of the cable and a selection of various-wattage loads. Stretch the cable out and attach a load. Precisely measure the voltage difference between the start and end of the run. Let the test sit for a while to see the difference that time and heating make. You might also try coiling the cable to see the difference made by concentrating the heat within a small area. For greater accuracy, add an AC current meter to monitor the actual amperage draw of the load under various conditions.
Try a variety of loads. Carefully note the results of each test, and of each variance of test parameters. Be aware that for large loads, the initial voltage feeding the test bench might become lowered. To lessen this possibility, use a high-current source. A voltage-compensating variable transformer might be employed if it can handle the currents involved. If neither is available, look at the starting and ending voltages and subtract the two. Even at lower voltages, the drop number will remain the same at a given current and wire resistance.
In closing, here are the standard Colour Codes for single-
and three-phase AC electrical purposes in Canada:
GREEN: Ground WHITE: Neutral BLACK: Hot RED: Hot BLUE: Hot |
Please remember that voltages, cable types, and
colour coding in this article are for Canada. Other
countries have their own standards which may differ.
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Alternating Current Electrical Power, Text Format
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