One of the foundational necessities of a successful gig is having adequate safe power. This can be a particular challenge at the Junior Varsity level. This article will be a high-level flyover, providing some basic insight into how the gear we use works. Future articles will go deeper.
What’s a Watt?
Power is measured in watts, and is the energy available to do work. The formula to calculate power in watts is P[SUB]watts[/SUB] = Voltage * Current. A 20A 120V circuit theoretically has 2400 watts of power available.
How much power do I have to work with?
All of the loads on a circuit count against the available power. For example, if a 20A 120V circuit – potentially 2400 watts of available power – powers a 300w light bulb, a 75w cash register, and a 1000w refrigerator, the consumption is 1375 watts, theoretically leaving 1025 watts available before the circuit is at maximum capacity. Exceeding this capacity will cause the circuit breaker to trip.
This is not an exact scenario. Most circuit breakers are “slow-blow”, meaning that a circuit breaker will allow more than its rated current to pass for a certain amount of time before enough heat builds up to trip the breaker.
A 20A breaker will pass 40A for up to 60 seconds before tripping, and 30A for up to 225 seconds before tripping. Manufacturing tolerances of the circuit breakers and other factors like ambient temperature in the breaker panel may alter the point at which the breaker trips, with higher ambient temperatures causing breakers to trip sooner and/or at a lower current than when temperatures are lower.
Continuous and Intermittent Power Consumption
The power consumption of a device can be broken down into the continuous component – the amount of power a device always draws whenever it is turned on, sometimes called idle current, and the intermittent component – additional energy required when the device is doing work, such as lighting a lamp, or amplifying a signal to be sent to a loudspeaker.
We saw above that the duration of the power consumption matters. If left long enough, most 20A circuit breakers will trip in the close vicinity of 20A, however we can actually draw significantly more than 20A on a circuit as long as we don’t draw it for very long. This behavior allows devices with high intermittent current requirements – greater than the circuit’s nameplate capacity – to still operate, as long as the amount and duration of that intermittent current does not exceed the breaker’s trip curve.
Power usage by device classification
Generally speaking, mixing consoles, signal processing equipment, backline instruments, and video projectors have almost no intermittent current requirement, so calculating the power required is as easy as reading the sticker on the back of the device.
Audio amplifiers and self-powered loudspeakers have a small continuous power requirement and a large intermittent power requirement when the amplifier is driven hard.
Lighting systems, including both tungsten and LED, have a small continuous power requirement to operate the control circuitry, and a comparatively large intermittent power requirement when the lamp is on.
Moving light fixtures with a discharge-type light source are somewhat similar to video projectors in that they have a high continuous power requirement, and a small intermittent power requirement when the fixture is moving. Tungsten-based moving light fixtures are similar to a tungsten dimmer system, as the lamp is the vast majority of a fixture’s power consumption.
This is the first half of the article. The second part is here: https://soundforums.net/entries/1192-Understanding-Power-Consumption-Part-2
Power is not strictly voltage x current for AC circuits. There is also a value called “power factor” that is used to convert between the volts and amps required for a device to function and the power converted by the device. For sizing circuits and generators, however, using the current (not the power) rating of the devices is the correct approach
[QUOTE=Rob Timmerman;bt1563]Power is not strictly voltage x current for AC circuits. There is also a value called “power factor” that is used to convert between the volts and amps required for a device to function and the power converted by the device. For sizing circuits and generators, however, using the current (not the power) rating of the devices is the correct approach[/QUOTE]
I hope to cover this in future writing. There’s only so much you can cover in a “high-level flyover”.
I will argue that I’ve not generally seen power factor actually calculated by a production company to determine circuit or generator sizing – that is not a trivial exercise to do with any granularity. Usually the upsizing required to prevent voltage sag for intermittent draws is sufficient to cover power factor issues.
As many devices are rated in watts, understanding where that number comes from is important. I’m not sure why calculating the current is any more accurate than calculating in power – neither intrinsically account for power factor.
[QUOTE][COLOR=#3E3E3E]As many devices are rated in watts, understanding where that number comes from is important. I’m not sure why calculating the current is any more accurate than calculating in power – neither intrinsically account for power factor.[/COLOR][/QUOTE]
TJ,
As Rob mentions, current is what matters here. True power, as result of the power factor, is academically interesting, but current is what is responsible for ohmic heating of all switchgear, bus, feeder, and branch circuits.
The current in each energized leg and/or the phasor sum of the legs in a neutral ultimately determines the ohmic heating and switchgear sizing, regardless how much of that current is in phase with the voltage and transferring power to the energized load.
[QUOTE=Phil Graham;bt1573]TJ,
As Rob mentions, current is what matters here. True power, as result of the power factor, is academically interesting, but current is what is responsible for ohmic heating of all switchgear, bus, feeder, and branch circuits.
The current in each energized leg and/or the phasor sum of the legs in a neutral ultimately determines the ohmic heating and switchgear sizing, regardless how much of that current is in phase with the voltage and transferring power to the energized load.[/QUOTE]
Phil,
Most of this article is written from the load side, where the power absolutely matters. Light bulbs, loudspeakers, and pretty much everything else are rated in wattage, as the current is voltage dependent, and not the whole story.
Take a 1000w light bulb. Ohms law tells us that will draw 8.3A if fed from a 120v supply. Is it therefore an 8.3A lightbulb? How about a low voltage lightbulb that draws 8.3A at 12v – is that equivalent to the bulb that draws 8.3A at 120v? Hardly.
I know what you are saying here, but you and Rob are getting ahead of me. This article isn’t about switchgear sizing; it is an intro to power consumption.