Demystifying VA (Volt Ampere) in Outdoor Lighting Systems

Understanding Power Dynamics and Design Considerations

Is VA a single static number? Not quite. In the world of outdoor lighting design, VA, or Volt-Ampere, is a dynamic measurement that fluctuates based on various factors. In this post, as we delve deeper, it becomes apparent that determining VA involves a nuanced process. Real power, measured in watts, and reactive power, known as VAR, come into play, with VA serving as a comprehensive measure of their combined effects. While the concept may seem straightforward, navigating the complexities of VA estimation and application requires a deeper understanding. Let's explore the intricacies of VA and its pivotal role in outdoor lighting design.

VA is a dynamic number that varies based on inputs. The base inputs are volts, ohms, and current. But to figure out current, you need to measure ohms (resistance) and voltage, then divide V by R. To get real power, you multiply voltage by the current (Watt (W) = Voltage (V) × Current (A)). Once you have Watts, you need to figure out the VAR (Volt Amp Reactive) these two numbers will equal VA (Volt Ampere) or apparent power.

How is VAR calculated? To do this, you need the Phase Angle measurement, and then you can calculate the VAR.  Then you can add Watts and VAR together to get VA. Super simple, right? Not really. Basically, there isn't an easy way of calculating the VAR in the field like you can for Watts. To do this, you need special equipment to measure phase angle, and it's not really practical for most installation crews, nor does it make much sense to spend time doing that. Most designers then depend on the manufactures to deliver the VA measurement.
To summarize, you need a number of mathematical formulas, a volt measurement, current measurement, and phase angle measurement to calculate the VA of a luminaire. VA is not a static number; not only does it change with volts (think multitap transformers or a luminaire's distance from the transformers), it changes with resistance (think wire long wire runs. An outdoor lighting system is a dynamic environment with many variables that happen at installation.

Can you correct for variables in resistance and volts? Yes, through proper planning, you can specify the length and number of runs, the number of luminaires per line, etc. The goal in using VA is to have the theoretical resistance and voltage set at every luminaire. As an example, say its 12VAC, you then specify wire gage, fixture number and number of fixtures per run to make sure 12VAC is delivered to every luminaire. This will allow you to use the VA given by a manufacturer in your calculations. Note: You do need to know what voltage is specified for a given VA to have an accurate theoretical estimates.

How do you use VA designing an outdoor system? Ultimately, any LED is going to pull Watts and VAR, with watts being the real power and VAR being power sent but not used to produce light. To accurately size a transformer, it's important to account for not only the power consumed by the luminaires but also for any losses in the distribution of energy in the system. VA is an easy number to use that accounts for what you're system pulling, so you can design a lighting system with safety in mind.

VA can be a valuable estimating tool for judging the ratio of luminaires to transformers in your lighting design, let's explore its significance. Essentially, VA combines both real power (Watts) and reactive power (VAR) into a single number, providing a holistic measure of the total power flow in the system when added together. Not long ago, in the era of halogen lighting, estimating luminaire to transformer ratios involved a simple yet effective method. For instance, if you had a 600W transformer, deducting 20% (80% remaining) accounted for losses in the system. Then, by dividing the remaining wattage by the wattage of your bulbs (say, a 35W halogen PAR36), you could determine the number of fixtures suitable for that transformer. It's worth noting that halogen bulbs typically had a power factor close to 1, meaning the estimated buffer primarily managed resistance in the system rather than reactive power (VAR).

Today, estimating an LED system, is where things get a little more complicated in estimation of the  luminaire to transformer ratio. Sometimes the assumption is that VA is a fixed number, but we now know resistance and voltage impact the VA at the luminaire. Many LEDs can operate at multiple voltages. For instance, Sterling Lighting's drivers can operate on AC or DC and anywhere from 9.5V to 22V. Additionally, many Sterling luminaires have adjustable drivers, which allows different watt values for the LED. Each voltage and each setting combination has a different VA. So to use VA in your hypothetical system planning, you need to know the voltage input a manufacturer is specifying when they give a VA value and then build your system to match that value.  This is one of the benefits of Sterling's Luminaire is they can operate across a wide range of input voltages, and wattage can be adjusted which makes your installation much easier.

What does variable VA numbers mean to you? Here is the deal... Every installer wants to design a safe system. They don't want to pull amperage so high that the transformer overheats, they don't want more current pulled through the lines than they are rated for, and they don't want to shorten the life of the LED by overpowering or underpowering the luminaire. VA is a measurement value that accounts for Watt and VAR but is only constant at consistent input voltage and resistance values. Your theoretical values are great to estimate the needs in a system you are designing, but they are theoretical. Just as any plan changes when in the field, every installer needs to test the results in the field. Be sure to check voltage at every fixture, and the current at the transformer for each line on every install. 

Hopefully this helps to demystify VA and how its used in outdoor lighting design. In the future we will detail the safety checks every install should go through to ensure a safe and reliable system. Stay tuned for that blog coming soon.